fix(auth): coerce empty-string optional URL fields to None in OAuthClientMetadata

src/mcp/shared/auth.py

@@ -67,6 +67,24 @@ class OAuthClientMetadata(BaseModel):
     software_id: str | None = None
     software_version: str | None = None
 
+    @field_validator(
+        "client_uri",
+        "logo_uri",
+        "tos_uri",
+        "policy_uri",
+        "jwks_uri",
+        mode="before",
+    )
+    @classmethod
+    def _empty_string_optional_url_to_none(cls, v: object) -> object:
+        # RFC 7591 §2 marks these URL fields OPTIONAL. Some authorization servers
+        # echo omitted metadata back as "" instead of dropping the keys, which
+        # AnyHttpUrl would otherwise reject — throwing away an otherwise valid
+        # registration response. Treat "" as absent.
+        if v == "":
+            return None
+        return v
+
     def validate_scope(self, requested_scope: str | None) -> list[str] | None:
         if requested_scope is None:
             return None

tests/client/test_auth.py

@@ -8,7 +8,7 @@
 import httpx
 import pytest
 from inline_snapshot import Is, snapshot
-from pydantic import AnyHttpUrl, AnyUrl
+from pydantic import AnyHttpUrl, AnyUrl, ValidationError
 
 from mcp.client.auth import OAuthClientProvider, PKCEParameters
 from mcp.client.auth.exceptions import OAuthFlowError
@@ -985,6 +985,79 @@ def text(self):
         assert "Registration failed: 400" in str(exc_info.value)
 
 
+class TestOAuthClientMetadataEmptyUrlCoercion:
+    """RFC 7591 §2 marks client_uri/logo_uri/tos_uri/policy_uri/jwks_uri as OPTIONAL.
+    Some authorization servers echo the client's omitted metadata back as ""
+    instead of dropping the keys; without coercion, AnyHttpUrl rejects "" and
+    the whole registration response is thrown away even though the server
+    returned a valid client_id."""
+
+    @pytest.mark.parametrize(
+        "empty_field",
+        ["client_uri", "logo_uri", "tos_uri", "policy_uri", "jwks_uri"],
+    )
+    def test_optional_url_empty_string_coerced_to_none(self, empty_field: str):
+        data = {
+            "redirect_uris": ["https://example.com/callback"],
+            empty_field: "",
+        }
+        metadata = OAuthClientMetadata.model_validate(data)
+        assert getattr(metadata, empty_field) is None
+
+    def test_all_optional_urls_empty_together(self):
+        data = {
+            "redirect_uris": ["https://example.com/callback"],
+            "client_uri": "",
+            "logo_uri": "",
+            "tos_uri": "",
+            "policy_uri": "",
+            "jwks_uri": "",
+        }
+        metadata = OAuthClientMetadata.model_validate(data)
+        assert metadata.client_uri is None
+        assert metadata.logo_uri is None
+        assert metadata.tos_uri is None
+        assert metadata.policy_uri is None
+        assert metadata.jwks_uri is None
+
+    def test_valid_url_passes_through_unchanged(self):
+        data = {
+            "redirect_uris": ["https://example.com/callback"],
+            "client_uri": "https://udemy.com/",
+        }
+        metadata = OAuthClientMetadata.model_validate(data)
+        assert str(metadata.client_uri) == "https://udemy.com/"
+
+    def test_information_full_inherits_coercion(self):
+        """OAuthClientInformationFull subclasses OAuthClientMetadata, so the
+        same coercion applies to DCR responses parsed via the full model."""
+        data = {
+            "client_id": "abc123",
+            "redirect_uris": ["https://example.com/callback"],
+            "client_uri": "",
+            "logo_uri": "",
+            "tos_uri": "",
+            "policy_uri": "",
+            "jwks_uri": "",
+        }
+        info = OAuthClientInformationFull.model_validate(data)
+        assert info.client_id == "abc123"
+        assert info.client_uri is None
+        assert info.logo_uri is None
+        assert info.tos_uri is None
+        assert info.policy_uri is None
+        assert info.jwks_uri is None
+
+    def test_invalid_non_empty_url_still_rejected(self):
+        """Coercion must only touch empty strings — garbage URLs still raise."""
+        data = {
+            "redirect_uris": ["https://example.com/callback"],
+            "client_uri": "not a url",
+        }
+        with pytest.raises(ValidationError):
+            OAuthClientMetadata.model_validate(data)
+
+
 class TestCreateClientRegistrationRequest:
     """Test client registration request creation."""