fix(auth): preserve root protected resource URI

src/mcp/shared/auth.py

@@ -1,4 +1,5 @@
 from typing import Any, Literal, cast
+from urllib.parse import urlsplit, urlunsplit
 
 from pydantic import AnyHttpUrl, AnyUrl, BaseModel, ConfigDict, Field, field_validator, model_validator
 
@@ -256,3 +257,19 @@ class ProtectedResourceMetadata(BaseModel):
     dpop_signing_alg_values_supported: list[str] | None = None
     # dpop_bound_access_tokens_required default is False, but omitted here for clarity
     dpop_bound_access_tokens_required: bool | None = None
+
+    @field_validator("resource", mode="before")
+    @classmethod
+    def _preserve_empty_resource_path(cls, value: object) -> object:
+        """Keep the RFC 9728 root resource URI free of a synthetic slash.
+
+        ``AnyHttpUrl`` normalizes ``https://example.com`` to
+        ``https://example.com/`` before the model's ``url_preserve_empty_path``
+        setting can preserve the distinction. This is especially visible when
+        the value arrives as an already-validated ``AnyHttpUrl`` from the
+        server settings.
+        """
+        parsed = urlsplit(str(value))
+        if parsed.path == "/":
+            return urlunsplit(parsed._replace(path=""))
+        return value

tests/shared/test_auth.py

@@ -1,9 +1,15 @@
 """Tests for OAuth 2.0 shared code."""
 
 import pytest
-from pydantic import AnyUrl, ValidationError
-
-from mcp.shared.auth import InvalidRedirectUriError, OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata
+from pydantic import AnyHttpUrl, AnyUrl, ValidationError
+
+from mcp.shared.auth import (
+    InvalidRedirectUriError,
+    OAuthClientInformationFull,
+    OAuthClientMetadata,
+    OAuthMetadata,
+    ProtectedResourceMetadata,
+)
 
 
 def test_oauth():
@@ -109,6 +115,27 @@ def test_valid_url_passes_through_unchanged():
     assert str(metadata.client_uri) == "https://udemy.com/"
 
 
+def test_protected_resource_metadata_preserves_empty_root_path():
+    metadata = ProtectedResourceMetadata.model_validate(
+        {
+            "resource": "https://example.com",
+            "authorization_servers": ["https://auth.example.com"],
+        }
+    )
+
+    assert str(metadata.resource) == "https://example.com"
+    assert '"resource":"https://example.com"' in metadata.model_dump_json()
+
+
+def test_protected_resource_metadata_strips_normalized_root_path():
+    metadata = ProtectedResourceMetadata(
+        resource=AnyHttpUrl("https://example.com"),
+        authorization_servers=[AnyHttpUrl("https://auth.example.com")],
+    )
+
+    assert str(metadata.resource) == "https://example.com"
+
+
 def test_information_full_inherits_coercion():
     """OAuthClientInformationFull shares the metadata base, so the same
     coercion applies to DCR responses parsed via the full model."""