Why are MCP resources called app-controlled and when should you use them?

Resources are controlled by your application code. Your app decides when to fetch resource data and how to use it, typically for UI elements or to add context to conversations. In a document management example, resources can be used in two ways: fetching data to populate autocomplete options in the UI, and retrieving content to augment prompts with additional context. Think of the Add from Google Drive feature in Claude's interface. The application code determines which documents to show and handles injecting their content into the chat context. The model is not deciding to fetch the data, the application is.