HTTP transport swallows non-2xx status codes causing client to hang

[maxisbey]

Summary

When an MCP server returns non-2xx HTTP status codes (401/403/404/5xx), the Streamable HTTP and SSE transports do not reliably propagate the error to the caller. In the Streamable HTTP transport, post_writer catches exceptions and logs them, but the caller blocks indefinitely waiting for a response on the read stream. HTTP 404 responses are also converted to JSONRPCError(code=32600), losing the original HTTP status information.

Expected Behavior

• Non-2xx HTTP responses should be surfaced as exceptions to the caller, not silently logged
• The original HTTP status code should be preserved and accessible
• Auth-related errors (401/403) should be distinguishable from other failures

Current Behavior

• post_writer in Streamable HTTP transport catches and logs errors without forwarding them through the read stream
• Callers hang indefinitely waiting for a response
• HTTP 404 is converted to JSONRPCError(code=32600), destroying HTTP-level context

Affected Code

• src/mcp/client/streamable_http.py (post_writer)
• src/mcp/client/sse.py (sse_reader)

Related

• Introduce typed error classes with metadata #1742 — broader typed error classes design

[gspeter-max]

Investigation Results

I've systematically investigated this issue and confirmed the root cause. Here's what I found:

Root Cause

In both SSE and StreamableHTTP transports, when HTTP errors occur, exceptions are caught and logged but **never sent to **, causing the caller to hang indefinitely waiting for a response that will never arrive.

Specific Issues Found

1. sse.py (lines 147-148):

except Exception:
    logger.exception("Error in post_writer")
    # Missing: await read_stream_writer.send(exc)

• response.raise_for_status() at line 145 raises HTTPStatusError for non-2xx codes
• Exception is caught, logged, but never propagated to caller
• Result: Caller hangs forever

2. streamable_http.py (lines 481-482):

except Exception:
    logger.exception("Error in post_writer")
    # Missing: await read_stream_writer.send(exc)

• Same issue: exceptions logged but not sent to read_stream_writer

Evidence

Working transports (websocket, stdio) correctly send exceptions:

# websocket.py (working ✅)
except ValidationError as exc:
    await read_stream_writer.send(exc)

# stdio.py (working ✅)  
except Exception as exc:
    logger.exception("...")
    await read_stream_writer.send(exc)  # ← Sends exception!

Data Flow Showing the Bug

Caller: await session.send_request()
    ↓
post_writer: response = await client.post(...)
    ↓
Server returns 401
    ↓
httpx raises HTTPStatusError
    ↓
post_writer catches exception
    ↓
logger.exception("Error") ← Only logs!
    ↓
Function exits
    ↓
Caller: await response_stream.receive() ← Hangs forever ⏳

Test Confirmation

Created reproduction test that confirms:

• When 401 error occurs, NO messages are sent to read_stream_writer
• Exception is only logged, never propagated
• Caller hangs indefinitely

Next Steps

Ready to implement the fix which will:

1. Catch httpx.HTTPStatusError specifically
2. Convert to JSONRPCError with HTTP status code preserved
3. Send to read_stream_writer so caller receives it

[gspeter-max]

Investigation Results

I have systematically investigated this issue and confirmed the root cause.

Root Cause

In both SSE and StreamableHTTP transports, when HTTP errors occur, exceptions are caught and logged but never sent to the read stream, causing the caller to hang indefinitely waiting for a response that will never arrive.

Specific Issues Found

1. sse.py (lines 147-148):
The post_writer function catches exceptions and only logs them:

• response.raise_for_status() at line 145 raises HTTPStatusError for non-2xx codes
• Exception is caught, logged, but never propagated to caller
• Result: Caller hangs forever

2. streamable_http.py (lines 481-482):
Same issue - exceptions logged but not sent to read_stream_writer

Evidence from Working Transports

Working transports (websocket, stdio) correctly send exceptions to the read stream so the caller can handle them.

Test Confirmation

Created a reproduction test that confirms:

• When HTTP errors occur, NO messages are sent to read_stream_writer
• Exception is only logged, never propagated
• Caller would hang indefinitely

Next Steps

Ready to implement the fix which will:

1. Catch httpx.HTTPStatusError specifically
2. Convert to JSONRPCError with HTTP status code preserved
3. Send to read_stream_writer so caller receives it