Function Calling & Tool Use

Function calling (also known as tool use) is one of the most powerful features of modern LLMs. It lets the model decide when and how to invoke external functions — enabling your AI application to interact with APIs, databases, and real-world systems.

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What Is Function Calling?

Instead of generating a text response, the model can output a structured request to call a function you have defined. Your application then executes the function and returns the result to the model.

┌──────────┐     "What's the weather     ┌──────────┐
│          │      in London?"             │          │
│   User   │─────────────────────────────▶│   LLM    │
│          │                              │          │
└──────────┘                              └────┬─────┘
                                               │
                              Function call:   │
                              get_weather(     │
                                city="London"  │
                              )                ▼
                                          ┌──────────┐
                                          │   Your   │
                              Execute     │   Code   │
                              function    │          │
                                          └────┬─────┘
                                               │
                              Result:          │
                              {"temp": 15,     │
                               "condition":    │
                               "cloudy"}       ▼
                                          ┌──────────┐
                                          │   LLM    │
                              Generate    │(with     │
                              response    │ result)  │
                                          └────┬─────┘
                                               │
                              "It's 15°C and   │
                               cloudy in       │
                               London."        ▼
                                          ┌──────────┐
                                          │   User   │
                                          └──────────┘

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Defining Tools (OpenAI)

Tools are defined using JSON Schema: