Client streamable-http: parse JSON-RPC errors from non-2xx response bodies

A spec-correct server may return the JSON-RPC error in an application/json body
at a non-2xx status (e.g. 400 for INVALID_PARAMS, 404 for METHOD_NOT_FOUND per
the 2026-07-28 transport binding). Surface that error to the caller instead of
synthesizing a generic INTERNAL_ERROR / 'Session terminated' from the status
alone. Falls back to the status-derived error if the body isn't valid JSON-RPC.

Unconditional (not version-gated): parsing a JSON-RPC body from a 4xx
application/json response is correct at any protocol version.

Author: maxisbey

src/mcp/client/streamable_http.py

@@ -314,16 +314,24 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
                 logger.debug("Received 202 Accepted")
                 return
 
-            if response.status_code == 404:
-                if isinstance(message, JSONRPCRequest):
-                    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
-
             if response.status_code >= 400:
                 if isinstance(message, JSONRPCRequest):
-                    error_data = ErrorData(code=INTERNAL_ERROR, message="Server returned an error response")
+                    # A spec-correct server may return the JSON-RPC error in the
+                    # body at a non-2xx status (e.g. 400 for INVALID_PARAMS, 404
+                    # for METHOD_NOT_FOUND). Surface that error rather than the
+                    # status-derived stand-in below.
+                    if response.headers.get("content-type", "").lower().startswith("application/json"):
+                        try:
+                            body = await response.aread()
+                            parsed = jsonrpc_message_adapter.validate_json(body, by_name=False)
+                            await ctx.read_stream_writer.send(SessionMessage(parsed))
+                            return
+                        except (httpx.StreamError, ValidationError):
+                            logger.debug("Non-2xx body was not a valid JSON-RPC message; using fallback error")
+                    if response.status_code == 404:
+                        error_data = ErrorData(code=INVALID_REQUEST, message="Session terminated")
+                    else:
+                        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