Improve OAuth protected resource metadata URL construction per RFC 9728 (#1407)

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

@@ -32,7 +32,7 @@ class ResourceServerSettings(BaseSettings):
     # Server settings
     host: str = "localhost"
     port: int = 8001
-    server_url: AnyHttpUrl = AnyHttpUrl("http://localhost:8001")
+    server_url: AnyHttpUrl = AnyHttpUrl("http://localhost:8001/mcp")
 
     # Authorization Server settings
     auth_server_url: AnyHttpUrl = AnyHttpUrl("http://localhost:9000")
@@ -137,7 +137,7 @@ def main(port: int, auth_server: str, transport: Literal["sse", "streamable-http
 
         # Create settings
         host = "localhost"
-        server_url = f"http://{host}:{port}"
+        server_url = f"http://{host}:{port}/mcp"
         settings = ResourceServerSettings(
             host=host,
             port=port,

src/mcp/server/auth/routes.py

@@ -1,5 +1,6 @@
 from collections.abc import Awaitable, Callable
 from typing import Any
+from urllib.parse import urlparse
 
 from pydantic import AnyHttpUrl
 from starlette.middleware.cors import CORSMiddleware
@@ -186,6 +187,25 @@ def build_metadata(
     return metadata
 
 
+def build_resource_metadata_url(resource_server_url: AnyHttpUrl) -> AnyHttpUrl:
+    """
+    Build RFC 9728 compliant protected resource metadata URL.
+
+    Inserts /.well-known/oauth-protected-resource between host and resource path
+    as specified in RFC 9728 §3.1.
+
+    Args:
+        resource_server_url: The resource server URL (e.g., https://example.com/mcp)
+
+    Returns:
+        The metadata URL (e.g., https://example.com/.well-known/oauth-protected-resource/mcp)
+    """
+    parsed = urlparse(str(resource_server_url))
+    # Handle trailing slash: if path is just "/", treat as empty
+    resource_path = parsed.path if parsed.path != "/" else ""
+    return AnyHttpUrl(f"{parsed.scheme}://{parsed.netloc}/.well-known/oauth-protected-resource{resource_path}")
+
+
 def create_protected_resource_routes(
     resource_url: AnyHttpUrl,
     authorization_servers: list[AnyHttpUrl],
@@ -218,9 +238,15 @@ def create_protected_resource_routes(
 
     handler = ProtectedResourceMetadataHandler(metadata)
 
+    # RFC 9728 §3.1: Register route at /.well-known/oauth-protected-resource + resource path
+    metadata_url = build_resource_metadata_url(resource_url)
+    # Extract just the path part for route registration
+    parsed = urlparse(str(metadata_url))
+    well_known_path = parsed.path
+
     return [
         Route(
-            "/.well-known/oauth-protected-resource",
+            well_known_path,
             endpoint=cors_middleware(handler.handle, ["GET", "OPTIONS"]),
             methods=["GET", "OPTIONS"],
         )

src/mcp/server/fastmcp/server.py

@@ -825,11 +825,10 @@ class FastMCP(Generic[LifespanResultT]):
             # Determine resource metadata URL
             resource_metadata_url = None
             if self.settings.auth and self.settings.auth.resource_server_url:
-                from pydantic import AnyHttpUrl
+                from mcp.server.auth.routes import build_resource_metadata_url
 
-                resource_metadata_url = AnyHttpUrl(
+                # Build compliant metadata URL for WWW-Authenticate header
-                    str(self.settings.auth.resource_server_url).rstrip("/") + "/.well-known/oauth-protected-resource"
+                resource_metadata_url = build_resource_metadata_url(self.settings.auth.resource_server_url)
-                )
 
             # Auth is enabled, wrap the endpoints with RequireAuthMiddleware
             routes.append(
@@ -938,11 +937,10 @@ class FastMCP(Generic[LifespanResultT]):
             # Determine resource metadata URL
             resource_metadata_url = None
             if self.settings.auth and self.settings.auth.resource_server_url:
-                from pydantic import AnyHttpUrl
+                from mcp.server.auth.routes import build_resource_metadata_url
 
