Support custom channel assistant IDs via lead_agent (#1500)

* Support custom channel assistant IDs via lead agent

* Normalize custom channel agent names

backend/app/channels/manager.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 import asyncio
 import logging
 import mimetypes
+import re
 import time
 from collections.abc import Mapping
 from typing import Any
@@ -17,6 +18,7 @@ logger = logging.getLogger(__name__)
 DEFAULT_LANGGRAPH_URL = "http://localhost:2024"
 DEFAULT_GATEWAY_URL = "http://localhost:8001"
 DEFAULT_ASSISTANT_ID = "lead_agent"
+CUSTOM_AGENT_NAME_PATTERN = re.compile(r"^[A-Za-z0-9-]+$")
 
 DEFAULT_RUN_CONFIG: dict[str, Any] = {"recursion_limit": 100}
 DEFAULT_RUN_CONTEXT: dict[str, Any] = {
@@ -33,6 +35,10 @@ CHANNEL_CAPABILITIES = {
 }
 
 
+class InvalidChannelSessionConfigError(ValueError):
+    """Raised when IM channel session overrides contain invalid agent config."""
+
+
 def _as_dict(value: Any) -> dict[str, Any]:
     return dict(value) if isinstance(value, Mapping) else {}
 
@@ -45,6 +51,21 @@ def _merge_dicts(*layers: Any) -> dict[str, Any]:
     return merged
 
 
+def _normalize_custom_agent_name(raw_value: str) -> str:
+    """Normalize legacy channel assistant IDs into valid custom agent names."""
+    normalized = raw_value.strip().lower().replace("_", "-")
+    if not normalized:
+        raise InvalidChannelSessionConfigError(
+            "Channel session assistant_id is empty. Use 'lead_agent' or a valid custom agent name."
+        )
+    if not CUSTOM_AGENT_NAME_PATTERN.fullmatch(normalized):
+        raise InvalidChannelSessionConfigError(
+            f"Invalid channel session assistant_id {raw_value!r}. "
+            "Use 'lead_agent' or a custom agent name containing only letters, digits, and hyphens."
+        )
+    return normalized
+
+
 def _extract_response_text(result: dict | list) -> str:
     """Extract the last AI message text from a LangGraph runs.wait result.
 
@@ -379,6 +400,13 @@ class ChannelManager:
             {"thread_id": thread_id},
         )
 
+        # Custom agents are implemented as lead_agent + agent_name context.
+        # Keep backward compatibility for channel configs that set
+        # assistant_id: <custom-agent-name> by routing through lead_agent.
+        if assistant_id != DEFAULT_ASSISTANT_ID:
+            run_context.setdefault("agent_name", _normalize_custom_agent_name(assistant_id))
+            assistant_id = DEFAULT_ASSISTANT_ID
+
         return assistant_id, run_config, run_context
 
     # -- LangGraph SDK client (lazy) ----------------------------------------
@@ -452,6 +480,14 @@ class ChannelManager:
                     await self._handle_command(msg)
                 else:
                     await self._handle_chat(msg)
+            except InvalidChannelSessionConfigError as exc:
+                logger.warning(
+                    "Invalid channel session config for %s (chat=%s): %s",
+                    msg.channel_name,
+                    msg.chat_id,
+                    exc,
+                )
+                await self._send_error(msg, str(exc))
             except Exception:
                 logger.exception(
                     "Error handling message from %s (chat=%s)",