gpt-5.4 is really, really good

GPT-5.4 (released as '5.4 Thinking') is reviewed after a week of hands-on use. Key highlights: 1M token context window, improved reasoning token efficiency, better mid-task steering, and significantly improved browser/computer use and vision capabilities. The model is praised as the best general-purpose AI for coding tasks, with Cursor internally endorsing it. However, it still lags behind Claude Opus and Gemini for front-end UI design. A notable security regression exists: prompt injection via function call return data succeeds ~2% of the time. GPT-5.4 Pro is expensive ($30/$180 per million tokens in/out) and often underperforms standard 5.4. The Codex model line appears to be discontinued in favor of 5.4 as the unified base. Prompting guidance from OpenAI is highlighted as more important than ever given the model's high steerability.