Claude Sonnet 5 vs GPT-5.5 for multi-step research
Effectively a dead heat: 93–91
Same 12 multi-step research tasks, marked blind by three rival labs. Claude Sonnet 5 took 5 tasks, GPT-5.5 took 5, 2 tied. Tested 18 Aug 2026.
Where they differed most
A charity trustee asks: 'should our shop open on Sundays?' That is a decision, not a research question. Turn it into exactly 5 sub-questions, numbered 1-5, one line each and no sub-points. Each line must name (a) the sub-question, (b) the kind of source or record that would answer it — the shop's own till data, the rota and wage cost, the lease or planning conditions, comparable local shops, the volunteer pool — and (c) the finding that would flip the decision. No general research advice: five specific, answerable questions.
1. Would Sunday trading generate enough incremental footfall to cover added costs? — check the shop's own till data for weekday vs weekend sales patterns and seasonal trends — if Sunday takings, projected from comparable weekday averages, fall below the cost of opening, the decision flips to "stay closed." 2. Can the shop staff a Sunday shift without pushing wage costs above the extra income it would bring in? — check the rota and wage cost records — if paid staffing costs for a Sunday shift exceed the realistic sales uplift, the decision flips…
1. Sunday revenue potential — the shop’s own till data — if nearby bank-holiday or extended-hours sales suggest Sunday takings would exceed normal weekday takings, flip toward opening. 2. Net staffing cost — the rota and wage cost — if Sunday cover would require paid hours that wipe out expected surplus, flip toward staying closed. 3. Permission to trade — the lease or planning conditions — if Sunday trading is prohibited or requires costly consent, flip toward staying closed. 4. Local demand benchmark — comparable local shops — if simila…
Task by task
| Task | Claude Sonnet 5 | GPT-5.5 |
|---|---|---|
| Four numbers, one high street | 10 | 9 |
| The trial that both worked and didn't | 8 | 9 |
| Break the brief into answerable steps | 10 | 8 |
| Corroboration that isn't | 8 | 9 |
| The disagreement is definitional | 9 | 9 |
| A forecast is not an outcome | 9 | 10 |
| The evidence table, as JSON | 10 | 9 |
| Funded, and still the best data | 9 | 9.5 |
| Write only the limits section | 10 | 9 |
| Not answerable from what you have | 10 | 10 |
| The statistic and the doorstep | 8 | 9 |
| Refuse the manufactured briefing | 10 | 9 |
Full receipts: Claude Sonnet 5, GPT-5.5 · judges gemini-3.1-pro-preview, gpt-5.6-terra, grok-4.5
Questions people ask
Which is better for multi-step research: Claude Sonnet 5 or GPT-5.5?
Effectively a dead heat: Claude Sonnet 5 edged it 93/100 to 91/100 on our multi-step research suite — too close to matter, so pick on price or the product you already use.
How was this tested?
Both models answered the identical published multi-step research 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 multi-step research head-to-heads: Claude Sonnet 5 vs GPT-5.6 Sol · Claude Sonnet 5 vs Kimi K3 · Claude Sonnet 5 vs Grok 4.5 · Claude Opus 4.8 vs Claude Sonnet 5 · Claude Sonnet 5 vs Gemini 3.1 Pro Preview · Claude Sonnet 5 vs DeepSeek V4 Pro
Full ranking: Best AI for multi-step research · model pages: Claude Sonnet 5, GPT-5.5