-                resource_metadata_url = AnyHttpUrl(
+                # Build compliant metadata URL for WWW-Authenticate header
-                    str(self.settings.auth.resource_server_url).rstrip("/") + "/.well-known/oauth-protected-resource"
+                resource_metadata_url = build_resource_metadata_url(self.settings.auth.resource_server_url)
-                )
 
             routes.append(
                 Route(
@@ -961,24 +959,14 @@ class FastMCP(Generic[LifespanResultT]):
 
         # 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 create_protected_resource_routes
-            from mcp.server.auth.routes import cors_middleware
-            from mcp.shared.auth import ProtectedResourceMetadata
 
-            protected_resource_metadata = ProtectedResourceMetadata(
+            routes.extend(
-                resource=self.settings.auth.resource_server_url,
+                create_protected_resource_routes(
+                    resource_url=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"],
-                )
             )
 
         routes.extend(self._custom_starlette_routes)

tests/server/auth/test_protected_resource.py

@@ -8,7 +8,7 @@ from inline_snapshot import snapshot
 from pydantic import AnyHttpUrl
 from starlette.applications import Starlette
 
-from mcp.server.auth.routes import create_protected_resource_routes
+from mcp.server.auth.routes import build_resource_metadata_url, create_protected_resource_routes
 
 
 @pytest.fixture
@@ -36,10 +36,11 @@ async def test_client(test_app: Starlette):
 
 
 @pytest.mark.anyio
-async def test_metadata_endpoint(test_client: httpx.AsyncClient):
+async def test_metadata_endpoint_with_path(test_client: httpx.AsyncClient):
-    """Test the OAuth 2.0 Protected Resource metadata endpoint."""
+    """Test the OAuth 2.0 Protected Resource metadata endpoint for path-based resource."""
 
