Response field last_id 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 last_id on ChatCompletionMessageList reachable from 1 operation. 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 ChatCompletionMessageList objects, used by 1 OpenAI operation.

Before and after

Code that breaks

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

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