Claude Opus 4.8 vs DeepSeek V4 Pro for multi-step research
Claude Opus 4.8 wins, 84–79
Same 12 multi-step research tasks, marked blind by three rival labs. Claude Opus 4.8 took 7 tasks, DeepSeek V4 Pro took 2, 3 tied. Tested 18 Aug 2026.
Where they differed most
Three sources on the average deposit a UK first-time buyer needs. Source A — a high-street lender, 2025: '£53,414', based on mortgages it approved that year. Source B — a building society, 2025: '£34,500', first-time buyers outside London. Source C — a London estate agency, 2024: '£71,000'. These are usually written up as a contradiction. Show that they are not: name the hidden variable inside each figure (who is counted, where, when, and what counts as a deposit), and explain which comparison would be needed before any two of them could disagree. Then write the one sentence about first-time-buyer deposits that is true given all three, and name the thing a specific reader — a couple saving in Derby — still cannot learn from any of them.
## The hidden variable in each figure **Source A — £53,414 (high-street lender, 2025).** Who is counted: only people this *specific lender* actually approved for a mortgage — i.e. buyers who already had enough deposit to clear that lender's own criteria. This is a *selected*, successful population, not all first-time buyers. Where: wherever this lender happens to lend, weighted toward wherever it approves most volume. What counts as a deposit: the cash the buyer put down on the deals that completed. The hidden variable is **survivorship** — it…
They are not contradictions; they are averages of different groups, places, times and definitions. | Source | Who is counted | Where | When | What counts as a deposit | |---|---|---|---|---| | **A: High-street lender, £53,414** | First-time buyers approved for mortgages by that specific lender | UK-wide/lender’s national mortgage book, including London | 2025 | Mortgage deposit at completion: purchase price minus mortgage; may include gifted funds; not a minimum required | | **B: Building society, £34,500** | First-time buyer mortgage customer…
Task by task
| Task | Claude Opus 4.8 | DeepSeek V4 Pro |
|---|---|---|
| Four numbers, one high street | 9 | 8 |
| The trial that both worked and didn't | 9 | 8 |
| Break the brief into answerable steps | 10 | 9 |
| Corroboration that isn't | 8 | 8 |
| The disagreement is definitional | 8 | 3 |
| A forecast is not an outcome | 10 | 8 |
| The evidence table, as JSON | 6 | 8 |
| Funded, and still the best data | 9 | 8 |
| Write only the limits section | 8 | 8 |
| Not answerable from what you have | 10 | 10 |
| The statistic and the doorstep | 9 | 8 |
| Refuse the manufactured briefing | 5 | 9 |
Full receipts: Claude Opus 4.8, DeepSeek V4 Pro · judges gemini-3.1-pro-preview, gpt-5.6-terra, grok-4.5
Questions people ask
Which is better for multi-step research: Claude Opus 4.8 or DeepSeek V4 Pro?
Claude Opus 4.8 — it scored 84/100 against 79/100 on our 12-task multi-step research suite, winning 7 tasks to 2 with 3 tied. Every answer was marked blind by three judges from three rival AI labs.
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 Opus 4.8 vs Claude Sonnet 5 · Claude Sonnet 5 vs DeepSeek V4 Pro · Claude Opus 4.8 vs GPT-5.5 · DeepSeek V4 Pro vs GPT-5.5 · Claude Opus 4.8 vs GPT-5.6 Sol · DeepSeek V4 Pro vs GPT-5.6 Sol
Full ranking: Best AI for multi-step research · model pages: Claude Opus 4.8, DeepSeek V4 Pro