If you have ever designed a REST endpoint, you already know most of what it takes to give a voice agent tools. Name it well, keep the parameters tight, validate inputs, return something useful. The difference is the client. Your endpoint is not called by a frontend developer reading your docs. It is called by a language model that is taking dictation from a stranger on a phone line, in real time, while that stranger waits in silence.

That one difference changes how I design every tool. I build production voice agents on Retell AI, with n8n and GoHighLevel behind them, and I went back through the nine client agent exports I keep in one folder to see what my tool definitions actually look like. They fall into two clear shapes. Comparing them taught me more than any guide I could have written up front.

What a custom function is, in API terms

In Retell, a custom function is four things: a name, a description, a JSON schema for the arguments, and a URL.…