.NET: Add support for Subworkflows and many threading fixes (#1066)

* feat: Add support for Workflow-as-Executor

* Fixes routing of 'object' compile-typed variables to properly take in type information
* Fixes a concurrency issue in StepTracer

* fix: Make Subworkflow ExternalRequests work properly

* fix: Threading and Concurrency fixes; prep for OffThread Mode

* refactor: Remove dead code around OffStreamRunEventStream

Currently not used, and will be replaced with a rewrite when brought back, so having it in the change is not valuable.

* ci: Work around issues with dotnet-format not properly analyzing the source

* fix: Fix the logic of AsyncCoordinator and AsyncBarrier

* Prevent individual wait cancellations from canceling the entire barrier
* Propagate information about whether the wait was completed or cancelled, and whether any waiters were present when released

* fix: Remove superfluous acces to .Keys in InProcStepTracer

* refactor: Clean up AsyncCoordinator's use of AsyncBarrier

dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/RequestPortAction.cs

@@ -2,8 +2,8 @@
 
 namespace Microsoft.Agents.AI.Workflows.Declarative.Interpreter;
 
-internal sealed class InputPortAction(InputPort port) : IModeledAction
+internal sealed class RequestPortAction(RequestPort port) : IModeledAction
 {
     public string Id => port.Id;
-    public InputPort InputPort => port;
+    public RequestPort RequestPort => port;
 }

dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/WorkflowActionVisitor.cs

@@ -252,7 +252,7 @@ internal sealed class WorkflowActionVisitor : DialogActionVisitor
 
         // Define input action
         string inputId = QuestionExecutor.Steps.Input(actionId);
-        InputPortAction inputPort = new(InputPort.Create<InputRequest, InputResponse>(inputId));
+        RequestPortAction inputPort = new(RequestPort.Create<InputRequest, InputResponse>(inputId));
         this._workflowModel.AddNode(inputPort, parentId);
         this._workflowModel.AddLinkFromPeer(parentId, inputId);
 

dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/WorkflowModelBuilder.cs

@@ -27,7 +27,7 @@ internal sealed class WorkflowModelBuilder : IModelBuilder<Func<object?, bool>>
     private static ExecutorIsh GetExecutorIsh(IModeledAction action) =>
         action switch
         {
-            InputPortAction port => port.InputPort,
+            RequestPortAction port => port.RequestPort,
             Executor executor => executor,
             _ => throw new DeclarativeModelException($"Unsupported modeled action: {action.GetType().Name}.")
         };

dotnet/src/Microsoft.Agents.AI.Workflows/AsyncBarrier.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Workflows.Execution;
+
+namespace Microsoft.Agents.AI.Workflows;
+
+internal sealed class AsyncBarrier()
+{
+    private readonly InitLocked<TaskCompletionSource<object>> _completionSource = new();
+
+    public async ValueTask<bool> JoinAsync(CancellationToken cancellation = default)
+    {
+        this._completionSource.Init(() => new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously));
+        TaskCompletionSource<object> completionSource = this._completionSource.Get()!;
+
+        // Create a new completion source to track cancellation, because cancelling a single waiter's join
+        // should not cancel the entire barrier.
+        TaskCompletionSource<object> cancellationSource = new();
+
+        using CancellationTokenRegistration registration = cancellation.Register(() => cancellationSource.SetResult(new()));
+
+        await Task.WhenAny(completionSource.Task, cancellationSource.Task).ConfigureAwait(false);
+        return !cancellation.IsCancellationRequested;
+    }
+
+    public bool ReleaseBarrier()
+    {
+        // If there is no completion source, then there are no waiters.
+        return this._completionSource.Get()?.TrySetResult(new()) ?? false;
+    }
+}

dotnet/src/Microsoft.Agents.AI.Workflows/AsyncCoordinator.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.Agents.AI.Workflows;
+
+internal sealed class AsyncCoordinator
+{
+    private AsyncBarrier? _coordinationBarrier;
+
+    /// <summary>
+    /// Wait for the Coordination owner to mark the next coordination point, then continue execution.
+    /// </summary>
+    /// <param name="cancellation">A cancellation token that can be used to cancel the wait.</param>
+    /// <returns>
+    /// A task that represents the asynchronous operation. The task result is <see langword="true"/>
+    /// if the wait was completed; otherwise, for example, if the wait was cancelled, <see langword="false"/>.
+    /// </returns>
+    public async ValueTask<bool> WaitForCoordinationAsync(CancellationToken cancellation = default)
+    {
+        // There is a chance that we might get a stale barrier that is getting released if there is a
+        // release happening concurrently with this call. This is by design, and should be considered
+        // when using this class.
+        AsyncBarrier actualBarrier = this._coordinationBarrier
+                                  ?? Interlocked.CompareExchange(ref this._coordinationBarrier, new(), null)
+                                  ?? this._coordinationBarrier!; // Re-read after setting
+
+        return await actualBarrier.JoinAsync(cancellation).ConfigureAwait(false);
+    }
+
+    /// <summary>
+    /// Marks the coordination point and releases any waiting operations if a coordination barrier is present.
+    /// </summary>
+    /// <returns>true if a coordination barrier was released; otherwise, false.</returns>
+    public bool MarkCoordinationPoint()
+    {
+        AsyncBarrier? maybeBarrier = Interlocked.Exchange(ref this._coordinationBarrier, null);
+        return maybeBarrier?.ReleaseBarrier() ?? false;
+    }
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointed.cs

@@ -16,9 +16,9 @@ namespace Microsoft.Agents.AI.Workflows;
 /// <seealso cref="StreamingRun"/>
 public class Checkpointed<TRun>
 {
-    private readonly ICheckpointingRunner _runner;
+    private readonly ICheckpointingHandle _runner;
 
-    internal Checkpointed(TRun run, ICheckpointingRunner runner)
+    internal Checkpointed(TRun run, ICheckpointingHandle runner)
     {
         this.Run = Throw.IfNull(run);
         this._runner = Throw.IfNull(runner);
@@ -31,7 +31,7 @@ public class Checkpointed<TRun>
     /// <seealso cref="StreamingRun"/>
     public TRun Run { get; }
 
-    /// <inheritdoc cref="ICheckpointingRunner.Checkpoints"/>
+    /// <inheritdoc cref="ICheckpointingHandle.Checkpoints"/>
     public IReadOnlyList<CheckpointInfo> Checkpoints => this._runner.Checkpoints;
 
     /// <summary>
@@ -46,7 +46,7 @@ public class Checkpointed<TRun>
         }
     }
 
-    /// <inheritdoc cref="ICheckpointingRunner.RestoreCheckpointAsync"/>
+    /// <inheritdoc cref="ICheckpointingHandle.RestoreCheckpointAsync"/>
     public ValueTask RestoreCheckpointAsync(CheckpointInfo checkpointInfo, CancellationToken cancellationToken = default)
         => this._runner.RestoreCheckpointAsync(checkpointInfo, cancellationToken);
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/ICheckpointingHandle.cs

@@ -6,7 +6,7 @@ using System.Threading.Tasks;
 
 namespace Microsoft.Agents.AI.Workflows.Checkpointing;
 
-internal interface ICheckpointingRunner
+internal interface ICheckpointingHandle
 {
     // TODO: Convert this to a multi-timeline (e.g.: Live timeline + forks for orphaned checkpoints due to timetravel)
     IReadOnlyList<CheckpointInfo> Checkpoints { get; }

dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/InMemoryCheckpointManager.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
 using System.Threading.Tasks;
 
 namespace Microsoft.Agents.AI.Workflows.Checkpointing;
@@ -12,6 +14,14 @@ internal sealed class InMemoryCheckpointManager : ICheckpointManager
 {
     private readonly Dictionary<string, RunCheckpointCache<Checkpoint>> _store = [];
 
+    public InMemoryCheckpointManager() { }
+
+    [JsonConstructor]
+    internal InMemoryCheckpointManager(Dictionary<string, RunCheckpointCache<Checkpoint>> store)
+    {
+        this._store = store;
+    }
+
     private RunCheckpointCache<Checkpoint> GetRunStore(string runId)
     {
         if (!this._store.TryGetValue(runId, out RunCheckpointCache<Checkpoint>? runStore))
@@ -44,4 +54,9 @@ internal sealed class InMemoryCheckpointManager : ICheckpointManager
 
         return new(value);
     }
+
+    internal bool HasCheckpoints(string runId) => this.GetRunStore(runId).HasCheckpoints;
+
+    public bool TryGetLastCheckpoint(string runId, [NotNullWhen(true)] out CheckpointInfo? checkpoint)
+        => this.GetRunStore(runId).TryGetLastCheckpointInfo(out checkpoint);
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/RepresentationExtensions.cs

@@ -27,7 +27,7 @@ internal static class RepresentationExtensions
         };
     }
 
-    public static InputPortInfo ToPortInfo(this InputPort port)
+    public static RequestPortInfo ToPortInfo(this RequestPort port)
     {
         Throw.IfNull(port);
         return new(new TypeId(port.Request), new TypeId(port.Response), port.Id);
@@ -46,7 +46,7 @@ internal static class RepresentationExtensions
             keySelector: sourceId => sourceId,
             elementSelector: sourceId => workflow.Edges[sourceId].Select(ToEdgeInfo).ToList());
 
-        HashSet<InputPortInfo> inputPorts = new(workflow.Ports.Values.Select(ToPortInfo));
+        HashSet<RequestPortInfo> inputPorts = new(workflow.Ports.Values.Select(ToPortInfo));
 
         return new WorkflowInfo(executors, edges, inputPorts, inputType, workflow.StartExecutorId, workflow.OutputExecutors);
     }

dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/RequestPortInfo.cs

@@ -8,4 +8,4 @@ namespace Microsoft.Agents.AI.Workflows.Checkpointing;
 /// <param name="RequestType"></param>
 /// <param name="ResponseType"></param>
 /// <param name="PortId"></param>
-public record class InputPortInfo(TypeId RequestType, TypeId ResponseType, string PortId);
+public record class RequestPortInfo(TypeId RequestType, TypeId ResponseType, string PortId);

dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/RunCheckpointCache.cs

@@ -2,17 +2,28 @@
 
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using System.Text.Json.Serialization;
 
 namespace Microsoft.Agents.AI.Workflows.Checkpointing;
 
 internal sealed class RunCheckpointCache<TStoreObject>
 {
-    private readonly HashSet<CheckpointInfo> _checkpointIndex = [];
+    private readonly List<CheckpointInfo> _checkpointIndex = [];
     private readonly Dictionary<CheckpointInfo, TStoreObject> _cache = [];
 
+    public RunCheckpointCache() { }
+
+    [JsonConstructor]
+    internal RunCheckpointCache(List<CheckpointInfo> checkpointIndex, Dictionary<CheckpointInfo, TStoreObject> cache)
+    {
+        this._checkpointIndex = checkpointIndex;
+        this._cache = cache;
+    }
+
+    [JsonIgnore]
     public IEnumerable<CheckpointInfo> Index => this._checkpointIndex;
 
-    public bool IsInIndex(CheckpointInfo key) => this._checkpointIndex.Contains(key);
+    public bool IsInIndex(CheckpointInfo key) => this._cache.ContainsKey(key);
     public bool TryGet(CheckpointInfo key, [MaybeNullWhen(false)] out TStoreObject value) => this._cache.TryGetValue(key, out value);
 
     public CheckpointInfo Add(string runId, TStoreObject value)
@@ -29,12 +40,26 @@ internal sealed class RunCheckpointCache<TStoreObject>
 
     public bool Add(CheckpointInfo key, TStoreObject value)
     {
-        bool added = this._checkpointIndex.Add(key);
-        if (added)
+        if (this.IsInIndex(key))
         {
-            this._cache[key] = value;
+            return false;
         }
 
-        return added;
+        this._cache[key] = value;
+        this._checkpointIndex.Add(key);
+        return true;
+    }
+
+    [JsonIgnore]
+    public bool HasCheckpoints => this._checkpointIndex.Count > 0;
+    public bool TryGetLastCheckpointInfo([NotNullWhen(true)] out CheckpointInfo? checkpointInfo)
+    {
+        if (this.HasCheckpoints)
+        {
+            checkpointInfo = this._checkpointIndex[this._checkpointIndex.Count - 1];
+            return true;
+        }
+        checkpointInfo = default;
+        return false;
     }
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/WorkflowInfo.cs

@@ -13,14 +13,14 @@ internal sealed class WorkflowInfo
     internal WorkflowInfo(
         Dictionary<string, ExecutorInfo> executors,
         Dictionary<string, List<EdgeInfo>> edges,
-        HashSet<InputPortInfo> inputPorts,
+        HashSet<RequestPortInfo> requestPorts,
         TypeId? inputType,
         string startExecutorId,
         HashSet<string>? outputExecutorIds)
     {
         this.Executors = Throw.IfNull(executors);
         this.Edges = Throw.IfNull(edges);
-        this.InputPorts = Throw.IfNull(inputPorts);
+        this.RequestPorts = Throw.IfNull(requestPorts);
 
         this.InputType = inputType;
         this.StartExecutorId = Throw.IfNullOrEmpty(startExecutorId);
@@ -29,7 +29,7 @@ internal sealed class WorkflowInfo
 
     public Dictionary<string, ExecutorInfo> Executors { get; }
     public Dictionary<string, List<EdgeInfo>> Edges { get; }
-    public HashSet<InputPortInfo> InputPorts { get; }
+    public HashSet<RequestPortInfo> RequestPorts { get; }
 
     public TypeId? InputType { get; }
     public string StartExecutorId { get; }
@@ -73,9 +73,9 @@ internal sealed class WorkflowInfo
         }
 
         // Validate the input ports
-        if (workflow.Ports.Count != this.InputPorts.Count ||
-            this.InputPorts.Any(portInfo =>
-                !workflow.Ports.TryGetValue(portInfo.PortId, out InputPort? port) ||
+        if (workflow.Ports.Count != this.RequestPorts.Count ||
+            this.RequestPorts.Any(portInfo =>
+                !workflow.Ports.TryGetValue(portInfo.PortId, out RequestPort? port) ||
                 !portInfo.RequestType.IsMatch(port.Request) ||
                 !portInfo.ResponseType.IsMatch(port.Response)))
         {
@@ -94,9 +94,4 @@ internal sealed class WorkflowInfo
 
     public bool IsMatch<TInput>(Workflow<TInput> workflow) =>
         this.IsMatch(workflow as Workflow) && this.InputType?.IsMatch<TInput>() == true;
-
-    //public bool IsMatch<TInput, TResult>(WorkflowWithOutput<TInput, TResult> workflow)
-    //    => this.IsMatch(workflow as Workflow)
-    //       && this.OutputType?.IsMatch(typeof(TResult)) is true
-    //       && this.OutputCollectorId is not null && this.OutputCollectorId == workflow.OutputCollectorId;
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Config.cs

@@ -6,7 +6,7 @@ namespace Microsoft.Agents.AI.Workflows;
 /// Represents a configuration for an object with a string identifier. For example, <see cref="IIdentified"/> object.
 /// </summary>
 /// <param name="id">A unique identifier for the configurable object.</param>
-public class Config(string? id = null)
+public class Config(string id)
 {
     /// <summary>
     /// Gets a unique identifier for the configurable object.
@@ -14,16 +14,16 @@ public class Config(string? id = null)
     /// <remarks>
     /// If not provided, the configured object will generate its own identifier.
     /// </remarks>
-    public string? Id => id;
+    public string Id => id;
 }
 
 /// <summary>
 /// Represents a configuration for an object with a string identifier and options of type <typeparamref name="TOptions"/>.
 /// </summary>
 /// <typeparam name="TOptions">The type of options for the configurable object.</typeparam>
-/// <param name="options">The options for the configurable object.</param>
 /// <param name="id">A unique identifier for the configurable object.</param>
-public class Config<TOptions>(TOptions? options = default, string? id = null) : Config(id)
+/// <param name="options">The options for the configurable object.</param>
+public class Config<TOptions>(string id, TOptions? options = default) : Config(id)
 {
     /// <summary>
     /// Gets the options for the configured object.

dotnet/src/Microsoft.Agents.AI.Workflows/ConfigurationExtensions.cs

@@ -16,7 +16,7 @@ public static class ConfigurationExtensions
     /// <param name="configured">The existing configuration for the subject type to be upcast to its parent type. Cannot be null.</param>
     /// <returns>A new <see cref="Configured{TParent}"/> instance that applies the original configuration logic to the parent type.</returns>
     public static Configured<TParent> Super<TSubject, TParent>(this Configured<TSubject> configured) where TSubject : TParent
-        => new(async config => await configured.FactoryAsync(config).ConfigureAwait(false), configured.Id, configured.Raw);
+        => new(async (config, runId) => await configured.FactoryAsync(config, runId).ConfigureAwait(false), configured.Id, configured.Raw);
 
     /// <summary>
     /// Creates a new configuration that treats the subject as its base type, allowing configuration to be applied at

dotnet/src/Microsoft.Agents.AI.Workflows/Configured.cs

@@ -34,7 +34,7 @@ public static class Configured
                 throw new ArgumentException($"Provided ID '{id}' does not match subject's ID '{identified.Id}'.", nameof(id));
             }
 
-            return new Configured<TSubject>(_ => new(subject), id: identified.Id, raw: raw ?? subject);
+            return new Configured<TSubject>((_, __) => new(subject), id: identified.Id, raw: raw ?? subject);
         }
 
         if (id is null)
@@ -42,7 +42,7 @@ public static class Configured
             throw new ArgumentNullException(nameof(id), "ID must be provided when the subject does not implement IIdentified.");
         }
 
-        return new Configured<TSubject>(_ => new(subject), id, raw: raw ?? subject);
+        return new Configured<TSubject>((_, __) => new(subject), id, raw: raw ?? subject);
     }
 }
 
@@ -53,7 +53,7 @@ public static class Configured
 /// <param name="factoryAsync">A factory to intantiate the subject when desired.</param>
 /// <param name="id">The unique identifier for the configured subject.</param>
 /// <param name="raw"></param>
-public class Configured<TSubject>(Func<Config, ValueTask<TSubject>> factoryAsync, string id, object? raw = null)
+public class Configured<TSubject>(Func<Config, string, ValueTask<TSubject>> factoryAsync, string id, object? raw = null)
 {
     /// <summary>
     /// Gets the raw representation of the configured object, if any.
@@ -68,7 +68,7 @@ public class Configured<TSubject>(Func<Config, ValueTask<TSubject>> factoryAsync
     /// <summary>
     /// Gets the factory function to create an instance of <typeparamref name="TSubject"/> given a <see cref="Config"/>.
     /// </summary>
-    public Func<Config, ValueTask<TSubject>> FactoryAsync => factoryAsync;
+    public Func<Config, string, ValueTask<TSubject>> FactoryAsync => factoryAsync;
 
     /// <summary>
     /// The configuration for this configured instance.
@@ -79,7 +79,7 @@ public class Configured<TSubject>(Func<Config, ValueTask<TSubject>> factoryAsync
     /// Gets a "partially" applied factory function that only requires no parameters to create an instance of
     /// <typeparamref name="TSubject"/> with the provided <see cref="Configuration"/> instance.
     /// </summary>
-    internal Func<ValueTask<TSubject>> BoundFactoryAsync => () => this.FactoryAsync(this.Configuration);
+    internal Func<string, ValueTask<TSubject>> BoundFactoryAsync => (runId) => this.FactoryAsync(this.Configuration, runId);
 }
 
