Python: Add factory pattern to concurrent orchestration builder (#2738)

* Add factory pattern to concurrent orchestration builder * Update readme * Address AI comments * Fix unit tests * Fix import * Prevent multiple calls to set participants or factories * Add comments * Mitigate warnings * Fix mypy * Address comments * Address Copilot comments * Fix tests
2026-06-16 21:04:09 +08:00 · 2025-12-10 17:23:28 -08:00
parent 638fbb5f03
commit 191779ce80
7 changed files with 702 additions and 35 deletions
@@ -3,6 +3,7 @@
 import asyncio
 import inspect
 import logging
+import uuid
 from collections.abc import Callable, Sequence
 from typing import Any

@@ -189,8 +190,11 @@ class ConcurrentBuilder:
    r"""High-level builder for concurrent agent workflows.

    - `participants([...])` accepts a list of AgentProtocol (recommended) or Executor.
+    - `register_participants([...])` accepts a list of factories for AgentProtocol (recommended)
+       or Executor factories
    - `build()` wires: dispatcher -> fan-out -> participants -> fan-in -> aggregator.
-    - `with_custom_aggregator(...)` overrides the default aggregator with an Executor or callback.
+    - `with_aggregator(...)` overrides the default aggregator with an Executor or callback.
+    - `register_aggregator(...)` accepts a factory for an Executor as custom aggregator.

    Usage:

@@ -201,14 +205,33 @@ class ConcurrentBuilder:
        # Minimal: use default aggregator (returns list[ChatMessage])
        workflow = ConcurrentBuilder().participants([agent1, agent2, agent3]).build()

+        # With agent factories
+        workflow = ConcurrentBuilder().register_participants([create_agent1, create_agent2, create_agent3]).build()
+

        # Custom aggregator via callback (sync or async). The callback receives
        # list[AgentExecutorResponse] and its return value becomes the workflow's output.
-        def summarize(results):
+        def summarize(results: list[AgentExecutorResponse]) -> str:
            return " | ".join(r.agent_run_response.messages[-1].text for r in results)


-        workflow = ConcurrentBuilder().participants([agent1, agent2, agent3]).with_custom_aggregator(summarize).build()
+        workflow = ConcurrentBuilder().participants([agent1, agent2, agent3]).with_aggregator(summarize).build()
+
+
+        # Custom aggregator via a factory
+        class MyAggregator(Executor):
+            @handler
+            async def aggregate(self, results: list[AgentExecutorResponse], ctx: WorkflowContext[Never, str]) -> None:
+                await ctx.yield_output(" | ".join(r.agent_run_response.messages[-1].text for r in results))
+
+
+        workflow = (
+            ConcurrentBuilder()
+            .register_participants([create_agent1, create_agent2, create_agent3])
+            .register_aggregator(lambda: MyAggregator(id="my_aggregator"))
+            .build()
+        )
+

        # Enable checkpoint persistence so runs can resume
        workflow = ConcurrentBuilder().participants([agent1, agent2, agent3]).with_checkpointing(storage).build()
@@ -219,10 +242,67 @@ class ConcurrentBuilder:

    def __init__(self) -> None:
        self._participants: list[AgentProtocol | Executor] = []
+        self._participant_factories: list[Callable[[], AgentProtocol | Executor]] = []
        self._aggregator: Executor | None = None
+        self._aggregator_factory: Callable[[], Executor] | None = None
        self._checkpoint_storage: CheckpointStorage | None = None
        self._request_info_enabled: bool = False

