fix(mcp,auth): address greptile review concerns

- handle_sse_mcp now calls _raise_preemptive_401_for_unauthenticated_servers
  so SSE clients to pass-through OAuth MCP servers receive the RFC 9728
  401 + WWW-Authenticate challenge that the streamable-HTTP path already emits.
- get_request_route strips a trailing slash from root_path before length-based
  prefix removal so non-canonical ASGI root_path values like "/litellm/"
  don't strip the leading slash from the returned route.
- _mcp_oauth_user_api_key_auth's cookie JWT decode now passes
  options={"verify_aud": False} so a future revision of the UI session
  JWT containing an aud claim cannot silently downgrade the request to
  unauthenticated.

Co-authored-by: Claude <claude@anthropic.com>

litellm/proxy/_experimental/mcp_server/server.py

@@ -3025,6 +3025,15 @@ if MCP_AVAILABLE:
             verbose_logger.debug(
                 f"MCP server auth headers: {list(mcp_server_auth_headers.keys()) if mcp_server_auth_headers else None}"
             )
+            # https://datatracker.ietf.org/doc/html/rfc9728#name-www-authenticate-response
+            await _raise_preemptive_401_for_unauthenticated_servers(
+                scope=scope,
+                mcp_servers=mcp_servers,
+                oauth2_headers=oauth2_headers,
+                mcp_server_auth_headers=mcp_server_auth_headers,
+                user_api_key_auth=user_api_key_auth,
+                client_ip=_sse_client_ip,
+            )
             set_auth_context(
                 user_api_key_auth=user_api_key_auth,
                 mcp_auth_header=mcp_auth_header,

litellm/proxy/auth/auth_utils.py

@@ -506,13 +506,16 @@ def get_request_route(request: Request) -> str:
         if not isinstance(scope, dict):
             return str(request.url.path)
         raw_path: str = str(scope.get("path", request.url.path))
-        root_path: str = str(scope.get("app_root_path", scope.get("root_path", "")))
+        root_path: str = str(
+            scope.get("app_root_path", scope.get("root_path", ""))
+        ).rstrip("/")
         if not isinstance(raw_path, str):
             return str(request.url.path)
-        # Only strip root_path when it is a meaningful prefix (not bare "/").
-        # Stripping bare "/" would remove the leading slash from every path
-        # e.g. "/team/new" → "team/new", breaking route matching.
-        if root_path and root_path != "/" and raw_path.startswith(root_path):
+        # Only strip root_path when it is a meaningful prefix. Trailing
+        # slashes are stripped above so the result always keeps its leading
+        # "/" — stripping a bare "/" or "/prefix/" would otherwise produce
+        # paths like "team/new" and break route matching.
+        if root_path and raw_path.startswith(root_path):
             return raw_path[len(root_path) :]
         return raw_path
     except Exception as e:

litellm/proxy/management_endpoints/mcp_management_endpoints.py

@@ -1522,6 +1522,7 @@ if MCP_AVAILABLE:
                         token_cookie,
                         master_key,
                         algorithms=["HS256"],
+                        options={"verify_aud": False},
                     )
                     if decoded.get("login_method") in ("sso", "username_password"):
                         cookie_key = decoded.get("key", "")