LLM Function Calling and Tool Use Guide 2026: OpenAI, Anthropic, Google
Function calling is the bridge between a language model’s text output and the real world. Instead of asking a model to guess what the weather is, you hand it a get_weather tool definition, and it decides when to call it, what arguments to pass, and how to incorporate the result. As of 2026, every major provider—OpenAI, Anthropic, and Google—supports this pattern, but the APIs look meaningfully different. This guide walks through each one with working Python code and covers parallel calls, agent loops, security, and how to pick the right approach. ...