fix(agentchat): validate participants type in BaseGroupChat with clear TypeError

python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_base_group_chat.py

@@ -78,6 +78,16 @@ def __init__(
     ):
         self._name = name
         self._description = description
+        if not isinstance(participants, list):
+            raise TypeError(
+                f"participants must be a list of ChatAgent or Team instances, got {type(participants).__name__!r}."
+            )
+        for i, participant in enumerate(participants):
+            if not isinstance(participant, (ChatAgent, Team)):
+                raise TypeError(
+                    f"participants[{i}] must be a ChatAgent or Team instance, "
+                    f"got {type(participant).__name__!r}."
+                )
         if len(participants) == 0:
             raise ValueError("At least one participant is required.")
         if len(participants) != len(set(participant.name for participant in participants)):

python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_round_robin_group_chat.py

@@ -123,6 +123,7 @@ class RoundRobinGroupChat(BaseGroupChat, Component[RoundRobinGroupChatConfig]):
         emit_team_events (bool, optional): Whether to emit team events through :meth:`BaseGroupChat.run_stream`. Defaults to False.
 
     Raises:
+        TypeError: If participants is not a list or contains non-agent/non-team items.
         ValueError: If no participants are provided or if participant names are not unique.
 
     Examples:

python/packages/autogen-agentchat/tests/test_group_chat.py

@@ -488,6 +488,26 @@ async def test_round_robin_group_chat_unknown_task_message_type(runtime: AgentRu
         )
 
 
+def test_group_chat_invalid_participants() -> None:
+    """Test that BaseGroupChat raises clear TypeError for invalid participants."""
+    agent = _EchoAgent("agent", "An echo agent.")
+
+    with pytest.raises(TypeError, match="participants must be a list"):
+        RoundRobinGroupChat(participants=None)  # type: ignore[arg-type]
+
+    with pytest.raises(TypeError, match="participants must be a list"):
+        RoundRobinGroupChat(participants="not a list")  # type: ignore[arg-type]
+
+    with pytest.raises(TypeError, match="participants must be a list"):
+        RoundRobinGroupChat(participants=42)  # type: ignore[arg-type]
+
+    with pytest.raises(TypeError, match=r"participants\[1\] must be a ChatAgent or Team instance"):
+        RoundRobinGroupChat(participants=[agent, "bad"])  # type: ignore[list-item]
+
+    with pytest.raises(TypeError, match=r"participants\[0\] must be a ChatAgent or Team instance"):
+        RoundRobinGroupChat(participants=[42])  # type: ignore[list-item]
+
+
 @pytest.mark.asyncio
 async def test_round_robin_group_chat_unknown_agent_message_type() -> None:
     model_client = ReplayChatCompletionClient(["Hello"])