What's the difference between an MCP and an API, and when should you use one versus the other?

MCP is used when you want to live and operate within an existing assistant environment like Claude or Codex — it lets that environment connect to and use another application's data or functions directly. An API is used when you want to build your own separate application or website on top of a company's platform (the example given was building a custom application on top of a product's API rather than just using it inside Claude). In short: if you want everything to happen within Claude, use the MCP; if you want to build your own standalone application/website with features on top of another platform, use the API.