Simon Willison • 11/20/2025

Building more with GPT-5.1-Codex-Max

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OpenAI has announced GPT-5.1-Codex-Max, a new model now the default in their Codex CLI coding agent. It is specifically designed for agentic coding tasks and features 'compaction,' a native training process allowing it to coherently work across millions of tokens, overcoming previous context-window limits for complex refactors and long agent loops. The article also covers its benchmark performance against competitors like Gemini 3 Pro and Claude Sonnet 4.5, and notes the broader rollout of GPT-5.1 Pro to ChatGPT users.

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#ai coding #Codex CLI #Gpt 51 Codex Max
Building more with GPT-5.1-Codex-Max

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