fix(core): preserve zero related request IDs

[franekmat]

Summary

• treat only an undefined relatedRequestId as absent when deciding whether a notification can be debounced
• add a same-tick regression proving two notifications associated with JSON-RPC request ID 0 are both sent
• add patch changesets for the affected client, server, and core packages

Fixes #2117.

Verification

• Regression test fails on unmodified origin/main with one send instead of two.
• pnpm --filter @modelcontextprotocol/core-internal exec vitest run test/shared/protocol.test.ts -t 'relatedRequestId 0 in the same tick' passes with the fix.
• pnpm --filter @modelcontextprotocol/core-internal check passes.
• pnpm lint:all passes.
• pnpm test:all: all test files passed on the second run, but Vitest exited non-zero because the existing protocol:timeout:max-total [sse 2025-11-25] E2E left two unhandled timeout rejections. Its direct targeted rerun passed (1 test, 411 skipped).

[changeset-bot]

🦋 Changeset detected

Latest commit: 01f75bf

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/server	Patch
@modelcontextprotocol/core-internal	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

Open in StackBlitz

@modelcontextprotocol/client

npm i https://pkg.pr.new/@modelcontextprotocol/client@2493

@modelcontextprotocol/codemod

npm i https://pkg.pr.new/@modelcontextprotocol/codemod@2493

@modelcontextprotocol/core

npm i https://pkg.pr.new/@modelcontextprotocol/core@2493

@modelcontextprotocol/server

npm i https://pkg.pr.new/@modelcontextprotocol/server@2493

@modelcontextprotocol/server-legacy

npm i https://pkg.pr.new/@modelcontextprotocol/server-legacy@2493

@modelcontextprotocol/express

npm i https://pkg.pr.new/@modelcontextprotocol/express@2493

@modelcontextprotocol/fastify

npm i https://pkg.pr.new/@modelcontextprotocol/fastify@2493

@modelcontextprotocol/hono

npm i https://pkg.pr.new/@modelcontextprotocol/hono@2493

@modelcontextprotocol/node

npm i https://pkg.pr.new/@modelcontextprotocol/node@2493

commit: 01f75bf

[franekmat]

Closing as a duplicate