Stateless streamable HTTP: tools/call with progress returns notification as response body #980

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1. DaleSeo

   added this to the 2026-07-28 spec milestone on Jul 13, 2026
2. DaleSeo

   added

   T-bugBug fixes and error correctionsBug fixes and error corrections

   T-transportTransport layer changesTransport layer changes

   P1High: significant functionality gap or spec violationHigh: significant functionality gap or spec violation

   2026-07-28

   on Jul 13, 2026
3. DaleSeo

   added a parent issue on Jul 13, 2026

   • Tracking: 2026-07-28 spec conformance #977
4. alexhancock

   closed this as completedin #990on Jul 14, 2026
5. DaleSeo commented on Jul 17, 2026

   

   DaleSeo

   on Jul 17, 2026

   Last edited by DaleSeo

   MemberAuthor

   More actions

   Reopening: this regressed with #973.

   tools-call-with-progress (2026-07-28 suite, 0.2.0-alpha.9) passes at 839922d (pre-#973) and fails at current main (2e2c791):

   Failed: HTTP 200: expected a JSON-RPC result for 'tools/call', got unexpected body
   {"jsonrpc":"2.0","method":"notifications/progress","params":{...,"progressToken":"progress-test-1"}}
   

   Root cause: #990 added the SSE fallback for non-terminal first messages to the legacy stateless path in StreamableHttpService (tower.rs, the if self.config.json_response branch that checks matches!(message, Response | Error)). #973 introduced a separate modern per-request path, serve_negotiated_request_directly, which the 2026-07-28 conformance scenario now hits (per-request _meta negotiation), and that path returns the first outbound message as the JSON body unconditionally when json_response is enabled:

   if self.config.json_response || jsonrpc_http_status(&first) != http::StatusCode::OK {
       return jsonrpc_message_response(first, true);
   }

   For a progress-emitting tool the first message is notifications/progress, so the client gets the notification as the response body and never receives the terminal result — the same failure mode #990 fixed for the legacy path. The modern path needs the same "JSON only when the first message is terminal, otherwise fall back to SSE" logic.

   Repro:

   cargo build -p mcp-conformance
   STATELESS=1 PORT=8002 ./target/debug/conformance-server &
   npx -y @modelcontextprotocol/conformance@0.2.0-alpha.9 server \
     --url http://127.0.0.1:8002/mcp --scenario tools-call-with-progress --spec-version 2026-07-28
6. DaleSeo

   reopened this on Jul 17, 2026
7. jamadeo

   closed this as completedin #1005on Jul 20, 2026