+    def register_participants(
+        self,
+        participant_factories: Sequence[Callable[[], AgentProtocol | Executor]],
+    ) -> "ConcurrentBuilder":
+        r"""Define the parallel participants for this concurrent workflow.
+
+        Accepts factories (callables) that return AgentProtocol instances (e.g., created
+        by a chat client) or Executor instances. Each participant created by a factory
+        is wired as a parallel branch using fan-out edges from an internal dispatcher.
+
+        Args:
+            participant_factories: Sequence of callables returning AgentProtocol or Executor instances
+
+        Raises:
+            ValueError: if `participant_factories` is empty or `.participants()`
+                       or `.register_participants()` were already called
+
+        Example:
+
+        .. code-block:: python
+
+            def create_researcher() -> ChatAgent:
+                return ...
+
+
+            def create_marketer() -> ChatAgent:
+                return ...
+
+
+            def create_legal() -> ChatAgent:
+                return ...
+
+
+            class MyCustomExecutor(Executor): ...
+
+
+            wf = ConcurrentBuilder().register_participants([create_researcher, create_marketer, create_legal]).build()
+
+            # Mixing agent(s) and executor(s) is supported
+            wf2 = ConcurrentBuilder().register_participants([create_researcher, MyCustomExecutor]).build()
+        """
+        if self._participants:
+            raise ValueError(
+                "Cannot mix .participants([...]) and .register_participants() in the same builder instance."
+            )
+
+        if self._participant_factories:
+            raise ValueError("register_participants() has already been called on this builder instance.")
+
+        if not participant_factories:
+            raise ValueError("participant_factories cannot be empty")
+
+        self._participant_factories = list(participant_factories)
+        return self
+
    def participants(self, participants: Sequence[AgentProtocol | Executor]) -> "ConcurrentBuilder":
        r"""Define the parallel participants for this concurrent workflow.

@@ -230,8 +310,12 @@ class ConcurrentBuilder:
        instances. Each participant is wired as a parallel branch using fan-out edges
        from an internal dispatcher.

+        Args:
+            participants: Sequence of AgentProtocol or Executor instances
+
        Raises:
-            ValueError: if `participants` is empty or contains duplicates
+            ValueError: if `participants` is empty, contains duplicates, or `.register_participants()`
+                       or `.participants()` were already called
            TypeError: if any entry is not AgentProtocol or Executor

        Example:
@@ -243,6 +327,14 @@ class ConcurrentBuilder:
            # Mixing agent(s) and executor(s) is supported
            wf2 = ConcurrentBuilder().participants([researcher_agent, my_custom_executor]).build()
        """
+        if self._participant_factories:
+            raise ValueError(
+                "Cannot mix .participants([...]) and .register_participants() in the same builder instance."
+            )
+
+        if self._participants:
+            raise ValueError("participants() has already been called on this builder instance.")
+
        if not participants:
            raise ValueError("participants cannot be empty")

@@ -265,38 +357,107 @@ class ConcurrentBuilder:
        self._participants = list(participants)
        return self

-    def with_aggregator(self, aggregator: Executor | Callable[..., Any]) -> "ConcurrentBuilder":
-        r"""Override the default aggregator with an Executor or a callback.
+    def register_aggregator(self, aggregator_factory: Callable[[], Executor]) -> "ConcurrentBuilder":
+        r"""Define a custom aggregator for this concurrent workflow.

-        - Executor: must handle `list[AgentExecutorResponse]` and
-            yield output using `ctx.yield_output(...)` and add a
-          output and the workflow becomes idle.
+        Accepts a factory (callable) that returns an Executor instance. The executor
+        should handle `list[AgentExecutorResponse]` and yield output using `ctx.yield_output(...)`.
+
+        Args:
+            aggregator_factory: Callable that returns an Executor instance
+
+        Example:
+        .. code-block:: python
+
+            class MyCustomExecutor(Executor): ...
+
+
+            wf = (
+                ConcurrentBuilder()
+                .register_participants([create_researcher, create_marketer, create_legal])
+                .register_aggregator(lambda: MyCustomExecutor(id="my_aggregator"))
+                .build()
+            )
+        """
+        if self._aggregator is not None:
+            raise ValueError(
+                "Cannot mix .with_aggregator(...) and .register_aggregator(...) in the same builder instance."
+            )
+
+        if self._aggregator_factory is not None:
+            raise ValueError("register_aggregator() has already been called on this builder instance.")
+
+        self._aggregator_factory = aggregator_factory
+        return self
+
+    def with_aggregator(
+        self,
+        aggregator: Executor
+        | Callable[[list[AgentExecutorResponse]], Any]
+        | Callable[[list[AgentExecutorResponse], WorkflowContext[Never, Any]], Any],
+    ) -> "ConcurrentBuilder":
+        r"""Override the default aggregator with an executor, an executor factory, or a callback.
+
+        - Executor: must handle `list[AgentExecutorResponse]` and yield output using `ctx.yield_output(...)`
        - Callback: sync or async callable with one of the signatures:
          `(results: list[AgentExecutorResponse]) -> Any | None` or
          `(results: list[AgentExecutorResponse], ctx: WorkflowContext) -> Any | None`.
          If the callback returns a non-None value, it becomes the workflow's output.

