feat(compat): McpError/ErrorCode/JSONRPCError/StreamableHTTPError + OAuth subclass aliases

[felixweinberger]

Part of the v2 backwards-compatibility series — see reviewer guide.

v2 split McpError into ProtocolError/SdkError and consolidated 17 OAuth error classes into OAuthError+OAuthErrorCode. This restores the v1 names as @deprecated aliases/subclasses.

Motivation and Context

v2 split McpError into ProtocolError/SdkError and consolidated 17 OAuth error classes into OAuthError+OAuthErrorCode. This restores the v1 names as @deprecated aliases/subclasses.

v1 vs v2 pattern & evidence

v1 pattern:

`if (err instanceof McpError && err.code === ErrorCode.InvalidParams)`; `throw new InvalidTokenError('...')`

v2-native:

`if (err instanceof ProtocolError && err.code === ProtocolErrorCode.InvalidParams)`; `throw new OAuthError(OAuthErrorCode.InvalidToken, '...')`

Evidence: GitHub code search: ~1,540 files reference McpError/ErrorCode. ErrorCode.ConnectionClosed/RequestTimeout shimmed to SdkErrorCode values so comparisons work (string-vs-number assignment to error.code: number is irreducible — 1-line consumer fix).

How Has This Been Tested?

• packages/core/test/errors/compat.test.ts — instanceof + code equivalence
• Integration: validated bump-only against 5 OSS repos via the v2-bc-integration validation branch
• pnpm typecheck:all && pnpm lint:all && pnpm test:all green

Breaking Changes

None — additive @deprecated shim.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added or updated documentation as needed

Additional context

Stacks on: C1

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🦋 Changeset detected

Latest commit: 6221eda

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/express	Patch
@modelcontextprotocol/fastify	Patch
@modelcontextprotocol/hono	Patch
@modelcontextprotocol/node	Patch

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commit: 6221eda

[felixweinberger]

@claude review

[felixweinberger]

@claude review

[felixweinberger]

@claude review

[felixweinberger]

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[felixweinberger]

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[claude]

🟡 User subclasses of CustomOAuthError get .name === 'OAuthError' instead of the subclass name — v1's base OAuthError constructor did this.name = this.constructor.name, so class MyErr extends CustomOAuthError { static errorCode = '…' } produced new MyErr('x').name === 'MyErr'. This is the same .name-fidelity gap already fixed for the 17 sub() classes in 473d2d8, but CustomOAuthError was missed; one-line fix: add this.name = new.target.name; after the super() call.

Extended reasoning...

What's the gap

v1's OAuthError base constructor (verified against src/server/auth/errors.ts @ 1.21.x) did this.name = this.constructor.name. So for the documented v1 pattern this file's JSDoc explicitly preserves —

class MyErr extends CustomOAuthError { static errorCode = 'my_code' }

— v1 produced new MyErr('x').name === 'MyErr', and err.toString() / stack traces showed MyErr: x.

In this PR, v2's OAuthError constructor (packages/core/src/auth/errors.ts:108) hardcodes this.name = 'OAuthError', and CustomOAuthError's constructor (lines 109-111) only does super((new.target as typeof CustomOAuthError).errorCode, message, errorUri) without resetting .name. So new MyErr('x').name === 'OAuthError'.

Why nothing else covers it

The 17 sub()-generated classes already handle this — line 23 sets this.name = name in each subclass constructor, and 473d2d8 added Object.defineProperty(Sub, 'name', …) for constructor.name. CustomOAuthError is the one OAuth compat class that doesn't go through sub(), and its constructor was simply not given the equivalent line. Note that CustomOAuthError is a named class declaration, so MyErr.name / err.constructor.name are already correct ('MyErr') — only the instance .name property (and therefore .toString() / stack-trace label) is wrong.

The existing test in compat.test.ts ("CustomOAuthError reads static errorCode from concrete subclass") only asserts e.code and instanceof, so it doesn't catch this.

Step-by-step proof

1. auth/errors.ts:108 — v2 OAuthError ctor: this.name = 'OAuthError' (hardcoded, not this.constructor.name).
2. oauthErrorsCompat.ts:109-111 — CustomOAuthError ctor: super((new.target as …).errorCode, message, errorUri); no this.name = … afterward.
3. User defines class MyErr extends CustomOAuthError { static override errorCode = 'my_code' }.
4. const e = new MyErr('nope') → super chain runs step 1 → e.name === 'OAuthError'.
5. e.toString() → 'OAuthError: nope'; stack trace header reads OAuthError: nope.
6. Under v1, step 1 instead did this.name = this.constructor.name → e.name === 'MyErr', e.toString() → 'MyErr: nope'.

Impact

Low — only affects err.name, err.toString(), and stack-trace labeling for the niche user-defined-subclass escape hatch on a @deprecated shim. instanceof MyErr, instanceof OAuthError, .code, .errorCode, and toResponseObject() all work correctly. But it's the same v1-fidelity category the author already chose to fix for sub() in 473d2d8 (resolved thread #3099495886), and this is the sibling code path that was overlooked.

Fix

export class CustomOAuthError extends OAuthError {
    static errorCode: string;
    constructor(message: string, errorUri?: string) {
        super((new.target as typeof CustomOAuthError).errorCode, message, errorUri);
        this.name = new.target.name;
    }
}

new.target already points at the user's concrete subclass (it's used on the line above for errorCode), so new.target.name gives 'MyErr' exactly as v1's this.constructor.name did. Optionally add expect(e.name).toBe('MyError') to the existing CustomOAuthError test.