GPT-5.1 Codex-Max Arrives: A New Chapter for Developers

OpenAI today announced GPT-5.1 Codex-Max, the next-generation model built specifically for long-horizon software engineering, deep debugging and agentic workflows. With a leap in reasoning power, context-window scale and token-efficiency, Codex-Max aims to transform how engineers build, refactor and collaborate on codebases.

What sets Codex-Max apart is its ability to work coherently across multiple context windows: the model automatically “compacts” its session-history when it nears the context-limit, thereby sustaining workflows over millions of tokens. That means it can stay in the loop on a large repo, iteratively improve tests, debug, refactor and ship — in one continuous agentic session.

On real-world benchmarks, it improves significantly over its predecessor. For example, on the TerminalBench 2.0 test Corpus (n = 89) it moves from ~52.8% (GPT-5.1 Codex) to ~58.1% accuracy for Codex-Max. In the SWE-Lancer/IC-SWE category, performance rises from ~66.3% to ~79.9%. OpenAI reports that many of the gains stem not just from raw accuracy, but from using fewer thinking-tokens and fewer tool-calls.

In practical terms, this means faster results at lower cost, but with higher reliability and deeper collaboration. The model is optimized for agentic coding tasks: PR generation, test-driven development, code reviews, refactors, front-end engineering, and multi-hour loops. OpenAI explicitly recommends using Codex-Max for these “agentic coding” workflows — while general purpose models may still serve chat or casual query contexts.

For developers working on Windows environments, there’s another first: this is the first model from OpenAI trained to operate natively in Windows. Its training now includes agent-tasks in Windows terminals, making it more usable in traditional dev-workflows.

When you factor in the competitive and rapidly evolving landscape — with Gemini 3 Pro from Google pushing ahead in multimodal reasoning and agent-use cases — Codex-Max keeps OpenAI firmly in the arena of engineering-first models. Gemini 3 Pro is already making waves for reasoning and multimodal tasks.

For the just4o.chat platform, this is exciting: as soon as OpenAI makes Codex-Max available via API, we will support it immediately. You’ll be able to plug Codex-Max into your just4o.chat workspace alongside our existing model-routing, memory system and persona tools — with no hidden engine-switching, no downgrades, and direct control over the model you use.

In short: if you build software, debug, refactor or coordinate agents at scale — this is the model you’ve been waiting for. And at just4o.chat, we’re ready to bring it into your workflow the moment it’s released via API.

Stay tuned — full rollout details forthcoming.