chat-completions-apilisted

# Chat Completions Compatibility Reference Chat Completions is the lingua franca of LLM serving — and because everyone has implemented and extended it longest, it carries the MOST undocumented divergence of the three open protocols. OpenAI keeps it fully supported but second-choice ("we recommend trying Responses") — and since 2026-07-29 OpenAI's own publications call it "our **legacy** Chat Completions API" (ARC-AGI-3 post; still no deprecation or sunset date); xAI, Groq, and Azure declare it legacy too. Legacy `/v1/completions` loses its last first-party OpenAI models **2026-09-28** and survives as a local/third-party surface. **Re-verified against the live deprecations page 2026-09-15 — date unchanged, and it is now 13 days out.** The four shutdowns that day, all replaced by `gpt-5.6-terra`: **`gpt-3.5-turbo-instruct`**, **`babbage-002`**, **`davinci-002`** (the three completions models) and `gpt-3.5-turbo-1106` (chat). **Their fine-tuned variants get longer**: `ft-babbage-002` and `ft-davinci-002` shut down **2026-10-23**, so a fleet running a fine-tune does not lose it on the 28th and should not be migrated on that assumption. > **This paragraph expires on a known date.** After 2026-09-28 the first-party > completions surface is gone and the tense here has to change; after 2026-10-23 > so does the fine-tune line. Re-read the deprecations page rather than editing > from memory — the page is JS-rendered, so fetch it and strip tags rather than > expecting a clean scrape.