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Everyone's telling me Codex 5.3 is better at coding than Opus 4.6, but my experience has been different. So here are my first impressions with both models in 3 different codebases: TypeScript, Rust, and Java.

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ForrestKnight

Claude Opus 4.6 and GPT-5.3 Codex were tested head-to-head across TypeScript, Rust, and Java codebases. Opus 4.6 generally produced better, more reliable code with cleaner first-try compilation, though it's slower. Codex 5.3 is faster and cheaper but sometimes requires iteration to fix errors. Both represent incremental improvements over predecessors, reducing back-and-forth iterations. Opus 4.6 handles larger codebases better with its 1M token context window, while Codex 5.3 excels at speed and cost-effectiveness. Real-world testing showed Opus solving bugs that previous versions couldn't, with more thoughtful implementation choices.

Coding with Opus 4.6 and Codex 5.3 is actually insane