+        Args:
+            aggregator: Executor instance, or callback function
+
        Example:

        .. code-block:: python
+            # Executor-based aggregator
+            class CustomAggregator(Executor):
+                @handler
+                async def aggregate(self, results: list[AgentExecutorResponse], ctx: WorkflowContext) -> None:
+                    await ctx.yield_output(" | ".join(r.agent_run_response.messages[-1].text for r in results))
+
+
+            wf = ConcurrentBuilder().participants([a1, a2, a3]).with_aggregator(CustomAggregator()).build()
+

            # Callback-based aggregator (string result)
-            async def summarize(results):
+            async def summarize(results: list[AgentExecutorResponse]) -> str:
                return " | ".join(r.agent_run_response.messages[-1].text for r in results)


-            wf = ConcurrentBuilder().participants([a1, a2, a3]).with_custom_aggregator(summarize).build()
+            wf = ConcurrentBuilder().participants([a1, a2, a3]).with_aggregator(summarize).build()
+
+
+            # Callback-based aggregator (yield result)
+            async def summarize(results: list[AgentExecutorResponse], ctx: WorkflowContext[Never, str]) -> None:
+                await ctx.yield_output(" | ".join(r.agent_run_response.messages[-1].text for r in results))
+
+
+            wf = ConcurrentBuilder().participants([a1, a2, a3]).with_aggregator(summarize).build()
        """
+        if self._aggregator_factory is not None:
+            raise ValueError(
+                "Cannot mix .with_aggregator(...) and .register_aggregator(...) in the same builder instance."
+            )
+
+        if self._aggregator is not None:
+            raise ValueError("with_aggregator() has already been called on this builder instance.")
+
        if isinstance(aggregator, Executor):
            self._aggregator = aggregator
        elif callable(aggregator):
            self._aggregator = _CallbackAggregator(aggregator)
        else:
            raise TypeError("aggregator must be an Executor or a callable")
+
        return self

    def with_checkpointing(self, checkpoint_storage: CheckpointStorage) -> "ConcurrentBuilder":
-        """Enable checkpoint persistence using the provided storage backend."""
+        """Enable checkpoint persistence using the provided storage backend.
+
+        Args:
+            checkpoint_storage: CheckpointStorage instance for persisting workflow state
+        """
        self._checkpoint_storage = checkpoint_storage
        return self

@@ -329,7 +490,7 @@ class ConcurrentBuilder:
            before sending the outputs to the aggregator
        - Aggregator yields output and the workflow becomes idle. The output is either:
          - list[ChatMessage] (default aggregator: one user + one assistant per agent)
-          - custom payload from the provided callback/executor
+          - custom payload from the provided aggregator

        Returns:
            Workflow: a ready-to-run workflow instance
@@ -343,26 +504,69 @@ class ConcurrentBuilder:

            workflow = ConcurrentBuilder().participants([agent1, agent2]).build()
        """
-        if not self._participants:
-            raise ValueError("No participants provided. Call .participants([...]) first.")
+        if not self._participants and not self._participant_factories:
+            raise ValueError(
+                "No participants provided. Call .participants([...]) or .register_participants([...]) first."
+            )

+        # Internal nodes
        dispatcher = _DispatchToAllParticipants(id="dispatcher")
-        aggregator = self._aggregator or _AggregateAgentConversations(id="aggregator")
+        aggregator = (
+            self._aggregator
+            if self._aggregator is not None
+            else (
+                self._aggregator_factory()
+                if self._aggregator_factory is not None
+                else _AggregateAgentConversations(id="aggregator")
+            )
+        )

        builder = WorkflowBuilder()
-        builder.set_start_executor(dispatcher)
-        builder.add_fan_out_edges(dispatcher, list(self._participants))
+        if self._participant_factories:
+            # Register executors/agents to avoid warnings from the workflow builder
+            # if factories are provided instead of direct instances. This doesn't
+            # break the factory pattern since the concurrent builder still creates
+            # new instances per workflow build.
+            factory_names: list[str] = []
+            for factory in self._participant_factories:
+                factory_name = uuid.uuid4().hex
+                factory_names.append(factory_name)
+                instance = factory()
+                if isinstance(instance, Executor):
+                    builder.register_executor(lambda executor=instance: executor, name=factory_name)  # type: ignore[misc]
+                else:
+                    builder.register_agent(lambda agent=instance: agent, name=factory_name)  # type: ignore[misc]
+            # Register the dispatcher and the aggregator
+            builder.register_executor(lambda: dispatcher, name="dispatcher")
+            builder.register_executor(lambda: aggregator, name="aggregator")

