MCP-AUT-001

Unauthenticated request returns 401 with WWW-Authenticate resource_metadata

“HTTP/1.1 401 Unauthorized. WWW-Authenticate: Bearer resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource".” — spec anchor

How to fix this

Return 401 with `WWW-Authenticate: Bearer resource_metadata="https://your-server/.well-known/oauth-protected-resource"`. A bare 401 is a dead end — the client has nowhere to go and authenticate.

Example: failing and passing

The 401 challenge has no resource_metadata parameter, so the client has no way to find the Protected Resource Metadata.

Violates MCP-AUT-001 · HTTP

POST /mcp HTTP/1.1
Host: mcp.example.com
Content-Type: application/json
MCP-Protocol-Version: 2026-07-28
Mcp-Method: tools/list
...

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="mcp"

The 401 challenge names the Protected Resource Metadata URL in resource_metadata, which the client fetches next.

Complies with MCP-AUT-001 · HTTP

POST /mcp HTTP/1.1
Host: mcp.example.com
Content-Type: application/json
MCP-Protocol-Version: 2026-07-28
Mcp-Method: tools/list
...

HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource"

How the validator checks this

Probe P8.1, against a streamable-http server on the modern protocol. What it reports:

pass: 401 carried WWW-Authenticate with resource_metadata="…"
fail: 401 without a WWW-Authenticate resource_metadata parameter (header: …), and no Protected Resource Metadata at the well-known URI fallback (…) either. The client has nowhere to authenticate.

Quoted from the probe that runs this check, so it cannot drift from what the validator actually reports.

This rule is checked deterministically: a fail here is a certain violation, not an inference.

Checked in the same request as MCP-AUT-006, MCP-AUT-007, MCP-AUT-008, MCP-SEC-007.

Check your own server against MCP-AUT-001

Probe P8.1 checks MCP-AUT-001 in the same request as 4 other rules, so one run shows your server's result for this rule next to the rest. Validate a server or read how the validator works.

Background on this topic: how to add OAuth to an MCP server.

Other Authorization rules

MCP-AUT-002RFC 9728 Protected Resource Metadata is served and valid
MCP-AUT-003PRM resource equals the canonical server URI
MCP-AUT-004Each listed authorization server exposes RFC 8414 or OIDC discovery metadata
MCP-AUT-005AS advertises authorization_response_iss_parameter_supported: true
MCP-AUT-006WWW-Authenticate includes a scope parameter
MCP-AUT-007Insufficient scope returns 403 with error="insufficient_scope"
MCP-AUT-008Tokens with a foreign audience are rejected
MCP-AUT-009offline_access absent from scopes_supported / challenge scope
MCP-AUT-010AS advertises S256 in code_challenge_methods_supported
MCP-AUT-011AS supports Client ID Metadata Documents, not DCR alone