 /// <summary>
@@ -91,7 +91,7 @@ public class Configured<TSubject>(Func<Config, ValueTask<TSubject>> factoryAsync
 /// <param name="id">The unique identifier for the configured subject.</param>
 /// <param name="options">Additional configuration options for the subject.</param>
 /// <param name="raw"></param>
-public class Configured<TSubject, TOptions>(Func<Config<TOptions>, ValueTask<TSubject>> factoryAsync, string id, TOptions? options = default, object? raw = null)
+public class Configured<TSubject, TOptions>(Func<Config<TOptions>, string, ValueTask<TSubject>> factoryAsync, string id, TOptions? options = default, object? raw = null)
 {
     /// <summary>
     /// The raw representation of the configured object, if any.
@@ -111,31 +111,31 @@ public class Configured<TSubject, TOptions>(Func<Config<TOptions>, ValueTask<TSu
     /// <summary>
     /// Gets the factory function to create an instance of <typeparamref name="TSubject"/> given a <see cref="Config{TOptions}"/>.
     /// </summary>
-    public Func<Config<TOptions>, ValueTask<TSubject>> FactoryAsync => factoryAsync;
+    public Func<Config<TOptions>, string, ValueTask<TSubject>> FactoryAsync => factoryAsync;
 
     /// <summary>
     /// The configuration for this configured instance.
     /// </summary>
-    public Config<TOptions> Configuration => new(this.Options, this.Id);
+    public Config<TOptions> Configuration => new(this.Id, this.Options);
 
     /// <summary>
     /// Gets a "partially" applied factory function that only requires no parameters to create an instance of
     /// <typeparamref name="TSubject"/> with the provided <see cref="Configuration"/> instance.
     /// </summary>
-    internal Func<ValueTask<TSubject>> BoundFactoryAsync => () => this.CreateValidatingMemoizedFactory()(this.Configuration);
+    internal Func<string, ValueTask<TSubject>> BoundFactoryAsync => (runId) => this.CreateValidatingMemoizedFactory()(this.Configuration, runId);
 
-    private Func<Config, ValueTask<TSubject>> CreateValidatingMemoizedFactory()
+    private Func<Config, string, ValueTask<TSubject>> CreateValidatingMemoizedFactory()
     {
         return FactoryAsync;
 
-        async ValueTask<TSubject> FactoryAsync(Config configuration)
+        async ValueTask<TSubject> FactoryAsync(Config configuration, string runId)
         {
             if (this.Id != configuration.Id)
             {
                 throw new InvalidOperationException($"Requested instance ID '{configuration.Id}' does not match configured ID '{this.Id}'.");
             }
 
-            TSubject subject = await this.FactoryAsync(this.Configuration).ConfigureAwait(false);
+            TSubject subject = await this.FactoryAsync(this.Configuration, runId).ConfigureAwait(false);
 
             if (this.Id is not null && subject is IIdentified identified && identified.Id != this.Id)
             {

dotnet/src/Microsoft.Agents.AI.Workflows/Edge.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System.Diagnostics;
 using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Agents.AI.Workflows;
@@ -32,6 +33,7 @@ public enum EdgeKind
 /// additional information relevant to the edge, and its concrete type depends on the value of <see
 /// cref="Kind"/>, functioning as a tagged union.
 /// </remarks>
+[DebuggerDisplay("[{Data.Id}]: {Kind}Edge({Data.Connection})")]
 public sealed class Edge
 {
     /// <summary>

dotnet/src/Microsoft.Agents.AI.Workflows/EdgeId.cs

@@ -56,4 +56,7 @@ public readonly struct EdgeId : IEquatable<EdgeId>
 
     /// <inheritdoc />
     public static bool operator !=(EdgeId left, EdgeId right) => !left.Equals(right);
+
+    /// <inheritdoc />
+    public override string ToString() => this.EdgeIndex.ToString();
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/AsyncRunHandle.cs

@@ -0,0 +1,157 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Workflows.Checkpointing;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal sealed class AsyncRunHandle : ICheckpointingHandle, IAsyncDisposable, IInputCoordinator
+{
+    private readonly AsyncCoordinator _waitForResponseCoordinator = new();
+    private readonly ISuperStepRunner _stepRunner;
+    private readonly ICheckpointingHandle _checkpointingHandle;
+
+    private readonly LockstepRunEventStream _eventStream;
+    private readonly CancellationTokenSource _endRunSource = new();
+    private int _isDisposed;
+    private int _isEventStreamTaken;
+
+    internal AsyncRunHandle(ISuperStepRunner stepRunner, ICheckpointingHandle checkpointingHandle, ExecutionMode mode)
+    {
+        this._stepRunner = Throw.IfNull(stepRunner);
+        this._checkpointingHandle = Throw.IfNull(checkpointingHandle);
+
+        this._eventStream = mode switch
+        {
+            //ExecutionMode.OffThread => Not supported yet
+            ExecutionMode.Lockstep => new LockstepRunEventStream(stepRunner),
+            _ => throw new ArgumentOutOfRangeException(nameof(mode), $"Unknown execution mode {mode}")
+        };
+        this._eventStream.Start();
+    }
+
+    public ValueTask<bool> WaitForNextInputAsync(CancellationToken cancellation = default)
+        => this._waitForResponseCoordinator.WaitForCoordinationAsync(cancellation);
+
+    public void ReleaseResponseWaiter() => this._waitForResponseCoordinator.MarkCoordinationPoint();
+
+    public string RunId => this._stepRunner.RunId;
+
+    public IReadOnlyList<CheckpointInfo> Checkpoints => this._checkpointingHandle.Checkpoints;
+
+    public ValueTask<RunStatus> GetStatusAsync(CancellationToken cancellation = default)
+        => this._eventStream.GetStatusAsync(cancellation);
+
+    public async IAsyncEnumerable<WorkflowEvent> TakeEventStreamAsync(bool breakOnHalt, [EnumeratorCancellation] CancellationToken cancellation = default)
+    {
+        // Create a linked cancellation token that combines the provided token with the end-run token
+        using var linkedSource = CancellationTokenSource.CreateLinkedTokenSource(cancellation, this._endRunSource.Token);
+
+        // Only one enumerator of this is allowed at a time
+        if (Interlocked.CompareExchange(ref this._isEventStreamTaken, 1, 0) != 0)
+        {
+            throw new InvalidOperationException("The event stream has already been taken. Only one enumerator is allowed at a time.");
+        }
+
+        try
+        {
+            await foreach (WorkflowEvent @event in this._eventStream.TakeEventStreamAsync(linkedSource.Token)
+                                                                    .ConfigureAwait(false))
+            {
+                yield return @event;
+            }
+        }
+        finally
+        {
+            Volatile.Write(ref this._isEventStreamTaken, 0);
+        }
+    }
+
+    public ValueTask<bool> IsValidInputTypeAsync<T>(CancellationToken cancellation = default)
+        => this._stepRunner.IsValidInputTypeAsync<T>(cancellation);
+
+    public async ValueTask<bool> EnqueueMessageAsync<T>(T message, CancellationToken cancellation = default)
+    {
+        if (message is ExternalResponse response)
+        {
+            // EnqueueResponseAsync marks the coordination point itself
+            await this.EnqueueResponseAsync(response, cancellation)
+                      .ConfigureAwait(false);
+
+            return true;
+        }
+
+        bool result = await this._stepRunner.EnqueueMessageAsync(message, cancellation)
+                                            .ConfigureAwait(false);
+
+        this._waitForResponseCoordinator.MarkCoordinationPoint();
+
+        return result;
+    }
+
+    public async ValueTask<bool> EnqueueMessageUntypedAsync([NotNull] object message, Type? declaredType = null, CancellationToken cancellation = default)
+    {
+        if (declaredType?.IsInstanceOfType(message) == false)
+        {
+            throw new ArgumentException($"Message is not of the declared type {declaredType}. Actual type: {message.GetType()}", nameof(message));
+        }
+
+        if (declaredType != null && typeof(ExternalResponse).IsAssignableFrom(declaredType))
+        {
+            // EnqueueResponseAsync marks the coordination point itself
+            await this.EnqueueResponseAsync((ExternalResponse)message, cancellation)
+                      .ConfigureAwait(false);
+
+            return true;
+        }
+        else if (declaredType == null && message is ExternalResponse response)
+        {
+            // EnqueueResponseAsync marks the coordination point itself
+            await this.EnqueueResponseAsync(response, cancellation)
+                      .ConfigureAwait(false);
+
+            return true;
+        }
+
+        bool result = await this._stepRunner.EnqueueMessageUntypedAsync(message, declaredType ?? message.GetType(), cancellation)
+                                            .ConfigureAwait(false);
+
+        this._waitForResponseCoordinator.MarkCoordinationPoint();
+
+        return result;
+    }
+
+    public async ValueTask EnqueueResponseAsync(ExternalResponse response, CancellationToken cancellation = default)
+    {
+        await this._stepRunner.EnqueueResponseAsync(response, cancellation).ConfigureAwait(false);
+
+        this._waitForResponseCoordinator.MarkCoordinationPoint();
+    }
+
+    public ValueTask RequestEndRunAsync()
+    {
+        this._endRunSource.Cancel();
+        return this._stepRunner.RequestEndRunAsync();
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (Interlocked.Exchange(ref this._isDisposed, 1) == 0)
+        {
+            this._endRunSource.Cancel();
+            await this.RequestEndRunAsync().ConfigureAwait(false);
+            this._endRunSource.Dispose();
+
+            await this._eventStream.DisposeAsync().ConfigureAwait(false);
+        }
+    }
+
+    public ValueTask RestoreCheckpointAsync(CheckpointInfo checkpointInfo, CancellationToken cancellationToken = default)
+        => this._checkpointingHandle.RestoreCheckpointAsync(checkpointInfo, cancellationToken);
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/AsyncRunHandleExtensions.cs

@@ -0,0 +1,46 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal static class AsyncRunHandleExtensions
+{
+    public async static ValueTask<Checkpointed<TRunType>> WithCheckpointingAsync<TRunType>(this AsyncRunHandle runHandle, Func<ValueTask<TRunType>> prepareFunc)
+    {
+        TRunType run = await prepareFunc().ConfigureAwait(false);
+        return new Checkpointed<TRunType>(run, runHandle);
+    }
+
+    public static async ValueTask<StreamingRun> EnqueueAndStreamAsync<TInput>(this AsyncRunHandle runHandle, TInput input, CancellationToken cancellation = default)
+    {
+        await runHandle.EnqueueMessageAsync(input, cancellation).ConfigureAwait(false);
+        return new(runHandle);
+    }
+
+    public static async ValueTask<StreamingRun> EnqueueUntypedAndStreamAsync(this AsyncRunHandle runHandle, object input, CancellationToken cancellation = default)
+    {
+        await runHandle.EnqueueMessageUntypedAsync(input, cancellation: cancellation).ConfigureAwait(false);
+        return new(runHandle);
+    }
+
+    public static async ValueTask<Run> EnqueueAndRunAsync<TInput>(this AsyncRunHandle runHandle, TInput input, CancellationToken cancellation = default)
+    {
+        await runHandle.EnqueueMessageAsync(input, cancellation).ConfigureAwait(false);
+        Run run = new(runHandle);
+
+        await run.RunToNextHaltAsync(cancellation).ConfigureAwait(false);
+        return run;
+    }
+
+    public static async ValueTask<Run> EnqueueUntypedAndRunAsync(this AsyncRunHandle runHandle, object input, CancellationToken cancellation = default)
+    {
+        await runHandle.EnqueueMessageUntypedAsync(input, cancellation: cancellation).ConfigureAwait(false);
+        Run run = new(runHandle);
+
+        await run.RunToNextHaltAsync(cancellation).ConfigureAwait(false);
+        return run;
+    }
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/ConcurrentEventSink.cs

@@ -0,0 +1,22 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Threading.Tasks;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal interface IEventSink
+{
+    ValueTask EnqueueAsync(WorkflowEvent workflowEvent);
+}
+
+internal class ConcurrentEventSink : IEventSink
+{
+    public ValueTask EnqueueAsync(WorkflowEvent workflowEvent)
+    {
+        return this.EventRaised?.Invoke(this, Throw.IfNull(workflowEvent)) ?? default;
+    }
+
+    public event Func<object?, WorkflowEvent, ValueTask>? EventRaised;
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/DeliveryMapping.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.Shared.Diagnostics;
@@ -29,7 +30,11 @@ internal sealed class DeliveryMapping
     {
         foreach (Executor target in this._targets)
         {
-            nextStep.MessagesFor(target.Id).AddRange(this._envelopes.Select(envelope => envelope));
+            ConcurrentQueue<MessageEnvelope> messageQueue = nextStep.MessagesFor(target.Id);
+            foreach (MessageEnvelope envelope in this._envelopes)
+            {
+                messageQueue.Enqueue(envelope);
+            }
         }
     }
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/EdgeConnection.cs

@@ -114,4 +114,10 @@ public sealed class EdgeConnection : IEquatable<EdgeConnection>
     /// The unique identifiers of the sinks connected by this edge.
     /// </summary>
     public List<string> SinkIds { get; }
+
+    /// <inheritdoc />
+    public override string ToString()
+    {
+        return $"[{string.Join(",", this.SourceIds)}] => [{string.Join(",", this.SinkIds)}]";
+    }
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/EdgeMap.cs

@@ -12,9 +12,9 @@ internal sealed class EdgeMap
 {
     private readonly Dictionary<EdgeId, EdgeRunner> _edgeRunners = [];
     private readonly Dictionary<EdgeId, IStatefulEdgeRunner> _statefulRunners = [];
-    private readonly Dictionary<string, InputEdgeRunner> _portEdgeRunners;
+    private readonly Dictionary<string, ResponseEdgeRunner> _portEdgeRunners;
 
-    private readonly InputEdgeRunner _inputRunner;
+    private readonly ResponseEdgeRunner _inputRunner;
     private readonly IStepTracer? _stepTracer;
 
     public EdgeMap(IRunnerContext runContext,
@@ -29,7 +29,7 @@ internal sealed class EdgeMap
 
     public EdgeMap(IRunnerContext runContext,
                    Dictionary<string, HashSet<Edge>> workflowEdges,
-                   IEnumerable<InputPort> workflowPorts,
+                   IEnumerable<RequestPort> workflowPorts,
                    string startExecutorId,
                    IStepTracer? stepTracer = null)
     {
@@ -53,10 +53,10 @@ internal sealed class EdgeMap
 
         this._portEdgeRunners = workflowPorts.ToDictionary(
             port => port.Id,
-            port => InputEdgeRunner.ForPort(runContext, port)
+            port => ResponseEdgeRunner.ForPort(runContext, port)
             );
 
-        this._inputRunner = new InputEdgeRunner(runContext, startExecutorId);
+        this._inputRunner = new ResponseEdgeRunner(runContext, startExecutorId);
         this._stepTracer = stepTracer;
     }
 
@@ -78,7 +78,7 @@ internal sealed class EdgeMap
 
     public ValueTask<DeliveryMapping?> PrepareDeliveryForResponseAsync(ExternalResponse response)
     {
-        if (!this._portEdgeRunners.TryGetValue(response.PortInfo.PortId, out InputEdgeRunner? portRunner))
+        if (!this._portEdgeRunners.TryGetValue(response.PortInfo.PortId, out ResponseEdgeRunner? portRunner))
         {
             throw new InvalidOperationException($"Port {response.PortInfo.PortId} not found in the edge map.");
         }

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/IInputCoordinator.cs

@@ -0,0 +1,11 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal interface IInputCoordinator
+{
+    ValueTask<bool> WaitForNextInputAsync(CancellationToken cancellation = default);
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/IRunEventStream.cs

@@ -0,0 +1,15 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal interface IRunEventStream : IAsyncDisposable
+{
+    void Start();
+    ValueTask<RunStatus> GetStatusAsync(CancellationToken cancellation = default);
+    IAsyncEnumerable<WorkflowEvent> TakeEventStreamAsync(CancellationToken cancellation = default);
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/IRunnerContext.cs

@@ -5,7 +5,7 @@ using System.Threading.Tasks;
 
 namespace Microsoft.Agents.AI.Workflows.Execution;
 
-internal interface IRunnerContext : IExternalRequestSink
+internal interface IRunnerContext : IExternalRequestSink, ISuperStepJoinContext
 {
     ValueTask AddEventAsync(WorkflowEvent workflowEvent);
     ValueTask SendMessageAsync(string sourceId, object message, string? targetId = null);

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/ISuperStepJoinContext.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal interface ISuperStepJoinContext
+{
+    bool WithCheckpointing { get; }
+
+    ValueTask ForwardWorkflowEventAsync(WorkflowEvent workflowEvent, CancellationToken cancellation = default);
+    ValueTask SendMessageAsync<TMessage>(string senderId, [DisallowNull] TMessage message, CancellationToken cancellation = default);
+
+    ValueTask AttachSuperstepAsync(ISuperStepRunner superStepRunner, CancellationToken cancellation = default);
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/ISuperStepRunner.cs

@@ -15,12 +15,16 @@ internal interface ISuperStepRunner
     bool HasUnservicedRequests { get; }
     bool HasUnprocessedMessages { get; }
 
-    ValueTask EnqueueResponseAsync(ExternalResponse response);
-    ValueTask<bool> EnqueueMessageAsync<T>(T message);
+    ValueTask EnqueueResponseAsync(ExternalResponse response, CancellationToken cancellation = default);
 
-    event EventHandler<WorkflowEvent>? WorkflowEvent;
+    ValueTask<bool> IsValidInputTypeAsync<T>(CancellationToken cancellation = default);
+    ValueTask<bool> EnqueueMessageAsync<T>(T message, CancellationToken cancellation = default);
+    ValueTask<bool> EnqueueMessageUntypedAsync(object message, Type declaredType, CancellationToken cancellation = default);
+
+    ConcurrentEventSink OutgoingEvents { get; }
 
     ValueTask<bool> RunSuperStepAsync(CancellationToken cancellationToken);
 
+    // This cannot be cancelled
     ValueTask RequestEndRunAsync();
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/InitLocked.cs

@@ -0,0 +1,45 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Threading;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal class InitLocked<T>() where T : class
+{
+    private int _writers;
+    private T? _value;
+
+    public T? Get()
+    {
+        return this._value;
+    }
+
+    public bool Init(Func<T> initializer)
+    {
+        if (Interlocked.Exchange(ref this._writers, 1) == 0)
+        {
+            try
+            {
+                if (this._value == null)
+                {
+                    this._value = initializer();
+                    return true;
+                }
+
+                return false;
+            }
+            finally
+            {
+                this._writers = 0;
+            }
+        }
+
+        return false;
+    }
+
+    public void Clear()
+    {
+        this._value = null;
+    }
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/LockstepRunEventStream.cs

