GPT-5.3-Codex
GPT-5.3-Codex is OpenAI’s most advanced agentic coding model, combining the frontier software engineering performance of GPT-5.2-Codex with the broader reasoning and professional knowledge capabilities of GPT-5.2. It achieves state-of-the-art results on SWE-Bench Pro and strong performance on Terminal-Bench 2.0 and OSWorld-Verified, reflecting improved multi-language coding, terminal proficiency, and real-world computer-use skills. The model is optimized for long-running, tool-using workflows and supports interactive steering during execution, making it suitable for complex development tasks, debugging, deployment, and iterative product work. Beyond coding, GPT-5.3-Codex performs strongly on structured knowledge-work benchmarks such as GDPval, supporting tasks like document drafting, spreadsheet analysis, slide creation, and operational research across domains. It is trained with enhanced cybersecurity awareness, including vulnerability identification capabilities, and deployed with additional safeguards for high-risk use cases. Compared to prior Codex models, it is more token-efficient and approximately 25% faster, targeting professional end-to-end workflows that span reasoning, execution, and computer interaction.
Specifications
Modalities
Supported Parameters
Max Output Tokens
128,000Reasoning Configuration
- Default
- Thinking on (effort: medium)
- Selectable effort levels
- xhighhighmediumlownone
- Turning thinking off
- Possible (reasoning.enabled: false)
Frequently asked questions
- Is GPT-5.3-Codex available on FastMetal?
- Not at the moment. GPT-5.6 Sol, from the same lab, is available on the FastMetal API today.
- What is the context window of GPT-5.3-Codex?
- 400,000 tokens, shared between the prompt and the response.