fix(core): send an error response when a request handler rejects with a nullish reason#2521
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… a nullish reason Protocol's inbound-request error path read error['code'] directly. When a handler rejects with null/undefined (a bare reject() or throw null), that indexing throws a TypeError inside the rejection callback, which propagates to the outer .catch — so no JSON-RPC error response is ever sent and the requester hangs until its own timeout. Coalesce the reason to a safe object before reading the code/message/data, so a -32603 Internal error response is always returned. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01N6RtoHuxrDqTUo9Mw9h4Cv
🦋 Changeset detectedLatest commit: e3cf478 The changes in this PR will be included in the next version bump. This PR includes changesets to release 6 packages
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@modelcontextprotocol/client
@modelcontextprotocol/codemod
@modelcontextprotocol/core
@modelcontextprotocol/server
@modelcontextprotocol/server-legacy
@modelcontextprotocol/express
@modelcontextprotocol/fastify
@modelcontextprotocol/hono
@modelcontextprotocol/node
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What's broken
Protocol's inbound-request error path readserror['code']directly. When a handler rejects withnull/undefined(a barereject()orthrow nullin library code), that indexing throws aTypeErrorinside the rejection callback, which propagates to the outer.catch— so no JSON-RPC error response is ever sent and the requester hangs until its own timeout.Fix
Coalesce the rejection reason to a safe object before reading
code/message/data, so a-32603Internal error response is always returned.Tests
New test in
packages/core-internal/test/shared/protocol.test.ts: a handler thatPromise.reject(undefined)now produces an error response for the request id (before the fix, none was sent). Full core-internal suite passes (1376 tests), typecheck + lint clean. Changeset included.🤖 Generated with Claude Code