I just found the answer here: https://platform.openai.com/docs/api-reference/chat/create
functions have been deprecated, and tools are the way forward.
Tools:
A list of tools the model may call. Currently, only functions are supported as a tool. Use this to provide a list of functions the model may generate JSON inputs for.
In the response object there is also a new finish_reason “tool_calls”, which also mentions the deprecated function_call.
It seems that most cookbooks have been updated, but the guides haven’t yet.