# Tool

A single action, such as running a task or a function, that an agent can call.

Where a tool lives depends on where it can be reached, not who built it. A centrally hosted tool that others can use is part of the stack. A local tool scoped to one person's harness, like an advanced developer's local MCP server, is part of that harness. An agent's permission to use a tool is always harness. Building a central tool is a stack contribution; wiring your agent to it is harness configuration.

Today tools are mostly central and shared; local, harness-only tools are the exception. Watch: personal agents may start creating tools on the fly for one person or task, which would shift part of the tool layer into the harness.

Also called: Tool use, Tool calling, Function calling

Often confused with: Agent skill

Related: Model Context Protocol

Source: https://redfinchadvisory.com/lexicon/tool (version 1.0, changed 2026-10-21)