---
published: true
id: tool
title: Tool
description: Knowledge captured in João Lopes' learning graph.
tags:
- ai-engineering
- agent-design
locale: en
resource: https://joaolopes.dev.br/knowledge/concepts/en/tool/
created: '2026-03-28'
---

# Tool
An action the harness can execute when the model asks: read a file, run a command, search for something.

## Definition
Each tool has a name and a description that the model receives in the context. When the model needs something done, it produces a tool call in its response. The harness reads that tool call, executes the action, and puts the result back in the context. Tools can be built-in (shipped with the agent, such as Read, Bash, Edit, Grep) or external (connected via MCP).

## Source
- [Tool use with Claude - Anthropic Docs](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview)

## Related
- [Tool Call](/concepts/en/tool-call.md)
- [Runtime](/concepts/en/runtime.md)
- [Skills](/concepts/en/skills.md)