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Damien Gallagher
Damien Gallagher

Posted on • Originally published at buildrlab.com

GPT‑5.3‑Codex vs Claude Opus 4.6: benchmarks, cost, and workflow tradeoffs

#ai #openai #anthropic #claude

GPT‑5.3‑Codex vs Claude Opus 4.6 is basically the question every builder is asking right now:

  • Do I want the best coding agent (Codex-first, computer-use oriented)?
  • Or the biggest-context, multi-workflow model (office work + coding + research)?

Below is a straight comparison based on what OpenAI and Anthropic are claiming today, plus the practical workflow implications.

TL;DR

  • If you live in agentic coding loops: GPT‑5.3‑Codex is the most directly targeted upgrade (OpenAI says it’s 25% faster and sets new highs on SWE‑Bench Pro + Terminal‑Bench 2.0).
  • If you need maximum context + broad knowledge work: Opus 4.6 has the standout feature: 1M token context (beta) and strong claims across Terminal‑Bench 2.0, GDPval-AA, BrowseComp.

What OpenAI claims for GPT‑5.3‑Codex

OpenAI describes GPT‑5.3‑Codex as:

  • the most capable agentic coding model to date
  • 25% faster than GPT‑5.2‑Codex
  • designed for long-running tasks that involve research + tool use + complex execution
  • steerable while it works “without losing context”

Benchmarks OpenAI calls out:

  • SWE‑Bench Pro (multi-language, contamination-resistant)
  • Terminal‑Bench 2.0
  • strong performance on OSWorld and GDPval

Source: https://openai.com/index/introducing-gpt-5-3-codex/

What Anthropic claims for Claude Opus 4.6

Anthropic positions Opus 4.6 as an upgrade to Opus 4.5 with:

  • improved agentic coding (planning, long tasks, large codebases)
  • better code review + debugging
  • 1M token context window (beta) (first for Opus-class models)

Benchmarks Anthropic calls out:

  • highest score on Terminal‑Bench 2.0 (agentic coding)
  • leads on Humanity’s Last Exam
  • on GDPval-AA: +144 Elo over the “next-best model” (OpenAI’s GPT‑5.2) and +190 over Opus 4.5
  • performs best on BrowseComp (hard-to-find info online)

Source: https://www.anthropic.com/news/claude-opus-4-6

Cost / workflow tradeoffs (what matters in practice)

1) Speed vs context

  • GPT‑5.3‑Codex: if it’s truly 25% faster, that’s a compounding gain in iterative build/test loops.
  • Opus 4.6: 1M context changes how you structure agents: fewer summarisation steps, fewer retrieval hops, larger “working set”.

2) “Agentic coding” isn’t one thing

There are at least two different workloads:

  • terminal execution + patch + verify (Codex-style)
  • big-context reasoning + cross-artifact work (docs/spreadsheets/presentations + code)

OpenAI is leaning hard into the first. Anthropic is trying to be excellent at both.

3) Token efficiency matters (even if you don’t think about it)

OpenAI explicitly mentions fewer tokens than prior models in some benchmark context.
Anthropic highlights compaction + effort controls.

These are two different paths to the same outcome: more work per dollar.

A sane decision rule (BuildrLab style)

If you’re choosing today, don’t overfit to benchmarks.

Run both models on:
1) a repo-wide refactor
2) a bug hunt + terminal repro
3) an end-to-end feature (UI + API + tests)

Score them on:

  • time to first working PR
  • number of iterations
  • how often they break unrelated code
  • how easy it is to steer mid-task

Sources

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