@@ -0,0 +1,117 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Workflows.Observability;
+
+namespace Microsoft.Agents.AI.Workflows.Execution;
+
+internal sealed class LockstepRunEventStream : IRunEventStream
+{
+    private static readonly string s_namespace = typeof(LockstepRunEventStream).Namespace!;
+    private static readonly ActivitySource s_activitySource = new(s_namespace);
+
+    public ValueTask<RunStatus> GetStatusAsync(CancellationToken cancellation = default) => new(this.RunStatus);
+
+    public LockstepRunEventStream(ISuperStepRunner stepRunner)
+    {
+        this.StepRunner = stepRunner;
+    }
+
+    private RunStatus RunStatus { get; set; } = RunStatus.NotStarted;
+    private ISuperStepRunner StepRunner { get; }
+
+    public void Start()
+    {
+        // No-op for lockstep execution
+    }
+
+    public async IAsyncEnumerable<WorkflowEvent> TakeEventStreamAsync([EnumeratorCancellation] CancellationToken cancellation = default)
+    {
+        ConcurrentQueue<WorkflowEvent> eventSink = [];
+
+        this.StepRunner.OutgoingEvents.EventRaised += OnWorkflowEventAsync;
+
+        using Activity? activity = s_activitySource.StartActivity(ActivityNames.WorkflowRun);
+        activity?.SetTag(Tags.WorkflowId, this.StepRunner.StartExecutorId).SetTag(Tags.RunId, this.StepRunner.RunId);
+
+        try
+        {
+            this.RunStatus = RunStatus.Running;
+            activity?.AddEvent(new ActivityEvent(EventNames.WorkflowStarted));
+
+            do
+            {
+                // Because we may be yielding out of this function, we need to ensure that the Activity.Current
+                // is set to our activity for the duration of this loop iteration.
+                Activity.Current = activity;
+
+                // Drain SuperSteps while there are steps to run
+                try
+                {
+                    await this.StepRunner.RunSuperStepAsync(cancellation).ConfigureAwait(false);
+                }
+                catch (Exception ex) when (activity is not null)
+                {
+                    activity.AddEvent(new ActivityEvent(EventNames.WorkflowError, tags: new() {
+                         { Tags.ErrorType, ex.GetType().FullName },
+                         { Tags.BuildErrorMessage, ex.Message },
+                    }));
+                    activity.CaptureException(ex);
+                    throw;
+                }
+
+                if (cancellation.IsCancellationRequested)
+                {
+                    yield break; // Exit if cancellation is requested
+                }
+
+                bool hadRequestHaltEvent = false;
+                foreach (WorkflowEvent raisedEvent in Interlocked.Exchange(ref eventSink, []))
+                {
+                    if (cancellation.IsCancellationRequested)
+                    {
+                        yield break; // Exit if cancellation is requested
+                    }
+
+                    // TODO: Do we actually want to interpret this as a termination request?
+                    if (raisedEvent is RequestHaltEvent)
+                    {
+                        hadRequestHaltEvent = true;
+                    }
+                    else
+                    {
+                        yield return raisedEvent;
+                    }
+                }
+
+                if (hadRequestHaltEvent)
+                {
+                    // If we had a completion event, we are done.
+                    yield break;
+                }
+            } while (this.StepRunner.HasUnprocessedMessages &&
+                     !cancellation.IsCancellationRequested);
+
+            activity?.AddEvent(new ActivityEvent(EventNames.WorkflowCompleted));
+        }
+        finally
+        {
+            this.RunStatus = this.StepRunner.HasUnservicedRequests ? RunStatus.PendingRequests : RunStatus.Idle;
+            this.StepRunner.OutgoingEvents.EventRaised -= OnWorkflowEventAsync;
+        }
+
+        ValueTask OnWorkflowEventAsync(object? sender, WorkflowEvent e)
+        {
+            eventSink.Enqueue(e);
+            return default;
+        }
+    }
+
+    public ValueTask DisposeAsync() => default;
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/ResponseEdgeRunner.cs

@@ -8,15 +8,15 @@ using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Agents.AI.Workflows.Execution;
 
-internal sealed class InputEdgeRunner(IRunnerContext runContext, string sinkId)
+internal sealed class ResponseEdgeRunner(IRunnerContext runContext, string sinkId)
     : EdgeRunner<string>(runContext, sinkId)
 {
-    public static InputEdgeRunner ForPort(IRunnerContext runContext, InputPort port)
+    public static ResponseEdgeRunner ForPort(IRunnerContext runContext, RequestPort port)
     {
         Throw.IfNull(port);
 
-        // The port is an input port, so we can use the port's ID as the sink ID.
-        return new InputEdgeRunner(runContext, port.Id);
+        // The port is an request port, so we can use the port's ID as the sink ID.
+        return new ResponseEdgeRunner(runContext, port.Id);
     }
 
     protected internal override async ValueTask<DeliveryMapping?> ChaseEdgeAsync(MessageEnvelope envelope, IStepTracer? stepTracer)
@@ -25,7 +25,7 @@ internal sealed class InputEdgeRunner(IRunnerContext runContext, string sinkId)
 
         using var activity = s_activitySource.StartActivity(ActivityNames.EdgeGroupProcess);
         activity?
-            .SetTag(Tags.EdgeGroupType, nameof(InputEdgeRunner))
+            .SetTag(Tags.EdgeGroupType, nameof(ResponseEdgeRunner))
             .SetTag(Tags.MessageSourceId, envelope.SourceId)
             .SetTag(Tags.MessageTargetId, this.EdgeData);
 

dotnet/src/Microsoft.Agents.AI.Workflows/Execution/StepContext.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.Agents.AI.Workflows.Checkpointing;
@@ -8,18 +9,13 @@ namespace Microsoft.Agents.AI.Workflows.Execution;
 
 internal sealed class StepContext
 {
-    public Dictionary<string, List<MessageEnvelope>> QueuedMessages { get; } = [];
+    public ConcurrentDictionary<string, ConcurrentQueue<MessageEnvelope>> QueuedMessages { get; } = [];
 
-    public bool HasMessages => this.QueuedMessages.Values.Any(messageList => messageList.Count > 0);
+    public bool HasMessages => this.QueuedMessages.Values.Any(messageQueue => !messageQueue.IsEmpty);
 
-    public List<MessageEnvelope> MessagesFor(string target)
+    public ConcurrentQueue<MessageEnvelope> MessagesFor(string target)
     {
-        if (!this.QueuedMessages.TryGetValue(target, out var messages))
-        {
-            this.QueuedMessages[target] = messages = [];
-        }
-
-        return messages;
+        return this.QueuedMessages.GetOrAdd(target, _ => new ConcurrentQueue<MessageEnvelope>());
     }
 
     // TODO: Create a MessageEnvelope class that extends from the ExportedState object (with appropriate rename) to avoid
@@ -29,7 +25,8 @@ internal sealed class StepContext
         return this.QueuedMessages.Keys.ToDictionary(
             keySelector: identity => identity,
             elementSelector: identity => this.QueuedMessages[identity]
-                                             .ConvertAll(v => new PortableMessageEnvelope(v))
+                                             .Select(v => new PortableMessageEnvelope(v))
+                                             .ToList()
         );
     }
 
@@ -37,7 +34,7 @@ internal sealed class StepContext
     {
         foreach (string identity in messages.Keys)
         {
-            this.QueuedMessages[identity] = messages[identity].ConvertAll(UnwrapExportedState);
+            this.QueuedMessages[identity] = new(messages[identity].Select(UnwrapExportedState));
         }
 
         static MessageEnvelope UnwrapExportedState(PortableMessageEnvelope es) => es.ToMessageEnvelope();

dotnet/src/Microsoft.Agents.AI.Workflows/ExecutionMode.cs

@@ -0,0 +1,9 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Microsoft.Agents.AI.Workflows;
+
+internal enum ExecutionMode
+{
+    OffThread,
+    Lockstep
+}

dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs

@@ -24,8 +24,6 @@ public abstract class Executor : IIdentified
     /// </summary>
     public string Id { get; }
 
-    private readonly ExecutorOptions _options;
-
     private static readonly string s_namespace = typeof(Executor).Namespace!;
     private static readonly ActivitySource s_activitySource = new(s_namespace);
 
@@ -37,9 +35,14 @@ public abstract class Executor : IIdentified
     protected Executor(string id, ExecutorOptions? options = null)
     {
         this.Id = id;
-        this._options = options ?? ExecutorOptions.Default;
+        this.Options = options ?? ExecutorOptions.Default;
     }
 
+    /// <summary>
+    /// Gets the configuration options for the executor.
+    /// </summary>
+    protected ExecutorOptions Options { get; }
+
     /// <summary>
     /// Override this method to register handlers for the executor.
     /// </summary>
@@ -57,7 +60,7 @@ public abstract class Executor : IIdentified
     /// <returns></returns>
     protected virtual ISet<Type> ConfigureYieldTypes()
     {
-        if (this._options.AutoYieldOutputHandlerResultObject)
+        if (this.Options.AutoYieldOutputHandlerResultObject)
         {
             return this.Router.DefaultOutputTypes;
         }
@@ -132,11 +135,11 @@ public abstract class Executor : IIdentified
         }
 
         // If we had a real return type, raise it as a SendMessage; TODO: Should we have a way to disable this behaviour?
-        if (result.Result is not null && this._options.AutoSendMessageHandlerResultObject)
+        if (result.Result is not null && this.Options.AutoSendMessageHandlerResultObject)
         {
             await context.SendMessageAsync(result.Result).ConfigureAwait(false);
         }
-        if (result.Result is not null && this._options.AutoYieldOutputHandlerResultObject)
+        if (result.Result is not null && this.Options.AutoYieldOutputHandlerResultObject)
         {
             await context.YieldOutputAsync(result.Result).ConfigureAwait(false);
         }

dotnet/src/Microsoft.Agents.AI.Workflows/ExecutorIsh.cs

@@ -29,7 +29,7 @@ public static class ExecutorIshConfigurationExtensions
     /// <param name="id">An id for the executor to be instantiated.</param>
     /// <param name="options">An optional parameter specifying the options.</param>
     /// <returns>An ExecutorIsh instance that resolves to the result of the factory call when messages get sent to it.</returns>
-    public static ExecutorIsh ConfigureFactory<TExecutor, TOptions>(this Func<Config<TOptions>, ValueTask<TExecutor>> factoryAsync, string id, TOptions? options = null)
+    public static ExecutorIsh ConfigureFactory<TExecutor, TOptions>(this Func<Config<TOptions>, string, ValueTask<TExecutor>> factoryAsync, string id, TOptions? options = null)
         where TExecutor : Executor
         where TOptions : ExecutorOptions
     {
@@ -48,6 +48,27 @@ public static class ExecutorIshConfigurationExtensions
                                typeof(FunctionExecutor<TInput, TOutput>),
                                ExecutorIsh.Type.Function);
 
+    /// <summary>
+    /// Configures a sub-workflow executor for the specified workflow, using the provided identifier and options.
+    /// </summary>
+    /// <param name="workflow">The workflow instance to be executed as a sub-workflow. Cannot be null.</param>
+    /// <param name="id">A unique identifier for the sub-workflow execution. Used to distinguish this sub-workflow instance.</param>
+    /// <param name="options">Optional configuration options for the sub-workflow executor. If null, default options are used.</param>
+    /// <returns>An ExecutorIsh instance representing the configured sub-workflow executor.</returns>
+    public static ExecutorIsh ConfigureSubWorkflow(this Workflow workflow, string id, ExecutorOptions? options = null)
+    {
+        object ownershipToken = new();
+        workflow.TakeOwnership(ownershipToken, subworkflow: true);
+
+        Configured<WorkflowHostExecutor, ExecutorOptions> configured = new(InitHostExecutorAsync, id, options, raw: workflow);
+        return new ExecutorIsh(configured.Super<WorkflowHostExecutor, Executor, ExecutorOptions>(), typeof(WorkflowHostExecutor), ExecutorIsh.Type.Workflow);
+
+        ValueTask<WorkflowHostExecutor> InitHostExecutorAsync(Config<ExecutorOptions> config, string runId)
+        {
+            return new(new WorkflowHostExecutor(config.Id, workflow, runId, ownershipToken, config.Options));
+        }
+    }
+
     /// <summary>
     /// Configures a function-based asynchronous message handler as an executor with the specified identifier and
     /// options.
@@ -114,13 +135,17 @@ public sealed class ExecutorIsh :
         /// </summary>
         Function,
         /// <summary>
-        /// An <see cref="InputPort"/> for servicing external requests.
+        /// An <see cref="RequestPort"/> for servicing external requests.
         /// </summary>
-        InputPort,
+        RequestPort,
         /// <summary>
         /// An <see cref="AIAgent"/> instance.
         /// </summary>
         Agent,
+        /// <summary>
+        /// A nested <see cref="Workflow"/> instance.
+        /// </summary>
+        Workflow,
     }
 
     /// <summary>
@@ -133,7 +158,7 @@ public sealed class ExecutorIsh :
     private readonly Configured<Executor>? _configuredExecutor;
     private readonly System.Type? _configuredExecutorType;
 
-    internal readonly InputPort? _inputPortValue;
+    internal readonly RequestPort? _requestPortValue;
     private readonly AIAgent? _aiAgentValue;
 
     /// <summary>
@@ -168,10 +193,10 @@ public sealed class ExecutorIsh :
     /// Initializes a new instance of the ExecutorIsh class using the specified input port.
     /// </summary>
     /// <param name="port">The input port to associate to be wrapped.</param>
-    public ExecutorIsh(InputPort port)
+    public ExecutorIsh(RequestPort port)
     {
-        this.ExecutorType = Type.InputPort;
-        this._inputPortValue = Throw.IfNull(port);
+        this.ExecutorType = Type.RequestPort;
+        this._requestPortValue = Throw.IfNull(port);
     }
 
     /// <summary>
@@ -191,9 +216,10 @@ public sealed class ExecutorIsh :
     {
         Type.Unbound => this._idValue ?? throw new InvalidOperationException("This ExecutorIsh is unbound and has no ID."),
         Type.Executor => this._configuredExecutor!.Id,
-        Type.InputPort => this._inputPortValue!.Id,
+        Type.RequestPort => this._requestPortValue!.Id,
         Type.Agent => this._aiAgentValue!.Id,
         Type.Function => this._configuredExecutor!.Id,
+        Type.Workflow => this._configuredExecutor!.Id,
         _ => throw new InvalidOperationException($"Unknown ExecutorIsh type: {this.ExecutorType}")
     };
 
@@ -201,9 +227,10 @@ public sealed class ExecutorIsh :
     {
         Type.Unbound => this._idValue,
         Type.Executor => this._configuredExecutor!.Raw ?? this._configuredExecutor,
-        Type.InputPort => this._inputPortValue,
+        Type.RequestPort => this._requestPortValue,
         Type.Agent => this._aiAgentValue,
         Type.Function => this._configuredExecutor!.Raw ?? this._configuredExecutor,
+        Type.Workflow => this._configuredExecutor!.Raw ?? this._configuredExecutor,
         _ => throw new InvalidOperationException($"Unknown ExecutorIsh type: {this.ExecutorType}")
     };
 
@@ -219,9 +246,10 @@ public sealed class ExecutorIsh :
     {
         Type.Unbound => throw new InvalidOperationException($"ExecutorIsh with ID '{this.Id}' is unbound."),
         Type.Executor => this._configuredExecutorType!,
-        Type.InputPort => typeof(RequestInfoExecutor),
+        Type.RequestPort => typeof(RequestInfoExecutor),
         Type.Agent => typeof(AIAgentHostExecutor),
         Type.Function => this._configuredExecutorType!,
+        Type.Workflow => this._configuredExecutorType!,
         _ => throw new InvalidOperationException($"Unknown ExecutorIsh type: {this.ExecutorType}")
     };
 
@@ -229,13 +257,14 @@ public sealed class ExecutorIsh :
     /// Gets an <see cref="Func{Executor}"/> that can be used to obtain an <see cref="Executor"/> instance
     /// corresponding to this <see cref="ExecutorIsh"/>.
     /// </summary>
-    private Func<ValueTask<Executor>> ExecutorProvider => this.ExecutorType switch
+    private Func<string, ValueTask<Executor>> ExecutorProvider => this.ExecutorType switch
     {
         Type.Unbound => throw new InvalidOperationException($"Executor with ID '{this.Id}' is unbound."),
         Type.Executor => this._configuredExecutor!.BoundFactoryAsync,
-        Type.InputPort => () => new(new RequestInfoExecutor(this._inputPortValue!)),
-        Type.Agent => () => new(new AIAgentHostExecutor(this._aiAgentValue!)),
+        Type.RequestPort => (runId) => new(new RequestInfoExecutor(this._requestPortValue!)),
+        Type.Agent => (runId) => new(new AIAgentHostExecutor(this._aiAgentValue!)),
         Type.Function => this._configuredExecutor!.BoundFactoryAsync,
+        Type.Workflow => this._configuredExecutor!.BoundFactoryAsync,
         _ => throw new InvalidOperationException($"Unknown ExecutorIsh type: {this.ExecutorType}")
     };
 
@@ -246,10 +275,10 @@ public sealed class ExecutorIsh :
     public static implicit operator ExecutorIsh(Executor executor) => new(executor);
 
     /// <summary>
-    /// Defines an implicit conversion from an <see cref="InputPort"/> to an <see cref="ExecutorIsh"/> instance.
+    /// Defines an implicit conversion from an <see cref="RequestPort"/> to an <see cref="ExecutorIsh"/> instance.
     /// </summary>
-    /// <param name="inputPort">The <see cref="InputPort"/> to convert to an <see cref="ExecutorIsh"/>.</param>
-    public static implicit operator ExecutorIsh(InputPort inputPort) => new(inputPort);
+    /// <param name="inputPort">The <see cref="RequestPort"/> to convert to an <see cref="ExecutorIsh"/>.</param>
+    public static implicit operator ExecutorIsh(RequestPort inputPort) => new(inputPort);
 
     /// <summary>
     /// Defines an implicit conversion from an <see cref="AIAgent"/> to an <see cref="ExecutorIsh"/> instance.
@@ -294,9 +323,10 @@ public sealed class ExecutorIsh :
     {
         Type.Unbound => $"'{this.Id}':<unbound>",
         Type.Executor => $"'{this.Id}':{this._configuredExecutorType!.Name}",
-        Type.InputPort => $"'{this.Id}':Input({this._inputPortValue!.Request.Name}->{this._inputPortValue!.Response.Name})",
+        Type.RequestPort => $"'{this.Id}':Input({this._requestPortValue!.Request.Name}->{this._requestPortValue!.Response.Name})",
         Type.Agent => $"{this.Id}':AIAgent(@{this._aiAgentValue!.GetType().Name})",
         Type.Function => $"'{this.Id}':{this._configuredExecutorType!.Name}",
+        Type.Workflow => $"'{this.Id}':{this._configuredExecutorType!.Name}",
         _ => $"'{this.Id}':<unknown[{this.ExecutorType}]>"
     };
 }

dotnet/src/Microsoft.Agents.AI.Workflows/ExecutorRegistration.cs

@@ -4,7 +4,7 @@ using System;
 using System.Threading.Tasks;
 using Microsoft.Shared.Diagnostics;
 
