Authorization Server Discovery

This document describes the mechanisms by which MCP servers advertise their associated authorization servers to MCP clients, as well as the discovery process through which MCP clients can determine authorization server endpoints and supported capabilities.

Authorization Server Location

MCP servers MUST implement the OAuth 2.0 Protected Resource Metadata (RFC9728) specification to indicate the locations of authorization servers. The Protected Resource Metadata document returned by the MCP server MUST include the authorization_servers field containing at least one authorization server.

The specific use of authorization_servers is beyond the scope of this specification; implementers should consult OAuth 2.0 Protected Resource Metadata (RFC9728) for guidance on implementation details.

Implementors should note that Protected Resource Metadata documents can define multiple authorization servers. The responsibility for selecting which authorization server to use lies with the MCP client, following the guidelines specified in RFC9728 Section 7.6 “Authorization Servers”.

When multiple authorization servers are listed in authorization_servers, each is an independent OAuth 2.0 authorization server. Consistent with RFC 6749 Section 2.2, client identifiers are unique to the authorization server that issued them. Clients MUST maintain separate registration state (client credentials, tokens) per authorization server and MUST NOT assume that credentials valid for one authorization server will be accepted by another. See Authorization Server Binding for the requirements on associating client credentials with the authorization server that issued them.

Protected Resource Metadata Discovery Requirements

MCP servers MUST implement one of the following discovery mechanisms to provide authorization server location information to MCP clients:

1. WWW-Authenticate Header: Include the resource metadata URL in the WWW-Authenticate HTTP header under resource_metadata when returning 401 Unauthorized responses, as described in RFC9728 Section 5.1.

2. Well-Known URI: Serve metadata at a well-known URI as specified in RFC9728. This can be either:

   • At the path of the server’s MCP endpoint: https://example.com/public/mcp could host metadata at https://example.com/.well-known/oauth-protected-resource/public/mcp
   • At the root: https://example.com/.well-known/oauth-protected-resource

MCP clients MUST support both discovery mechanisms and use the resource metadata URL from the parsed WWW-Authenticate headers when present; otherwise, they MUST fall back to constructing and requesting the well-known URIs in the order listed above.

MCP clients MUST be able to parse WWW-Authenticate headers and respond appropriately to HTTP 401 Unauthorized responses from the MCP server.

Servers can also include a scope parameter in the WWW-Authenticate challenge to indicate the scopes required for accessing the resource; the scope semantics and the associated client behavior are defined in the Scope Selection Strategy section.

Authorization Server Metadata Discovery

MCP uses the default oauth-authorization-server well-known URI suffix defined in RFC 8414 Section 3.1 for authorization server metadata discovery. MCP does not define an application-specific well-known URI suffix.

To handle different issuer URL formats and ensure interoperability with both OAuth 2.0 Authorization Server Metadata and OpenID Connect Discovery 1.0 specifications, MCP clients MUST attempt multiple well-known endpoints when discovering authorization server metadata.

The discovery approach is based on RFC 8414 Section 3.1 “Authorization Server Metadata Request” for OAuth 2.0 Authorization Server Metadata discovery and RFC 8414 Section 5 “Compatibility Notes” for OpenID Connect Discovery 1.0 interoperability.

For issuer URLs with path components (e.g., https://auth.example.com/tenant1), clients MUST try endpoints in the following priority order:

1. OAuth 2.0 Authorization Server Metadata with path insertion: https://auth.example.com/.well-known/oauth-authorization-server/tenant1
2. OpenID Connect Discovery 1.0 with path insertion: https://auth.example.com/.well-known/openid-configuration/tenant1
3. OpenID Connect Discovery 1.0 path appending: https://auth.example.com/tenant1/.well-known/openid-configuration

For issuer URLs without path components (e.g., https://auth.example.com), clients MUST try:

1. OAuth 2.0 Authorization Server Metadata: https://auth.example.com/.well-known/oauth-authorization-server
2. OpenID Connect Discovery 1.0: https://auth.example.com/.well-known/openid-configuration

After retrieving a metadata document, MCP clients MUST validate it as required by RFC8414 Section 3.3 or OpenID Connect Discovery Section 4.3: the issuer value in the document MUST be identical to the issuer identifier used to construct the well-known URL. If they differ, the client MUST NOT use the metadata. For example, a document fetched from https://attacker.example/.well-known/oauth-authorization-server that contains "issuer": "https://honest.example" MUST be rejected.

Sequence Diagram

The following diagram outlines an example flow:

sequenceDiagram
    participant C as Client
    participant M as MCP Server (Resource Server)
    participant A as Authorization Server

    Note over C: Attempt unauthenticated MCP request
    C->>M: MCP request without token
    M-->>C: HTTP 401 Unauthorized (may include WWW-Authenticate header)

    alt Header includes resource_metadata
        Note over C: Extract resource_metadata URL from header
        C->>M: GET resource_metadata URI
        M-->>C: Resource metadata with authorization server URL
    else No resource_metadata in header
        Note over C: Fallback to well-known URI probing
        Note over M: _Not applicable if the MCP server is at the root_
        C->>M: GET /.well-known/oauth-protected-resource/mcp
        alt Sub-path metadata found
            M-->>C: Resource metadata with authorization server URL
        else Sub-path not found
            C->>M: GET /.well-known/oauth-protected-resource
            alt Root metadata found
                M-->>C: Resource metadata with authorization server URL
            else Root metadata not found
                Note over C: Abort or use pre-configured values
            end
        end
    end

    Note over C: Validate RS metadata,<br />build AS metadata URL

    C->>A: GET Authorization server metadata endpoint
    Note over C,A: Try OAuth 2.0 and OpenID Connect<br/>discovery endpoints in priority order
    A-->>C: Authorization server metadata

    Note over C,A: OAuth 2.1 authorization flow happens here

    C->>A: Token request
    A-->>C: Access token

    C->>M: MCP request with access token
    M-->>C: MCP response
    Note over C,M: MCP communication continues with valid token

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Last updated Oct 08, 2026