feat: add agent type registry and create_agent() factory

backend/src/agents/main_agent/__init__.py

@@ -30,6 +30,7 @@
 from .main_agent import MainAgent
 from .base_agent import BaseAgent
 from .chat_agent import ChatAgent
+from .agent_types import create_agent, register_agent_type, get_available_types
 from .core import ModelConfig, SystemPromptBuilder
 from .session import SessionFactory
 from .tools import ToolRegistry, ToolFilter, GatewayIntegration, create_default_registry
@@ -56,6 +57,9 @@
     "BaseAgent",
     "ChatAgent",
     "MainAgent",
+    "create_agent",
+    "register_agent_type",
+    "get_available_types",
 
     # Core components
     "ModelConfig",

backend/src/agents/main_agent/agent_types.py

@@ -0,0 +1,60 @@
+"""
+Agent type registry and factory function.
+
+Provides a single entry point for creating agents by type string.
+New agent types register here as they're implemented.
+"""
+
+import logging
+from typing import Optional, List
+
+from agents.main_agent.base_agent import BaseAgent
+
+logger = logging.getLogger(__name__)
+
+
+# Registry of agent type string → class
+# New agent types add themselves here as they're implemented
+_AGENT_TYPES = {}
+
+
+def register_agent_type(type_name: str, agent_class: type) -> None:
+    """Register an agent class for a given type string."""
+    _AGENT_TYPES[type_name] = agent_class
+
+
+def get_available_types() -> List[str]:
+    """Return list of registered agent type names."""
+    return list(_AGENT_TYPES.keys())
+
+
+def create_agent(agent_type: str = "chat", **kwargs) -> BaseAgent:
+    """
+    Create an agent by type string.
+
+    Args:
+        agent_type: Agent type ("chat", "skill", "voice"). Default: "chat".
+        **kwargs: Passed directly to the agent constructor (session_id, user_id, etc.)
+
+    Returns:
+        BaseAgent subclass instance
+
+    Raises:
+        ValueError: If agent_type is not registered
+    """
+    agent_class = _AGENT_TYPES.get(agent_type)
+    if agent_class is None:
+        available = ", ".join(sorted(_AGENT_TYPES.keys()))
+        raise ValueError(f"Unknown agent_type '{agent_type}'. Available: {available}")
+
+    logger.info(f"Creating {agent_type} agent ({agent_class.__name__})")
+    return agent_class(**kwargs)
+
+
+# Register built-in agent types
+def _register_defaults():
+    from agents.main_agent.chat_agent import ChatAgent
+    register_agent_type("chat", ChatAgent)
+
+
+_register_defaults()

backend/tests/agents/main_agent/test_agent_types.py

@@ -0,0 +1,72 @@
+"""Tests for agent type registry and factory function."""
+
+import pytest
+from agents.main_agent.agent_types import (
+    create_agent,
+    register_agent_type,
+    get_available_types,
+    _AGENT_TYPES,
+)
+from agents.main_agent.base_agent import BaseAgent
+from agents.main_agent.chat_agent import ChatAgent
+from agents.main_agent.main_agent import MainAgent
+
+
+class TestGetAvailableTypes:
+    """Req AT-1: Available types discovery."""
+
+    def test_chat_is_registered_by_default(self):
+        assert "chat" in get_available_types()
+
+    def test_returns_list_of_strings(self):
+        types = get_available_types()
+        assert isinstance(types, list)
+        assert all(isinstance(t, str) for t in types)
+
+
+class TestRegisterAgentType:
+    """Req AT-2: Dynamic agent type registration."""
+
+    def test_register_new_type(self):
+        class DummyAgent(BaseAgent):
+            def _create_agent(self): pass
+            async def stream_async(self, message, **kwargs): yield ""
+
+        register_agent_type("dummy", DummyAgent)
+        assert "dummy" in get_available_types()
+
+        # Cleanup
+        del _AGENT_TYPES["dummy"]
+
+    def test_register_overwrites_existing(self):
+        original = _AGENT_TYPES.get("chat")
+        try:
+            register_agent_type("chat", MainAgent)
+            assert _AGENT_TYPES["chat"] is MainAgent
+        finally:
+            _AGENT_TYPES["chat"] = original
+
+
+class TestCreateAgent:
+    """Req AT-3: Factory function routing."""
+
+    def test_unknown_type_raises_value_error(self):
+        with pytest.raises(ValueError, match="Unknown agent_type 'nonexistent'"):
+            create_agent("nonexistent", session_id="test")
+
+    def test_error_message_lists_available_types(self):
+        with pytest.raises(ValueError, match="chat"):
+            create_agent("nonexistent", session_id="test")
+
+
+class TestMainAgentBackwardCompat:
+    """Req AT-4: MainAgent remains a valid ChatAgent subclass."""
+
+    def test_mainagent_is_subclass_of_chatagent(self):
+        assert issubclass(MainAgent, ChatAgent)
+
+    def test_mainagent_is_subclass_of_baseagent(self):
+        assert issubclass(MainAgent, BaseAgent)
+
+    def test_chatagent_is_subclass_of_baseagent(self):
+        assert issubclass(ChatAgent, BaseAgent)