-using ExecutorFactoryF = System.Func<System.Threading.Tasks.ValueTask<Microsoft.Agents.AI.Workflows.Executor>>;
+using ExecutorFactoryF = System.Func<string, System.Threading.Tasks.ValueTask<Microsoft.Agents.AI.Workflows.Executor>>;
 
 namespace Microsoft.Agents.AI.Workflows;
 
@@ -12,7 +12,7 @@ internal sealed class ExecutorRegistration(string id, Type executorType, Executo
 {
     public string Id { get; } = Throw.IfNullOrEmpty(id);
     public Type ExecutorType { get; } = Throw.IfNull(executorType);
-    public ExecutorFactoryF ProviderAsync { get; } = Throw.IfNull(provider);
+    private ExecutorFactoryF ProviderAsync { get; } = Throw.IfNull(provider);
     public bool IsNotExecutorInstance { get; } = rawData is not Executor;
     public bool IsUnresettableSharedInstance { get; } = rawData is Executor && rawData is not IResettableExecutor;
 
@@ -58,5 +58,5 @@ internal sealed class ExecutorRegistration(string id, Type executorType, Executo
         return executor;
     }
 
-    public async ValueTask<Executor> CreateInstanceAsync() => this.CheckId(await this.ProviderAsync().ConfigureAwait(false));
+    public async ValueTask<Executor> CreateInstanceAsync(string runId) => this.CheckId(await this.ProviderAsync(runId).ConfigureAwait(false));
 }

dotnet/src/Microsoft.Agents.AI.Workflows/ExternalRequest.cs

@@ -13,7 +13,7 @@ namespace Microsoft.Agents.AI.Workflows;
 /// <param name="PortInfo">The port to invoke.</param>
 /// <param name="RequestId">A unique identifier for this request instance.</param>
 /// <param name="Data">The data contained in the request.</param>
-public record ExternalRequest(InputPortInfo PortInfo, string RequestId, PortableValue Data)
+public record ExternalRequest(RequestPortInfo PortInfo, string RequestId, PortableValue Data)
 {
     /// <summary>
     /// Attempts to retrieve the underlying data as the specified type.
@@ -29,6 +29,13 @@ public record ExternalRequest(InputPortInfo PortInfo, string RequestId, Portable
     /// <returns>true if the underlying data is of type TValue; otherwise, false.</returns>
     public bool DataIs<TValue>() => this.Data.Is<TValue>();
 
+    /// <summary>
+    /// Determines whether the underlying data is of the specified type and outputs the value if it is.
+    /// </summary>
+    /// <typeparam name="TValue">The type to compare with the underlying data.</typeparam>
+    /// <returns>true if the underlying data is of type TValue; otherwise, false.</returns>
+    public bool DataIs<TValue>([NotNullWhen(true)] out TValue? value) => this.Data.Is(out value);
+
     /// <summary>
     /// Creates a new <see cref="ExternalRequest"/> for the specified input port and data payload.
     /// </summary>
@@ -37,7 +44,7 @@ public record ExternalRequest(InputPortInfo PortInfo, string RequestId, Portable
     /// <param name="requestId">An optional unique identifier for this request instance. If <c>null</c>, a UUID will be generated.</param>
     /// <returns>An <see cref="ExternalRequest"/> instance containing the specified port, data, and request identifier.</returns>
     /// <exception cref="InvalidOperationException">Thrown when the input data object does not match the expected request type.</exception>
-    public static ExternalRequest Create(InputPort port, [NotNull] object data, string? requestId = null)
+    public static ExternalRequest Create(RequestPort port, [NotNull] object data, string? requestId = null)
     {
         if (!port.Request.IsInstanceOfType(Throw.IfNull(data)))
         {
@@ -58,7 +65,7 @@ public record ExternalRequest(InputPortInfo PortInfo, string RequestId, Portable
     /// <param name="data">The data payload to include in the request. Must not be <c>null</c>.</param>
     /// <param name="requestId">An optional identifier for the request. If <c>null</c>, a default identifier may be assigned.</param>
     /// <returns>An <see cref="ExternalRequest"/> instance containing the specified port, data, and request identifier.</returns>
-    public static ExternalRequest Create<T>(InputPort port, T data, string? requestId = null) => Create(port, (object)Throw.IfNull(data), requestId);
+    public static ExternalRequest Create<T>(RequestPort port, T data, string? requestId = null) => Create(port, (object)Throw.IfNull(data), requestId);
 
     /// <summary>
     /// Creates a new <see cref="ExternalResponse"/> corresponding to the request, with the speicified data payload.

dotnet/src/Microsoft.Agents.AI.Workflows/ExternalResponse.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Microsoft.Agents.AI.Workflows.Checkpointing;
 
 namespace Microsoft.Agents.AI.Workflows;
@@ -11,7 +12,7 @@ namespace Microsoft.Agents.AI.Workflows;
 /// <param name="PortInfo">The port invoked.</param>
 /// <param name="RequestId">The unique identifier of the corresponding request.</param>
 /// <param name="Data">The data contained in the response.</param>
-public record ExternalResponse(InputPortInfo PortInfo, string RequestId, PortableValue Data)
+public record ExternalResponse(RequestPortInfo PortInfo, string RequestId, PortableValue Data)
 {
     /// <summary>
     /// Attempts to retrieve the underlying data as the specified type.
@@ -27,6 +28,15 @@ public record ExternalResponse(InputPortInfo PortInfo, string RequestId, Portabl
     /// <returns>true if the underlying data is of type TValue; otherwise, false.</returns>
     public bool DataIs<TValue>() => this.Data.Is<TValue>();
 
+    /// <summary>
+    /// Determines whether the underlying data can be retrieved as the specified type.
+    /// </summary>
+    /// <typeparam name="TValue">The type to which the underlying data is to be cast if available.</typeparam>
+    /// <param name="value">When this method returns, contains the value of type <typeparamref name="TValue"/> if the data is
+    /// available and compatible.</param>
+    /// <returns>true if the data is present and can be cast to <typeparamref name="TValue"/>; otherwise, false.</returns>
+    public bool DataIs<TValue>([NotNullWhen(true)] out TValue? value) => this.Data.Is(out value);
+
     /// <summary>
     /// Attempts to retrieve the underlying data as the specified type.
     /// </summary>

dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcStepTracer.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
@@ -16,11 +17,11 @@ internal sealed class InProcStepTracer : IStepTracer
     public bool StateUpdated { get; private set; }
     public CheckpointInfo? Checkpoint { get; private set; }
 
-    public HashSet<string> Instantiated { get; } = [];
-    public HashSet<string> Activated { get; } = [];
+    public ConcurrentDictionary<string, string> Instantiated { get; } = new();
+    public ConcurrentDictionary<string, string> Activated { get; } = new();
 
-    public void TraceIntantiated(string executorId) => this.Instantiated.Add(executorId);
-    public void TraceActivated(string executorId) => this.Activated.Add(executorId);
+    public void TraceIntantiated(string executorId) => this.Instantiated.TryAdd(executorId, executorId);
+    public void TraceActivated(string executorId) => this.Activated.TryAdd(executorId, executorId);
     public void TraceStatePublished() => this.StateUpdated = true;
     public void TraceCheckpointCreated(CheckpointInfo checkpoint) => this.Checkpoint = checkpoint;
 
@@ -60,7 +61,7 @@ internal sealed class InProcStepTracer : IStepTracer
         });
     }
 
-    public SuperStepCompletedEvent Complete(bool nextStepHasActions, bool hasPendingRequests) => new(this.StepNumber, new SuperStepCompletionInfo(this.Activated, this.Instantiated)
+    public SuperStepCompletedEvent Complete(bool nextStepHasActions, bool hasPendingRequests) => new(this.StepNumber, new SuperStepCompletionInfo(this.Activated.Keys, this.Instantiated.Keys)
     {
         HasPendingMessages = nextStepHasActions,
         HasPendingRequests = hasPendingRequests,
@@ -72,19 +73,19 @@ internal sealed class InProcStepTracer : IStepTracer
     {
         StringBuilder sb = new();
 
-        if (this.Instantiated.Count != 0)
+        if (!this.Instantiated.IsEmpty)
         {
-            sb.Append("Instantiated: ").Append(string.Join(", ", this.Instantiated.OrderBy(id => id, StringComparer.Ordinal)));
+            sb.Append("Instantiated: ").Append(string.Join(", ", this.Instantiated.Keys.OrderBy(id => id, StringComparer.Ordinal)));
         }
 
-        if (this.Activated.Count != 0)
+        if (!this.Activated.IsEmpty)
         {
             if (sb.Length != 0)
             {
                 sb.AppendLine();
             }
 
-            sb.Append("Activated: ").Append(string.Join(", ", this.Activated.OrderBy(id => id, StringComparer.Ordinal)));
+            sb.Append("Activated: ").Append(string.Join(", ", this.Activated.Keys.OrderBy(id => id, StringComparer.Ordinal)));
         }
 
         return sb.ToString();

dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcessRunner.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -18,18 +19,20 @@ namespace Microsoft.Agents.AI.Workflows.InProc;
 /// <remarks><para> <see cref="InProcessRunner"/> enables step-by-step execution of a workflow graph entirely
 /// within the current process, without distributed coordination. It is primarily intended for testing, debugging, or
 /// scenarios where workflow execution does not require executor distribution. </para></remarks>
-internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
+internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingHandle
 {
-    public InProcessRunner(Workflow workflow, ICheckpointManager? checkpointManager, string? runId = null, params Type[] knownValidInputTypes)
+    public InProcessRunner(Workflow workflow, ICheckpointManager? checkpointManager, string? runId = null, object? workflowOwnership = null, bool subworkflow = false, IEnumerable<Type>? knownValidInputTypes = null)
     {
         this.RunId = runId ?? Guid.NewGuid().ToString("N");
         this.StartExecutorId = workflow.StartExecutorId;
 
         this.Workflow = Throw.IfNull(workflow);
-        this.RunContext = new InProcessRunnerContext(workflow, this.RunId, this.StepTracer);
+        this.RunContext = new InProcessRunnerContext(workflow, this.RunId, withCheckpointing: checkpointManager != null, this.OutgoingEvents, this.StepTracer, workflowOwnership, subworkflow);
         this.CheckpointManager = checkpointManager;
 
-        this._knownValidInputTypes = [.. knownValidInputTypes];
+        this._knownValidInputTypes = knownValidInputTypes != null
+                                   ? [.. knownValidInputTypes]
+                                   : [];
 
         // Initialize the runners for each of the edges, along with the state for edges that
         // need it.
@@ -43,7 +46,7 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
     public string StartExecutorId { get; }
 
     private readonly HashSet<Type> _knownValidInputTypes;
-    public async ValueTask<bool> IsValidInputTypeAsync(Type messageType)
+    public async ValueTask<bool> IsValidInputTypeAsync(Type messageType, CancellationToken cancellation = default)
     {
         if (this._knownValidInputTypes.Contains(messageType))
         {
@@ -60,7 +63,10 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
         return false;
     }
 
-    private async ValueTask<bool> EnqueueMessageInternalAsync(object message, Type messageType)
+    public ValueTask<bool> IsValidInputTypeAsync<T>(CancellationToken cancellation = default)
+        => this.IsValidInputTypeAsync(typeof(T), cancellation);
+
+    public async ValueTask<bool> EnqueueMessageUntypedAsync(object message, Type declaredType, CancellationToken cancellation = default)
     {
         this.RunContext.CheckEnded();
         Throw.IfNull(message);
@@ -72,22 +78,22 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
 
         // Check that the type of the incoming message is compatible with the starting executor's
         // input type.
-        if (!await this.IsValidInputTypeAsync(messageType).ConfigureAwait(false))
+        if (!await this.IsValidInputTypeAsync(declaredType, cancellation).ConfigureAwait(false))
         {
             return false;
         }
 
-        await this.RunContext.AddExternalMessageAsync(message, messageType).ConfigureAwait(false);
+        await this.RunContext.AddExternalMessageAsync(message, declaredType).ConfigureAwait(false);
         return true;
     }
 
-    public ValueTask<bool> EnqueueMessageAsync<T>(T message)
-        => this.EnqueueMessageInternalAsync(Throw.IfNull(message), typeof(T));
+    public ValueTask<bool> EnqueueMessageAsync<T>(T message, CancellationToken cancellation = default)
+        => this.EnqueueMessageUntypedAsync(Throw.IfNull(message), typeof(T), cancellation);
 
-    public ValueTask<bool> EnqueueMessageAsync(object message)
-        => this.EnqueueMessageInternalAsync(Throw.IfNull(message), message.GetType());
+    public ValueTask<bool> EnqueueMessageAsync(object message, CancellationToken cancellation = default)
+        => this.EnqueueMessageUntypedAsync(Throw.IfNull(message), message.GetType(), cancellation);
 
-    ValueTask ISuperStepRunner.EnqueueResponseAsync(ExternalResponse response)
+    ValueTask ISuperStepRunner.EnqueueResponseAsync(ExternalResponse response, CancellationToken cancellation)
     {
         // TODO: Check that there exists a corresponding input port?
         return this.RunContext.AddExternalResponseAsync(response);
@@ -95,78 +101,32 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
 
     private InProcStepTracer StepTracer { get; } = new();
     private Workflow Workflow { get; init; }
-    private InProcessRunnerContext RunContext { get; init; }
+    internal InProcessRunnerContext RunContext { get; init; }
     private ICheckpointManager? CheckpointManager { get; }
     private EdgeMap EdgeMap { get; init; }
 
-    event EventHandler<WorkflowEvent>? ISuperStepRunner.WorkflowEvent
-    {
-        add => this.WorkflowEvent += value;
-        remove => this.WorkflowEvent -= value;
-    }
+    public ConcurrentEventSink OutgoingEvents { get; } = new();
 
-    private event EventHandler<WorkflowEvent>? WorkflowEvent;
+    private ValueTask RaiseWorkflowEventAsync(WorkflowEvent workflowEvent)
+        => this.OutgoingEvents.EnqueueAsync(workflowEvent);
 
-    private void RaiseWorkflowEvent(WorkflowEvent workflowEvent)
-    {
-        this.WorkflowEvent?.Invoke(this, workflowEvent);
-    }
-
-    public async ValueTask<StreamingRun> ResumeStreamAsync(CheckpointInfo checkpoint, CancellationToken cancellationToken = default)
+    public ValueTask<AsyncRunHandle> BeginStreamAsync(ExecutionMode mode, CancellationToken cancellation = default)
     {
         this.RunContext.CheckEnded();
-        Throw.IfNull(checkpoint);
+        return new(new AsyncRunHandle(this, this, mode));
+    }
+
+    public async ValueTask<AsyncRunHandle> ResumeStreamAsync(ExecutionMode mode, CheckpointInfo fromCheckpoint, CancellationToken cancellation = default)
+    {
+        this.RunContext.CheckEnded();
+        Throw.IfNull(fromCheckpoint);
         if (this.CheckpointManager is null)
         {
             throw new InvalidOperationException("This runner was not configured with a CheckpointManager, so it cannot restore checkpoints.");
         }
 
-        await this.RestoreCheckpointAsync(checkpoint, cancellationToken).ConfigureAwait(false);
-
-        return new StreamingRun(this);
-    }
-
-    public async ValueTask<StreamingRun> StreamAsync(object input, CancellationToken cancellationToken = default)
-    {
-        this.RunContext.CheckEnded();
-        await this.EnqueueMessageAsync(input).ConfigureAwait(false);
-
-        return new StreamingRun(this);
-    }
-
-    public async ValueTask<StreamingRun> StreamAsync<TInput>(TInput input, CancellationToken cancellationToken = default)
-    {
-        this.RunContext.CheckEnded();
-        await this.EnqueueMessageAsync(input).ConfigureAwait(false);
-
-        return new StreamingRun(this);
-    }
-
-    internal async ValueTask<Run> ResumeAsync(CheckpointInfo checkpoint, CancellationToken cancellationToken = default)
-    {
-        this.RunContext.CheckEnded();
-        StreamingRun streamingRun = await this.ResumeStreamAsync(checkpoint, cancellationToken).ConfigureAwait(false);
-        cancellationToken.ThrowIfCancellationRequested();
-
-        return await Run.CaptureStreamAsync(streamingRun, cancellationToken).ConfigureAwait(false);
-    }
-
-    public async ValueTask<Run> RunAsync(object input, CancellationToken cancellationToken = default)
-    {
-        this.RunContext.CheckEnded();
-        StreamingRun streamingRun = await this.StreamAsync(input, cancellationToken).ConfigureAwait(false);
-        cancellationToken.ThrowIfCancellationRequested();
-
-        return await Run.CaptureStreamAsync(streamingRun, cancellationToken).ConfigureAwait(false);
-    }
-
-    public async ValueTask<Run> RunAsync<TInput>(TInput input, CancellationToken cancellationToken = default)
-    {
-        this.RunContext.CheckEnded();
-        StreamingRun streamingRun = await this.StreamAsync(input, cancellationToken).ConfigureAwait(false);
-        cancellationToken.ThrowIfCancellationRequested();
-
-        return await Run.CaptureStreamAsync(streamingRun, cancellationToken).ConfigureAwait(false);
+        await this.RestoreCheckpointAsync(fromCheckpoint, cancellation).ConfigureAwait(false);
+        return new AsyncRunHandle(this, this, mode);
     }
 
     bool ISuperStepRunner.HasUnservicedRequests => this.RunContext.HasUnservicedRequests;
@@ -184,34 +144,23 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
 
         StepContext currentStep = await this.RunContext.AdvanceAsync().ConfigureAwait(false);
 
-        if (currentStep.HasMessages)
+        if (currentStep.HasMessages ||
+            this.RunContext.HasQueuedExternalDeliveries ||
+            this.RunContext.JoinedRunnersHaveActions)
         {
             await this.RunSuperstepAsync(currentStep).ConfigureAwait(false);
             return true;
         }
 
-        this.EmitPendingEvents();
         return false;
     }
 
-    private void EmitPendingEvents()
-    {
-        if (this.RunContext.QueuedEvents.Count > 0)
-        {
-            foreach (WorkflowEvent @event in this.RunContext.QueuedEvents)
-            {
-                this.RaiseWorkflowEvent(@event);
-            }
-            this.RunContext.QueuedEvents.Clear();
-        }
-    }
-
-    private async ValueTask DeliverMessagesAsync(string receiverId, List<MessageEnvelope> envelopes)
+    private async ValueTask DeliverMessagesAsync(string receiverId, ConcurrentQueue<MessageEnvelope> envelopes)
     {
         Executor executor = await this.RunContext.EnsureExecutorAsync(receiverId, this.StepTracer).ConfigureAwait(false);
 
         this.StepTracer.TraceActivated(receiverId);
-        foreach (MessageEnvelope envelope in envelopes)
+        while (envelopes.TryDequeue(out var envelope))
         {
             await executor.ExecuteAsync(
                 envelope.Message,
@@ -223,7 +172,7 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
 
     private async ValueTask RunSuperstepAsync(StepContext currentStep)
     {
-        this.RaiseWorkflowEvent(this.StepTracer.Advance(currentStep));
+        await this.RaiseWorkflowEventAsync(this.StepTracer.Advance(currentStep)).ConfigureAwait(false);
 
