GPT-5.3-Codex

A coding-focused model from OpenAI's GPT-5.3 generation with a 400k token context window, released after GPT-5.3 Chat as a specialized variant for software engineering tasks.

Strengths

Optimized for code generation, debugging, and working with large codebases across extended development sessions thanks to its large context window.

Best for

Long-form code work, multi-file refactoring, and maintaining context across substantial engineering projects.

Limitations

Positioned as a specialized coding model; use GPT-5.4 or later generations for frontier reasoning capability and broader professional workloads beyond software development.