mcp >= 2.0's Streamable HTTP client folds any non-2xx whose body it cannot parse
as a JSON-RPC error into the opaque `-32603 Server returned an error response`.
Hermes printed that text verbatim in the SSE-fallback warning and in the
"both transports failed" ConnectionError, so users saw no status, no URL and
none of the server's own words (e.g. `400 {"code":-32020,"message":"Unsupported
MCP-Protocol-Version"}`) and had to reach for curl to learn what the server
actually said (#114350, #113359).
- `_make_http_rejection_recorder`: response hook on the owned SDK-httpx client
that remembers the last 4xx/5xx (status, method, URL, head of the body; SSE
bodies are never read). Sibling of the redirect-header stripper hook.
- `_describe_http_failure`: appends that detail only when the root cause is
the SDK's opaque -32603 text, so a real JSON-RPC error or an httpx status
error is never duplicated.
- `_run_http`: the fallback warning and both raise paths (both-transports
ConnectionError; the no-fallback re-raise after a proven session, strict
redirect headers or a non-rejection) carry the detail. Debug-logs the
endpoint each connect attempt uses.
- Docs: troubleshooting entry for reading the new message.
Verified live against a real Streamable HTTP server (`hermes mcp test`, temp
HERMES_HOME): base prints `Streamable HTTP: Server returned an error response`;
fixed head prints `... (HTTP 400 from POST http://127.0.0.1:PORT/mcp:
{"jsonrpc":"2.0","id":null,"error":{"code":-32020,...}})`. Control: a 400
served as application/json already surfaces the JSON-RPC message and gets no
appendix; servers negotiating `initialize` down to 2025-06-18 (fixture and a
real FastMCP on mcp 1.12.4) connect and list tools on base and fix alike — the
pinned mcp 2.0.0 stamps the negotiated version on every post-handshake request
(wire-recorded), so the sticky seed in `_run_http` is not the cause of the
reported 400 and stays as designed (#14816).
Co-authored-by: JoaoMarcos44 <joaomarcosdias444@gmail.com>
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