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fix: Add exception handling for POST request failures in StreamableHTTP #1008

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fix: Add exception handling for POST request failures in StreamableHTTP #1008

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fix: add exception handling for POST request failures in StreamableHT…
Jun 23, 2025
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Merge branch 'main' into main
Kulaxyz Jun 27, 2025
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Kulaxyz Jun 27, 2025
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Kulaxyz Jul 15, 2025
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Kulaxyz Jul 17, 2025
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70 changes: 40 additions & 30 deletions src/mcp/client/streamable_http.py
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Original file line number Diff line number Diff line change
Expand Up @@ -255,40 +255,50 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
headers = self._prepare_request_headers(ctx.headers)
message = ctx.session_message.message
is_initialization = self._is_initialization_request(message)
try:
async with ctx.client.stream(
"POST",
self.url,
json=message.model_dump(by_alias=True, mode="json", exclude_none=True),
headers=headers,
) as response:
if response.status_code == 202:
logger.debug("Received 202 Accepted")
return

async with ctx.client.stream(
"POST",
self.url,
json=message.model_dump(by_alias=True, mode="json", exclude_none=True),
headers=headers,
) as response:
if response.status_code == 202:
logger.debug("Received 202 Accepted")
return

if response.status_code == 404:
if isinstance(message.root, JSONRPCRequest):
await self._send_session_terminated_error(
ctx.read_stream_writer,
message.root.id,
)
return
if response.status_code == 404:
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@felixweinberger felixweinberger Sep 22, 2025 •
edited
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We should instead expand this or add a case like

if response.status_code in (500, 502, 503, ...):

That does specific handling of only those specific errors you're encountering and that are expected that we want to handle. That would prevent us from masking other errors that might occur.

Other unexpected HTTP errors should be handled by raise_for_status() lower down.

if isinstance(message.root, JSONRPCRequest):
await self._send_session_terminated_error(
ctx.read_stream_writer,
message.root.id,
)
return

response.raise_for_status()
if is_initialization:
self._maybe_extract_session_id_from_response(response)
response.raise_for_status()
if is_initialization:
self._maybe_extract_session_id_from_response(response)

content_type = response.headers.get(CONTENT_TYPE, "").lower()
content_type = response.headers.get(CONTENT_TYPE, "").lower()

if content_type.startswith(JSON):
await self._handle_json_response(response, ctx.read_stream_writer, is_initialization)
elif content_type.startswith(SSE):
await self._handle_sse_response(response, ctx, is_initialization)
else:
await self._handle_unexpected_content_type(
content_type,
ctx.read_stream_writer,
)
if content_type.startswith(JSON):
await self._handle_json_response(response, ctx.read_stream_writer, is_initialization)
elif content_type.startswith(SSE):
await self._handle_sse_response(response, ctx, is_initialization)
else:
await self._handle_unexpected_content_type(
content_type,
ctx.read_stream_writer,
)
except Exception as exc:
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@felixweinberger felixweinberger Sep 22, 2025 •
edited
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This is too broad for the specific case you mention - we shouldn't be doing this broad exception handling specifically for 5xx= failures as we might mask other issues this way.

logger.error(f"Error in _handle_post_request: {exc}")
request_id = "Unknown"
if isinstance(message.root, JSONRPCRequest):
request_id = message.root.id
error_response = JSONRPCError(
jsonrpc="2.0", id=request_id, error=ErrorData(code=-32603, message=f"Transport error: {str(exc)}")
)
error_session_message = SessionMessage(JSONRPCMessage(error_response))
await ctx.read_stream_writer.send(error_session_message)

async def _handle_json_response(
self,
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