fix(auth): coerce empty-string optional URL fields to None in OAuthClientMetadata (#2404)

Author: felixweinberger

AGENTS.md

@@ -45,7 +45,9 @@
 
 - Framework: `uv run --frozen pytest`
 - Async testing: use anyio, not asyncio
-- Do not use `Test` prefixed classes, use functions
+- Do not use `Test` prefixed classes — write plain top-level `test_*` functions.
+  Legacy files still contain `Test*` classes; do NOT follow that pattern for new
+  tests even when adding to such a file.
 - IMPORTANT: Tests should be fast and deterministic. Prefer in-memory async execution;
   reach for threads only when necessary, and subprocesses only as a last resort.
 - For end-to-end behavior, an in-memory `Client(server)` is usually the

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/shared/test_auth.py

@@ -1,6 +1,9 @@
 """Tests for OAuth 2.0 shared code."""
 
-from mcp.shared.auth import OAuthMetadata
+import pytest
+from pydantic import ValidationError
+
+from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata
 
 
 def test_oauth():
@@ -58,3 +61,80 @@ def test_oauth_with_jarm():
             "token_endpoint_auth_methods_supported": ["client_secret_basic", "client_secret_post"],
         }
     )
+
+
+# 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(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():
+    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():
+    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():
+    """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():
+    """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)