fix: validate participants early in BaseGroupChat

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

@@ -1,7 +1,7 @@
 import asyncio
 import uuid
 from abc import ABC, abstractmethod
-from typing import Any, AsyncGenerator, Callable, Dict, List, Mapping, Sequence
+from typing import Any, AsyncGenerator, Callable, Dict, List, Mapping, Sequence, Tuple
 
 from autogen_core import (
     AgentId,
@@ -67,7 +67,7 @@ def __init__(
         self,
         name: str,
         description: str,
-        participants: List[ChatAgent | Team],
+        participants: List[ChatAgent | Team] | Tuple[ChatAgent | Team, ...],
         group_chat_manager_name: str,
         group_chat_manager_class: type[SequentialRoutedAgent],
         termination_condition: TerminationCondition | None = None,
@@ -78,8 +78,19 @@ def __init__(
     ):
         self._name = name
         self._description = description
+        if participants is None:
+            raise TypeError("participants must be a list or tuple of ChatAgent or Team instances, got None")
+        if not isinstance(participants, (list, tuple)):
+            raise TypeError(
+                f"participants must be a list or tuple of ChatAgent or Team instances, got {type(participants).__name__}"
+            )
         if len(participants) == 0:
             raise ValueError("At least one participant is required.")
+        for i, participant in enumerate(participants):
+            if not isinstance(participant, (ChatAgent, Team)):
+                raise TypeError(
+                    f"participants[{i}] must be a ChatAgent or Team instance, got {type(participant).__name__}"
+                )
         if len(participants) != len(set(participant.name for participant in participants)):
             raise ValueError("The participant names must be unique.")
         self._participants = participants

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

@@ -1,5 +1,5 @@
 import asyncio
-from typing import Any, Callable, List, Mapping, Sequence
+from typing import Any, Callable, List, Mapping, Sequence, Tuple
 
 from autogen_core import AgentRuntime, Component, ComponentModel
 from pydantic import BaseModel
@@ -241,7 +241,7 @@ async def main() -> None:
 
     def __init__(
         self,
-        participants: List[ChatAgent | Team],
+        participants: List[ChatAgent | Team] | Tuple[ChatAgent | Team, ...],
         *,
         name: str | None = None,
         description: str | None = None,

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

@@ -1944,3 +1944,35 @@ async def test_selector_group_chat_streaming(runtime: AgentRuntime | None) -> No
 
     # Content-based verification instead of index-based
     # Note: The streaming test verifies the streaming behavior, not the final result content
+
+
+@pytest.mark.asyncio
+async def test_round_robin_group_chat_validates_participants_none() -> None:
+    """Test that participants=None raises a clear TypeError."""
+    with pytest.raises(TypeError, match="participants must be a list or tuple"):
+        RoundRobinGroupChat(participants=None)  # type: ignore
+
+
+@pytest.mark.asyncio
+async def test_round_robin_group_chat_validates_participants_not_sequence() -> None:
+    """Test that non-sequence participants raises a clear TypeError."""
+    with pytest.raises(TypeError, match="participants must be a list or tuple"):
+        RoundRobinGroupChat(participants="not a list")  # type: ignore
+
+
+@pytest.mark.asyncio
+async def test_round_robin_group_chat_validates_participants_invalid_type() -> None:
+    """Test that participants containing non-agent objects raises a clear TypeError."""
+    from autogen_agentchat.agents import UserProxyAgent
+
+    with pytest.raises(TypeError, match=r"participants\[1\] must be a ChatAgent or Team"):
+        RoundRobinGroupChat(participants=[UserProxyAgent(name="valid"), "invalid"])  # type: ignore
+
+
+@pytest.mark.asyncio
+async def test_round_robin_group_chat_accepts_tuple_participants() -> None:
+    """Test that participants passed as a tuple is accepted."""
+    from autogen_agentchat.agents import UserProxyAgent
+
+    team = RoundRobinGroupChat(participants=(UserProxyAgent(name="agent1"), UserProxyAgent(name="agent2")))
+    assert len(team._participants) == 2