GPT-3.5 Turbo Instruct

The last model on the completions endpoint: no roles, no messages, just text in and text out, the interface the whole field started with. It was shipped as a bridge for developers who had not moved to chat yet, and it has outlived nearly every model that was meant to replace it. Its grave is dug and waiting.

Hand it half a sentence and it finished the sentence, rather than answering a question about it.