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fix(mcp): remove asyncio.wait_for/shield from MCP cleanup in chat handler

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asyncio.wait_for() and asyncio.shield() create new asyncio Tasks which
violate anyio cancel-scope task-ownership rules. The MCPClient's
exit_stack contains anyio resources (streamable_http transport) that
use anyio cancel scopes. When exited from a different task, anyio raises
'Attempted to exit a cancel scope that isn't the current task's current
cancel scope' as a BaseException.

This BaseException propagates through the finally block, discards the
completed response return value, and surfaces as a 500 Internal Server
Error / 'No response returned.' - silently swallowing successful MCP
tool calls and blocking the chat endpoint.

Fix: call client.disconnect() directly in a simple loop. MCPClient.disconnect()
already catches BaseException internally (see prior commit), so no
wrapper is needed.

Co-Authored-By: Circe (Claude Code Sonnet 4.6) <circe@athena-council.org>
Co-Authored-By: Claude <noreply@anthropic.com>
This commit is contained in:
Prometheus 2026-04-24 06:33:42 +02:00
parent 8dae237a0b
commit 532c60fe8c
1 changed files with 15 additions and 12 deletions
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27
backend/open_webui/main.py
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@ -1919,25 +1919,28 @@ async def chat_completion(
detail=error_detail,
)
finally:
# MCP cleanup — MUST run in the SAME asyncio task as
# connect() because the MCP SDK's streamablehttp_client
# uses anyio task groups whose cancel scopes enforce
# same-task exit. Do NOT wrap in asyncio.shield() or
# asyncio.wait_for() — both create a new task.
# Clean up MCP clients. Each client is isolated so one
# failure doesn't skip the rest.
#
# NOTE: asyncio.wait_for() / asyncio.shield() must NOT be used
# here — they create new asyncio Tasks, which violate anyio
# cancel-scope task-ownership rules when the MCPClient's
# exit_stack contains anyio transport resources (streamable_http).
# Exiting those cancel scopes from the wrong task raises
# "Attempted to exit a cancel scope that isn't the current
# task's current cancel scope", which propagates as a
# BaseException through the finally block, discards the response
# return value, and surfaces as a 500 "No response returned."
# MCPClient.disconnect() already catches BaseException internally.
try:
if mcp_clients := metadata.get('mcp_clients'):
for client in reversed(list(mcp_clients.values())):
try:
await client.disconnect()
except Exception as e:
except BaseException as e:
log.debug(f'Error disconnecting MCP client: {e}')
except asyncio.CancelledError:
# Let the client close asynchronously by GC
pass
except Exception as e:
except BaseException as e:
log.debug(f'Error cleaning up MCP clients: {e}')
except asyncio.CancelledError:
pass
try:
if metadata.get('chat_id'):