Summary

When a server returns a non-2xx HTTP status code (e.g. 500 Internal Server Error, 403 Forbidden), the streamable HTTP client previously called response.raise_for_status(), which raised an unhandled httpx.HTTPStatusError. This exception was never caught, so the pending request would hang indefinitely — the caller's await session.list_tools() (or similar) would never resolve.

This PR replaces raise_for_status() with explicit status code handling that sends a JSONRPCError back through the read stream, matching the pattern already established for 404 (session terminated) responses. For requests (messages with an id), the client now sends an INTERNAL_ERROR JSONRPCError so the pending request resolves immediately with an MCPError. For notifications (no id), the error is silently discarded since there is no caller waiting for a response.

Follow-up to

• fix: send JSONRPCError instead of bare exceptions in streamable HTTP client #2005 — Fixed similar issues for unexpected content types, JSON parse failures, and SSE parse failures, but did not address plain HTTP error responses.

Related issues

• Fixes BUG REPORT: HTTP errors are being silently swallowed in the StreamableHTTP client #1010 — HTTP errors silently swallowed in the StreamableHTTP client
• Related to Client tool call hangs forever if server crashes or connection dies when using streamable-http #1577 — Client tool call hangs forever if server crashes
• Related to ClientSession Error Handling #1401 — ClientSession error handling
• Related to When throwing the HTTP status exception, MCP client is disregarding the exception message! #1212 — raise_for_status() discards error response body
• Related to Lazy HTTP connections seem to make error handling difficult. #1393 — Lazy HTTP connection error handling
• Related to Http errors are not handled properly by streamable_http #1835 — HTTP 403 causing client to hang (closed as dup)
• Related to Cannot handle Unexpected content type when initializing MCP session #1382 — Unexpected content type hanging (fixed by fix: send JSONRPCError instead of bare exceptions in streamable HTTP client #2005)
• Related to streamable_http client call_tool hangs when receiving invalid JSONRPCMessage #1144 — Invalid JSON hanging (fixed by fix: send JSONRPCError instead of bare exceptions in streamable HTTP client #2005)

Related PRs

• fix: Catch http errors  #1308 — Proposed fix to catch HTTP errors (open)
• fix: Add exception handling for POST request failures in StreamableHTTP #1008 — Earlier attempt at exception handling for POST failures (closed)
• Add httpx.HTTPStatusError error handling to tool run #1119 — Earlier attempt at HTTPStatusError handling (closed)

Test plan

• Added test_http_error_status_sends_jsonrpc_error — verifies that an HTTP 500 from the server results in an MCPError raised to the caller instead of a hang
• All existing tests pass
• Pre-commit hooks pass (ruff, pyright, prettier)