# How Tool Use Works — Claude API / OpenAI API Basics

Source: https://www.geekswithgeeks.com/en/llm-apis/t-concept

> Follow the loop of request, tool call and result.

## The model never runs your code

A model cannot look up your orders or send an email by itself. With **tool use (function calling)** you describe functions (name, purpose and a JSON Schema for the arguments) in the request. If the model decides a tool would help, its reply is not final text but a **structured request**: "call `get_order_status` with `order_id = 481516`", and the stop reason says so (`tool_use` for Anthropic, `tool_calls` for OpenAI). **Your code** validates the arguments, runs the function, and sends the **result** back in the next request, together with the earlier messages. The model then continues, possibly asking for more tools, until it returns a normal answer. You own the loop, so you can enforce permissions, limits and logging.

## The model asks, your code acts

You describe tools; the model requests a call; your code runs it and sends back the result.

![Four steps: define, request, execute, return.](assets/figures/llm-apis/section-4-map.svg) — Figure 4.1 — Define, request, execute and return.

## The tool loop in one picture

Two API calls and one local function call answer one user question.

```text
user: "Where is order 481516?"
  -> API call 1 (with tool definitions)
  <- model: stop_reason = tool_use / tool_calls
         "call get_order_status(order_id='481516')"
  -> YOUR CODE validates args, runs get_order_status -> {"status": "shipped"}
  -> API call 2 (history + the tool result)
  <- model: "Your order 481516 has shipped."   (stop_reason = end_turn / stop)
```

## Several tools in one reply

A single model reply can request several tool calls. Return a result for every one before the next request.

**Quiz:** Who actually executes a tool call?

- [ ] The browser
- [ ] The model
- [ ] The provider's GPU
- [x] Your application code

*Answer:* Your application code. The model only requests the call; you decide whether and how to run it.