-    response = await test_client.get("/.well-known/oauth-protected-resource")
+    # For resource with path "/resource", metadata should be accessible at the path-aware location
+    response = await test_client.get("/.well-known/oauth-protected-resource/resource")
     assert response.json() == snapshot(
         {
             "resource": "https://example.com/resource",
@@ -50,3 +51,148 @@ async def test_metadata_endpoint(test_client: httpx.AsyncClient):
             "bearer_methods_supported": ["header"],
         }
     )
+
+
+@pytest.mark.anyio
+async def test_metadata_endpoint_root_path_returns_404(test_client: httpx.AsyncClient):
+    """Test that root path returns 404 for path-based resource."""
+
+    # Root path should return 404 for path-based resources
+    response = await test_client.get("/.well-known/oauth-protected-resource")
+    assert response.status_code == 404
+
+
+@pytest.fixture
+def root_resource_app():
+    """Fixture to create protected resource routes for root-level resource."""
+
+    # Create routes for a resource without path component
+    protected_resource_routes = create_protected_resource_routes(
+        resource_url=AnyHttpUrl("https://example.com"),
+        authorization_servers=[AnyHttpUrl("https://auth.example.com")],
+        scopes_supported=["read"],
+        resource_name="Root Resource",
+    )
+
+    app = Starlette(routes=protected_resource_routes)
+    return app
+
+
+@pytest.fixture
+async def root_resource_client(root_resource_app: Starlette):
+    """Fixture to create an HTTP client for the root resource app."""
+    async with httpx.AsyncClient(
+        transport=httpx.ASGITransport(app=root_resource_app), base_url="https://mcptest.com"
+    ) as client:
+        yield client
+
+
+@pytest.mark.anyio
+async def test_metadata_endpoint_without_path(root_resource_client: httpx.AsyncClient):
+    """Test metadata endpoint for root-level resource."""
+
+    # For root resource, metadata should be at standard location
+    response = await root_resource_client.get("/.well-known/oauth-protected-resource")
+    assert response.status_code == 200
+    assert response.json() == snapshot(
+        {
+            "resource": "https://example.com/",
+            "authorization_servers": ["https://auth.example.com/"],
+            "scopes_supported": ["read"],
+            "resource_name": "Root Resource",
+            "bearer_methods_supported": ["header"],
+        }
+    )
+
+
+class TestMetadataUrlConstruction:
+    """Test URL construction utility function."""
+
+    def test_url_without_path(self):
+        """Test URL construction for resource without path component."""
+        resource_url = AnyHttpUrl("https://example.com")
+        result = build_resource_metadata_url(resource_url)
+        assert str(result) == "https://example.com/.well-known/oauth-protected-resource"
+
+    def test_url_with_path_component(self):
+        """Test URL construction for resource with path component."""
+        resource_url = AnyHttpUrl("https://example.com/mcp")
+        result = build_resource_metadata_url(resource_url)
+        assert str(result) == "https://example.com/.well-known/oauth-protected-resource/mcp"
+
+    def test_url_with_trailing_slash_only(self):
+        """Test URL construction for resource with trailing slash only."""
+        resource_url = AnyHttpUrl("https://example.com/")
+        result = build_resource_metadata_url(resource_url)
+        # Trailing slash should be treated as empty path
+        assert str(result) == "https://example.com/.well-known/oauth-protected-resource"
+
+    @pytest.mark.parametrize(
+        "resource_url,expected_url",
+        [
+            ("https://example.com", "https://example.com/.well-known/oauth-protected-resource"),
+            ("https://example.com/", "https://example.com/.well-known/oauth-protected-resource"),
+            ("https://example.com/mcp", "https://example.com/.well-known/oauth-protected-resource/mcp"),
+            ("http://localhost:8001/mcp", "http://localhost:8001/.well-known/oauth-protected-resource/mcp"),
+        ],
+    )
+    def test_various_resource_configurations(self, resource_url: str, expected_url: str):
+        """Test URL construction with various resource configurations."""
+        result = build_resource_metadata_url(AnyHttpUrl(resource_url))
+        assert str(result) == expected_url
+
+
+class TestRouteConsistency:
+    """Test consistency between URL generation and route registration."""
+
+    def test_route_path_matches_metadata_url(self):
+        """Test that route path matches the generated metadata URL."""
+        resource_url = AnyHttpUrl("https://example.com/mcp")
+
+        # Generate metadata URL
+        metadata_url = build_resource_metadata_url(resource_url)
+
+        # Create routes
+        routes = create_protected_resource_routes(
+            resource_url=resource_url,
+            authorization_servers=[AnyHttpUrl("https://auth.example.com")],
+        )
+
+        # Extract path from metadata URL
+        from urllib.parse import urlparse
+
+        metadata_path = urlparse(str(metadata_url)).path
+
+        # Verify consistency
+        assert len(routes) == 1
+        assert routes[0].path == metadata_path
+
+    @pytest.mark.parametrize(
+        "resource_url,expected_path",
+        [
+            ("https://example.com", "/.well-known/oauth-protected-resource"),
+            ("https://example.com/", "/.well-known/oauth-protected-resource"),
+            ("https://example.com/mcp", "/.well-known/oauth-protected-resource/mcp"),
+        ],
+    )
+    def test_consistent_paths_for_various_resources(self, resource_url: str, expected_path: str):
+        """Test that URL generation and route creation are consistent."""
+        resource_url_obj = AnyHttpUrl(resource_url)
+
+        # Test URL generation
+        metadata_url = build_resource_metadata_url(resource_url_obj)
+        from urllib.parse import urlparse
+
+        url_path = urlparse(str(metadata_url)).path
+
+        # Test route creation
+        routes = create_protected_resource_routes(
+            resource_url=resource_url_obj,
+            authorization_servers=[AnyHttpUrl("https://auth.example.com")],
+        )
+        route_path = routes[0].path
+
+        # Both should match expected path
+        assert url_path == expected_path
+        assert route_path == expected_path
+        assert url_path == route_path