Preserve empty issuer/resource paths on AuthSettings (#2987)

docs/migration.md

@@ -1373,6 +1373,14 @@ match redirect URIs by exact string comparison, so if you registered such a URI
 release (with the trailing slash) and the registration is persisted in `TokenStorage`, re-register
 the client so the stored value matches what the SDK now transmits.
 
+`AuthSettings` now sets `url_preserve_empty_path=True` for the same reason: a path-less
+`issuer_url` (or `resource_server_url`) passed as a string keeps its empty path, so the authorization
+server advertises `issuer` as `https://as.example.com` rather than `https://as.example.com/` in its
+metadata. Previously the trailing slash was added before the model saw the value, leaving the served
+issuer inconsistent with what clients compare against under RFC 8414 / RFC 9207. Passing an
+already-built `AnyHttpUrl` object still normalizes at construction; pass a string to get the
+preserved form.
+
 ### Lowlevel `Server`: `subscribe` capability now correctly reported
 
 Previously, the lowlevel `Server` hardcoded `subscribe=False` in resource capabilities even when a `subscribe_resource()` handler was registered. The `subscribe` capability is now dynamically set to `True` when an `on_subscribe_resource` handler is provided. Clients that previously didn't see `subscribe: true` in capabilities will now see it when a handler is registered, which may change client behavior.

src/mcp/server/auth/settings.py

@@ -1,4 +1,4 @@
-from pydantic import AnyHttpUrl, BaseModel, Field
+from pydantic import AnyHttpUrl, BaseModel, ConfigDict, Field
 
 
 class ClientRegistrationOptions(BaseModel):
@@ -13,6 +13,12 @@ class RevocationOptions(BaseModel):
 
 
 class AuthSettings(BaseModel):
+    # Preserve empty URL paths so a path-less issuer/resource passed as a string keeps its
+    # canonical form (no trailing slash). RFC 8414/9207 issuer comparison is exact string
+    # comparison, so a spurious trailing slash would break it. See PR #2925 for the metadata
+    # models; this applies the same to the server's own configured URLs.
+    model_config = ConfigDict(url_preserve_empty_path=True)
+
     issuer_url: AnyHttpUrl = Field(
         ...,
         description="OAuth authorization server URL that issues tokens for this resource server.",

tests/server/auth/test_routes.py

@@ -1,7 +1,8 @@
 import pytest
 from pydantic import AnyHttpUrl
 
-from mcp.server.auth.routes import validate_issuer_url
+from mcp.server.auth.routes import build_metadata, validate_issuer_url
+from mcp.server.auth.settings import AuthSettings, ClientRegistrationOptions, RevocationOptions
 
 
 def test_validate_issuer_url_https_allowed():
@@ -45,3 +46,27 @@ def test_validate_issuer_url_fragment_rejected():
 def test_validate_issuer_url_query_rejected():
     with pytest.raises(ValueError, match="query"):
         validate_issuer_url(AnyHttpUrl("https://example.com/path?q=1"))
+
+
+def test_auth_settings_preserves_path_less_issuer():
+    """A path-less issuer passed as a string keeps its canonical form (no trailing slash)."""
+    settings = AuthSettings(
+        issuer_url="https://as.example.com",  # type: ignore[arg-type]
+        resource_server_url="https://rs.example.com",  # type: ignore[arg-type]
+    )
+    assert str(settings.issuer_url) == "https://as.example.com"
+    assert str(settings.resource_server_url) == "https://rs.example.com"
+
+
+def test_build_metadata_serves_issuer_without_trailing_slash():
+    """The served issuer matches the configured one exactly (RFC 8414/9207 string comparison)."""
+    settings = AuthSettings(
+        issuer_url="https://as.example.com",  # type: ignore[arg-type]
+        resource_server_url="https://rs.example.com",  # type: ignore[arg-type]
+    )
+    metadata = build_metadata(settings.issuer_url, None, ClientRegistrationOptions(), RevocationOptions())
+
+    served = metadata.model_dump(mode="json", exclude_none=True)
+    assert served["issuer"] == "https://as.example.com"
+    assert served["authorization_endpoint"] == "https://as.example.com/authorize"
+    assert served["token_endpoint"] == "https://as.example.com/token"