-        if self._request_info_enabled:
-            # Insert interceptor between fan-in and aggregator
-            # participants -> fan-in -> interceptor -> aggregator
-            request_info_interceptor = RequestInfoInterceptor(executor_id="request_info")
-            builder.add_fan_in_edges(list(self._participants), request_info_interceptor)
-            builder.add_edge(request_info_interceptor, aggregator)
+            builder.set_start_executor("dispatcher")
+            builder.add_fan_out_edges("dispatcher", factory_names)
+            if self._request_info_enabled:
+                # Insert interceptor between fan-in and aggregator
+                # participants -> fan-in -> interceptor -> aggregator
+                builder.register_executor(
+                    lambda: RequestInfoInterceptor(executor_id="request_info"),
+                    name="request_info_interceptor",
+                )
+                builder.add_fan_in_edges(factory_names, "request_info_interceptor")
+                builder.add_edge("request_info_interceptor", "aggregator")
+            else:
+                # Direct fan-in to aggregator
+                builder.add_fan_in_edges(factory_names, "aggregator")
        else:
-            # Direct fan-in to aggregator
-            builder.add_fan_in_edges(list(self._participants), aggregator)
+            builder.set_start_executor(dispatcher)
+            builder.add_fan_out_edges(dispatcher, self._participants)

+            if self._request_info_enabled:
+                # Insert interceptor between fan-in and aggregator
+                # participants -> fan-in -> interceptor -> aggregator
+                request_info_interceptor = RequestInfoInterceptor(executor_id="request_info")
+                builder.add_fan_in_edges(self._participants, request_info_interceptor)
+                builder.add_edge(request_info_interceptor, aggregator)
+            else:
+                # Direct fan-in to aggregator
+                builder.add_fan_in_edges(self._participants, aggregator)
        if self._checkpoint_storage is not None:
            builder = builder.with_checkpointing(self._checkpoint_storage)

@@ -374,7 +374,7 @@ class WorkflowBuilder:
                )
        """
        if name in self._executor_registry:
-            raise ValueError(f"An executor factory with the name '{name}' is already registered.")
+            raise ValueError(f"An agent factory with the name '{name}' is already registered.")

        def wrapped_factory() -> AgentExecutor:
            agent = factory_func()
@@ -1148,21 +1148,29 @@ class WorkflowBuilder:
        if isinstance(self._start_executor, Executor):
            start_executor = self._start_executor

-        executors: dict[str, Executor] = {}
+        # Maps registered factory names to created executor instances for edge resolution
+        factory_name_to_instance: dict[str, Executor] = {}
+        # Maps executor IDs to created executor instances to prevent duplicates
+        executor_id_to_instance: dict[str, Executor] = {}
        deferred_edge_groups: list[EdgeGroup] = []
        for name, exec_factory in self._executor_registry.items():
            instance = exec_factory()
+            if instance.id in executor_id_to_instance:
+                raise ValueError(f"Executor with ID '{instance.id}' has already been created.")
+            executor_id_to_instance[instance.id] = instance
+
            if isinstance(self._start_executor, str) and name == self._start_executor:
                start_executor = instance
+
            # All executors will get their own internal edge group for receiving system messages
            deferred_edge_groups.append(InternalEdgeGroup(instance.id))  # type: ignore[call-arg]
-            executors[name] = instance
+            factory_name_to_instance[name] = instance

        def _get_executor(name: str) -> Executor:
            """Helper to get executor by the registered name. Raises if not found."""
-            if name not in executors:
-                raise ValueError(f"Executor with name '{name}' has not been registered.")
-            return executors[name]
+            if name not in factory_name_to_instance:
+                raise ValueError(f"Factory '{name}' has not been registered.")
+            return factory_name_to_instance[name]

        for registration in self._edge_registry:
            match registration:
@@ -1179,7 +1187,7 @@ class WorkflowBuilder:
                    cases_converted: list[SwitchCaseEdgeGroupCase | SwitchCaseEdgeGroupDefault] = []
                    for case in cases:
                        if not isinstance(case.target, str):
-                            raise ValueError("Switch case target must be a registered executor name (str) if deferred.")
+                            raise ValueError("Switch case target must be a registered factory name (str) if deferred.")
                        target_exec = _get_executor(case.target)
                        if isinstance(case, Default):
                            cases_converted.append(SwitchCaseEdgeGroupDefault(target_id=target_exec.id))
@@ -1201,7 +1209,7 @@ class WorkflowBuilder:
        if start_executor is None:
            raise ValueError("Failed to resolve starting executor from registered factories.")

