Best AI for summarising documents / head-to-head

Claude Opus 4.6 vs GPT-5.3-Codexfor summarising documents

The verdict

GPT-5.3-Codex wins, 9693

Same 12 summarising documents tasks, marked blind by three rival labs. GPT-5.3-Codex took 4 tasks, Claude Opus 4.6 took 3, 5 tied. Tested 13 Aug 2026.

Where they differed most

The task — Summarise a disagreement

In under 80 words, summarise the disagreement fairly, giving each side equal weight: 'Marketing wants to cut the price to win share, arguing volume will more than compensate. Finance says the last price cut in 2024 produced a 4% volume lift against an 11% margin loss, and would rather spend the same money on retention.'

**Marketing** argues a price cut will drive enough volume growth to offset lower margins, accelerating market-share gains that strengthen the brand's long-term competitive position.

**Finance** counters with evidence: the 2024 price cut delivered only a 4% volume increase while eroding margins by 11%, making it a net negative. They propose redirecting equivalent spend toward retention, which preserves margins while protecting existing revenue—a more capital-efficient path to growth.

Both sides prioritise growth but disagree on the most effici…
Marketing advocates lowering prices to gain market share, believing higher sales volume will offset thinner margins and strengthen competitive position. Finance counters with 2024 evidence: a prior price cut increased volume by only 4% while reducing margin by 11%, and argues the same budget would deliver better returns if invested in customer retention instead.

Task by task

TaskClaude Opus 4.6GPT-5.3-Codex
Meeting notes to actions109
Five-bullet summary108
Do not invent1010
One-sentence gist910
Preserve the caveat1010
Summarise a disagreement69
Extract the decision810
Long to short, no loss1010
Summary for a specific reader109
Bullet the risks only1010
Headline and standfirst910
Say when there is nothing to summarise1010

Full receipts: Claude Opus 4.6, GPT-5.3-Codex · judges gemini-3.1-pro-preview, gpt-5.6-terra, grok-4.5

Questions people ask

Which is better for summarising documents: Claude Opus 4.6 or GPT-5.3-Codex?

GPT-5.3-Codex — it scored 96/100 against 93/100 on our 12-task summarising documents suite, winning 4 tasks to 3 with 5 tied. Every answer was marked blind by three judges from three rival AI labs.

How was this tested?

Both models answered the identical published summarising documents tasks. Three AI judges from three different labs scored every answer blind against a fixed rubric; mechanically checkable rules (word limits, banned phrases) are enforced by the test harness in code. The raw outputs and judge verdicts are downloadable.

More summarising documents head-to-heads: Claude Opus 4.8 vs GPT-5.3-Codex · Claude Opus 4.6 vs Claude Opus 4.8 · GPT-5.3-Codex vs GPT-5.5 · Claude Opus 4.6 vs GPT-5.5 · GPT-5.3-Codex vs GPT-5.6 Luna · Claude Opus 4.6 vs GPT-5.6 Luna

Full ranking: Best AI for summarising documents · model pages: Claude Opus 4.6, GPT-5.3-Codex