I have created a tool with an express endpoint as a server url (well, using ngrok for local development). The tool's purpose is simple - to save a message left by the caller. It has a single property "clientMessage". In the payload that is received there is a lot of data and to access the property I have to drill down with

req.body.message.toolCalls[0].function

This has the correct message:

{
  name: 'saveClientMessage',
  arguments: { clientMessage: "David, I'll meet you tomorrow at 9." }
}

My question is - is the intended approach for accessing arguments sent in tool functions? As I mentioned, there is a lot of data in the payload and I couldn't see anywhere in the docs which described the shape of the payload and what is the intended way to access values of properties which come with tool calls.

Yes, accessing the tool function arguments through req.body.message.toolCalls\[0\].function is the correct approach for Vapi tool function calls. This path follows Vapi's payload structure for custom tool calls sent to your server URL