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fix(core): make instanceof on SDK error classes work across bundled copies (#2384)

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Felix Weinberger 2026-07-06 15:53:12 +01:00 • committed by GitHub
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---
'@modelcontextprotocol/client': patch
'@modelcontextprotocol/server': patch
---
`instanceof` on the SDK error classes (`ProtocolError` and its typed subclasses, `SdkError`/`SdkHttpError`, `OAuthError`, and the client's `SseError`, `UnauthorizedError`, and OAuth-client-flow error family — `OAuthClientFlowError` and its subclasses) now works across separately bundled copies of the SDK. The classes match by a stable brand (via `Symbol.hasInstance` and a registry symbol) instead of prototype identity, so a process that uses both `@modelcontextprotocol/client` and `@modelcontextprotocol/server` - a gateway, host, or in-process test - can check errors constructed by either package against the class re-exported by the other. Ordinary prototype-based `instanceof` is preserved as a fallback; user-defined subclasses keep plain prototype semantics. Notes: cross-bundle matching requires both copies to be at or after this release; brands assert identity, not field shape, across versions - keep reading fields defensively. As a side effect, a foreign-bundle `SdkError` used as an abort reason is now rethrown as-is instead of being wrapped as a `RequestTimeout`. Branded hierarchies additionally expose an explicit static guard, `X.isInstance(value)`, that reads the same brand and narrows in TypeScript — an alternative for codebases that prefer predicate-style checks over `instanceof`. Also: `UnauthorizedError` now sets `error.name` to `'UnauthorizedError'` (previously `'Error'`), and per-package conformance tests enforce that every exported error class participates in branding. Version-negotiation probing now recognizes `UnauthorizedError` (previously a dead name-string check) and propagates it unchanged, so `connect()` on an auth-gated server rejects with the original `UnauthorizedError` (previously wrapped as the `cause` of an `SdkError(EraNegotiationFailed)`) — run `finishAuth()` and reconnect, and the retry probes with the token.