fix: JSON Serialization issue with MultiPartyConversation (#5653)

When MultiPartyConversation gets saved during checkpointing, the data for the chat history is not persisted, resulting in failures to deserialize after. The fix is to make the history visible to the source generated serialization code.

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

@@ -36,7 +36,7 @@ internal sealed class HandoffEndExecutor(bool returnToPrevious) : Executor(Execu
                     sharedState.PreviousAgentId = handoff.PreviousAgentId;
                 }
 
-                await context.YieldOutputAsync(sharedState.Conversation.CloneAllMessages(), cancellationToken).ConfigureAwait(false);
+                await context.YieldOutputAsync(sharedState.Conversation.CloneHistory(), cancellationToken).ConfigureAwait(false);
 
                 return sharedState;
             }, context, cancellationToken).ConfigureAwait(false);

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

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Collections.Generic;
+using System.Text.Json.Serialization;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.AI;
@@ -23,7 +24,20 @@ internal static class HandoffConstants
 
 internal sealed class HandoffSharedState
 {
-    public MultiPartyConversation Conversation { get; } = new();
+    [JsonConstructor]
+    internal HandoffSharedState(MultiPartyConversation conversation, string? previousAgentId)
+    {
+        this.Conversation = conversation;
+        this.PreviousAgentId = previousAgentId;
+    }
+
+    public HandoffSharedState()
+    {
+        this.Conversation = new([]);
+    }
+
+    [JsonInclude]
+    public MultiPartyConversation Conversation { get; internal set; }
 
     public string? PreviousAgentId { get; set; }
 }

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

@@ -3,20 +3,33 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Text.Json.Serialization;
 using Microsoft.Extensions.AI;
 
 namespace Microsoft.Agents.AI.Workflows.Specialized;
 
 internal sealed class MultiPartyConversation
 {
-    private readonly List<ChatMessage> _history = [];
     private readonly object _mutex = new();
 
-    public List<ChatMessage> CloneAllMessages()
+    [JsonConstructor]
+    internal MultiPartyConversation(List<ChatMessage> history)
+    {
+        this.History = history ?? [];
+    }
+
+    /// <summary>
+    /// In order to support JSON serializaiton, this property must be internally visible. However, it should not be used
+    /// in concurrent contexts without proper locking, as the underlying list is not thread safe.
+    /// </summary>
+    [JsonInclude]
+    internal List<ChatMessage> History { get; }
+
+    public List<ChatMessage> CloneHistory()
     {
         lock (this._mutex)
         {
-            return this._history.ToList();
+            return this.History.ToList();
         }
     }
 
@@ -24,23 +37,24 @@ internal sealed class MultiPartyConversation
     {
         lock (this._mutex)
         {
-            int count = this._history.Count - bookmark;
+            int count = this.History.Count - bookmark;
             if (count < 0)
             {
                 throw new InvalidOperationException($"Bookmark value too large: {bookmark} vs count={count}");
             }
 
-            return (this._history.Skip(bookmark).ToArray(), this.CurrentBookmark);
+            return (this.History.Skip(bookmark).ToArray(), this.CurrentBookmark);
         }
     }
 
-    private int CurrentBookmark => this._history.Count;
+    [JsonIgnore]
+    private int CurrentBookmark => this.History.Count;
 
     public int AddMessages(IEnumerable<ChatMessage> messages)
     {
         lock (this._mutex)
         {
-            this._history.AddRange(messages);
+            this.History.AddRange(messages);
             return this.CurrentBookmark;
         }
     }
@@ -49,7 +63,7 @@ internal sealed class MultiPartyConversation
     {
         lock (this._mutex)
         {
-            this._history.Add(message);
+            this.History.Add(message);
             return this.CurrentBookmark;
         }
     }

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

@@ -95,6 +95,8 @@ internal static partial class WorkflowsJsonUtilities
 
     // Built-in Executor State Types
     [JsonSerializable(typeof(AIAgentHostState))]
+    [JsonSerializable(typeof(HandoffSharedState))]
+    [JsonSerializable(typeof(HandoffAgentHostState))]
 
     // Event Types
     //[JsonSerializable(typeof(WorkflowEvent))]