fix(agentchat): raise clear errors for invalid group chat participants

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

@@ -1,6 +1,7 @@
 import asyncio
 import uuid
 from abc import ABC, abstractmethod
+from collections.abc import Sequence as ABCSequence
 from typing import Any, AsyncGenerator, Callable, Dict, List, Mapping, Sequence
 
 from autogen_core import (
@@ -63,6 +64,24 @@ class BaseGroupChat(Team, ABC, ComponentBase[BaseModel]):
 
     component_type = "team"
 
+    @staticmethod
+    def _validate_participants(participants: object) -> List[ChatAgent | Team]:
+        if not isinstance(participants, ABCSequence) or isinstance(participants, (str, bytes, bytearray)):
+            raise TypeError("participants must be a non-empty sequence of ChatAgent or Team instances.")
+
+        validated_participants = list(participants)
+        if len(validated_participants) == 0:
+            raise ValueError("At least one participant is required.")
+
+        invalid_participant = next(
+            (participant for participant in validated_participants if not isinstance(participant, (ChatAgent, Team))),
+            None,
+        )
+        if invalid_participant is not None:
+            raise TypeError("participants must be a non-empty sequence of ChatAgent or Team instances.")
+
+        return validated_participants
+
     def __init__(
         self,
         name: str,
@@ -78,11 +97,10 @@ def __init__(
     ):
         self._name = name
         self._description = description
-        if len(participants) == 0:
-            raise ValueError("At least one participant is required.")
-        if len(participants) != len(set(participant.name for participant in participants)):
+        validated_participants = self._validate_participants(participants)
+        if len(validated_participants) != len(set(participant.name for participant in validated_participants)):
             raise ValueError("The participant names must be unique.")
-        self._participants = participants
+        self._participants = validated_participants
         self._base_group_chat_manager_class = group_chat_manager_class
         self._termination_condition = termination_condition
         self._max_turns = max_turns
@@ -91,7 +109,7 @@ def __init__(
             for message_type in custom_message_types:
                 self._message_factory.register(message_type)
 
-        for agent in participants:
+        for agent in validated_participants:
             if isinstance(agent, ChatAgent):
                 for message_type in agent.produced_message_types:
                     try:
@@ -112,15 +130,15 @@ def __init__(
         # The names are used to identify the agents within the team.
         # The names may not be unique across different teams.
         self._group_chat_manager_name = group_chat_manager_name
-        self._participant_names: List[str] = [participant.name for participant in participants]
-        self._participant_descriptions: List[str] = [participant.description for participant in participants]
+        self._participant_names: List[str] = [participant.name for participant in validated_participants]
+        self._participant_descriptions: List[str] = [participant.description for participant in validated_participants]
         # The group chat topic type is used for broadcast communication among all participants and the group chat manager.
         self._group_topic_type = f"group_topic_{self._team_id}"
         # The group chat manager topic type is used for direct communication with the group chat manager.
         self._group_chat_manager_topic_type = f"{self._group_chat_manager_name}_{self._team_id}"
         # The participant topic types are used for direct communication with each participant.
         self._participant_topic_types: List[str] = [
-            f"{participant.name}_{self._team_id}" for participant in participants
+            f"{participant.name}_{self._team_id}" for participant in validated_participants
         ]
         # The output topic type is used for emitting streaming messages from the group chat.
         # The group chat manager will relay the messages to the output message queue.

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

@@ -1732,6 +1732,23 @@ async def test_round_robin_group_chat_with_message_list(runtime: AgentRuntime |
         await team.run(task=[])
 
 
+def test_round_robin_group_chat_rejects_non_sequence_participants() -> None:
+    with pytest.raises(TypeError, match="participants must be a non-empty sequence of ChatAgent or Team instances"):
+        RoundRobinGroupChat(participants=None)  # type: ignore[arg-type]
+
+
+def test_round_robin_group_chat_rejects_string_participants() -> None:
+    with pytest.raises(TypeError, match="participants must be a non-empty sequence of ChatAgent or Team instances"):
+        RoundRobinGroupChat(participants="not a list")  # type: ignore[arg-type]
+
+
+def test_round_robin_group_chat_rejects_non_agent_participants() -> None:
+    agent = _EchoAgent("Agent1", "First agent")
+
+    with pytest.raises(TypeError, match="participants must be a non-empty sequence of ChatAgent or Team instances"):
+        RoundRobinGroupChat(participants=[agent, "bad"])  # type: ignore[list-item]
+
+
 @pytest.mark.asyncio
 async def test_declarative_groupchats_with_config(runtime: AgentRuntime | None) -> None:
     # Create basic agents and components for testing