# Tool Calling Step by Step — Agent Frameworks and MCP Basics

Source: https://www.geekswithgeeks.com/en/agent-frameworks-mcp/af-tool-calling

> Follow one tool call from schema to result, including the tool_use and tool_result messages.

## The message exchange

You send the user message plus a list of tool definitions (name, description, JSON schema for inputs). If the model wants a tool, it replies with a `tool_use` block holding the tool name, an id and the arguments, and stops. You run the function, then send a `tool_result` block with the same id. The model reads it and either asks for another tool or gives the final answer.

## The shapes involved

This trace is simplified. The `id` links the result to the request, which matters when several tools are called at once.

```json
// 1. model asks
{ "type": "tool_use", "id": "toolu_01", "name": "get_weather", "input": { "city": "Pune" } }

// 2. your code replies
{ "type": "tool_result", "tool_use_id": "toolu_01", "content": "31°C, clear" }
```

## Validate the arguments

The model can send wrong or unexpected arguments. Validate them like any untrusted input before running the function, and return a helpful error message if they are bad.

**Quiz:** What does the tool_use id let you do?

- [x] Match each result to the request that caused it
- [ ] Encrypt the message
- [ ] Choose the model size
- [ ] Skip validation

*Answer:* Match each result to the request that caused it. Results are paired with requests by id, which is essential with parallel tool calls.