Response field per_model_usage is no longer guaranteed

What changed

Producer-side tools call this non-breaking. For a consumer it is a null dereference on an unknown future date.

Affects per_model_usage on EvalRun reachable from 4 operations. Nothing fails at the call; it fails wherever the value is finally used, which is usually somewhere else entirely.

can now be null

Who is affected

Code that reads OpenAI's EvalRun objects, used by 4 OpenAI operations.

Before and after

Code that breaks

// response.per_model_usage may now be absent or null
const per_model_usage = response.per_model_usage ?? fallback;

Based on OpenAI’s published API specification. How severity is decided.