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docs(migration): dual-role instanceof caveat in the Errors section (#2385)

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Felix Weinberger 2026-06-29 15:39:32 +01:00 • committed by GitHub
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docs/migration/upgrade-to-v2.md
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@ -560,6 +560,17 @@ Replace the literal with the named code. Loud (`TS2367`) when the compared value
typed `SdkErrorCode`; silent when the left side is `unknown` or a cast — grep for
`=== -32000` / `=== -32001`.
**Dual-role processes: `instanceof` does not cross the packages.**
`@modelcontextprotocol/client` and `@modelcontextprotocol/server` each bundle their own
copy of these error classes, so in a process that uses both — a gateway, a host, an
in-process test — an error constructed by one package fails `instanceof` against the
class imported from the other, silently. When an error may originate from the other
package, match on stable fields instead of class identity: `error.code` values
(`SdkErrorCode` strings for SDK errors, numeric JSON-RPC codes for protocol errors,
`OAuthErrorCode` strings for OAuth errors) plus presence checks like `'status' in e`,
or reconstruct typed protocol errors with `ProtocolError.fromError(code, message, data)`
— it exists precisely because `instanceof` does not survive bundle boundaries.
**Constructing the error (test stubs, custom transports).** v1
`new StreamableHTTPError(code, message)` becomes
`new SdkHttpError(code, message, data)`: the first argument is now a `SdkErrorCode`