-        return start_executor, list(executors.values()), deferred_edge_groups
+        return start_executor, list(executor_id_to_instance.values()), deferred_edge_groups

    def build(self) -> Workflow:
        """Build and return the constructed workflow.
@@ -3,6 +3,7 @@
 from typing import Any, cast

 import pytest
+from typing_extensions import Never

 from agent_framework import (
    AgentExecutorRequest,
@@ -52,6 +53,55 @@ def test_concurrent_builder_rejects_duplicate_executors() -> None:
        ConcurrentBuilder().participants([a, b])


+def test_concurrent_builder_rejects_duplicate_executors_from_factories() -> None:
+    """Test that duplicate executor IDs from factories are detected at build time."""
+
+    def create_dup1() -> Executor:
+        return _FakeAgentExec("dup", "A")
+
+    def create_dup2() -> Executor:
+        return _FakeAgentExec("dup", "B")  # same executor id
+
+    builder = ConcurrentBuilder().register_participants([create_dup1, create_dup2])
+    with pytest.raises(ValueError, match="Executor with ID 'dup' has already been created."):
+        builder.build()
+
+
+def test_concurrent_builder_rejects_mixed_participants_and_factories() -> None:
+    """Test that mixing .participants() and .register_participants() raises an error."""
+    # Case 1: participants first, then register_participants
+    with pytest.raises(ValueError, match="Cannot mix .participants"):
+        (
+            ConcurrentBuilder()
+            .participants([_FakeAgentExec("a", "A")])
+            .register_participants([lambda: _FakeAgentExec("b", "B")])
+        )
+
+    # Case 2: register_participants first, then participants
+    with pytest.raises(ValueError, match="Cannot mix .participants"):
+        (
+            ConcurrentBuilder()
+            .register_participants([lambda: _FakeAgentExec("a", "A")])
+            .participants([_FakeAgentExec("b", "B")])
+        )
+
+
+def test_concurrent_builder_rejects_multiple_calls_to_participants() -> None:
+    """Test that multiple calls to .participants() raises an error."""
+    with pytest.raises(ValueError, match=r"participants\(\) has already been called"):
+        (ConcurrentBuilder().participants([_FakeAgentExec("a", "A")]).participants([_FakeAgentExec("b", "B")]))
+
+
+def test_concurrent_builder_rejects_multiple_calls_to_register_participants() -> None:
+    """Test that multiple calls to .register_participants() raises an error."""
+    with pytest.raises(ValueError, match=r"register_participants\(\) has already been called"):
+        (
+            ConcurrentBuilder()
+            .register_participants([lambda: _FakeAgentExec("a", "A")])
+            .register_participants([lambda: _FakeAgentExec("b", "B")])
+        )
+
+
 async def test_concurrent_default_aggregator_emits_single_user_and_assistants() -> None:
    # Three synthetic agent executors
    e1 = _FakeAgentExec("agentA", "Alpha")
@@ -159,6 +209,138 @@ def test_concurrent_custom_aggregator_uses_callback_name_for_id() -> None:
    assert aggregator.id == "summarize"


