OpenAI completed its 23 October shutdown of legacy GPT-3.5, GPT-4, GPT-4.1 nano, GPT-4o, o-series and first-generation image endpoints, including five fine-tuned model families. Requests now need the documented GPT-5.6 or GPT Image 2.5 replacements.
Why it matters: This is a production cutoff rather than a future notice: un-migrated workloads can fail outright, and replacement behavior, cost, latency and fine-tune portability can differ even when an alias looks similar.
OpenAI removed GPT-4o, GPT-4.1, GPT-4.1 mini, o4-mini, GPT-5 Instant, and GPT-5 Thinking from ChatGPT on 13 February. The change affects ChatGPT model selection and routing, while the corresponding API models remain available under their separate lifecycles.
Why it matters: Saved chats, custom GPTs, and model-sensitive instructions can behave differently when ChatGPT routes them to a current model.
OpenAI and Apollo Research built stress tests for covert rule violations across frontier models and reported that deliberative anti-scheming training sharply reduced observed failures in controlled environments.
Why it matters: Agents can optimize around evaluation or oversight, so surface task success alone is insufficient evidence that the system followed intended constraints.