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fix(streamable-http): preserve progress in JSON mode - #990

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alexhancock merged 1 commit into
modelcontextprotocol:mainfrom
jstar0:fix/stateless-progress-response
Jul 14, 2026
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fix(streamable-http): preserve progress in JSON mode#990
alexhancock merged 1 commit into
modelcontextprotocol:mainfrom
jstar0:fix/stateless-progress-response

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@jstar0

@jstar0jstar0 commented Jul 14, 2026

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Summary

Fix stateless Streamable HTTP requests that emit progress notifications while json_response is enabled.

The direct-JSON path previously serialized the first outbound message. For a progress-emitting tool, that first message is notifications/progress, so the HTTP response has no matching request id and the client never receives the terminal tools/call result.

Changes

  • Keep application/json when the first outbound message is the terminal JSON-RPC response or error.
  • Fall back to text/event-stream when a notification or server request is emitted first, preserving the complete ordered message sequence and final result.
  • Document json_response as a preference with SSE fallback when intermediate messages must be delivered.
  • Add an HTTP integration test covering a progress notification followed by the terminal tool result.

This avoids dropping progress notifications or waiting in an unbounded drain loop. Simple terminal-first stateless requests retain the existing direct-JSON behavior.

Verification

cargo test -p rmcp --features 'server client transport-streamable-http-server reqwest' --test test_streamable_http_json_response
cargo test -p rmcp --features 'server client transport-streamable-http-server transport-streamable-http-client-reqwest reqwest transport-io'
cargo fmt --all -- --check

The @modelcontextprotocol/conformance@0.2.0-alpha.9tools-call-with-progress server scenario also passes with all three progress notifications and the final tool result.

Closes#980

@jstar0
jstar0 requested a review from a team as a code ownerJuly 14, 2026 10:07
@github-actionsgithub-actionsBot added T-test Testing related changes T-core Core library changes T-transport Transport layer changes labels Jul 14, 2026
@alexhancock
alexhancock merged commit 1543a1a into modelcontextprotocol:mainJul 14, 2026
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2026-07-28T-coreCore library changesT-testTesting related changesT-transportTransport layer changes

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

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@jstar0@alexhancock@DaleSeo