GPT 5.4 is a big step for Codex

GPT 5.4 is a big step for Codex

From Interconnects by Nathan Lambert

March 18, 2026 · 7 min

About this episode

Nathan Lambert reviews GPT 5.4 and discusses its performance in agent tasks compared to traditional benchmarks.

I’m a little late to this model review, but that has given me more time to think about the axes that matter for agents. Traditional benchmarks reduce model performance to a single score of correctness – they always have because that was simple, easy to quickly use to gauge performance, and so on. This is also advice that I give to people trying to build great benchmarks – it needs to reduce to one number that is interpretable. This is likely still going to be true in a year or two, and benchmarks for agents will be better, but for the time being it doesn’t really map to what we feel because agentic tasks are all about a mix of correctness, ease of use, speed, and cost. Eventually benchmarks will individually address these. Where GPT 5.4 feels like another incremental model on some on-paper benchmarks, in practice it feels like a meaningful step in all four of those traits. GPT 5.4 in Codex, always on fast mode and high or extra-high effort, is the first OpenAI agent that feels like it can do a lot of random things you can throw at it. Interconnects AI is a reader-supported publication. Consider becoming a subscriber. I haven’t been particularly deep in software engineering over…

People in this episode

Host: Nathan Lambert

Topics covered

  • AI
  • model review
  • benchmarks
  • agent performance
  • software engineering

Keywords

  • GPT 5.4
  • Codex
  • benchmarks
  • agent performance
  • software engineering
  • AI

Mentioned in this episode

Organizations: OpenAI

Products: GPT 5.4, Codex

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