OpenAI deprecates entire GPT-5.1 line — all conversations auto-routed to GPT-5.3/5.4

As of March 11, GPT-5.1 Instant, GPT-5.1 Thinking, and GPT-5.1 Pro are no longer available in ChatGPT. Existing conversations that used GPT-5.1 models are automatically continued on the corresponding current equivalent: GPT-5.1 Instant → GPT-5.3 Instant, GPT-5.1 Thinking → GPT-5.4 Thinking, GPT-5.1 Pro → GPT-5.4 Pro. The deprecation was implemented with no separate announcement — it appeared in ChatGPT's release notes.

Two data points emerge from this. First, the pace of model cycling is accelerating sharply. GPT-5.1 is being fully deprecated within weeks of GPT-5.4's release. The gap between "current" and "legacy" is now measured in weeks, not quarters.