What Do Tool Calls and Function Calling Cost?
Tool calling looks like a single feature but bills as several: the tool schemas you send, the arguments the model writes, and the extra turn to use the result. This guide unpacks where those tokens come from.
The schema tax
Every request that offers tools includes their definitions as input tokens. A rich toolset with verbose JSON schemas can quietly add hundreds of input tokens to each call, whether or not a tool is actually used.
The extra round trip
A tool call is rarely one request: the model emits arguments, your code runs the tool, then you send the result back for a final answer. That second turn re-sends context, so budget for at least two billed calls per tool use.
Calculators for this topic
Frequently asked questions
Do tool definitions cost tokens?+
Yes — tool and function schemas are sent as input on every call that offers them, even if no tool is invoked.
Why does one tool call cost two requests?+
The model first returns the tool arguments, then you send the tool result back for a final answer — two billed round trips.