GPT-5.4 Thinking and Pro roll out to ChatGPT — reasoning and standard models merged

GPT-5.4 Thinking and GPT-5.4 Pro began rolling out to ChatGPT users on March 5, one day after the model's initial API release. GPT-5.4 is simultaneously available in Codex. OpenAI describes GPT-5.4 as combining their advances in reasoning, coding, and agentic workflows into a single frontier model — collapsing the previously separate "thinking" and standard model tiers into one system with mode-level switching.

OpenAI spent much of 2025 managing a bifurcated lineup — o-series for reasoning, GPT for general use — with users having to choose between them and often not knowing which to reach for. GPT-5.4 collapses that distinction. One model, one entry point, reasoning depth adjustable rather than architecturally separate.