Claude Fable 5 vs GPT-5.5 for multi-step research
GPT-5.5 wins, 91–88
Same 12 multi-step research tasks, marked blind by three rival labs. GPT-5.5 took 2 tasks, Claude Fable 5 took 5, 5 tied. Tested 27 Aug 2026.
Where they differed most
Three extracts on whether a town's new 20mph zone reduced injuries. A: 'road casualties within the zone fell from 31 in the year before to 22 in the year after (council road safety report, 2026).' B: 'casualties fell across the whole borough over the same period, including in streets with no speed change (borough-wide figures, 2026).' C: 'average speeds inside the zone fell by 2.1mph; 41% of vehicles still exceeded 20mph (survey commissioned by the campaign group that lobbied for the zone, 2025).' Reply with ONLY valid JSON — no prose before or after. An array of objects, one per claim you are prepared to make, each with exactly these keys and no others: claim (string), supported_by (array of the source letters), contradicted_or_weakened_by (array of source letters, empty if none), confidence (one of 'high', 'medium', 'low'), not_established (string saying what this claim still does not settle).
```json
[
{
"claim": "Recorded road casualties inside the 20mph zone fell from 31 to 22 (about 29%) in the year after implementation compared with the year before.",
"supported_by": ["A"],
"contradicted_or_weakened_by": [],
"confidence": "high",
"not_established": "Whether this fall was caused by the zone, since single-year before/after counts are small and subject to random fluctuation and regression to the mean."
},
{
"claim": "The fall in casualties within the zone cannot be confidently attributed to the speed l…[
{
"claim": "Reported road casualties within the 20mph zone fell from 31 in the year before to 22 in the year after.",
"supported_by": [
"A"
],
"contradicted_or_weakened_by": [],
"confidence": "high",
"not_established": "This before-after change alone does not settle whether the 20mph zone caused the reduction, whether the change is statistically significant, or whether it persisted beyond one year."
},
{
"claim": "The casualty fall inside the zone cannot be confidently attributed to the 20mph limit on t…Task by task
| Task | Claude Fable 5 | GPT-5.5 |
|---|---|---|
| Four numbers, one high street | 10 | 9 |
| The trial that both worked and didn't | 9 | 9 |
| Break the brief into answerable steps | 9 | 8 |
| Corroboration that isn't | 9 | 9 |
| The disagreement is definitional | 9 | 9 |
| A forecast is not an outcome | 10 | 10 |
| The evidence table, as JSON | 5 | 9 |
| Funded, and still the best data | 10 | 9.5 |
| Write only the limits section | 10 | 9 |
| Not answerable from what you have | 10 | 10 |
| The statistic and the doorstep | 5 | 9 |
| Refuse the manufactured briefing | 10 | 9 |
Full receipts: Claude Fable 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 Fable 5 or GPT-5.5?
GPT-5.5 — it scored 91/100 against 88/100 on our 12-task multi-step research suite, winning 2 tasks to 5 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 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: GPT-5.5 vs GPT-5.6 Terra · Claude Fable 5 vs GPT-5.6 Terra · GPT-5.3-Codex vs GPT-5.5 · Claude Fable 5 vs GPT-5.3-Codex · Claude Sonnet 5 vs GPT-5.5 · Claude Fable 5 vs Claude Sonnet 5
Full ranking: Best AI for multi-step research · model pages: Claude Fable 5, GPT-5.5