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fix: preserve HTTP status codes in streamable HTTP transport error responses #2176

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giulio-leone wants to merge 2 commits into from
+50 −38
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fix: preserve HTTP status codes in streamable HTTP transport error responses #2176

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fix: preserve HTTP status codes in streamable HTTP transport error re…
giulio-leone Feb 28, 2026
0176b50
style: apply ruff format
giulio-leone Feb 28, 2026
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88 changes: 50 additions & 38 deletions src/mcp/client/streamable_http.py
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Original file line number Diff line number Diff line change
Expand Up @@ -257,48 +257,60 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
message = ctx.session_message.message
is_initialization = self._is_initialization_request(message)

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

if response.status_code == 404: # pragma: no branch
if isinstance(message, JSONRPCRequest): # pragma: no branch
error_data = ErrorData(code=INVALID_REQUEST, message="Session terminated")
session_message = SessionMessage(JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data))
await ctx.read_stream_writer.send(session_message)
return
try:
async with ctx.client.stream(
"POST",
self.url,
json=message.model_dump(by_alias=True, mode="json", exclude_unset=True),
headers=headers,
) as response:
if response.status_code == 202:
logger.debug("Received 202 Accepted")
return

if response.status_code >= 400:
if isinstance(message, JSONRPCRequest):
error_data = ErrorData(code=INTERNAL_ERROR, message="Server returned an error response")
session_message = SessionMessage(JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data))
await ctx.read_stream_writer.send(session_message)
return
if response.status_code >= 400:
# Read body for error detail
await response.aread()
body_text = response.text[:200] if response.text else ""
error_msg = (
f"HTTP {response.status_code}: {body_text}" if body_text else f"HTTP {response.status_code}"
)
if isinstance(message, JSONRPCRequest):
error_data = ErrorData(
code=INTERNAL_ERROR,
message=error_msg,
)
session_message = SessionMessage(JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data))
await ctx.read_stream_writer.send(session_message)
return

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

# Per https://modelcontextprotocol.io/specification/2025-06-18/basic#notifications:
# The server MUST NOT send a response to notifications.
# Per https://modelcontextprotocol.io/specification/2025-06-18/basic#notifications:
# The server MUST NOT send a response to notifications.
if isinstance(message, JSONRPCRequest):
content_type = response.headers.get("content-type", "").lower()
if content_type.startswith("application/json"):
await self._handle_json_response(
response, ctx.read_stream_writer, is_initialization, request_id=message.id
)
elif content_type.startswith("text/event-stream"):
await self._handle_sse_response(response, ctx, is_initialization)
else:
logger.error(f"Unexpected content type: {content_type}")
error_data = ErrorData(code=INVALID_REQUEST, message=f"Unexpected content type: {content_type}")
error_msg = SessionMessage(JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data))
await ctx.read_stream_writer.send(error_msg)
except Exception as exc:
# Propagate connection/transport errors to the caller via the read stream
# so they don't hang waiting for a response that will never arrive.
if isinstance(message, JSONRPCRequest):
content_type = response.headers.get("content-type", "").lower()
if content_type.startswith("application/json"):
await self._handle_json_response(
response, ctx.read_stream_writer, is_initialization, request_id=message.id
)
elif content_type.startswith("text/event-stream"):
await self._handle_sse_response(response, ctx, is_initialization)
else:
logger.error(f"Unexpected content type: {content_type}")
error_data = ErrorData(code=INVALID_REQUEST, message=f"Unexpected content type: {content_type}")
error_msg = SessionMessage(JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data))
await ctx.read_stream_writer.send(error_msg)
error_data = ErrorData(code=INTERNAL_ERROR, message=str(exc))
session_message = SessionMessage(JSONRPCError(jsonrpc="2.0", id=message.id, error=error_data))
with contextlib.suppress(Exception):
await ctx.read_stream_writer.send(session_message)
raise

async def _handle_json_response(
self,
Expand Down
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