Make "resource" optional on earlier protocols

src/mcp/client/auth.py

@@ -95,6 +95,7 @@ class OAuthContext:
     protected_resource_metadata: ProtectedResourceMetadata | None = None
     oauth_metadata: OAuthMetadata | None = None
     auth_server_url: str | None = None
+    protocol_version: str | None = None
 
     # Client registration
     client_info: OAuthClientInformationFull | None = None
@@ -154,6 +155,25 @@ def get_resource_url(self) -> str:
 
         return resource
 
+    def should_include_resource_param(self, protocol_version: str | None = None) -> bool:
+        """Determine if the resource parameter should be included in OAuth requests.
+
+        Returns True if:
+        - Protected resource metadata is available, OR
+        - MCP-Protocol-Version header is 2025-06-18 or later
+        """
+        # If we have protected resource metadata, include the resource param
+        if self.protected_resource_metadata is not None:
+            return True
+
+        # If no protocol version provided, don't include resource param
+        if not protocol_version:
+            return False
+
+        # Check if protocol version is 2025-06-18 or later
+        # Version format is YYYY-MM-DD, so string comparison works
+        return protocol_version >= "2025-06-18"
+
 
 class OAuthClientProvider(httpx.Auth):
     """
@@ -320,9 +340,12 @@ async def _perform_authorization(self) -> tuple[str, str]:
             "state": state,
             "code_challenge": pkce_params.code_challenge,
             "code_challenge_method": "S256",
-            "resource": self.context.get_resource_url(),  # RFC 8707
         }
 
+        # Only include resource param if conditions are met
+        if self.context.should_include_resource_param(self.context.protocol_version):
+            auth_params["resource"] = self.context.get_resource_url()  # RFC 8707
+
         if self.context.client_metadata.scope:
             auth_params["scope"] = self.context.client_metadata.scope
 
@@ -358,9 +381,12 @@ async def _exchange_token(self, auth_code: str, code_verifier: str) -> httpx.Req
             "redirect_uri": str(self.context.client_metadata.redirect_uris[0]),
             "client_id": self.context.client_info.client_id,
             "code_verifier": code_verifier,
-            "resource": self.context.get_resource_url(),  # RFC 8707
         }
 
+        # Only include resource param if conditions are met
+        if self.context.should_include_resource_param(self.context.protocol_version):
+            token_data["resource"] = self.context.get_resource_url()  # RFC 8707
+
         if self.context.client_info.client_secret:
             token_data["client_secret"] = self.context.client_info.client_secret
 
@@ -409,9 +435,12 @@ async def _refresh_token(self) -> httpx.Request:
             "grant_type": "refresh_token",
             "refresh_token": self.context.current_tokens.refresh_token,
             "client_id": self.context.client_info.client_id,
-            "resource": self.context.get_resource_url(),  # RFC 8707
         }
 
+        # Only include resource param if conditions are met
+        if self.context.should_include_resource_param(self.context.protocol_version):
+            refresh_data["resource"] = self.context.get_resource_url()  # RFC 8707
+
         if self.context.client_info.client_secret:
             refresh_data["client_secret"] = self.context.client_info.client_secret
 
@@ -457,6 +486,9 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
             if not self._initialized:
                 await self._initialize()
 
+            # Capture protocol version from request headers
+            self.context.protocol_version = request.headers.get(MCP_PROTOCOL_VERSION)
+
             # Perform OAuth flow if not authenticated
             if not self.context.is_token_valid():
                 try:

tests/client/test_auth.py

@@ -13,6 +13,7 @@
     OAuthClientInformationFull,
     OAuthClientMetadata,
     OAuthToken,
+    ProtectedResourceMetadata,
 )
 
 
@@ -434,6 +435,87 @@ async def test_refresh_token_request(self, oauth_provider, valid_tokens):
         assert "client_secret=test_secret" in content
 
 
+class TestProtectedResourceMetadata:
+    """Test protected resource handling."""
+
+    @pytest.mark.anyio
+    async def test_resource_param_included_with_recent_protocol_version(self, oauth_provider: OAuthClientProvider):
+        """Test resource parameter is included for protocol version >= 2025-06-18."""
+        # Set protocol version to 2025-06-18
+        oauth_provider.context.protocol_version = "2025-06-18"
+        oauth_provider.context.client_info = OAuthClientInformationFull(
+            client_id="test_client",
+            client_secret="test_secret",
+            redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+        )
+
+        # Test in token exchange
+        request = await oauth_provider._exchange_token("test_code", "test_verifier")
+        content = request.content.decode()
+        assert "resource=" in content
+        # Check URL-encoded resource parameter
+        from urllib.parse import quote
+
+        expected_resource = quote(oauth_provider.context.get_resource_url(), safe="")
+        assert f"resource={expected_resource}" in content
+
+        # Test in refresh token
+        oauth_provider.context.current_tokens = OAuthToken(
+            access_token="test_access",
+            token_type="Bearer",
+            refresh_token="test_refresh",
+        )
+        refresh_request = await oauth_provider._refresh_token()
+        refresh_content = refresh_request.content.decode()
+        assert "resource=" in refresh_content
+
+    @pytest.mark.anyio
+    async def test_resource_param_excluded_with_old_protocol_version(self, oauth_provider: OAuthClientProvider):
+        """Test resource parameter is excluded for protocol version < 2025-06-18."""
+        # Set protocol version to older version
+        oauth_provider.context.protocol_version = "2025-03-26"
+        oauth_provider.context.client_info = OAuthClientInformationFull(
+            client_id="test_client",
+            client_secret="test_secret",
+            redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+        )
+
+        # Test in token exchange
+        request = await oauth_provider._exchange_token("test_code", "test_verifier")
+        content = request.content.decode()
+        assert "resource=" not in content
+
+        # Test in refresh token
+        oauth_provider.context.current_tokens = OAuthToken(
+            access_token="test_access",
+            token_type="Bearer",
+            refresh_token="test_refresh",
+        )
+        refresh_request = await oauth_provider._refresh_token()
+        refresh_content = refresh_request.content.decode()
+        assert "resource=" not in refresh_content
+
+    @pytest.mark.anyio
+    async def test_resource_param_included_with_protected_resource_metadata(self, oauth_provider: OAuthClientProvider):
+        """Test resource parameter is always included when protected resource metadata exists."""
+        # Set old protocol version but with protected resource metadata
+        oauth_provider.context.protocol_version = "2025-03-26"
+        oauth_provider.context.protected_resource_metadata = ProtectedResourceMetadata(
+            resource=AnyHttpUrl("https://api.example.com/v1/mcp"),
+            authorization_servers=[AnyHttpUrl("https://api.example.com")],
+        )
+        oauth_provider.context.client_info = OAuthClientInformationFull(
+            client_id="test_client",
+            client_secret="test_secret",
+            redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+        )
+
+        # Test in token exchange
+        request = await oauth_provider._exchange_token("test_code", "test_verifier")
+        content = request.content.decode()
+        assert "resource=" in content
+
+
 class TestAuthFlow:
     """Test the auth flow in httpx."""