Function Calling is fine-tuned to a certain output format, but it very often strays from that format. My function-calling-handling code has a mess of edge case handlers that catch when GPT-4 is calling functions incorrectly.

It’s not though, they even say it in their docs that sending a schema does not guarantee that the model will actually adhere to the scheme or even produce valid JSON

Surprisingly the function calling mechanism doesn't appear to use this trick - apparently it's still possible to get the wrong JSON structure back from it occasionally.