The token overhead of tool/function definitions on every call.
Tool definitions ride along as input tokens on each request. Ten verbose tools can add a thousand tokens per call you pay for constantly. Cache them — see the prompt caching calculator.
Free function-calling cost calculator — tool definitions add tokens to every request; see what that overhead costs.
Yes. The JSON schema for your tools is sent as input tokens on every request, whether or not a tool is used. With many tools or verbose descriptions, that overhead is significant across high volume.
Expose only the tools a call might need, keep descriptions terse, and cache the static tool block with prompt caching so you aren't billed full price to resend it each time.