Tools
Definition
Tools are the discrete capabilities a model can invoke to act on the world. A model on its own can only emit text; tools turn text emissions into real effects — reading files, running shell commands, editing code, searching the web, querying databases, calling APIs. Common examples in coding agents include bash, read, edit, grep, web_search, web_fetch, and any MCP server exposed to the runtime.
Mechanically, tool use works via function calling. The harness advertises a set of tool schemas (name, description, input parameters) in the model's context. When the model decides a tool is needed, it emits a structured tool call; the harness executes it, captures the result, and feeds that result back as a new turn. The model then either calls another tool or produces a final response. This repeating cycle is the agentic loop.
Tools are where agents meet reality. Their design determines how capable, safe, and efficient an agent can be: too few tools and the model is blind; too many and its context fills with schemas it will never use. Permission scoping on tools — which tools a given agent may call, against which resources — is one of the main safety levers in any agentic system.
Tools can be built in to a harness, loaded from local plugins, or exposed over Model Context Protocol (MCP) by external servers, which is what makes the MCP ecosystem important for composable agents.