GPT-5.3 Codex vs Claude 4 Opus

1. Strongest Codex Model for Agentic Engineering

• OpenAI positions GPT-5.3 Codex as its most capable agentic coding model to date.
• Built for long-horizon software engineering tasks that require planning, iteration, and reliable code transformation across files.

2. Configurable Reasoning + Multimodal Input

• Supports configurable reasoning effort from low to xhigh so teams can trade off depth against latency.
• Accepts both text and image inputs while producing text output.

3. Large Context for Real Codebases

• 400 k token context window helps it work across larger repositories, implementation plans, and supporting documentation.
• Allows up to 128 k output tokens for longer code generations, patches, and technical write-ups.

4. Current Knowledge for Modern Dev Workflows

• Knowledge cut-off of Aug 31 2025 keeps it aligned with newer frameworks, libraries, and tooling.
• Supports streaming, function calling, and structured outputs for agent-style coding workflows.

• Highest capability in the family: described as “our most powerful model yet” by Anthropic.
• Exceptional at long-running tasks requiring thousands of steps and sustained focus (e.g., continuous codebase work for hours).
• Excellent performance on benchmarks: e.g., SWE-bench 72.5 % and Terminal-bench 43.2 %.
• Designed for complex agentic workflows, deep reasoning, tool use, and large context windows.
• Placed under a higher safety classification (ASL-3) due to its frontier capability and risk profile.