Tool
A function the model is allowed to call to affect the world.
An external capability exposed to the model: search the web, read a file, run a query, send an email. The model does not execute anything itself — it requests a call and the harness performs it.
Tools are the boundary between talking and doing. A model with no tools can only produce text; its entire ability to change the environment comes from the tools you expose.
Think of every tool as a contract between deterministic software and a non-deterministic caller. The agent might call it with the wrong arguments, at the wrong time, twice, or not at all. So a good tool validates its inputs, fails with a message a model can act on, and returns results that are cheap to read.
An error message written for a model
A search tool returns "Error 400" when the query is malformed.
The agent cannot act on that, so it retries the same thing. Returning "query must be a string shorter than 200 characters; you sent an array" lets it fix itself on the next step. Error messages are prompts in disguise.
Replacing twelve tools with one
A platform exposes twelve narrowly-scoped lookup tools.
The agent spends its reasoning choosing between them and picks wrong. Consolidating into one tool with a parameter, or exposing a searchable catalogue, reduces the action-selection problem — fewer, sharper tools usually beat many precise ones.
- blogWriting effective tools for AI agents — using AI agents ↗
Treats tools as contracts between deterministic software and a non-deterministic caller: clear boundaries, token-efficient results, evaluation against realistic tasks.
- paperToolformer: language models can teach themselves to use tools ↗
Self-supervised learning of when to call an API, which one, with what arguments, and how to use the result — tool use as a learned behaviour rather than a prompt trick.
- docsTools — Agents SDK ↗
The full tool taxonomy in one place: hosted tools, runtime tools, function tools, agents-as-tools, shell, computer use, and programmatic tool calling.
Tool Call
The model asking for a specific function with specific arguments.
Tool Result
What came back from the call, appended to context.
Function Calling · Tool use
The model-provider feature that makes structured tool calls reliable.
MCP · Model Context Protocol
An open protocol for plugging tools and data into any agent.
Harness · Scaffold
Everything around the model that makes it an agent.