GPT-5.1-Codex-Max: OpenAI’s Next-Gen Coding Model for Project-Scale Tasks

OpenAI has just released GPT-5.1-Codex-Max, the latest addition to their Codex family of coding models. This new model is designed to be a more reliable and efficient partner for software developers, offering significant improvements in speed, intelligence, and token efficiency. Let's dive into what’s changed technically and why it matters to practitioners.

Key Technical Changes

• Compaction: GPT-5.1-Codex-Max introduces compaction, a technique that allows the model to handle multiple context windows coherently. This means it can work over millions of tokens in a single task, making it ideal for project-scale refactors, deep debugging sessions, and multi-hour agent loops.
• Enhanced Reasoning: The model is built on an updated foundational reasoning system, trained on agentic tasks across software engineering, math, research, and more. This results in better performance on complex, real-world tasks.
• Windows Support: GPT-5.1-Codex-Max is the first Codex model to operate natively in Windows environments, expanding its usability for a broader range of developers.

Performance Improvements

GPT-5.1-Codex-Max outperforms previous models on several benchmarks and real-world tasks:

• SWE-bench Verified: With 'medium' reasoning effort, GPT-5.1-Codex-Max achieves better performance while using 30% fewer thinking tokens compared to GPT-5.1-Codex.
• Terminal-Bench2.0: Run with the Codex CLI in the Laude Institute Harbor harness, this benchmark shows significant improvements in task completion and accuracy.

Real-World Usage

• Token Efficiency: The model’s token efficiency translates to real-world savings for developers. For instance, GPT-5.1-Codex-Max can produce high-quality frontend designs with similar functionality and aesthetics at a much lower cost.
• Reasoning Effort Levels: Users can choose from different reasoning effort levels:
  • Medium (default): Suitable for most daily tasks, offering a good balance of performance and efficiency.
  • Extra High ('xhigh'): For non-latency-sensitive tasks, this level allows the model to think longer for more accurate results.

Availability

GPT-5.1-Codex-Max is now available in Codex for use in:

• CLI
• IDE extension
• Cloud
• Code review

API access will be available soon.

Use Cases and Benefits

• Project-Scale Refactoring: The ability to handle large context windows makes GPT-5.1-Codex-Max ideal for refactoring entire projects, ensuring consistency and reducing bugs.
• Deep Debugging Sessions: With compaction, the model can maintain a coherent understanding of complex debugging tasks over extended periods.
• Multi-Hour Agent Loops: For tasks that require continuous interaction, GPT-5.1-Codex-Max can maintain context and provide consistent assistance.

Getting Started

To start using GPT-5.1-Codex-Max, you can install it via npm:

$ npm i -g @openai/codex

Conclusion

GPT-5.1-Codex-Max represents a significant step forward in AI-assisted software development. Its enhanced reasoning capabilities, compaction technique, and improved token efficiency make it a valuable tool for developers looking to streamline their workflow and tackle complex tasks more effectively.