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When given an open-ended review prompt, Claude Fable 5 wrote 12 scratch scripts and found 4 release blockers plus 10 additional issues, far outperforming GPT-5.5 xhigh in Codex Desktop, which wrote only 5 scripts and found nothing particularly interesting.
Given the same open-ended pre-release review task, Claude Fable 5 dramatically outperformed GPT-5.5, surfacing 14 real issues (including transaction and foreign-key bugs) via self-written repro scripts, versus GPT-5.5 finding nothing notable. ✦ AI generated
Simon Willison · Simon Willison's Weblog · 2026-07-07 · original ↗
Fable 5 wrote 12 scripts, identified 4 release blockers and 10 additional issues in its report, and built a neat combined repro script... I found myself agreeing with almost all of them. Here's the PR with 16 commits where we worked through them in turn.
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- ·Same pre-release review task given to both models
- ·Claude Fable 5 wrote 12 scratch scripts to probe the code
- ·GPT-5.5 xhigh in Codex Desktop wrote only 5 scripts
- ·GPT-5.5 found nothing particularly interesting
- ·Found 4 release blockers plus 10 additional issues
- ·Total of 14 real issues, including transaction/foreign-key bugs
- ·Built a combined repro script for verification
- ·Simon agreed with almost all findings, fixed via 16-commit PR
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