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What is Function calling?

Updated May 2026

Function calling is when an AI model picks a function from a list, fills in the inputs from the conversation, and asks the runtime to execute it.

Same idea as tool calling. The AI doesn't run the function itself — it returns a structured request ('call function X with these arguments') and the host runtime executes it. The result then goes back to the model for the next turn.

Frequently asked questions

How is function calling implemented?

The model returns a structured tool_use block in its response. The runtime parses it, executes the named function with the provided arguments, and feeds the result back as a tool_result block.

Related terms

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