+async def test_concurrent_with_aggregator_executor_instance() -> None:
+    """Test with_aggregator using an Executor instance (not factory)."""
+
+    class CustomAggregator(Executor):
+        @handler
+        async def aggregate(self, results: list[AgentExecutorResponse], ctx: WorkflowContext[Never, str]) -> None:
+            texts: list[str] = []
+            for r in results:
+                msgs: list[ChatMessage] = r.agent_run_response.messages
+                texts.append(msgs[-1].text if msgs else "")
+            await ctx.yield_output(" & ".join(sorted(texts)))
+
+    e1 = _FakeAgentExec("agentA", "One")
+    e2 = _FakeAgentExec("agentB", "Two")
+
+    aggregator_instance = CustomAggregator(id="instance_aggregator")
+    wf = ConcurrentBuilder().participants([e1, e2]).with_aggregator(aggregator_instance).build()
+
+    completed = False
+    output: str | None = None
+    async for ev in wf.run_stream("prompt: instance test"):
+        if isinstance(ev, WorkflowStatusEvent) and ev.state == WorkflowRunState.IDLE:
+            completed = True
+        elif isinstance(ev, WorkflowOutputEvent):
+            output = cast(str, ev.data)
+        if completed and output is not None:
+            break
+
+    assert completed
+    assert output is not None
+    assert isinstance(output, str)
+    assert output == "One & Two"
+
+
+async def test_concurrent_with_aggregator_executor_factory() -> None:
+    """Test with_aggregator using an Executor factory."""
+
+    class CustomAggregator(Executor):
+        @handler
+        async def aggregate(self, results: list[AgentExecutorResponse], ctx: WorkflowContext[Never, str]) -> None:
+            texts: list[str] = []
+            for r in results:
+                msgs: list[ChatMessage] = r.agent_run_response.messages
+                texts.append(msgs[-1].text if msgs else "")
+            await ctx.yield_output(" | ".join(sorted(texts)))
+
+    e1 = _FakeAgentExec("agentA", "One")
+    e2 = _FakeAgentExec("agentB", "Two")
+
+    wf = (
+        ConcurrentBuilder()
+        .participants([e1, e2])
+        .register_aggregator(lambda: CustomAggregator(id="custom_aggregator"))
+        .build()
+    )
+
+    completed = False
+    output: str | None = None
+    async for ev in wf.run_stream("prompt: factory test"):
+        if isinstance(ev, WorkflowStatusEvent) and ev.state == WorkflowRunState.IDLE:
+            completed = True
+        elif isinstance(ev, WorkflowOutputEvent):
+            output = cast(str, ev.data)
+        if completed and output is not None:
+            break
+
+    assert completed
+    assert output is not None
+    assert isinstance(output, str)
+    assert output == "One | Two"
+
+
+async def test_concurrent_with_aggregator_executor_factory_with_default_id() -> None:
+    """Test with_aggregator using an Executor class directly as factory (with default __init__ parameters)."""
+
+    class CustomAggregator(Executor):
+        def __init__(self, id: str = "default_aggregator") -> None:
+            super().__init__(id)
+
+        @handler
+        async def aggregate(self, results: list[AgentExecutorResponse], ctx: WorkflowContext[Never, str]) -> None:
+            texts: list[str] = []
+            for r in results:
+                msgs: list[ChatMessage] = r.agent_run_response.messages
+                texts.append(msgs[-1].text if msgs else "")
+            await ctx.yield_output(" | ".join(sorted(texts)))
+
+    e1 = _FakeAgentExec("agentA", "One")
+    e2 = _FakeAgentExec("agentB", "Two")
+
+    wf = ConcurrentBuilder().participants([e1, e2]).register_aggregator(CustomAggregator).build()
+
+    completed = False
+    output: str | None = None
+    async for ev in wf.run_stream("prompt: factory test"):
+        if isinstance(ev, WorkflowStatusEvent) and ev.state == WorkflowRunState.IDLE:
+            completed = True
+        elif isinstance(ev, WorkflowOutputEvent):
+            output = cast(str, ev.data)
+        if completed and output is not None:
+            break
+
+    assert completed
+    assert output is not None
+    assert isinstance(output, str)
+    assert output == "One | Two"
+
+
+def test_concurrent_builder_rejects_multiple_calls_to_with_aggregator() -> None:
+    """Test that multiple calls to .with_aggregator() raises an error."""
+
+    def summarize(results: list[AgentExecutorResponse]) -> str:  # type: ignore[override]
+        return str(len(results))
+
+    with pytest.raises(ValueError, match=r"with_aggregator\(\) has already been called"):
+        (ConcurrentBuilder().with_aggregator(summarize).with_aggregator(summarize))
+
+
+def test_concurrent_builder_rejects_multiple_calls_to_register_aggregator() -> None:
+    """Test that multiple calls to .register_aggregator() raises an error."""
+
+    class CustomAggregator(Executor):
+        pass
+
+    with pytest.raises(ValueError, match=r"register_aggregator\(\) has already been called"):
+        (
+            ConcurrentBuilder()
+            .register_aggregator(lambda: CustomAggregator(id="agg1"))
+            .register_aggregator(lambda: CustomAggregator(id="agg2"))
+        )
+
+
 async def test_concurrent_checkpoint_resume_round_trip() -> None:
    storage = InMemoryCheckpointStorage()

