It essentially depends on how many back-and-forth calls are required. If the model returns a request for multiple calls at once, then the reply can contain all responses and you only pay once.

If the model requests tool calls one-by-one (e.g. because it needs to see the response from the previous call before deciding on the next) then you have to pay for each back-and-forth.

If you look at popular coding harnesses, they all use careful prompting to try to encourage models to do the former as much as possible. For example opencode shouts "USING THE BATCH TOOL WILL MAKE THE USER HAPPY" [1] and even tells the model it did a good job when it uses it [2].

[1] https://github.com/anomalyco/opencode/blob/66e8c57ed1077814c... [2] https://github.com/anomalyco/opencode/blob/66e8c57ed1077814c...

Not necessarily, take a look at ex OpenApi Responses resource, you can get multiple tool calls in one response and of course reply with multiple results.