OpenAI API Update Introduces GPT-5.1 as Default Reasoning Engine, New Codex Variants, and Assistants API Sunset Plan

OpenAI has restructured its API lineup with GPT-5.1 now serving as the default reasoning engine, a move that signals a strategic shift toward optimizing for logic-heavy and multi-step workflows. The update also introduces GPT-5.4 mini and nano Codex variants specifically designed for agentic and hig

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