fix(channel): reject concurrent same-thread runs (#1465) (#1475)

* fix(channel): reject concurrent same-thread runs (#1465)

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(lint): sort imports in manager.py and test_channels.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(channel): widen _is_thread_busy_error to BaseException and downgrade busy log to warning

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/channels/manager.py

@@ -10,6 +10,8 @@ import time
 from collections.abc import Mapping
 from typing import Any
 
+from langgraph_sdk.errors import ConflictError
+
 from app.channels.message_bus import InboundMessage, InboundMessageType, MessageBus, OutboundMessage, ResolvedAttachment
 from app.channels.store import ChannelStore
 
@@ -27,6 +29,7 @@ DEFAULT_RUN_CONTEXT: dict[str, Any] = {
     "subagent_enabled": False,
 }
 STREAM_UPDATE_MIN_INTERVAL_SECONDS = 0.35
+THREAD_BUSY_MESSAGE = "This conversation is already processing another request. Please wait for it to finish and try again."
 
 CHANNEL_CAPABILITIES = {
     "feishu": {"supports_streaming": True},
@@ -39,6 +42,14 @@ class InvalidChannelSessionConfigError(ValueError):
     """Raised when IM channel session overrides contain invalid agent config."""
 
 
+def _is_thread_busy_error(exc: BaseException | None) -> bool:
+    if exc is None:
+        return False
+    if isinstance(exc, ConflictError):
+        return True
+    return "already running a task" in str(exc)
+
+
 def _as_dict(value: Any) -> dict[str, Any]:
     return dict(value) if isinstance(value, Mapping) else {}
 
@@ -606,6 +617,7 @@ class ChannelManager:
                 config=run_config,
                 context=run_context,
                 stream_mode=["messages-tuple", "values"],
+                multitask_strategy="reject",
             ):
                 event = getattr(chunk, "event", "")
                 data = getattr(chunk, "data", None)
@@ -641,7 +653,10 @@ class ChannelManager:
                 last_publish_at = now
         except Exception as exc:
             stream_error = exc
-            logger.exception("[Manager] streaming error: thread_id=%s", thread_id)
+            if _is_thread_busy_error(exc):
+                logger.warning("[Manager] thread busy (concurrent run rejected): thread_id=%s", thread_id)
+            else:
+                logger.exception("[Manager] streaming error: thread_id=%s", thread_id)
         finally:
             result = last_values if last_values is not None else {"messages": [{"type": "ai", "content": latest_text}]}
             response_text = _extract_response_text(result)
@@ -652,7 +667,10 @@ class ChannelManager:
                 if attachments:
                     response_text = _format_artifact_text([attachment.virtual_path for attachment in attachments])
                 elif stream_error:
-                    response_text = "An error occurred while processing your request. Please try again."
+                    if _is_thread_busy_error(stream_error):
+                        response_text = THREAD_BUSY_MESSAGE
+                    else:
+                        response_text = "An error occurred while processing your request. Please try again."
                 else:
                     response_text = latest_text or "(No response from agent)"