Fix validate_scope rejecting scopes when client scope is None

When OAuthClientMetadata.scope is None (no scopes registered),
validate_scope() was converting it to an empty list, causing all
requested scopes to be rejected with InvalidScopeError. Now treats
None as "no restrictions" and allows any requested scope through.

Fixes #2216

Author: Christian-Sidak

src/mcp/shared/auth.py

@@ -89,11 +89,14 @@ def validate_scope(self, requested_scope: str | None) -> list[str] | None:
         if requested_scope is None:
             return None
         requested_scopes = requested_scope.split(" ")
-        allowed_scopes = [] if self.scope is None else self.scope.split(" ")
+        if self.scope is None:
+            # No registered scopes means no restrictions
+            return requested_scopes
+        allowed_scopes = self.scope.split(" ")
         for scope in requested_scopes:
-            if scope not in allowed_scopes:  # pragma: no branch
+            if scope not in allowed_scopes:
                 raise InvalidScopeError(f"Client was not registered with scope {scope}")
-        return requested_scopes  # pragma: no cover
+        return requested_scopes
 
     def validate_redirect_uri(self, redirect_uri: AnyUrl | None) -> AnyUrl:
         if redirect_uri is not None:

tests/shared/test_auth.py

@@ -138,3 +138,52 @@ def test_invalid_non_empty_url_still_rejected():
     }
     with pytest.raises(ValidationError):
         OAuthClientMetadata.model_validate(data)
+
+
+class TestValidateScope:
+    """Tests for OAuthClientMetadata.validate_scope()."""
+
+    def _make_client(self, scope: str | None = None) -> OAuthClientMetadata:
+        return OAuthClientMetadata.model_validate(
+            {
+                "redirect_uris": ["https://example.com/callback"],
+                "scope": scope,
+            }
+        )
+
+    def test_requested_scope_none_returns_none(self):
+        client = self._make_client(scope="read write")
+        assert client.validate_scope(None) is None
+
+    def test_registered_scope_none_allows_any_requested_scope(self):
+        """When the client has no registered scopes (scope=None),
+        any requested scope should be allowed through."""
+        client = self._make_client(scope=None)
+        result = client.validate_scope("read write admin")
+        assert result == ["read", "write", "admin"]
+
+    def test_registered_scope_none_allows_single_scope(self):
+        client = self._make_client(scope=None)
+        result = client.validate_scope("read")
+        assert result == ["read"]
+
+    def test_valid_scope_subset(self):
+        client = self._make_client(scope="read write admin")
+        result = client.validate_scope("read write")
+        assert result == ["read", "write"]
+
+    def test_valid_scope_exact_match(self):
+        client = self._make_client(scope="read write")
+        result = client.validate_scope("read write")
+        assert result == ["read", "write"]
+
+    def test_invalid_scope_raises_error(self):
+        from mcp.shared.auth import InvalidScopeError
+
+        client = self._make_client(scope="read write")
+        with pytest.raises(InvalidScopeError, match="delete"):
+            client.validate_scope("read delete")
+
+    def test_no_registered_scope_and_no_requested_scope(self):
+        client = self._make_client(scope=None)
+        assert client.validate_scope(None) is None