What is MCP resource?

Resources let a server share data that provides context to a language model. Each resource is identified by a URI. Typical examples are files, database schemas and other application-specific information. A resource definition carries the URI, a name, and optionally a title, description, MIME type and size. Content comes back as either text or base64-encoded binary data.

Clients find resources with resources/list and fetch them with resources/read. Servers can also publish resource templates, which use URI templates so a client can build addresses for a whole family of resources. A server may support subscriptions, so a client that has asked to watch specific URIs gets notified when one of them changes, and it can also announce when the overall list changes.

The specification describes resources as application-driven. The host decides how to bring them into the conversation: through a picker in the interface, through search and filtering, or through automatic inclusion based on heuristics or the model's own selection. That differs from tools, which the model controls, and prompts, which the user controls.

Security duties named in the spec include validating all resource URIs, checking permissions before operations, and sanitizing file paths to block directory traversal when serving file:// resources. If a requested resource does not exist, the server must return an error and not an empty contents array.