fix(client): accept a protected resource that differs only by a trailing slash

[AmineBenzaazaa]

Motivation and Context

checkResourceAllowed returned false early when the requested path was shorter than the configured one. A client pointed at https://host/mcp therefore failed auth() against a server whose protected resource metadata advertises https://host/mcp/, while the reverse case passed. The prefix check right below already normalises both paths to a trailing slash, so the early return was only adding the asymmetry.

Fixes #3041

What changed

• packages/core-internal/src/shared/authUtils.ts: removed the length early-return and noted in the doc comment that a single trailing slash is not significant.
• packages/core-internal/test/shared/authUtils.test.ts: the trailing-slash test now expects a match in both directions. A new test checks that a shorter, different path is still rejected (/folder vs /folder/sub/, / vs /mcp/, /mc vs /mcp/).
• Changeset (patch: client).

How Has This Been Tested?

The updated test fails on main (expected false to be true) and passes with the change. core-internal (1526 tests) and client (998 tests) pass, and typecheck and lint are clean.

Breaking Changes

None. Only previously rejected /x vs /x/ pairs are now accepted; origin and path-prefix checks are unchanged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 91cf8df

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

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commit: 91cf8df

[koriyoshi2041]

Reviewed 91cf8dff against the resource/path boundary. The focused auth-utils suite plus an independent matrix passed (25/25 total): both trailing-slash directions and a child path are accepted, while /mcp123, /mc, the parent path, and scheme/host/port changes remain rejected. I also checked the existing query/fragment behavior stayed unchanged.

core-internal typecheck, changed-file Prettier, and diff hygiene pass. I don't see the two-line change widening access beyond the intended path boundary.

[0xamlab]

The asymmetry is real: on main checkResourceAllowed rejects https://example.com/mcp against https://example.com/mcp/ while accepting the reverse, because the length check returned false when the requested path was shorter. Removing that early return and normalizing both paths with a trailing slash at packages/core-internal/src/shared/authUtils.ts:57-60 makes the check symmetric. I verified test/shared/authUtils.test.ts passes all 13 tests on this branch, including the new bidirectional trailing-slash case, and the regression cases for /folder vs /folder/sub/ still fail as expected.