         // Deliver the messages and queue the next step
         List<Task> receiverTasks =
@@ -236,12 +185,22 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
         // that we would need to avoid firing the tasks when we call InvokeEdgeAsync, or RouteExternalMessageAsync.
         await Task.WhenAll(receiverTasks).ConfigureAwait(false);
 
-        // After the message handler invocations, we may have some events to deliver
-        this.EmitPendingEvents();
+        // When we have sub-workflows, sending a message to the WorkflowHostExecutor will only queue it into the
+        // subworkflow's input queue. In order to actually process the message and align the supersteps correctly,
+        // we need to drive the superstep of the subworkflow here.
+        // TODO: Investigate if we can fully pull in the subworkflow execution into the WorkflowHostExecutor itself.
+        List<Task> subworkflowTasks = new();
+        foreach (ISuperStepRunner subworkflowRunner in this.RunContext.JoinedSubworkflowRunners)
+        {
+            subworkflowTasks.Add(subworkflowRunner.RunSuperStepAsync(CancellationToken.None).AsTask());
+        }
+
+        await Task.WhenAll(subworkflowTasks).ConfigureAwait(false);
 
         await this.CheckpointAsync().ConfigureAwait(false);
 
-        this.RaiseWorkflowEvent(this.StepTracer.Complete(this.RunContext.NextStepHasActions, this.RunContext.HasUnservicedRequests));
+        await this.RaiseWorkflowEventAsync(this.StepTracer.Complete(this.RunContext.NextStepHasActions, this.RunContext.HasUnservicedRequests))
+                  .ConfigureAwait(false);
     }
 
     private WorkflowInfo? _workflowInfoCache;
@@ -310,5 +269,5 @@ internal sealed class InProcessRunner : ISuperStepRunner, ICheckpointingRunner
     private bool CheckWorkflowMatch(Checkpoint checkpoint) =>
         checkpoint.Workflow.IsMatch(this.Workflow);
 
-    ValueTask ISuperStepRunner.RequestEndRunAsync() => this.RunContext.EndRunAsync();
+    public ValueTask RequestEndRunAsync() => this.RunContext.EndRunAsync();
 }

dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcessRunnerContext.cs

@@ -4,10 +4,10 @@ using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
-
 using Microsoft.Agents.AI.Workflows.Checkpointing;
 using Microsoft.Agents.AI.Workflows.Execution;
 using Microsoft.Agents.AI.Workflows.Observability;
@@ -32,17 +32,29 @@ internal sealed class InProcessRunnerContext : IRunnerContext
 
     private readonly ConcurrentDictionary<string, Task<Executor>> _executors = new();
     private readonly ConcurrentQueue<Func<ValueTask>> _queuedExternalDeliveries = new();
+    private readonly ConcurrentQueue<ISuperStepRunner> _joinedSubworkflowRunners = new();
 
-    private readonly Dictionary<string, ExternalRequest> _externalRequests = [];
+    private readonly ConcurrentDictionary<string, ExternalRequest> _externalRequests = new();
 
-    public InProcessRunnerContext(Workflow workflow, string runId, IStepTracer? stepTracer, ILogger? logger = null)
+    public InProcessRunnerContext(
+        Workflow workflow,
+        string runId,
+        bool withCheckpointing,
+        IEventSink outgoingEvents,
+        IStepTracer? stepTracer,
+        object? workflowOwnership = null,
+        bool subworkflow = false,
+        ILogger? logger = null)
     {
-        workflow.TakeOwnership(this);
+        workflow.TakeOwnership(this, existingOwnershipSignoff: workflowOwnership);
         this._workflow = workflow;
         this._runId = runId;
 
         this._edgeMap = new(this, this._workflow, stepTracer);
         this._outputFilter = new(workflow);
+
+        this.WithCheckpointing = withCheckpointing;
+        this.OutgoingEvents = outgoingEvents;
     }
 
     public async ValueTask<Executor> EnsureExecutorAsync(string executorId, IStepTracer? tracer)
@@ -57,7 +69,7 @@ internal sealed class InProcessRunnerContext : IRunnerContext
                 throw new InvalidOperationException($"Executor with ID '{executorId}' is not registered.");
             }
 
-            Executor executor = await registration.ProviderAsync().ConfigureAwait(false);
+            Executor executor = await registration.CreateInstanceAsync(this._runId).ConfigureAwait(false);
             tracer?.TraceActivated(executorId);
 
             if (executor is RequestInfoExecutor requestInputExecutor)
@@ -65,12 +77,25 @@ internal sealed class InProcessRunnerContext : IRunnerContext
                 requestInputExecutor.AttachRequestSink(this);
             }
 
+            if (executor is WorkflowHostExecutor workflowHostExecutor)
+            {
+                await workflowHostExecutor.AttachSuperStepContextAsync(this).ConfigureAwait(false);
+            }
+
             return executor;
         }
 
         return await executorTask.ConfigureAwait(false);
     }
 
+    public async ValueTask<IEnumerable<Type>> GetStartingExecutorInputTypesAsync(CancellationToken cancellation = default)
+    {
+        Executor startingExecutor = await this.EnsureExecutorAsync(this._workflow.StartExecutorId, tracer: null)
+                                              .ConfigureAwait(false);
+
+        return startingExecutor.InputTypes;
+    }
+
     public ValueTask AddExternalMessageAsync(object message, Type declaredType)
     {
         this.CheckEnded();
@@ -112,8 +137,13 @@ internal sealed class InProcessRunnerContext : IRunnerContext
         }
     }
 
-    public bool NextStepHasActions => this._nextStep.HasMessages || !this._queuedExternalDeliveries.IsEmpty;
-    public bool HasUnservicedRequests => this._externalRequests.Count > 0;
+    public bool HasQueuedExternalDeliveries => !this._queuedExternalDeliveries.IsEmpty;
+    public bool JoinedRunnersHaveActions => this._joinedSubworkflowRunners.Any(joinedRunner => joinedRunner.HasUnprocessedMessages);
+    public bool NextStepHasActions => this._nextStep.HasMessages ||
+                                      this.HasQueuedExternalDeliveries ||
+                                      this.JoinedRunnersHaveActions;
+    public bool HasUnservicedRequests => !this._externalRequests.IsEmpty ||
+                                      this._joinedSubworkflowRunners.Any(joinedRunner => joinedRunner.HasUnservicedRequests);
 
     public async ValueTask<StepContext> AdvanceAsync()
     {
@@ -132,8 +162,7 @@ internal sealed class InProcessRunnerContext : IRunnerContext
     public ValueTask AddEventAsync(WorkflowEvent workflowEvent)
     {
         this.CheckEnded();
-        this.QueuedEvents.Add(workflowEvent);
-        return default;
+        return this.OutgoingEvents.EnqueueAsync(workflowEvent);
     }
 
     private static readonly string s_namespace = typeof(IWorkflowContext).Namespace!;
@@ -178,17 +207,21 @@ internal sealed class InProcessRunnerContext : IRunnerContext
     public ValueTask PostAsync(ExternalRequest request)
     {
         this.CheckEnded();
-        this._externalRequests.Add(request.RequestId, request);
+        if (!this._externalRequests.TryAdd(request.RequestId, request))
+        {
+            throw new ArgumentException($"Pending request with id '{request.RequestId}' already exists.");
+        }
+
         return this.AddEventAsync(new RequestInfoEvent(request));
     }
 
     public bool CompleteRequest(string requestId)
     {
         this.CheckEnded();
-        return this._externalRequests.Remove(requestId);
+        return this._externalRequests.TryRemove(requestId, out _);
     }
 
-    public readonly List<WorkflowEvent> QueuedEvents = [];
+    private IEventSink OutgoingEvents { get; }
 
     internal StateManager StateManager { get; } = new();
 
@@ -239,7 +272,9 @@ internal sealed class InProcessRunnerContext : IRunnerContext
         public IReadOnlyDictionary<string, string>? TraceContext => traceContext;
     }
 
-    internal Task PrepareForCheckpointAsync(CancellationToken cancellationToken = default)
+    public bool WithCheckpointing { get; }
+
+    internal Task PrepareForCheckpointAsync(CancellationToken cancellation = default)
     {
         this.CheckEnded();
 
@@ -248,7 +283,7 @@ internal sealed class InProcessRunnerContext : IRunnerContext
         async Task InvokeCheckpointingAsync(Task<Executor> executorTask)
         {
             Executor executor = await executorTask.ConfigureAwait(false);
-            await executor.OnCheckpointingAsync(this.Bind(executor.Id), cancellationToken).ConfigureAwait(false);
+            await executor.OnCheckpointingAsync(this.Bind(executor.Id), cancellation).ConfigureAwait(false);
         }
     }
 
@@ -269,11 +304,6 @@ internal sealed class InProcessRunnerContext : IRunnerContext
     {
         this.CheckEnded();
 
-        if (this.QueuedEvents.Count > 0)
-        {
-            throw new InvalidOperationException("Cannot export state when there are queued events. Please process or clear the events before exporting state.");
-        }
-
         Dictionary<string, List<PortableMessageEnvelope>> queuedMessages = this._nextStep.ExportMessages();
         RunnerStateData result = new(instantiatedExecutors: [.. this._executors.Keys],
                                      queuedMessages,
@@ -300,11 +330,6 @@ internal sealed class InProcessRunnerContext : IRunnerContext
     {
         this.CheckEnded();
 
-        if (this.QueuedEvents.Count > 0)
-        {
-            throw new InvalidOperationException("Cannot import state when there are queued events. Please process or clear the events before importing state.");
-        }
-
         RunnerStateData importedState = checkpoint.RunnerData;
 
         Task<Executor>[] executorTasks = importedState.InstantiatedExecutors
@@ -328,7 +353,7 @@ internal sealed class InProcessRunnerContext : IRunnerContext
         await Task.WhenAll(executorTasks).ConfigureAwait(false);
     }
 
-    [System.Diagnostics.CodeAnalysis.SuppressMessage("Maintainability", "CA1513:Use ObjectDisposedException throw helper",
+    [SuppressMessage("Maintainability", "CA1513:Use ObjectDisposedException throw helper",
         Justification = "Does not exist in NetFx 4.7.2")]
     internal void CheckEnded()
     {
@@ -345,4 +370,20 @@ internal sealed class InProcessRunnerContext : IRunnerContext
             await this._workflow.ReleaseOwnershipAsync(this).ConfigureAwait(false);
         }
     }
+
+    public IEnumerable<ISuperStepRunner> JoinedSubworkflowRunners => this._joinedSubworkflowRunners;
+
+    public ValueTask AttachSuperstepAsync(ISuperStepRunner superStepRunner, CancellationToken cancellation = default)
+    {
+        // This needs to be a thread-safe ordered collection because we can potentially instantiate executors
+        // in parallel, which means multiple sub-workflows could be attaching at the same time.
+        this._joinedSubworkflowRunners.Enqueue(superStepRunner);
+        return default;
+    }
+
+    ValueTask ISuperStepJoinContext.ForwardWorkflowEventAsync(WorkflowEvent workflowEvent, CancellationToken cancellation)
+        => this.AddEventAsync(workflowEvent);
+
+    ValueTask ISuperStepJoinContext.SendMessageAsync<TMessage>(string senderId, [DisallowNull] TMessage message, CancellationToken cancellation)
+        => this.SendMessageAsync(senderId, Throw.IfNull(message));
 }

dotnet/src/Microsoft.Agents.AI.Workflows/InProcessExecution.cs

@@ -1,7 +1,11 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.Agents.AI.Workflows.Checkpointing;
+using Microsoft.Agents.AI.Workflows.Execution;
 using Microsoft.Agents.AI.Workflows.InProc;
 
 namespace Microsoft.Agents.AI.Workflows;
@@ -10,71 +14,28 @@ namespace Microsoft.Agents.AI.Workflows;
 /// Provides methods to initiate and manage in-process workflow executions, supporting both streaming and
 /// non-streaming modes with asynchronous operations.
 /// </summary>
-public static class InProcessExecution
+public sealed class InProcessExecution
 {
-    internal static InProcessRunner CreateRunner(Workflow workflow, CheckpointManager? checkpointManager, string? runId)
-        => new(workflow, checkpointManager, runId);
+    private static InProcessExecution DefaultInstance { get; } = new(ExecutionMode.Lockstep);
 
-    internal static InProcessRunner CreateRunner<TInput>(Workflow<TInput> checkedWorkflow, CheckpointManager? checkpointManager, string? runId)
-        where TInput : notnull
-        => new(checkedWorkflow, checkpointManager, runId, [typeof(TInput)]);
-
-    private static ValueTask<StreamingRun> StreamAsync<TInput>(InProcessRunner runner, TInput input, CancellationToken cancellationToken = default)
-        => runner.StreamAsync(input, cancellationToken);
-
-    private static ValueTask<StreamingRun> StreamAsync(InProcessRunner runner, object input, CancellationToken cancellationToken = default)
-        => runner.StreamAsync(input, cancellationToken);
-
-    private static ValueTask<Run> RunAsync<TInput>(InProcessRunner runner, TInput input, CancellationToken cancellationToken = default)
-        => runner.RunAsync(input, cancellationToken);
-
-    private static ValueTask<Run> RunAsync(InProcessRunner runner, object input, CancellationToken cancellationToken = default)
-        => runner.RunAsync(input, cancellationToken);
-
-    private static async ValueTask<Checkpointed<StreamingRun>> StreamCheckpointedAsync<TInput>(InProcessRunner runner, TInput input, CancellationToken cancellationToken = default)
-        where TInput : notnull
+    private readonly ExecutionMode _executionMode;
+    private InProcessExecution(ExecutionMode mode)
     {
-        StreamingRun run = await StreamAsync(runner, input, cancellationToken).ConfigureAwait(false);
-        await runner.CheckpointAsync(cancellationToken).ConfigureAwait(false);
-
-        return new(run, runner);
+        this._executionMode = mode;
     }
 
-    private static async ValueTask<Checkpointed<StreamingRun>> StreamCheckpointedAsync(InProcessRunner runner, object input, CancellationToken cancellationToken = default)
-    {
-        StreamingRun run = await StreamAsync(runner, input, cancellationToken).ConfigureAwait(false);
-        await runner.CheckpointAsync(cancellationToken).ConfigureAwait(false);
+    internal static ExecutionMode DefaultMode => DefaultInstance._executionMode;
 
-        return new(run, runner);
+    internal ValueTask<AsyncRunHandle> BeginRunAsync(Workflow workflow, ICheckpointManager? checkpointManager, string? runId, IEnumerable<Type> knownValidInputTypes, CancellationToken cancellationToken)
+    {
+        InProcessRunner runner = new(workflow, checkpointManager, runId, knownValidInputTypes: knownValidInputTypes);
+        return runner.BeginStreamAsync(this._executionMode, cancellationToken);
     }
 
-    private static async ValueTask<Checkpointed<Run>> RunCheckpointedAsync<TInput>(InProcessRunner runner, TInput input, CancellationToken cancellationToken = default)
-        where TInput : notnull
+    internal ValueTask<AsyncRunHandle> ResumeRunAsync(Workflow workflow, ICheckpointManager? checkpointManager, string? runId, CheckpointInfo fromCheckpoint, IEnumerable<Type> knownValidInputTypes, CancellationToken cancellationToken)
     {
-        Run run = await RunAsync(runner, input, cancellationToken).ConfigureAwait(false);
-        await runner.CheckpointAsync(cancellationToken).ConfigureAwait(false);
-
-        return new(run, runner);
-    }
-
-    private static async ValueTask<Checkpointed<Run>> RunCheckpointedAsync(InProcessRunner runner, object input, CancellationToken cancellationToken = default)
-    {
-        Run run = await RunAsync(runner, input, cancellationToken).ConfigureAwait(false);
-        await runner.CheckpointAsync(cancellationToken).ConfigureAwait(false);
-
-        return new(run, runner);
-    }
-
-    private static async ValueTask<Checkpointed<StreamingRun>> ResumeStreamCheckpointedAsync(InProcessRunner runner, CheckpointInfo fromCheckpoint, CancellationToken cancellationToken = default)
-    {
-        StreamingRun run = await runner.ResumeStreamAsync(fromCheckpoint, cancellationToken).ConfigureAwait(false);
-        return new(run, runner);
-    }
-
-    private static async ValueTask<Checkpointed<Run>> ResumeRunCheckpointedAsync(InProcessRunner runner, CheckpointInfo fromCheckpoint, CancellationToken cancellationToken = default)
-    {
-        Run run = await runner.ResumeAsync(fromCheckpoint, cancellationToken).ConfigureAwait(false);
-        return new(run, runner);
+        InProcessRunner runner = new(workflow, checkpointManager, runId, knownValidInputTypes: knownValidInputTypes);
+        return runner.ResumeStreamAsync(this._executionMode, fromCheckpoint, cancellationToken);
     }
 
     /// <summary>
@@ -87,17 +48,19 @@ public static class InProcessExecution
     /// <param name="workflow">The workflow to be executed. Must not be <c>null</c>.</param>
     /// <param name="input">The input message to be processed as part of the streaming run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{StreamingRun}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="StreamingRun"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<StreamingRun> StreamAsync<TInput>(
+    public static async ValueTask<StreamingRun> StreamAsync<TInput>(
         Workflow workflow,
         TInput input,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager: null, runId);
-        return StreamAsync(runner, (object)input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager: null, runId: runId, [], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.EnqueueAndStreamAsync(input, cancellationToken).ConfigureAwait(false);
     }
 
     /// <summary>
@@ -110,17 +73,19 @@ public static class InProcessExecution
     /// <param name="workflow">The workflow to be executed. Must not be <c>null</c>.</param>
     /// <param name="input">The input message to be processed as part of the streaming run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{StreamingRun}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="StreamingRun"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<StreamingRun> StreamAsync<TInput>(
+    public static async ValueTask<StreamingRun> StreamAsync<TInput>(
         Workflow<TInput> workflow,
         TInput input,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager: null, runId);
-        return StreamAsync(runner, input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager: null, runId: runId, [typeof(TInput)], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.EnqueueAndStreamAsync(input, cancellationToken).ConfigureAwait(false);
     }
 
     /// <summary>
@@ -134,18 +99,21 @@ public static class InProcessExecution
     /// <param name="input">The input message to be processed as part of the streaming run.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{StreamingRun}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="StreamingRun"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Checkpointed<StreamingRun>> StreamAsync<TInput>(
+    public static async ValueTask<Checkpointed<StreamingRun>> StreamAsync<TInput>(
         Workflow workflow,
         TInput input,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager, runId);
-        return StreamCheckpointedAsync(runner, (object)input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager, runId: runId, [], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync(() => runHandle.EnqueueAndStreamAsync(input, cancellationToken))
+                              .ConfigureAwait(false);
     }
 
     /// <summary>
@@ -159,18 +127,21 @@ public static class InProcessExecution
     /// <param name="input">The input message to be processed as part of the streaming run.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{StreamingRun}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="StreamingRun"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Checkpointed<StreamingRun>> StreamAsync<TInput>(
+    public static async ValueTask<Checkpointed<StreamingRun>> StreamAsync<TInput>(
         Workflow<TInput> workflow,
         TInput input,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager, runId);
-        return StreamCheckpointedAsync(runner, input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager, runId: runId, [typeof(TInput)], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync(() => runHandle.EnqueueAndStreamAsync(input, cancellationToken))
+                              .ConfigureAwait(false);
     }
 
