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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
b51f34f
Fix MCP initialization hang: ensure future resolution and improve err…
aryanyk Apr 30, 2026
1ec87f0
Ruff formatting
aryanyk Apr 30, 2026
cd88d44
Merge branch 'main' into fix-mcp-hang-on-init
aryanyk Apr 30, 2026
2a94a07
Merge branch 'main' into fix-mcp-hang-on-init
aryanyk Apr 30, 2026
b78dbd7
Merge branch 'main' into fix-mcp-hang-on-init
aryanyk May 1, 2026
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119 changes: 75 additions & 44 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))
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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 +293,24 @@ 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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exc.__cause__ = ex
exc.__suppress_context__ = True
ready_future.set_exception(exc)
try:
await self._exit_stack.aclose()
except Exception:
logger.warning("Failed to close exit stack during error handling")
return

try:
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session = await self._exit_stack.enter_async_context(
ClientSession(
Expand All @@ -315,36 +323,60 @@ 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 connect to the MCP server. Please check your configuration."
)
exc.__cause__ = ex
exc.__suppress_context__ = True
ready_future.set_exception(exc)
try:
await self._exit_stack.aclose()
except Exception:
logger.warning("Failed to close exit stack during error handling")
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
exc.__suppress_context__ = True
ready_future.set_exception(exc)
try:
await self._exit_stack.aclose()
except Exception:
logger.warning("Failed to close exit stack during error handling")
return
self.session = session
elif self.session._request_id == 0:
# If the session is not initialized, we need to reinitialize it
await self.session.initialize()
logger.debug("Connected to MCP server: %s", self.session)
if self.load_tools_flag:
await self.load_tools()
if self.load_prompts_flag:
await self.load_prompts()

if logger.level != logging.NOTSET:
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)
if not ready_future.done():
ready_future.set_result(None)
except Exception as ex:
if not ready_future.done():
ready_future.set_exception(ex)
try:
await self.session.set_logging_level(
next(level for level, value in LOG_LEVEL_MAPPING.items() if value == logger.level)
)
await self._exit_stack.aclose()
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
ready_event.set()
logger.warning("Failed to close exit stack during error handling")
return
# 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 @@ -460,17 +492,20 @@ async def message_handler(
if isinstance(message, types.ServerNotification):
match message.root.method:
case "notifications/tools/list_changed":
await self.load_tools()
try:
await self.load_tools()
except Exception as ex:
logger.warning("Failed to reload tools on notification: %s", ex)
case "notifications/prompts/list_changed":
await self.load_prompts()
try:
await self.load_prompts()
except Exception as ex:
logger.warning("Failed to reload prompts on notification: %s", ex)

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 +515,8 @@ 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
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