Not all clients support that—currently limited to ChatGPT custom GPT actions. It’s not a standard. Fortunately, Anthropic, Google, and OpenAI have agreed to adopt MCP as a shared protocol to enable models to use tools. This protocol mainly exists to simplify things for those building LLM-powered clients like Claude, ChatGPT, Cursor, etc. If you want an LLM (through API calls) to interact with an your API, you can’t just hand it an API key and expect it to work—you need to build an Agent for that.