     /// <summary>
@@ -182,17 +153,20 @@ public static class InProcessExecution
     /// <param name="fromCheckpoint">The <see cref="CheckpointInfo"/> corresponding to the checkpoint from which to resume.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="StreamingRun"/> that provides access to the results of the streaming run.</returns>
-    public static ValueTask<Checkpointed<StreamingRun>> ResumeStreamAsync(
+    public static async ValueTask<Checkpointed<StreamingRun>> ResumeStreamAsync(
         Workflow workflow,
         CheckpointInfo fromCheckpoint,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default)
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager, runId);
-        return ResumeStreamCheckpointedAsync(runner, fromCheckpoint, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.ResumeRunAsync(workflow, checkpointManager, runId: runId, fromCheckpoint, [], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync<StreamingRun>(() => new(new StreamingRun(runHandle)))
+                              .ConfigureAwait(false);
     }
 
     /// <summary>
@@ -205,17 +179,20 @@ public static class InProcessExecution
     /// <param name="fromCheckpoint">The <see cref="CheckpointInfo"/> corresponding to the checkpoint from which to resume.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="StreamingRun"/> that provides access to the results of the streaming run.</returns>
-    public static ValueTask<Checkpointed<StreamingRun>> ResumeStreamAsync<TInput>(
+    public static async ValueTask<Checkpointed<StreamingRun>> ResumeStreamAsync<TInput>(
         Workflow<TInput> workflow,
         CheckpointInfo fromCheckpoint,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager, runId);
-        return ResumeStreamCheckpointedAsync(runner, fromCheckpoint, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.ResumeRunAsync(workflow, checkpointManager, runId: runId, fromCheckpoint, [typeof(TInput)], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync<StreamingRun>(() => new(new StreamingRun(runHandle)))
+                              .ConfigureAwait(false);
     }
 
     /// <summary>
@@ -227,17 +204,19 @@ public static class InProcessExecution
     /// <param name="workflow">The workflow to be executed. Must not be <c>null</c>.</param>
     /// <param name="input">The input message to be processed as part of the run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{Run}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="Run"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Run> RunAsync<TInput>(
+    public static async ValueTask<Run> RunAsync<TInput>(
         Workflow workflow,
         TInput input,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager: null, runId);
-        return RunAsync(runner, (object)input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager: null, runId: runId, [], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.EnqueueAndRunAsync(input, cancellationToken).ConfigureAwait(false);
     }
 
     /// <summary>
@@ -249,17 +228,19 @@ public static class InProcessExecution
     /// <param name="workflow">The workflow to be executed. Must not be <c>null</c>.</param>
     /// <param name="input">The input message to be processed as part of the run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{Run}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="Run"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Run> RunAsync<TInput>(
+    public static async ValueTask<Run> RunAsync<TInput>(
         Workflow<TInput> workflow,
         TInput input,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager: null, runId);
-        return RunAsync(runner, input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager: null, runId: runId, [typeof(TInput)], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.EnqueueAndRunAsync(input, cancellationToken).ConfigureAwait(false);
     }
 
     /// <summary>
@@ -272,18 +253,21 @@ public static class InProcessExecution
     /// <param name="input">The input message to be processed as part of the run.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{Run}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="Run"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Checkpointed<Run>> RunAsync<TInput>(
+    public static async ValueTask<Checkpointed<Run>> RunAsync<TInput>(
         Workflow workflow,
         TInput input,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager: checkpointManager, runId);
-        return RunCheckpointedAsync(runner, (object)input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager, runId: runId, [], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync(() => runHandle.EnqueueAndRunAsync(input, cancellationToken))
+                              .ConfigureAwait(false);
     }
 
     /// <summary>
@@ -296,18 +280,21 @@ public static class InProcessExecution
     /// <param name="input">The input message to be processed as part of the run.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{Run}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="Run"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Checkpointed<Run>> RunAsync<TInput>(
+    public static async ValueTask<Checkpointed<Run>> RunAsync<TInput>(
         Workflow<TInput> workflow,
         TInput input,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager: checkpointManager, runId);
-        return RunCheckpointedAsync(runner, input, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.BeginRunAsync(workflow, checkpointManager, runId: runId, [typeof(TInput)], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync(() => runHandle.EnqueueAndRunAsync(input, cancellationToken))
+                              .ConfigureAwait(false);
     }
 
     /// <summary>
@@ -319,18 +306,21 @@ public static class InProcessExecution
     /// <param name="fromCheckpoint">The <see cref="CheckpointInfo"/> corresponding to the checkpoint from which to resume.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{Run}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="Run"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Checkpointed<Run>> ResumeAsync(
+    public static async ValueTask<Checkpointed<Run>> ResumeAsync(
         Workflow workflow,
         CheckpointInfo fromCheckpoint,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default)
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager, runId);
-        return ResumeRunCheckpointedAsync(runner, fromCheckpoint, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.ResumeRunAsync(workflow, checkpointManager, runId: runId, fromCheckpoint, [], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync<Run>(() => new(new Run(runHandle)))
+                              .ConfigureAwait(false);
     }
 
     /// <summary>
@@ -342,17 +332,20 @@ public static class InProcessExecution
     /// <param name="fromCheckpoint">The <see cref="CheckpointInfo"/> corresponding to the checkpoint from which to resume.</param>
     /// <param name="checkpointManager">The <see cref="CheckpointManager"/> to use with this run.</param>
     /// <param name="runId">An optional unique identifier for the run. If not provided, a new identifier will be generated.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellationToken requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ValueTask{Run}"/> that represents the asynchronous operation. The result contains a <see
     /// cref="Run"/> for managing and interacting with the streaming run.</returns>
-    public static ValueTask<Checkpointed<Run>> ResumeAsync<TInput>(
+    public static async ValueTask<Checkpointed<Run>> ResumeAsync<TInput>(
         Workflow<TInput> workflow,
         CheckpointInfo fromCheckpoint,
         CheckpointManager checkpointManager,
         string? runId = null,
         CancellationToken cancellationToken = default) where TInput : notnull
     {
-        InProcessRunner runner = CreateRunner(workflow, checkpointManager, runId);
-        return ResumeRunCheckpointedAsync(runner, fromCheckpoint, cancellationToken);
+        AsyncRunHandle runHandle = await DefaultInstance.ResumeRunAsync(workflow, checkpointManager, runId: runId, fromCheckpoint, [typeof(TInput)], cancellationToken)
+                                                        .ConfigureAwait(false);
+
+        return await runHandle.WithCheckpointingAsync<Run>(() => new(new Run(runHandle)))
+                              .ConfigureAwait(false);
     }
 }

dotnet/src/Microsoft.Agents.AI.Workflows/InputPort.cs

@@ -1,24 +0,0 @@
-// Copyright (c) Microsoft. All rights reserved.
-
-using System;
-
-namespace Microsoft.Agents.AI.Workflows;
-
-/// <summary>
-/// An external request port for a <see cref="Workflow"/> with the specified request and response types.
-/// </summary>
-/// <param name="Id"></param>
-/// <param name="Request"></param>
-/// <param name="Response"></param>
-public sealed record InputPort(string Id, Type Request, Type Response)
-{
-    /// <summary>
-    /// Creates a new <see cref="InputPort"/> instance configured for the specified request and response types.
-    /// </summary>
-    /// <typeparam name="TRequest">The type of the request messages that the input port will accept.</typeparam>
-    /// <typeparam name="TResponse">The type of the response messages that the input port will produce.</typeparam>
-    /// <param name="id">The unique identifier for the input port.</param>
-    /// <returns>An <see cref="InputPort"/> instance associated with the specified <paramref name="id"/>, configured to handle
-    /// requests of type <typeparamref name="TRequest"/> and responses of type <typeparamref name="TResponse"/>.</returns>
-    public static InputPort Create<TRequest, TResponse>(string id) => new(id, typeof(TRequest), typeof(TResponse));
-};

dotnet/src/Microsoft.Agents.AI.Workflows/PortableValue.cs

@@ -150,7 +150,7 @@ public sealed class PortableValue
     /// <param name="value">When this method returns, contains the value cast or deserialized to type TValue
     /// if the conversion succeeded, or null if the conversion failed.</param>
     /// <returns>true if the current instance can be assigned to targetType; otherwise, false.</returns>
-    public bool IsType(Type targetType, out object? value)
+    public bool IsType(Type targetType, [NotNullWhen(true)] out object? value)
     {
         Throw.IfNull(targetType);
         if (this.Value is IDelayedDeserialization delayedDeserialization)

dotnet/src/Microsoft.Agents.AI.Workflows/RequestPort.cs

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+
+namespace Microsoft.Agents.AI.Workflows;
+
+/// <summary>
+/// An external request port for a <see cref="Workflow"/> with the specified request and response types.
+/// </summary>
+/// <param name="Id"></param>
+/// <param name="Request"></param>
+/// <param name="Response"></param>
+public record RequestPort(string Id, Type Request, Type Response)
+{
+    /// <summary>
+    /// Creates a new <see cref="RequestPort"/> instance configured for the specified request and response types.
+    /// </summary>
+    /// <typeparam name="TRequest">The type of the request messages that the input port will accept.</typeparam>
+    /// <typeparam name="TResponse">The type of the response messages that the input port will produce.</typeparam>
+    /// <param name="id">The unique identifier for the input port.</param>
+    /// <returns>An <see cref="RequestPort"/> instance associated with the specified <paramref name="id"/>, configured to handle
+    /// requests of type <typeparamref name="TRequest"/> and responses of type <typeparamref name="TResponse"/>.</returns>
+    public static RequestPort<TRequest, TResponse> Create<TRequest, TResponse>(string id) => new(id, typeof(TRequest), typeof(TResponse));
+};
+
+/// <summary>
+/// An external request port for a <see cref="Workflow"/> with the specified request and response types.
+/// </summary>
+/// <param name="Id"></param>
+/// <param name="Request"></param>
+/// <param name="Response"></param>
+/// <param name="AllowWrapped"></param>
+public sealed record RequestPort<TRequest, TResponse>(string Id, Type Request, Type Response, bool AllowWrapped = false) : RequestPort(Id, Request, Response);

dotnet/src/Microsoft.Agents.AI.Workflows/RouteBuilder.cs

@@ -73,7 +73,7 @@ public class RouteBuilder
         return this;
     }
 
-    internal RouteBuilder AddHandler(Type type, Func<object, IWorkflowContext, ValueTask> handler, bool overwrite = false)
+    internal RouteBuilder AddHandlerUntyped(Type type, Func<object, IWorkflowContext, ValueTask> handler, bool overwrite = false)
     {
         Throw.IfNull(handler);
 
@@ -86,7 +86,7 @@ public class RouteBuilder
         }
     }
 
-    internal RouteBuilder AddHandler<TResult>(Type type, Func<object, IWorkflowContext, ValueTask<TResult>> handler, bool overwrite = false)
+    internal RouteBuilder AddHandlerUntyped<TResult>(Type type, Func<object, IWorkflowContext, ValueTask<TResult>> handler, bool overwrite = false)
     {
         Throw.IfNull(handler);
 

dotnet/src/Microsoft.Agents.AI.Workflows/Run.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.Agents.AI.Workflows.Execution;
 
 namespace Microsoft.Agents.AI.Workflows;
 
@@ -12,6 +13,11 @@ namespace Microsoft.Agents.AI.Workflows;
 /// </summary>
 public enum RunStatus
 {
+    /// <summary>
+    /// The run has not yet started. This only occurs when running in "lockstep" mode.
+    /// </summary>
+    NotStarted,
+
     /// <summary>
     /// The run has halted, has no outstanding requets, but has not received a <see cref="RequestHaltEvent"/>.
     /// </summary>
@@ -22,11 +28,12 @@ public enum RunStatus
     /// </summary>
     PendingRequests,
 
-    // TODO: Figure out if we want to have some way to have a true "converged" state
-    ///// <summary>
-    ///// The run has halted after converging.
-    ///// </summary>
-    //Completed,
+    /// <summary>
+    /// The user has ended the run. No further events will be emitted, and no messages can be sent to it.
+    /// </summary>
+    /// <seealso cref="StreamingRun.EndRunAsync"/>
+    /// <seealso cref="Run.EndRunAsync"/>
+    Ended,
 
     /// <summary>
     /// The workflow is currently running, and may receive events or requests.
@@ -40,49 +47,35 @@ public enum RunStatus
 /// </summary>
 public sealed class Run
 {
-    internal static async ValueTask<Run> CaptureStreamAsync(StreamingRun run, CancellationToken cancellationToken = default)
-    {
-        Run result = new(run);
-        await result.RunToNextHaltAsync(cancellationToken).ConfigureAwait(false);
-        return result;
-    }
-
     private readonly List<WorkflowEvent> _eventSink = [];
-    private readonly StreamingRun _streamingRun;
-    internal Run(StreamingRun streamingRun)
+    private readonly AsyncRunHandle _runHandle;
+    internal Run(AsyncRunHandle _runHandle)
     {
-        this._streamingRun = streamingRun;
+        this._runHandle = _runHandle;
     }
 
     internal async ValueTask<bool> RunToNextHaltAsync(CancellationToken cancellationToken = default)
     {
         bool hadEvents = false;
-        this.Status = RunStatus.Running;
-        await foreach (WorkflowEvent evt in this._streamingRun.WatchStreamAsync(blockOnPendingRequest: false, cancellationToken).ConfigureAwait(false))
+        await foreach (WorkflowEvent evt in this._runHandle.TakeEventStreamAsync(breakOnHalt: true, cancellationToken).ConfigureAwait(false))
         {
             hadEvents = true;
             this._eventSink.Add(evt);
         }
 
-        // TODO: bookmark every halt for history visualization?
-
-        this.Status =
-            this._streamingRun.HasUnservicedRequests
-              ? RunStatus.PendingRequests
-              : RunStatus.Idle;
-
         return hadEvents;
     }
 
     /// <summary>
     /// A unique identifier for the run. Can be provided at the start of the run, or auto-generated.
     /// </summary>
-    public string RunId => this._streamingRun.RunId;
+    public string RunId => this._runHandle.RunId;
 
     /// <summary>
     /// Gets the current execution status of the workflow run.
     /// </summary>
-    public RunStatus Status { get; private set; }
+    public ValueTask<RunStatus> GetStatusAsync(CancellationToken cancellation = default)
+        => this._runHandle.GetStatusAsync(cancellation);
 
     /// <summary>
     /// Gets all events emitted by the workflow.
@@ -120,7 +113,7 @@ public sealed class Run
     {
         foreach (ExternalResponse response in responses)
         {
-            await this._streamingRun.SendResponseAsync(response).ConfigureAwait(false);
+            await this._runHandle.EnqueueResponseAsync(response, cancellationToken).ConfigureAwait(false);
         }
 
         return await this.RunToNextHaltAsync(cancellationToken).ConfigureAwait(false);
@@ -129,25 +122,36 @@ public sealed class Run
     /// <summary>
     /// Resume execution of the workflow with the provided external responses.
     /// </summary>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <param name="messages">An array of messages to send to the workflow. Messages will only be sent if they are valid
     /// input types to the starting executor or a <see cref="ExternalResponse"/>.</param>
-    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns><c>true</c> if the workflow had any output events, <c>false</c> otherwise.</returns>
-    public async ValueTask<bool> ResumeAsync<T>(IEnumerable<T> messages, CancellationToken cancellationToken = default)
+    public async ValueTask<bool> ResumeAsync<T>(CancellationToken cancellationToken = default, params IEnumerable<T> messages)
+        where T : notnull
     {
         if (messages is IEnumerable<ExternalResponse> responses)
         {
             return await this.ResumeAsync(responses, cancellationToken).ConfigureAwait(false);
         }
 
-        foreach (T message in messages)
+        if (typeof(T) == typeof(object))
         {
-            await this._streamingRun.TrySendMessageAsync(message).ConfigureAwait(false);
+            foreach (object? message in messages)
+            {
+                await this._runHandle.EnqueueMessageUntypedAsync(message, cancellation: cancellationToken).ConfigureAwait(false);
+            }
+        }
+        else
+        {
+            foreach (T message in messages)
+            {
+                await this._runHandle.EnqueueMessageAsync(message, cancellationToken).ConfigureAwait(false);
+            }
         }
 
         return await this.RunToNextHaltAsync(cancellationToken).ConfigureAwait(false);
     }
 
     /// <inheritdoc cref="StreamingRun.EndRunAsync"/>
-    public ValueTask EndRunAsync() => this._streamingRun.EndRunAsync();
+    public ValueTask EndRunAsync() => this._runHandle.RequestEndRunAsync();
 }

dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/RequestInfoExecutor.cs

@@ -9,21 +9,26 @@ using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Agents.AI.Workflows.Specialized;
 
+internal sealed class RequestPortOptions
+{
+}
+
 internal sealed class RequestInfoExecutor : Executor
 {
-    private readonly Dictionary<string, ExternalRequest> _wrappedRequests = [];
-    private InputPort Port { get; }
+    private readonly Dictionary<string, ExternalRequest> _wrappedRequests = new();
+    private RequestPort Port { get; }
     private IExternalRequestSink? RequestSink { get; set; }
 
     private static ExecutorOptions DefaultOptions => new()
     {
         // We need to be able to return the ExternalRequest/Result objects so they can be bubbled up
         // through the event system, but we do not want to forward the Request message.
-        AutoSendMessageHandlerResultObject = false
+        AutoSendMessageHandlerResultObject = false,
+        AutoYieldOutputHandlerResultObject = false
     };
 
     private readonly bool _allowWrapped;
-    public RequestInfoExecutor(InputPort port, bool allowWrapped = true) : base(port.Id, DefaultOptions)
+    public RequestInfoExecutor(RequestPort port, bool allowWrapped = true) : base(port.Id, DefaultOptions)
     {
         this.Port = port;
 
@@ -34,7 +39,7 @@ internal sealed class RequestInfoExecutor : Executor
     {
         routeBuilder = routeBuilder
             // Handle incoming requests (as raw request payloads)
-            .AddHandler(this.Port.Request, this.HandleAsync)
+            .AddHandlerUntyped(this.Port.Request, this.HandleAsync)
             .AddCatchAll(this.HandleCatchAllAsync);
 
         if (this._allowWrapped)
@@ -45,12 +50,12 @@ internal sealed class RequestInfoExecutor : Executor
 
         return routeBuilder
             // Handle incoming responses (as wrapped Response object)
-            .AddHandler<ExternalResponse, ExternalResponse>(this.HandleAsync);
+            .AddHandler<ExternalResponse, ExternalResponse?>(this.HandleAsync);
     }
 
     internal void AttachRequestSink(IExternalRequestSink requestSink) => this.RequestSink = Throw.IfNull(requestSink);
 