@@ -278,3 +460,92 @@ async def test_concurrent_checkpoint_runtime_overrides_buildtime() -> None:

        assert len(runtime_checkpoints) > 0, "Runtime storage should have checkpoints"
        assert len(buildtime_checkpoints) == 0, "Build-time storage should have no checkpoints when overridden"
+
+
+def test_concurrent_builder_rejects_empty_participant_factories() -> None:
+    with pytest.raises(ValueError):
+        ConcurrentBuilder().register_participants([])
+
+
+async def test_concurrent_builder_reusable_after_build_with_participants() -> None:
+    """Test that the builder can be reused to build multiple identical workflows with participants()."""
+    e1 = _FakeAgentExec("agentA", "One")
+    e2 = _FakeAgentExec("agentB", "Two")
+
+    builder = ConcurrentBuilder().participants([e1, e2])
+
+    builder.build()
+
+    assert builder._participants[0] is e1  # type: ignore
+    assert builder._participants[1] is e2  # type: ignore
+    assert builder._participant_factories == []  # type: ignore
+
+
+async def test_concurrent_builder_reusable_after_build_with_factories() -> None:
+    """Test that the builder can be reused to build multiple workflows with register_participants()."""
+    call_count = 0
+
+    def create_agent_executor_a() -> Executor:
+        nonlocal call_count
+        call_count += 1
+        return _FakeAgentExec("agentA", "One")
+
+    def create_agent_executor_b() -> Executor:
+        nonlocal call_count
+        call_count += 1
+        return _FakeAgentExec("agentB", "Two")
+
+    builder = ConcurrentBuilder().register_participants([create_agent_executor_a, create_agent_executor_b])
+
+    # Build the first workflow
+    wf1 = builder.build()
+
+    assert builder._participants == []  # type: ignore
+    assert len(builder._participant_factories) == 2  # type: ignore
+    assert call_count == 2
+
+    # Build the second workflow
+    wf2 = builder.build()
+    assert call_count == 4
+
+    # Verify that the two workflows have different executor instances
+    assert wf1.executors["agentA"] is not wf2.executors["agentA"]
+    assert wf1.executors["agentB"] is not wf2.executors["agentB"]
+
+
+async def test_concurrent_with_register_participants() -> None:
+    """Test workflow creation using register_participants with factories."""
+
+    def create_agent1() -> Executor:
+        return _FakeAgentExec("agentA", "Alpha")
+
+    def create_agent2() -> Executor:
+        return _FakeAgentExec("agentB", "Beta")
+
+    def create_agent3() -> Executor:
+        return _FakeAgentExec("agentC", "Gamma")
+
+    wf = ConcurrentBuilder().register_participants([create_agent1, create_agent2, create_agent3]).build()
+
+    completed = False
+    output: list[ChatMessage] | None = None
+    async for ev in wf.run_stream("test prompt"):
+        if isinstance(ev, WorkflowStatusEvent) and ev.state == WorkflowRunState.IDLE:
+            completed = True
+        elif isinstance(ev, WorkflowOutputEvent):
+            output = cast(list[ChatMessage], ev.data)
+        if completed and output is not None:
+            break
+
+    assert completed
+    assert output is not None
+    messages: list[ChatMessage] = output
+
+    # Expect one user message + one assistant message per participant
+    assert len(messages) == 1 + 3
+    assert messages[0].role == Role.USER
+    assert "test prompt" in messages[0].text
+
+    assistant_texts = {m.text for m in messages[1:]}
+    assert assistant_texts == {"Alpha", "Beta", "Gamma"}
+    assert all(m.role == Role.ASSISTANT for m in messages[1:])
@@ -293,6 +293,20 @@ def test_register_duplicate_name_raises_error():
        builder.register_executor(lambda: MockExecutor(id="executor_2"), name="MyExecutor")


+def test_register_duplicate_id_raises_error():
+    """Test that registering duplicate id raises an error."""
+    builder = WorkflowBuilder()
+
+    # Register first executor
+    builder.register_executor(lambda: MockExecutor(id="executor"), name="MyExecutor1")
+    builder.register_executor(lambda: MockExecutor(id="executor"), name="MyExecutor2")
+    builder.set_start_executor("MyExecutor1")
+
+    # Registering second executor with same ID should raise ValueError
+    with pytest.raises(ValueError, match="Executor with ID 'executor' has already been created."):
+        builder.build()
+
+
 def test_register_agent_basic():
    """Test basic agent registration with lazy initialization."""
    builder = WorkflowBuilder()