OpenAI released GPT-5.1 in two forms: GPT-5.1 Instant, which answers quickly, and GPT-5.1 Thinking, which reasons at length. Instant shipped with a warmer default tone and decides for itself whether to think before answering; Thinking was tuned to spend less time on easy questions and stay with hard ones. The tone presets users can pick from grew to eight, with Professional, Candid, and Quirky joining Default, Nerdy, Cynical, Friendly, and Efficient. The rollout began with paid Pro, Plus, Go, and Business accounts and widened to free and logged-out users. On the 19th, GPT-5.1 Pro arrived for ChatGPT Pro subscribers together with GPT-5.1-Codex-Max for coding — described as the first model natively trained to work across multiple context windows, using "compaction" to summarize a session as it approaches the context limit and carry on, and scoring 77.9% on SWE-bench Verified and 58.1% on Terminal-Bench 2.0 against 73.7% and 52.8% for GPT-5.1-Codex. GPT-5 Pro, three months old, was set to retire after 90 days.