-    public async ValueTask<ExternalRequest> HandleCatchAllAsync(PortableValue message, IWorkflowContext context)
+    public async ValueTask<ExternalRequest?> HandleCatchAllAsync(PortableValue message, IWorkflowContext context)
     {
         Throw.IfNull(message);
 
@@ -61,9 +66,14 @@ internal sealed class RequestInfoExecutor : Executor
 
             ExternalRequest request = ExternalRequest.Create(this.Port, maybeRequest!);
             await this.RequestSink!.PostAsync(request).ConfigureAwait(false);
+            return request;
+        }
+        else if (message.Is(out ExternalRequest? request))
+        {
+            return await this.HandleAsync(request, context).ConfigureAwait(false);
         }
 
-        throw new InvalidOperationException($"Message type {message.TypeId} could not be interpreted as a value of Request Type {this.Port.Request}");
+        return null;
     }
 
     public async ValueTask<ExternalRequest> HandleAsync(ExternalRequest message, IWorkflowContext context)
@@ -71,7 +81,7 @@ internal sealed class RequestInfoExecutor : Executor
         Debug.Assert(this._allowWrapped);
         Throw.IfNull(message);
 
-        if (!message.Data.IsType(this.Port.Request))
+        if (!message.Data.IsType(this.Port.Request, out var requestData))
         {
             throw new InvalidOperationException($"Message type {message.Data.TypeId} could not be interpreted as a value of Request Type {this.Port.Request}");
         }
@@ -81,9 +91,9 @@ internal sealed class RequestInfoExecutor : Executor
             throw new InvalidOperationException($"Response type {this.Port.Response} is not a valid response for original request, whose expected response is {message.PortInfo.ResponseType}");
         }
 
-        ExternalRequest request = ExternalRequest.Create(this.Port, message);
+        ExternalRequest request = ExternalRequest.Create(this.Port, requestData, message.RequestId);
 
-        this._wrappedRequests.Add(request.RequestId, message);
+        this._wrappedRequests.Add(message.RequestId, message);
 
         await this.RequestSink!.PostAsync(request).ConfigureAwait(false);
 
@@ -101,11 +111,16 @@ internal sealed class RequestInfoExecutor : Executor
         return request;
     }
 
-    public async ValueTask<ExternalResponse> HandleAsync(ExternalResponse message, IWorkflowContext context)
+    public async ValueTask<ExternalResponse?> HandleAsync(ExternalResponse message, IWorkflowContext context)
     {
         Throw.IfNull(message);
         Throw.IfNull(message.Data);
 
+        if (message.PortInfo.PortId != this.Port.Id)
+        {
+            return null;
+        }
+
         object data = message.DataAs(this.Port.Response) ??
             throw new InvalidOperationException(
                 $"Message type {message.Data.TypeId} is not assignable to the response type {this.Port.Response.Name} of input port {this.Port.Id}.");

dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/WorkflowHostExecutor.cs

@@ -0,0 +1,266 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Workflows.Checkpointing;
+using Microsoft.Agents.AI.Workflows.Execution;
+using Microsoft.Agents.AI.Workflows.InProc;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Workflows.Specialized;
+
+internal class WorkflowHostExecutor : Executor, IResettableExecutor
+{
+    private readonly string _runId;
+    private readonly Workflow _workflow;
+    private readonly object _ownershipToken;
+
+    private InProcessRunner? _activeRunner;
+    private InMemoryCheckpointManager? _checkpointManager;
+    private readonly ExecutorOptions _options;
+
+    private ISuperStepJoinContext? _joinContext;
+    private StreamingRun? _run;
+
+    [MemberNotNullWhen(true, nameof(_checkpointManager))]
+    private bool WithCheckpointing => this._checkpointManager != null;
+
+    public WorkflowHostExecutor(string id, Workflow workflow, string runId, object ownershipToken, ExecutorOptions? options = null) : base(id, options)
+    {
+        this._options = options ?? new();
+
+        Throw.IfNull(workflow);
+        this._runId = Throw.IfNull(runId);
+        this._ownershipToken = Throw.IfNull(ownershipToken);
+        this._workflow = Throw.IfNull(workflow);
+    }
+
+    protected override RouteBuilder ConfigureRoutes(RouteBuilder routeBuilder)
+    {
+        return routeBuilder.AddCatchAll(this.QueueExternalMessageAsync);
+    }
+
+    private async ValueTask QueueExternalMessageAsync(PortableValue portableValue, IWorkflowContext context)
+    {
+        if (portableValue.Is(out ExternalResponse? response))
+        {
+            response = this.CheckAndUnqualifyResponse(response);
+            await this.EnsureRunSendMessageAsync(response).ConfigureAwait(false);
+        }
+        else
+        {
+            InProcessRunner runner = await this.EnsureRunnerAsync().ConfigureAwait(false);
+            IEnumerable<Type> validInputTypes = await runner.RunContext.GetStartingExecutorInputTypesAsync().ConfigureAwait(false);
+            foreach (Type candidateType in validInputTypes)
+            {
+                if (portableValue.IsType(candidateType, out object? message))
+                {
+                    await this.EnsureRunSendMessageAsync(message, candidateType).ConfigureAwait(false);
+                    return;
+                }
+            }
+        }
+    }
+
+    private ISuperStepJoinContext JoinContext => Throw.IfNull(this._joinContext, "Must attach to a join context before starting the run.");
+
+    internal async ValueTask<InProcessRunner> EnsureRunnerAsync()
+    {
+        if (this._activeRunner == null)
+        {
+            if (this.JoinContext.WithCheckpointing)
+            {
+                // Use a seprate in-memory checkpoint manager for scoping purposes. We do not need to worry about
+                // serialization because we will be relying on the parent workflow's checkpoint manager to do that,
+                // if needed. For our purposes, all we need is to keep a faithful representation of the checkpointed
+                // objects so we can emit them back to the parent workflow on checkpoint creation.
+                this._checkpointManager = new InMemoryCheckpointManager();
+            }
+
+            this._activeRunner = new(this._workflow, this._checkpointManager, this._runId, this._ownershipToken, subworkflow: true);
+        }
+
+        return this._activeRunner;
+    }
+
+    internal async ValueTask<StreamingRun> EnsureRunSendMessageAsync(object? incomingMessage = null, Type? incomingMessageType = null, bool resume = false, CancellationToken cancellation = default)
+    {
+        Debug.Assert(this._joinContext != null, "Must attach to a join context before starting the run.");
+
+        if (this._run != null)
+        {
+            if (incomingMessage != null)
+            {
+                await this._run.TrySendMessageUntypedAsync(incomingMessage, incomingMessageType ?? incomingMessage.GetType()).ConfigureAwait(false);
+            }
+
+            return this._run;
+        }
+
+        InProcessRunner activeRunner = await this.EnsureRunnerAsync().ConfigureAwait(false);
+        AsyncRunHandle runHandle;
+
+        if (this.WithCheckpointing)
+        {
+            if (resume)
+            {
+                // Attempting to resume from checkpoint
+                if (!this._checkpointManager.TryGetLastCheckpoint(this._runId, out CheckpointInfo? lastCheckpoint))
+                {
+                    throw new InvalidOperationException("No checkpoints available to resume from.");
+                }
+
+                runHandle = await activeRunner.ResumeStreamAsync(InProcessExecution.DefaultMode, lastCheckpoint!, cancellation)
+                                                             .ConfigureAwait(false);
+                if (incomingMessage != null)
+                {
+                    await runHandle.EnqueueUntypedAndRunAsync(incomingMessage, cancellation).ConfigureAwait(false);
+                }
+            }
+            else if (incomingMessage != null)
+            {
+                runHandle = await activeRunner.BeginStreamAsync(InProcessExecution.DefaultMode, cancellation)
+                                                             .ConfigureAwait(false);
+
+                await runHandle.EnqueueUntypedAndRunAsync(incomingMessage, cancellation).ConfigureAwait(false);
+            }
+            else
+            {
+                throw new InvalidOperationException("Cannot start a checkpointed workflow run without an incoming message or resume flag.");
+            }
+        }
+        else
+        {
+            runHandle = await activeRunner.BeginStreamAsync(InProcessExecution.DefaultMode, cancellation).ConfigureAwait(false);
+
+            await runHandle.EnqueueMessageUntypedAsync(Throw.IfNull(incomingMessage), cancellation: cancellation).ConfigureAwait(false);
+        }
+
+        this._run = new(runHandle);
+
+        await this._joinContext.AttachSuperstepAsync(activeRunner, cancellation).ConfigureAwait(false);
+        activeRunner.OutgoingEvents.EventRaised += this.ForwardWorkflowEventAsync;
+
+        return this._run;
+    }
+
+    private ExternalResponse? CheckAndUnqualifyResponse([DisallowNull] ExternalResponse response)
+    {
+        if (!Throw.IfNull(response).PortInfo.PortId.StartsWith($"{this.Id}.", StringComparison.Ordinal))
+        {
+            return null;
+        }
+
+        RequestPortInfo unqualifiedPort = response.PortInfo with { PortId = response.PortInfo.PortId.Substring(this.Id.Length + 1) };
+        return response with { PortInfo = unqualifiedPort };
+    }
+
+    private ExternalRequest QualifyRequestPortId(ExternalRequest internalRequest)
+    {
+        RequestPortInfo requestPort = internalRequest.PortInfo with { PortId = $"{this.Id}.{internalRequest.PortInfo.PortId}" };
+        return internalRequest with { PortInfo = requestPort };
+    }
+
+    private async ValueTask ForwardWorkflowEventAsync(object? sender, WorkflowEvent evt)
+    {
+        // Note that we are explicitly not using the checked JoinContext property here, because this is an async callback.
+        try
+        {
+            Task resultTask = Task.CompletedTask;
+            switch (evt)
+            {
+                case WorkflowStartedEvent:
+                case SuperStepStartedEvent:
+                case SuperStepCompletedEvent:
+                    // These events are internal to the subworkflow and do not need to be forwarded.
+                    break;
+                case RequestInfoEvent requestInfoEvt:
+                    ExternalRequest request = requestInfoEvt.Request;
+                    resultTask = this._joinContext?.SendMessageAsync(this.Id, this.QualifyRequestPortId(request)).AsTask() ?? Task.CompletedTask;
+                    break;
+                case WorkflowErrorEvent errorEvent:
+                    resultTask = this._joinContext?.ForwardWorkflowEventAsync(new SubworkflowErrorEvent(this.Id, errorEvent.Data as Exception)).AsTask() ?? Task.CompletedTask;
+                    break;
+                case WorkflowOutputEvent outputEvent:
+                    if (this._joinContext != null &&
+                        this._options.AutoSendMessageHandlerResultObject
+                        && outputEvent.Data != null)
+                    {
+                        resultTask = this._joinContext.SendMessageAsync(this.Id, outputEvent.Data).AsTask();
+                    }
+                    break;
+                case RequestHaltEvent requestHaltEvent:
+                    resultTask = this._joinContext?.ForwardWorkflowEventAsync(new RequestHaltEvent()).AsTask() ?? Task.CompletedTask;
+                    break;
+                case WorkflowWarningEvent warningEvent:
+                    if (warningEvent.Data is string warningMessage)
+                    {
+                        resultTask = this._joinContext?.ForwardWorkflowEventAsync(new SubworkflowWarningEvent(this.Id, warningMessage)).AsTask() ?? Task.CompletedTask;
+                    }
+                    break;
+                default:
+                    resultTask = this._joinContext?.ForwardWorkflowEventAsync(evt).AsTask() ?? Task.CompletedTask;
+                    break;
+            }
+
+            await resultTask.ConfigureAwait(false);
+        }
+        catch (Exception ex)
+        {
+            try
+            {
+                _ = this._joinContext?.ForwardWorkflowEventAsync(new SubworkflowErrorEvent(this.Id, ex)).AsTask();
+            }
+            catch
+            { }
+        }
+    }
+
+    internal async ValueTask AttachSuperStepContextAsync(ISuperStepJoinContext joinContext)
+    {
+        this._joinContext = Throw.IfNull(joinContext);
+    }
+
+    protected internal override async ValueTask OnCheckpointingAsync(IWorkflowContext context, CancellationToken cancellationToken = default)
+    {
+        await context.QueueStateUpdateAsync(nameof(CheckpointManager), this._checkpointManager).ConfigureAwait(false);
+
+        await base.OnCheckpointingAsync(context, cancellationToken).ConfigureAwait(false);
+    }
+
+    protected internal override async ValueTask OnCheckpointRestoredAsync(IWorkflowContext context, CancellationToken cancellationToken = default)
+    {
+        await base.OnCheckpointRestoredAsync(context, cancellationToken).ConfigureAwait(false);
+
+        InMemoryCheckpointManager manager = await context.ReadStateAsync<InMemoryCheckpointManager>(nameof(InMemoryCheckpointManager)).ConfigureAwait(false) ?? new();
+        if (this._checkpointManager == manager)
+        {
+            // We are restoring in the context of the same run; not need to rebuild the entire execution stack.
+        }
+        else
+        {
+            this._checkpointManager = manager;
+
+            await this.ResetAsync().ConfigureAwait(false);
+        }
+
+        StreamingRun run = await this.EnsureRunSendMessageAsync(cancellation: cancellationToken).ConfigureAwait(false);
+    }
+
+    public async ValueTask ResetAsync()
+    {
+        this._run = null;
+
+        if (this._activeRunner != null)
+        {
+            this._activeRunner.OutgoingEvents.EventRaised -= this.ForwardWorkflowEventAsync;
+            await this._activeRunner.RequestEndRunAsync().ConfigureAwait(false);
+
+            this._activeRunner = new(this._workflow, this._checkpointManager, this._runId);
+        }
+    }
+}

dotnet/src/Microsoft.Agents.AI.Workflows/StreamingRun.cs

@@ -2,13 +2,11 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 
 using Microsoft.Agents.AI.Workflows.Execution;
-using Microsoft.Agents.AI.Workflows.Observability;
 using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Agents.AI.Workflows;
@@ -19,27 +17,26 @@ namespace Microsoft.Agents.AI.Workflows;
 /// </summary>
 public sealed class StreamingRun
 {
-    private TaskCompletionSource<object>? _waitForResponseSource;
-    private readonly ISuperStepRunner _stepRunner;
+    private readonly AsyncRunHandle _runHandle;
 
-    private static readonly string s_namespace = typeof(StreamingRun).Namespace!;
-    private static readonly ActivitySource s_activitySource = new(s_namespace);
-
-    /// <summary>
-    /// Gets a value indicating whether there are any outstanding <see cref="ExternalRequest"/>s for which a
-    /// <see cref="ExternalResponse"/> has not been sent.
-    /// </summary>
-    public bool HasUnservicedRequests => this._stepRunner.HasUnservicedRequests;
-
-    internal StreamingRun(ISuperStepRunner stepRunner)
+    internal StreamingRun(AsyncRunHandle runHandle)
     {
-        this._stepRunner = Throw.IfNull(stepRunner);
+        this._runHandle = Throw.IfNull(runHandle);
     }
 
+    private ValueTask<bool> WaitOnInputAsync(CancellationToken cancellation = default)
+        => this._runHandle.WaitForNextInputAsync(cancellation);
+
     /// <summary>
     /// A unique identifier for the run. Can be provided at the start of the run, or auto-generated.
     /// </summary>
-    public string RunId => this._stepRunner.RunId;
+    public string RunId => this._runHandle.RunId;
+
+    /// <summary>
+    /// Gets the current execution status of the workflow run.
+    /// </summary>
+    public ValueTask<RunStatus> GetStatusAsync(CancellationToken cancellation = default)
+        => this._runHandle.GetStatusAsync(cancellation);
 
     /// <summary>
     /// Asynchronously sends the specified response to the external system and signals completion of the current
@@ -49,11 +46,7 @@ public sealed class StreamingRun
     /// <param name="response">The <see cref="ExternalResponse"/> to send. Must not be <c>null</c>.</param>
     /// <returns>A <see cref="ValueTask"/> that represents the asynchronous send operation.</returns>
     public ValueTask SendResponseAsync(ExternalResponse response)
-    {
-        this._waitForResponseSource?.TrySetResult(new());
-
-        return this._stepRunner.EnqueueResponseAsync(response);
-    }
+        => this._runHandle.EnqueueResponseAsync(response);
 
     /// <summary>
     /// Attempts to send the specified message asynchronously and returns a value indicating whether the operation was
@@ -65,18 +58,11 @@ public sealed class StreamingRun
     /// <returns>A <see cref="ValueTask{Boolean}"/> that represents the asynchronous send operation. It's
     /// <see cref="ValueTask{Boolean}.Result"/> is <see langword="true"/> if the message was sent
     /// successfully; otherwise, <see langword="false"/>.</returns>
-    public async ValueTask<bool> TrySendMessageAsync<TMessage>(TMessage message)
-    {
-        Throw.IfNull(message);
+    public ValueTask<bool> TrySendMessageAsync<TMessage>(TMessage message)
+        => this._runHandle.EnqueueMessageAsync(message);
 
-        if (message is ExternalResponse response)
-        {
-            await this.SendResponseAsync(response).ConfigureAwait(false);
-            return true;
-        }
-
-        return await this._stepRunner.EnqueueMessageAsync(message).ConfigureAwait(false);
-    }
+    internal ValueTask<bool> TrySendMessageUntypedAsync(object message, Type? declaredType = null)
+        => this._runHandle.EnqueueMessageUntypedAsync(message, declaredType);
 
     /// <summary>
     /// Asynchronously streams workflow events as they occur during workflow execution.
@@ -96,94 +82,35 @@ public sealed class StreamingRun
         bool blockOnPendingRequest,
         [EnumeratorCancellation] CancellationToken cancellationToken = default)
     {
-        List<WorkflowEvent> eventSink = [];
+        RunStatus runStatus;
 
-        this._stepRunner.WorkflowEvent += OnWorkflowEvent;
-
-        using Activity? activity = s_activitySource.StartActivity(ActivityNames.WorkflowRun);
-        activity?.SetTag(Tags.WorkflowId, this._stepRunner.StartExecutorId).SetTag(Tags.RunId, this.RunId);
-
-        try
+        do
         {
-            activity?.AddEvent(new ActivityEvent(EventNames.WorkflowStarted));
-            do
+            await foreach (WorkflowEvent @event in this._runHandle.TakeEventStreamAsync(breakOnHalt: true, cancellationToken)
+                                                                  .WithCancellation(cancellationToken)
+                                                                  .ConfigureAwait(false))
             {
-                // Because we may be yielding out of this function, we need to ensure that the Activity.Current
-                // is set to our activity for the duration of this loop iteration.
-                Activity.Current = activity;
+                yield return @event;
+            }
 
-                // Drain SuperSteps while there are steps to run
-                try
-                {
-                    await this._stepRunner.RunSuperStepAsync(cancellationToken).ConfigureAwait(false);
-                }
-                catch (Exception ex) when (activity is not null)
-                {
-                    activity.AddEvent(new ActivityEvent(EventNames.WorkflowError, tags: new() {
-                        { Tags.ErrorType, ex.GetType().FullName },
-                        { Tags.BuildErrorMessage, ex.Message },
-                    }));
-                    activity.CaptureException(ex);
-                    throw;
-                }
+            if (cancellationToken.IsCancellationRequested)
+            {
+                yield break; // We are done.
+            }
 
