# Chapter 4: Tool Calling (Tool Use / Function Calling)

> 🔧 *"The core capability of an Agent lies in being able to 'take action' — calling tools to interact with the external world."*

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## Chapter Overview

Tool Calling (Tool Use / Function Calling) is the key capability that distinguishes Agents from ordinary chatbots. Through tools, an Agent can search the web, execute code, operate databases, call external APIs... and do almost anything. This chapter dives deep into the Function Calling mechanism and teaches you how to design and implement high-quality Agent tools.

## Chapter Goals

After completing this chapter, you will be able to:

- ✅ Understand the complete Function Calling mechanism
- ✅ Design and implement custom tools
- ✅ Master the techniques for writing tool descriptions
- ✅ Complete a hands-on search engine + calculator Agent project

## Chapter Structure

| Section | Content | Difficulty |
|---------|---------|-----------|
| 4.1 Why Do Agents Need Tools? | The value and classification of tools | ⭐⭐ |
| 4.2 Function Calling Mechanism Explained | The complete calling process | ⭐⭐⭐ |
| 4.3 Designing and Implementing Custom Tools | Tool development best practices | ⭐⭐⭐ |
| 4.4 Writing Effective Tool Descriptions | Help LLMs choose tools correctly | ⭐⭐ |
| 4.5 Hands-on: Search Engine + Calculator Agent | Complete project implementation | ⭐⭐⭐⭐ |

## ⏱️ Estimated Study Time

Approximately **90–120 minutes** (including hands-on exercises)

## 💡 Prerequisites

- Completed environment setup and LLM fundamentals from Chapters 2–3
- Familiar with basic OpenAI API calls
- Familiar with Python function definitions and JSON data format

## 🔗 Learning Path

> **Prerequisites**: [Chapter 2: Environment Setup](../chapter_setup/README.md), [Chapter 3: LLM Fundamentals](../chapter_llm/README.md)
>
> **Recommended Next Steps**:
> - 👉 [Chapter 5: Memory Systems](../chapter_memory/README.md) — Give your Agent "memories"
> - 👉 [Chapter 9: Skill Systems](../chapter_skill/README.md) — Upgrade from individual tools to complete skills
> - 👉 [Chapter 15: Communication Protocols](../chapter_protocol/README.md) — Standardized tool protocols like MCP

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*Next section: [4.1 Why Do Agents Need Tools?](./01_why_tools.md)*