As of March 11, 2026, GPT-5.1 models are no longer available in ChatGPT.This applies to GPT-5.1 Instant, GPT-5.1 Thinking, and GPT-5.1 Pro. Existing conversations that used GPT-5.1 will automatically continue on the corresponding current model: GPT-5.3 Instant, GPT-5.4 Thinking, or GPT-5.4 Pro.