Function Calling — Structured Tool Use

Tool schema

{
  "name": "get_weather",
  "description": "Get current weather for a city",
  "input_schema": {
    "type": "object",
    "properties": {
      "city": {"type": "string"}
    },
    "required": ["city"]
  }
}

Best practices

Descriptions matter — model uses them to pick tool. Enum types for restricted choices. Provide examples in system prompt for complex tools.

Tool selection failures

Wrong tool chosen. Args hallucinated. Mitigate: fewer tools (<20), clear descriptions, similar-named tools disambiguated.

Parallel tool calls

Modern APIs support batch calls per turn. Use when tools are independent (2 lookups, 2 API calls). Reduces round trips.