OpenAI flipped the default model for ChatGPT on May 5, 2026. GPT-5.5 Instant replaced GPT-5.3. If you have workflows, integrations, or API calls pointing at chat-latest, your behaviour changed that day. Here is a practical breakdown of what is different, what the benchmark numbers actually mean, and the specific hallucination failure modes you need to account for before trusting this model in production. The API change you need to know about first The chat-latest alias now resolves to GPT-5.5 Instant. If you are using this alias in any production API call, you are already on the new model. GPT-5.3 Instant remains available as an explicit model ID for paid API users. OpenAI has confirmed a three-month transition window before it is retired. If you have a workflow that was tuned specifically to GPT-5.3's behaviour, you have until roughly early August 2026 before you must migrate.…