-                if (cancellationToken.IsCancellationRequested)
-                {
-                    yield break; // Exit if cancellation is requested
-                }
+            runStatus = await this._runHandle.GetStatusAsync(cancellationToken).ConfigureAwait(false);
+            if (runStatus == RunStatus.Idle)
+            {
+                yield break; // We are done.
+            }
 
-                bool hadCompletionEvent = false;
-                foreach (WorkflowEvent raisedEvent in Interlocked.Exchange(ref eventSink, []))
-                {
-                    if (cancellationToken.IsCancellationRequested)
-                    {
-                        yield break; // Exit if cancellation is requested
-                    }
-
-                    // TODO: Do we actually want to interpret this as a termination request?
-                    if (raisedEvent is RequestHaltEvent)
-                    {
-                        hadCompletionEvent = true;
-                    }
-                    else
-                    {
-                        yield return raisedEvent;
-                    }
-                }
-
-                if (hadCompletionEvent)
-                {
-                    // If we had a completion event, we are done.
-                    yield break;
-                }
-
-                // If we do not have any actions to take on the Workflow, but have unprocessed
-                // requests, wait for the responses to come in before exiting out of the workflow
-                // execution.
-                if (blockOnPendingRequest &&
-                    !this._stepRunner.HasUnprocessedMessages &&
-                    this._stepRunner.HasUnservicedRequests)
-                {
-                    this._waitForResponseSource ??= new();
-
-                    using CancellationTokenRegistration registration = cancellationToken.Register(() => this._waitForResponseSource?.SetResult(new()));
-
-                    await this._waitForResponseSource.Task.ConfigureAwait(false);
-                    this._waitForResponseSource = null;
-                }
-            } while (this._stepRunner.HasUnprocessedMessages);
-
-            activity?.AddEvent(new ActivityEvent(EventNames.WorkflowCompleted));
-        }
-        finally
-        {
-            this._stepRunner.WorkflowEvent -= OnWorkflowEvent;
-        }
-
-        void OnWorkflowEvent(object? sender, WorkflowEvent e)
-        {
-            eventSink.Add(e);
-        }
+            if (blockOnPendingRequest && runStatus == RunStatus.PendingRequests)
+            {
+                // Although we are only doing this while there are pending requests, any input allows us to continue
+                // running, so we should not wait until the input is specifically an ExternalResponse.
+                await this.WaitOnInputAsync(cancellationToken).ConfigureAwait(false);
+            }
+        } while (runStatus == RunStatus.Running);
     }
 
     /// <summary>
@@ -192,7 +119,10 @@ public sealed class StreamingRun
     /// </summary>
     /// <returns>A ValueTask that represents the asynchronous operation. The task is complete when the run has
     /// ended and cleanup is finished.</returns>
-    public ValueTask EndRunAsync() => this._stepRunner.RequestEndRunAsync();
+    public async ValueTask EndRunAsync()
+    {
+        await this._runHandle.DisposeAsync().ConfigureAwait(false);
+    }
 }
 
 /// <summary>

dotnet/src/Microsoft.Agents.AI.Workflows/SubworkflowErrorEvent.cs

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+
+namespace Microsoft.Agents.AI.Workflows;
+
+/// <summary>
+/// Event triggered when a workflow encounters an error.
+/// </summary>
+/// <param name="subworkflowId">The ID of the subworkflow that encountered the error.</param>
+/// <param name="e">Optionally, the <see cref="Exception"/> representing the error.</param>
+public sealed class SubworkflowErrorEvent(string subworkflowId, Exception? e) : WorkflowErrorEvent(e)
+{
+    /// <summary>
+    /// Gets the ID of the subworkflow that encountered the error.
+    /// </summary>
+    public string SubworkflowId { get; } = subworkflowId;
+}

dotnet/src/Microsoft.Agents.AI.Workflows/SubworkflowWarningEvent.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Microsoft.Agents.AI.Workflows;
+
+/// <summary>
+/// Event triggered when a subworkflow encounters a warning-confition.
+/// sub-workflow.
+/// </summary>
+/// <param name="message">The warning message.</param>
+/// <param name="subWorkflowId">The unique identifier of the sub-workflow that triggered the warning. Cannot be null or empty.</param>
+public sealed class SubworkflowWarningEvent(string message, string subWorkflowId) : WorkflowWarningEvent(message)
+{
+    /// <summary>
+    /// The unique identifier of the sub-workflow that triggered the warning.
+    /// </summary>
+    public string SubWorkflowId { get; } = subWorkflowId;
+}

dotnet/src/Microsoft.Agents.AI.Workflows/SuperStepCompletionInfo.cs

@@ -8,17 +8,17 @@ namespace Microsoft.Agents.AI.Workflows;
 /// <summary>
 /// Debug information about the SuperStep that finished running.
 /// </summary>
-public sealed class SuperStepCompletionInfo(HashSet<string> activatedExecutors, HashSet<string>? instantiatedExecutors = null)
+public sealed class SuperStepCompletionInfo(IEnumerable<string> activatedExecutors, IEnumerable<string>? instantiatedExecutors = null)
 {
     /// <summary>
     /// The unique identifiers of <see cref="Executor"/> instances that processed messages during this SuperStep
     /// </summary>
-    public HashSet<string> ActivatedExecutors { get; } = Throw.IfNull(activatedExecutors);
+    public HashSet<string> ActivatedExecutors { get; } = [.. Throw.IfNull(activatedExecutors)];
 
     /// <summary>
     /// The unique identifiers of <see cref="Executor"/> instances newly created during this SuperStep
     /// </summary>
-    public HashSet<string> InstantiatedExecutors { get; } = instantiatedExecutors ?? [];
+    public HashSet<string> InstantiatedExecutors { get; } = [.. instantiatedExecutors ?? []];
 
     /// <summary>
     /// A flag indicating whether the managed state was written to during this SuperStep. If the run was started

dotnet/src/Microsoft.Agents.AI.Workflows/Workflow.cs

@@ -35,7 +35,7 @@ public class Workflow
         );
     }
 
-    internal Dictionary<string, InputPort> Ports { get; init; } = [];
+    internal Dictionary<string, RequestPort> Ports { get; init; } = [];
 
     /// <summary>
     /// Gets the collection of external request ports, keyed by their ID.
@@ -43,7 +43,7 @@ public class Workflow
     /// <remarks>
     /// Each port has a corresponding entry in the <see cref="Registrations"/> dictionary.
     /// </remarks>
-    public Dictionary<string, InputPortInfo> ReflectPorts()
+    public Dictionary<string, RequestPortInfo> ReflectPorts()
     {
         return this.Ports.Keys.ToDictionary(
             keySelector: key => key,
@@ -84,7 +84,7 @@ public class Workflow
 
         // TODO: Can we cache this somehow to avoid having to instantiate a new one when running?
         // Does that break some user expectations?
-        Executor startExecutor = await startRegistration.ProviderAsync().ConfigureAwait(false);
+        Executor startExecutor = await startRegistration.CreateInstanceAsync(string.Empty).ConfigureAwait(false);
 
         if (!startExecutor.InputTypes.Any(t => t.IsAssignableFrom(typeof(TInput))))
         {
@@ -125,9 +125,16 @@ public class Workflow
     }
 
     private object? _ownerToken;
-    internal void TakeOwnership(object ownerToken)
+    private bool _ownedAsSubworkflow;
+    internal void TakeOwnership(object ownerToken, bool subworkflow = false, object? existingOwnershipSignoff = null)
     {
-        object? maybeToken = Interlocked.CompareExchange(ref this._ownerToken, ownerToken, null);
+        object? maybeToken = Interlocked.CompareExchange(ref this._ownerToken, ownerToken, existingOwnershipSignoff);
+        if (maybeToken == null && existingOwnershipSignoff != null)
+        {
+            // We expected to already be owned, but we were not
+            throw new InvalidOperationException("Existing ownership token was provided, but the workflow is unowned.");
+        }
+
         if (maybeToken == null && this._needsReset)
         {
             // There is no owner, but the workflow failed to reset on ownership release (because there are
@@ -137,14 +144,22 @@ public class Workflow
                 );
         }
 
-        if (maybeToken != null && !ReferenceEquals(maybeToken, ownerToken))
+        if (!ReferenceEquals(maybeToken, existingOwnershipSignoff) && !ReferenceEquals(maybeToken, ownerToken))
         {
             // Someone else owns the workflow
             Debug.Assert(maybeToken != null);
-            throw new InvalidOperationException("Cannot use a Workflow in multiple simultaneous (Streaming)Runs.");
+            throw new InvalidOperationException(
+                (subworkflow, this._ownedAsSubworkflow) switch
+                {
+                    (true, true) => "Cannot use a Workflow as a subworkflow of multiple parent workflows.",
+                    (true, false) => "Cannot use a running Workflow as a subworkflow.",
+                    (false, true) => "Cannot directly run a Workflow that is a subworkflow of another workflow.",
+                    (false, false) => "Cannot use a Workflow that is already owned by another runner or parent workflow.",
+                });
         }
 
         this._needsReset = true;
+        this._ownedAsSubworkflow = subworkflow;
     }
 
     [System.Diagnostics.CodeAnalysis.SuppressMessage("Maintainability", "CA1513:Use ObjectDisposedException throw helper",

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowBuilder.cs

@@ -32,7 +32,7 @@ public class WorkflowBuilder
     private readonly Dictionary<string, HashSet<Edge>> _edges = [];
     private readonly HashSet<string> _unboundExecutors = [];
     private readonly HashSet<EdgeConnection> _conditionlessConnections = [];
-    private readonly Dictionary<string, InputPort> _inputPorts = [];
+    private readonly Dictionary<string, RequestPort> _inputPorts = [];
     private readonly HashSet<string> _outputExecutors = [];
 
     private readonly string _startExecutorId;
@@ -88,9 +88,9 @@ public class WorkflowBuilder
             }
         }
 
-        if (executorish.ExecutorType == ExecutorIsh.Type.InputPort)
+        if (executorish.ExecutorType == ExecutorIsh.Type.RequestPort)
         {
-            InputPort port = executorish._inputPortValue!;
+            RequestPort port = executorish._requestPortValue!;
             this._inputPorts[port.Id] = port;
         }
 
@@ -151,11 +151,13 @@ public class WorkflowBuilder
     /// </summary>
     /// <param name="source">The executor that acts as the source node of the edge. Cannot be null.</param>
     /// <param name="target">The executor that acts as the target node of the edge. Cannot be null.</param>
+    /// <param name="idempotent">If set to <see langword="true"/>, adding the same edge multiple times will be a NoOp,
+    /// rather than an error.</param>
     /// <returns>The current instance of <see cref="WorkflowBuilder"/>.</returns>
     /// <exception cref="InvalidOperationException">Thrown if an unconditional edge between the specified source and target
     /// executors already exists.</exception>
-    public WorkflowBuilder AddEdge(ExecutorIsh source, ExecutorIsh target)
-        => this.AddEdge<object>(source, target, null);
+    public WorkflowBuilder AddEdge(ExecutorIsh source, ExecutorIsh target, bool idempotent = false)
+        => this.AddEdge<object>(source, target, null, idempotent);
 
     internal static Func<object?, bool>? CreateConditionFunc<T>(Func<T?, bool>? condition)
     {
@@ -185,7 +187,13 @@ public class WorkflowBuilder
             {
                 maybeObj = portableValue.AsType(typeof(T));
             }
-            return condition(maybeObj);
+
+            if (maybeObj is T typed)
+            {
+                return condition(typed);
+            }
+
+            return condition(null);
         };
     }
 
@@ -198,11 +206,13 @@ public class WorkflowBuilder
     /// <param name="source">The executor that acts as the source node of the edge. Cannot be null.</param>
     /// <param name="target">The executor that acts as the target node of the edge. Cannot be null.</param>
     /// <param name="condition">An optional predicate that determines whether the edge should be followed based on the input.
+    /// <param name="idempotent">If set to <see langword="true"/>, adding the same edge multiple times will be a NoOp,
+    /// rather than an error.</param>
     /// If null, the edge is always activated when the source sends a message.</param>
     /// <returns>The current instance of <see cref="WorkflowBuilder"/>.</returns>
     /// <exception cref="InvalidOperationException">Thrown if an unconditional edge between the specified source and target
     /// executors already exists.</exception>
-    public WorkflowBuilder AddEdge<T>(ExecutorIsh source, ExecutorIsh target, Func<T?, bool>? condition = null)
+    public WorkflowBuilder AddEdge<T>(ExecutorIsh source, ExecutorIsh target, Func<T?, bool>? condition = null, bool idempotent = false)
     {
         // Add an edge from source to target with an optional condition.
         // This is a low-level builder method that does not enforce any specific executor type.
@@ -213,6 +223,11 @@ public class WorkflowBuilder
         EdgeConnection connection = new(source.Id, target.Id);
         if (condition is null && this._conditionlessConnections.Contains(connection))
         {
+            if (idempotent)
+            {
+                return this;
+            }
+
             throw new InvalidOperationException(
                 $"An edge from '{source.Id}' to '{target.Id}' already exists without a condition. " +
                 "You cannot add another edge without a condition for the same source and target.");

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowBuilderExtensions.cs

@@ -25,10 +25,24 @@ public static class WorkflowBuilderExtensions
     /// <param name="executors">The target executors to which messages will be forwarded.</param>
     /// <returns>The updated <see cref="WorkflowBuilder"/> instance.</returns>
     public static WorkflowBuilder ForwardMessage<TMessage>(this WorkflowBuilder builder, ExecutorIsh source, params IEnumerable<ExecutorIsh> executors)
+        => builder.ForwardMessage<TMessage>(source, condition: null, executors);
+
+    /// <summary>
+    /// Adds edges to the workflow that forward messages of the specified type from the source executor to
+    /// one or more target executors.
+    /// </summary>
+    /// <typeparam name="TMessage">The type of message to forward.</typeparam>
+    /// <param name="builder">The <see cref="WorkflowBuilder"/> to which the edges will be added.</param>
+    /// <param name="source">The source executor from which messages will be forwarded.</param>
+    /// <param name="condition">An optional condition that messages must satisfy to be forwarded. If <see langword="null"/>,
+    /// all messages of type <typeparamref name="TMessage"/> will be forwarded.</param>
+    /// <param name="executors">The target executors to which messages will be forwarded.</param>
+    /// <returns>The updated <see cref="WorkflowBuilder"/> instance.</returns>
+    public static WorkflowBuilder ForwardMessage<TMessage>(this WorkflowBuilder builder, ExecutorIsh source, Func<TMessage, bool>? condition = null, params IEnumerable<ExecutorIsh> executors)
     {
         Throw.IfNull(executors);
 
-        Func<object?, bool> predicate = WorkflowBuilder.CreateConditionFunc<TMessage>((Func<object?, bool>)IsAllowedType)!;
+        Func<object?, bool> predicate = WorkflowBuilder.CreateConditionFunc<TMessage>(IsAllowedTypeAndMatchingCondition)!;
 
 #if NET
         if (executors.TryGetNonEnumeratedCount(out int count) && count == 1)
@@ -43,7 +57,7 @@ public static class WorkflowBuilderExtensions
 
         // The reason we can check for "not null" here is that CreateConditionFunc<T> will do the correct unwrapping
         // logic for PortableValues.
-        static bool IsAllowedType(object? message) => message is not null;
+        bool IsAllowedTypeAndMatchingCondition(TMessage? message) => message != null && (condition == null || condition(message));
     }
 
     /// <summary>
@@ -84,10 +98,11 @@ public static class WorkflowBuilderExtensions
     /// executor in the order provided.</remarks>
     /// <param name="builder">The workflow builder to which the executor chain will be added. </param>
     /// <param name="source">The initial executor in the chain. Cannot be null.</param>
+    /// <param name="allowRepetition">If set to <see langword="true"/>, the same executor can be added to the chain multiple times.</param>
     /// <param name="executors">An ordered array of executors to be added to the chain after the source.</param>
     /// <returns>The original workflow builder instance with the specified executor chain added.</returns>
     /// <exception cref="ArgumentException">Thrown if there is a cycle in the chain.</exception>
-    public static WorkflowBuilder AddChain(this WorkflowBuilder builder, ExecutorIsh source, params IEnumerable<ExecutorIsh> executors)
+    public static WorkflowBuilder AddChain(this WorkflowBuilder builder, ExecutorIsh source, bool allowRepetition = false, params IEnumerable<ExecutorIsh> executors)
     {
         Throw.IfNull(builder);
         Throw.IfNull(source);
@@ -98,13 +113,13 @@ public static class WorkflowBuilderExtensions
         {
             Throw.IfNull(executor, nameof(executors));
 
-            if (seenExecutors.Contains(executor.Id))
+            if (!allowRepetition && seenExecutors.Contains(executor.Id))
             {
                 throw new ArgumentException($"Executor '{executor.Id}' is already in the chain.", nameof(executors));
             }
             seenExecutors.Add(executor.Id);
 
-            builder.AddEdge(source, executor);
+            builder.AddEdge(source, executor, idempotent: true);
             source = executor;
         }
 
@@ -130,7 +145,7 @@ public static class WorkflowBuilderExtensions
         Throw.IfNull(source);
         Throw.IfNull(portId);
 
-        InputPort port = new(portId, typeof(TRequest), typeof(TResponse));
+        RequestPort port = new(portId, typeof(TRequest), typeof(TResponse));
         return builder.AddEdge(source, port)
                       .AddEdge(port, source);
     }

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowErrorEvent.cs

@@ -10,4 +10,4 @@ namespace Microsoft.Agents.AI.Workflows;
 /// <param name="e">
 /// Optionally, the <see cref="Exception"/> representing the error.
 /// </param>
-public sealed class WorkflowErrorEvent(Exception? e) : WorkflowEvent(e);
+public class WorkflowErrorEvent(Exception? e) : WorkflowEvent(e);

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowJsonDefintion.cs

@@ -15,6 +15,6 @@ internal class WorkflowJsonDefinitionData
 {
     public string StartExecutorId { get; set; } = string.Empty;
     public IEnumerable<Edge> Edges { get; set; } = [];
-    public IEnumerable<InputPort> Ports { get; set; } = [];
+    public IEnumerable<RequestPort> Ports { get; set; } = [];
     public IEnumerable<string> OutputExecutors { get; set; } = [];
 }

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowWarningEvent.cs

@@ -6,4 +6,4 @@ namespace Microsoft.Agents.AI.Workflows;
 /// Event triggered when a workflow encounters a warning-condition.
 /// </summary>
 /// <param name="message">The warning message.</param>
-public sealed class WorkflowWarningEvent(string message) : WorkflowEvent(message);
+public class WorkflowWarningEvent(string message) : WorkflowEvent(message);

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowsJsonUtilities.cs

@@ -67,6 +67,7 @@ internal static partial class WorkflowsJsonUtilities
     [JsonSerializable(typeof(CheckpointInfo))]
     [JsonSerializable(typeof(PortableValue))]
     [JsonSerializable(typeof(PortableMessageEnvelope))]
+    [JsonSerializable(typeof(InMemoryCheckpointManager))]
 
     // Runtime State Types
     [JsonSerializable(typeof(ScopeKey))]