fix: accept null introspection_endpoint in OAuthMetadataSchema

[billyriaz]

Problem

Some OAuth authorization servers return introspection_endpoint: null in their /.well-known/oauth-authorization-server metadata rather than omitting the field entirely. This is valid per RFC 7662 — the field is optional and null should be treated the same as absent.

The current schema uses z.string().optional() which accepts undefined but rejects null, causing OAuth flow initiation to fail with:

Invalid input: expected string, received null
  path: ["introspection_endpoint"]

Fix

Change z.string().optional() to z.string().nullish() for introspection_endpoint. This accepts string | null | undefined, correctly handling servers that explicitly set the field to null.

Reproduction

Connect any MCP client to an OAuth server whose /.well-known/oauth-authorization-server returns "introspection_endpoint": null. The OAuth flow fails immediately at metadata validation before the authorization redirect is attempted.

Example metadata that triggers the bug:

{
  "issuer": "https://example.com",
  "authorization_endpoint": "https://example.com/oauth/authorize",
  "token_endpoint": "https://example.com/oauth/token",
  "introspection_endpoint": null,
  "registration_endpoint": "https://example.com/oauth/register"
}

[changeset-bot]

⚠️ No Changeset found

Latest commit: 9f3448f

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[pkg-pr-new]

Open in StackBlitz

@modelcontextprotocol/client

npm i https://pkg.pr.new/@modelcontextprotocol/client@1923

@modelcontextprotocol/server

npm i https://pkg.pr.new/@modelcontextprotocol/server@1923

@modelcontextprotocol/express

npm i https://pkg.pr.new/@modelcontextprotocol/express@1923

@modelcontextprotocol/fastify

npm i https://pkg.pr.new/@modelcontextprotocol/fastify@1923

@modelcontextprotocol/hono

npm i https://pkg.pr.new/@modelcontextprotocol/hono@1923

@modelcontextprotocol/node

npm i https://pkg.pr.new/@modelcontextprotocol/node@1923

commit: 9f3448f