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Stateless streamable HTTP: tools/call with progress returns notification as response body #980
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- added a parent issue
on Jul 13, 2026 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, theif self.config.json_responsebranch that checksmatches!(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_metanegotiation), and that path returns the first outbound message as the JSON body unconditionally whenjson_responseis 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
Part of #977.
On the 2026-07-28 suite (
0.2.0-alpha.9),tools-call-with-progressfails against the stateless conformance server:In stateless mode the progress notification is emitted as the POST response body instead of (or before) the final
tools/callresult. The scenario passes against the stateful server on the 2025-11-25 suite, so this is specific to the stateless lifecycle.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 -o results