fix: resolve MCP OAuth authorize_url discovery and fail clearly (#26647) (#26654)

backend/open_webui/routers/configs.py

@@ -205,7 +205,7 @@ async def register_oauth_client(
         log.debug(f'Failed to register OAuth client: {e}')
         raise HTTPException(
             status_code=400,
-            detail=f'Failed to register OAuth client',
+            detail=f'Failed to register OAuth client: {e}',
         )