[JosephDoUrden]

Fixes #3133.

connect() documents that timeout and signal both apply to the handshake or probe, but only the timeout reached the server/discover probe. Aborting the signal did nothing until the probe timed out (60 s by default), and an already-aborted signal still sent the probe.

What changed in packages/client:

• NegotiationDeps carries the connect signal. ProbeWindow.exchange returns early if it is already aborted, listens for abort before send(), and passes it to the transport as requestSignal, so an in-flight HTTP POST is torn down too.
• negotiateEra checks the signal before opening the window, after every exchange whatever the reply was, and once more before handing the transport over, so a reply landing with the abort still rejects, no retry or corrective probe goes out after an abort, and a failed connect leaves the transport's start untouched.
• negotiateStdioViaSibling checks it after the sibling is reaped, next to the existing caller-close() check, so an abort landing during disposal never starts the session transport.
• client.ts only passes options.signal into the negotiation deps.
• The error is the one the legacy path produces for the same signal: the SdkError reason as is, otherwise SdkError(RequestTimeout, String(reason)), mirroring Protocol.request. fix: distinguish AbortSignal cancellation from a real timeout #3046 wants to change that mapping; the helper here is one place to update with it.

Not in this PR: connect({ prior: { kind: 'modern', … }, signal }) still ignores the signal. There is no probe or handshake on that path, so it is arguably documented out, but it is a separate decision.

Tests: test/client/connectSignalProbe.test.ts, built on the cases in @Floriferous's repro branch (legacy control, auto, pin, pre-aborted, stdio sibling, real HTTP) plus same-tick reply-then-abort, abort between verdict and handover, abort during sibling disposal, abort inside send() under maxRetries, listener removal, requestSignal forwarding and the error mapping. On real stdio the rejection waits for the sibling to be reaped (up to about 1 s if it ignores SIGTERM); the changeset says so.

• pnpm typecheck:all, pnpm lint:all
• pnpm test:all: all packages green; one e2e timing scenario (protocol:timeout:max-total [sse 2025-11-25], fake timers) failed once under full load and passed on rerun, unrelated to this change

AI Disclosure

AI assistance (Claude) was used for issue research and test scaffolding. The implementation was written and reviewed by the author.

[changeset-bot]

🦋 Changeset detected

Latest commit: b7111c5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/client	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core-internal	Patch

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