RFC 9728 compliance: protected resource metadata URL mismatches when resource identifier has a path

[shulkx]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

Environment

• Client: VS Code MCP (http)
• Server: examples/servers/simple-auth
• Config:

  {
    "servers": {
      "Get-Time": {
        "url": "http://localhost:8001/mcp",
        "type": "http"
      }
    }
  }

Problem 1: Protected Resource Metadata “resource” mismatch

On startup, VS Code shows:

Connection state: Error Error sending message to http://localhost:8001/mcp: Error: Protected Resource Metadata resource "http://localhost:8001/" does not match MCP server resolved resource "http://localhost:8001/mcp". The MCP server must follow OAuth spec https://datatracker.ietf.org/doc/html/rfc9728#PRConfigurationValidation

Per RFC 9728 validation, the example should set the resource identifier to include the path component:

• Expected in example: resource_server_url=AnyHttpUrl(f"{settings.server_url}mcp"), # <--- add mcp
• File:

  python-sdk/examples/servers/simple-auth/mcp_simple_auth/server.py

  Lines 71 to 84 in 71889d7

  	app = FastMCP(
  	name="MCP Resource Server",
  	instructions="Resource Server that validates tokens via Authorization Server introspection",
  	host=settings.host,
  	port=settings.port,
  	debug=True,
  	# Auth configuration for RS mode
  	token_verifier=token_verifier,
  	auth=AuthSettings(
  	issuer_url=settings.auth_server_url,
  	required_scopes=[settings.mcp_scope],
  	resource_server_url=settings.server_url,
  	),
  	)

When I include /mcp in resource_server_url, this error goes away.

Problem 2: WWW-Authenticate resource_metadata vs actual route

The server builds resource_metadata_url for the WWW-Authenticate header and also registers the well-known route. These two do not match when the resource identifier has a path.

• Header construction (resource_metadata_url):

  • Code:

    python-sdk/src/mcp/server/fastmcp/server.py

    Lines 942 to 951 in 71889d7

    	resource_metadata_url = AnyHttpUrl(
    	str(self.settings.auth.resource_server_url).rstrip("/") + "/.well-known/oauth-protected-resource"
    	)
    	routes.append(
    	Route(
    	self.settings.streamable_http_path,
    	endpoint=RequireAuthMiddleware(streamable_http_app, required_scopes, resource_metadata_url),
    	)
    	)

  • Behavior:
    • resource_server_url = http://localhost:8001 → resource_metadata_url = http://localhost:8001/.well-known/oauth-protected-resource
    • resource_server_url = http://localhost:8001/mcp → resource_metadata_url = http://localhost:8001/mcp/.well-known/oauth-protected-resource

• Actual route registration (well-known endpoint):

  • Code:

    python-sdk/src/mcp/server/fastmcp/server.py

    Lines 961 to 981 in 71889d7

    	# Add protected resource metadata endpoint if configured as RS
    	if self.settings.auth and self.settings.auth.resource_server_url:
    	from mcp.server.auth.handlers.metadata import ProtectedResourceMetadataHandler
    	from mcp.server.auth.routes import cors_middleware
    	from mcp.shared.auth import ProtectedResourceMetadata
    	protected_resource_metadata = ProtectedResourceMetadata(
    	resource=self.settings.auth.resource_server_url,
    	authorization_servers=[self.settings.auth.issuer_url],
    	scopes_supported=self.settings.auth.required_scopes,
    	)
    	routes.append(
    	Route(
    	"/.well-known/oauth-protected-resource",
    	endpoint=cors_middleware(
    	ProtectedResourceMetadataHandler(protected_resource_metadata).handle,
    	["GET", "OPTIONS"],
    	),
    	methods=["GET", "OPTIONS"],
    	)
    	)

  • Behavior:
    • resource_server_url = http://localhost:8001 → endpoint = http://localhost:8001/.well-known/oauth-protected-resource (matches)
    • resource_server_url = http://localhost:8001/mcp → endpoint = http://localhost:8001/.well-known/oauth-protected-resource (mismatch)

Result: the client follows the WWW-Authenticate header to http://localhost:8001/mcp/.well-known/oauth-protected-resource, but the server only serves the route at the root.

Problem 3: Well-known URL construction (RFC 9728 §3.1)

RFC 9728 §3.1 states that when the resource identifier contains a path, the well-known suffix must be inserted between the host and the path:

If the resource identifier value contains a path or query component, any terminating slash (/) following the host component MUST be removed before inserting /.well-known/ and the well-known URI path suffix between the host component and the path and/or query components.

• If resource = http://localhost:8001 → well-known = http://localhost:8001/.well-known/oauth-protected-resource
• If resource = http://localhost:8001/mcp → well-known = http://localhost:8001/.well-known/oauth-protected-resource/mcp

Current implementation:

• Header builds: http://localhost:8001/mcp/.well-known/oauth-protected-resource (appends after the path)
• Route registers only: http://localhost:8001/.well-known/oauth-protected-resource (at the root)
• Neither matches the RFC-required path-aware form.

Expected behavior (RFC 9728)

RFC 9728 §3.1: insert /.well-known/oauth-protected-resource between the host and the resource path.

• If resource_server_url = http://localhost:8001 (no path):

  • Header should point to: http://localhost:8001/.well-known/oauth-protected-resource
  • Route should be served at: http://localhost:8001/.well-known/oauth-protected-resource

• If resource_server_url = http://localhost:8001/mcp (has path /mcp):

  • Header should point to: http://localhost:8001/.well-known/oauth-protected-resource/mcp
  • Route should be served at: http://localhost:8001/.well-known/oauth-protected-resource/mcp

Example Code

Python & MCP Python SDK

Python 3.11
MCP Python SDK v1.15.0