fix: use discoverOAuthFromWWWAuthenticate for reactive OAuth flow (#18760)

[echoVic]

Problem

handleAutomaticOAuth() passes the resource_metadata URI (already a .well-known URL extracted from the WWW-Authenticate header) to discoverOAuthConfig(), which expects a server URL and internally constructs .well-known paths from it. This causes:

1. Double-nested .well-known paths → 404 on resource metadata fetch
2. Fallback to base URL → fetches metadata without path context
3. ResourceMismatchError → the resource parameter from fallback metadata doesn't match the path-based server URL

This affects all OAuth-protected MCP servers that serve resources under paths (gateways, multi-tenant setups).

Root Cause

// BEFORE (broken): resourceMetadataUri is already a .well-known URL
const resourceMetadataUri = OAuthUtils.parseWWWAuthenticateHeader(wwwAuthenticate);
if (resourceMetadataUri) {
  // discoverOAuthConfig() will try to build .well-known URLs from this,
  // resulting in double-nested paths like:
  // /.well-known/oauth-protected-resource/.well-known/oauth-protected-resource/path
  oauthConfig = await OAuthUtils.discoverOAuthConfig(resourceMetadataUri);
}

Fix

Use the existing discoverOAuthFromWWWAuthenticate() utility which correctly:

1. Parses the resource_metadata URI from the WWW-Authenticate header
2. Directly fetches the resource metadata (no double .well-known construction)
3. Validates the resource parameter against the actual server URL (RFC 9728 §7.3)
4. Discovers the authorization server from the metadata

// AFTER (fixed): use the correct utility that handles the full flow
const serverUrl = mcpServerConfig.httpUrl || mcpServerConfig.url;
let oauthConfig = await OAuthUtils.discoverOAuthFromWWWAuthenticate(
  wwwAuthenticate,
  serverUrl,
);

Fallback to base URL discovery via discoverOAuthConfig() is preserved for servers that don't include resource_metadata in the WWW-Authenticate header.

Fixes #18760

[gemini-code-assist]

Summary of Changes

Hello @echoVic, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the automatic OAuth handling for MCP servers, particularly those with path-based resources. The previous implementation caused failures in OAuth configuration discovery due to incorrect URI handling, resulting in authentication issues. The fix refactors the discovery process to use a more robust utility, ensuring proper resource metadata fetching and validation, thereby improving the reliability of OAuth authentication for affected server configurations.

Highlights

• OAuth Discovery Fix: Resolved an issue where handleAutomaticOAuth() incorrectly passed an already .well-known URI to discoverOAuthConfig(), leading to double-nested .well-known paths, 404 errors, and ResourceMismatchError for OAuth-protected MCP servers.
• Corrected Utility Usage: Switched to using OAuthUtils.discoverOAuthFromWWWAuthenticate() which correctly parses the resource_metadata URI, directly fetches metadata, validates the resource parameter against the server URL, and discovers the authorization server.
• Fallback Preservation: Ensured that the fallback mechanism to discover OAuth configuration from the base URL via discoverOAuthConfig() is preserved for servers that do not include resource_metadata in the WWW-Authenticate header.

Changelog

• packages/core/src/tools/mcp-client.ts
  • Refactored handleAutomaticOAuth to use discoverOAuthFromWWWAuthenticate for initial OAuth configuration discovery.
  • Adjusted the declaration of serverUrl to be available earlier in the handleAutomaticOAuth function.
  • Modified the fallback logic for OAuth discovery to correctly use discoverOAuthConfig when discoverOAuthFromWWWAuthenticate does not yield a configuration.

Activity

• The author, echoVic, created this pull request to fix a reported issue (fix(oauth): 401-triggered OAuth discovery fails for path-based MCP servers due to misrouted resource_metadata URI #18760).

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[gemini-code-assist]

Code Review

This pull request fixes a bug in the automatic OAuth flow where a resource_metadata URI was incorrectly passed to a function expecting a server URL. The change to use discoverOAuthFromWWWAuthenticate correctly handles the discovery process, making the implementation more robust by preserving fallback logic and removing redundant serverUrl variable declarations. The security analysis did not find any vulnerabilities, although it noted that the content of several files, such as packages/core/src/tools/mcp-client.ts and packages/core/src/mcp/oauth-utils.ts, could not be read, which prevented SAST reconnaissance passes on them.