MCP and AI basics
Tool
A single action an MCP server offers, such as 'run a report' or 'create a payment link'. The model chooses when to call a tool, and the host usually asks the user to approve sensitive calls.
A tool is one action an MCP server offers to the model. 'List accessible accounts', 'run a GA4 report', 'create a payment link' and 'update a cart' are all tools. Each has a name, a description and an input schema that says which parameters it takes. The model reads those descriptions, decides which tool fits the question and fills in the parameters.
Tools are where risk lives. A tool that only reads data can at worst return a wrong or incomplete answer. A tool that writes can change spend, prices, inventory or customer records. Vendors mark this in different ways: a separate read-only mode, a restricted key, or annotations on the tool itself. The directory records each server's access level so you can see it before connecting.
Good tools return focused results rather than whole data dumps, because everything a tool returns takes up the model's context window. When an assistant seems to lose track in a long analysis, the volume of tool output is often the reason.