ChatGPT connector: api_tool.list_resources finds no tools or throws a ValidationError
Error: api_tool.list_resources returned no resources
What causes it
api_tool is ChatGPT's internal layer for apps and MCP connectors. Before it can run a Notion, Slack or GitHub action, ChatGPT calls api_tool.list_resources to discover what the connector exposes, then calls the chosen tool (older chats show api_tool.call_tool with a path like /Connector/link_<id>/tool). If discovery comes back empty, ChatGPT never reaches the real action. Reports on the OpenAI developer forum point to three causes: the connector is not enabled in the current chat, custom MCP connectors need Developer mode, and a tool input schema ChatGPT cannot parse (for example "ValidationError: Input should be 'object'"). We have not reproduced this inside ChatGPT ourselves. Our tests use a plain MCP client, so use this page to tell a ChatGPT-side discovery problem apart from a server problem.
How to fix it
- Start a new chat and enable the connector for that chat from the + menu. Connectors are added per chat, not globally.
- For a custom MCP server, turn on Developer mode under Settings > Apps & Connectors > Advanced, then create or re-create the connector.
- Name the app in your request ("Use Notion to search for ...") and try one read-only action first.
- If list_resources throws a ValidationError, make sure every tool's inputSchema is a JSON Schema object with type "object" at the top level. Forum users fixed it by removing optional() fields their schema library generated.
- After renaming or removing tools on the server, disconnect and reconnect the connector so ChatGPT refreshes its cached tool list.
- Rule out the server: open the connector's ConnectorReport page for its latest handshake reading from outside ChatGPT, or run tools/list in MCP Inspector. If tools list fine there, the problem is on the ChatGPT side.
Connector reports people check for this
We tested these outside ChatGPT, so their reports show whether the server itself answers.
Related fixes
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- MCP Inspector keeps failing after a bad OAuth attempt
- Notion MCP: the consent Continue button stays disabled
- GitHub MCP issue_write shows a form and creates nothing
- MCP error -32601 Method not found on resources/list
- Slack MCP search fails: needs keywords and natural_language_query
- GitHub MCP: "403 Resource not accessible by personal access token" on create_pull_request
- GitHub MCP sub_issue_write: "404 Not Found" when adding a sub-issue
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