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Python: Fix MCP plugin hang when initialization fails #13414 #13437

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93b77d3
Fix MCP plugin hang when initialization fails
aryanyk Jan 3, 2026
96b3a6a
Merge branch 'main' into fix-mcp-hang-on-init
aryanyk Feb 25, 2026
13115ae
Merge branch 'main' into fix-mcp-hang-on-init
moonbox3 Apr 8, 2026
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62 changes: 34 additions & 28 deletions python/semantic_kernel/connectors/mcp.py
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Original file line number Diff line number Diff line change
Expand Up @@ -269,15 +269,15 @@ async def __aexit__(

async def connect(self) -> None:
"""Connect to the MCP server."""
ready_event = asyncio.Event()
loop = asyncio.get_running_loop()
ready_future: asyncio.Future[None] = loop.create_future()

try:
self._current_task = asyncio.create_task(self._inner_connect(ready_event))
await ready_event.wait()
self._current_task = asyncio.create_task(self._inner_connect(ready_future))
await ready_future
except KernelPluginInvalidConfigurationError:
ready_event.clear()
raise
except Exception as ex:
ready_event.clear()
await self.close()
raise FunctionExecutionException("Failed to enter context manager.") from ex

Expand All @@ -293,16 +293,20 @@ async def close(self) -> None:
self._current_task = None
self.session = None

async def _inner_connect(self, ready_event: asyncio.Event) -> None:
async def _inner_connect(self, ready_future: asyncio.Future) -> None:
if not self.session:
try:
transport = await self._exit_stack.enter_async_context(self.get_mcp_client())
except Exception as ex:
await self._exit_stack.aclose()
ready_event.set()
raise KernelPluginInvalidConfigurationError(
"Failed to connect to the MCP server. Please check your configuration."
) from ex
if not ready_future.done():
exc = KernelPluginInvalidConfigurationError(
"Failed to connect to the MCP server. Please check your configuration."
)
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@github-actions github-actions bot Apr 8, 2026

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Unlike raise KernelPluginInvalidConfigurationError(...) from ex, manually setting __cause__ does not set __suppress_context__ = True. This means Python will print 'During handling of the above exception…' instead of 'The above exception was the direct cause of…'. Add exc.__suppress_context__ = True after setting __cause__.

Suggested change
)
exc.__cause__ = ex
exc.__suppress_context__ = True
ready_future.set_exception(exc)

exc.__cause__ = ex
ready_future.set_exception(exc)
return

try:
session = await self._exit_stack.enter_async_context(
ClientSession(
Expand All @@ -316,16 +320,24 @@ async def _inner_connect(self, ready_event: asyncio.Event) -> None:
)
except Exception as ex:
await self._exit_stack.aclose()
raise KernelPluginInvalidConfigurationError(
"Failed to create a session. Please check your configuration."
) from ex
if not ready_future.done():
exc = KernelPluginInvalidConfigurationError(
"Failed to create a session. Please check your configuration."
)
exc.__cause__ = ex
ready_future.set_exception(exc)
return
try:
await session.initialize()
except Exception as ex:
await self._exit_stack.aclose()
raise KernelPluginInvalidConfigurationError(
"Failed to initialize session. Please check your configuration."
) from ex
if not ready_future.done():
exc = KernelPluginInvalidConfigurationError(
"Failed to initialize session. Please check your configuration."
)
exc.__cause__ = ex
ready_future.set_exception(exc)
return
self.session = session
elif self.session._request_id == 0:
# If the session is not initialized, we need to reinitialize it
Expand All @@ -344,7 +356,8 @@ async def _inner_connect(self, ready_event: asyncio.Event) -> None:
except Exception:
logger.warning("Failed to set log level to %s", logger.level)
# Setting up is complete, will now signal the main loop that we are ready
Comment on lines 334 to 358
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Deadlock bug: If load_tools(), load_prompts(), session.initialize() (line 332), or set_logging_level() raises an unhandled exception, the task exits but ready_future is never resolved—connect() hangs forever on await ready_future. Wrap this entire block in a try/except that resolves the future on failure, consistent with the earlier error paths in _inner_connect.

Suggested change
try:
if self.load_tools_flag:
await self.load_tools()
if self.load_prompts_flag:
await self.load_prompts()
if logger.level != logging.NOTSET:
try:
await self.session.set_logging_level(
next(level for level, value in LOG_LEVEL_MAPPING.items() if value == logger.level)
)
except Exception:
logger.warning("Failed to set log level to %s", logger.level)
# Setting up is complete, will now signal the main loop that we are ready
if not ready_future.done():
ready_future.set_result(None)
except Exception as ex:
await self._exit_stack.aclose()
if not ready_future.done():
ready_future.set_exception(ex)
return

ready_event.set()
if not ready_future.done():
ready_future.set_result(None)
# Create a stop event to signal the exit stack to close
self._stop_event = asyncio.Event()
await self._stop_event.wait()
Expand Down Expand Up @@ -466,11 +479,8 @@ async def message_handler(

async def load_prompts(self):
"""Load prompts from the MCP server."""
try:
prompt_list = await self.session.list_prompts()
except Exception:
prompt_list = None
for prompt in prompt_list.prompts if prompt_list else []:
prompt_list = await self.session.list_prompts()
for prompt in prompt_list.prompts:
local_name = _normalize_mcp_name(prompt.name)
func = kernel_function(name=local_name, description=prompt.description)(
partial(self.get_prompt, prompt.name)
Expand All @@ -480,12 +490,8 @@ async def load_prompts(self):

async def load_tools(self):
"""Load tools from the MCP server."""
try:
tool_list = await self.session.list_tools()
except Exception:
tool_list = None
# Create methods with the kernel_function decorator for each tool
for tool in tool_list.tools if tool_list else []:
tool_list = await self.session.list_tools()
for tool in tool_list.tools:
local_name = _normalize_mcp_name(tool.name)
func = kernel_function(name=local_name, description=tool.description)(partial(self.call_tool, tool.name))
func.__kernel_function_parameters__ = _get_parameter_dicts_from_mcp_tool(tool)
Expand Down