fix(mcp): use negotiated protocol version in transport request headers#15128
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This was referenced May 8, 2026
lgrammel
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…est headers (#15164) This is an automated backport of #15128 to the release-v6.0 branch. FYI @aayush-kapoor This backport has conflicts that need to be resolved manually. ### `git cherry-pick` output ``` Auto-merging packages/mcp/src/tool/mcp-client.test.ts Auto-merging packages/mcp/src/tool/mcp-client.ts CONFLICT (content): Merge conflict in packages/mcp/src/tool/mcp-client.ts Auto-merging packages/mcp/src/tool/mcp-http-transport.test.ts Auto-merging packages/mcp/src/tool/mcp-http-transport.ts Auto-merging packages/mcp/src/tool/mcp-sse-transport.test.ts Auto-merging packages/mcp/src/tool/mcp-sse-transport.ts Auto-merging packages/mcp/src/tool/mcp-transport.ts Auto-merging packages/mcp/src/tool/mock-mcp-transport.ts error: could not apply 2655da8... fix(mcp): use negotiated protocol version in transport request headers (#15128) hint: After resolving the conflicts, mark them with hint: "git add/rm <pathspec>", then run hint: "git cherry-pick --continue". hint: You can instead skip this commit with "git cherry-pick --skip". hint: To abort and get back to the state before "git cherry-pick", hint: run "git cherry-pick --abort". hint: Disable this message with "git config set advice.mergeConflict false" ``` --------- Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: Aayush Kapoor <aayushkapoor34@gmail.com>
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Background
#14413
after a successful initialize handshake,
HttpMCPTransportandSseMCPTransportwere sending mcp-protocol-version: 2025-11-25 (LATEST_PROTOCOL_VERSION) in every subsequent request header, ignoring the version the server actually negotiated down toSummary
added optional
protocolVersion?: stringfield to the transports so the client can inject the negotiated version into any transport after initManual Verification
na
Checklist
pnpm changesetin the project root)Related Issues
fixes #14413