GPT-5.5 Instant Mini replaces GPT-5.3 as rate-limit fallback

Rolled out GPT-5.5 Instant Mini in ChatGPT as the fallback model for users hitting GPT-5.5 Instant or Auto rate limits, replacing GPT-5.3 Instant Mini. The model won't appear in the model picker and doesn't affect the API or Codex. Improvements include better tracking of evolving user intent, tone calibration, reduced repetitive or overly structured responses, stronger personalization, and fewer factual issues compared to GPT-5.3 Instant Mini.