# Tool Use

> An LLM's ability to call external functions — search, code execution, APIs — by emitting a structured request the host executes.

_The Agentic Web Lexicon · /glossary/tool-use · [JSON](/api/glossary/tool-use) · [all The Agentic Web Lexicon](/glossary)_

- **term:** Tool Use
- **category:** core
- **short_def:** An LLM's ability to call external functions — search, code execution, APIs — by emitting a structured request the host executes.
- **long_def:** The model does not run the tool itself; it outputs a tool call, the host runs it and returns the result, and the model continues. Tool use (also called function calling) is what turns a chat model into an agent.
- **see_also:** ai-agent, mcp, agentic-loop
- **etymology_origin:** — (verify-against-primary-at-build)
- **related_to:** ai-agent, mcp, agentic-loop
- **contrast_with:** Unlike plain text generation, tool use emits a structured, machine-parseable call that the host actually executes — the model proposes the action; the host performs it.
- **example:** OpenAI shipped function calling for tool use in its API in June 2023; MCP later standardized how those tools are discovered and described across vendors.
- **source:** https://platform.openai.com/docs/guides/function-calling
- **status:** active
- **why_it_matters:** Tool use is the mechanism by which an agent acts on your site; exposing clean, well-described tools (e.g. via MCP) is what makes a site actionable to agents.
- **sameAs:** —
- **bridge_entity:** protocols/capability/mcp
- **last_verified:** 2026-06-15
- **md_twin:** /glossary/tool-use.md