Today · post October 12

Codex tops Claude Code as Fable 5.1's harness; it's one test

A test that investor Chamath Palihapitiya commissioned from 8090 says the same Claude Fable 5.1 solved 61% of tasks inside OpenAI's Codex and 41% inside Claude Code. Claude Code and Codex are both harnesses, the program wrapped around a model that calls tools and manages steps for it. Put the same model in a different harness and it solves different problems, so part of the score measures the harness. The test used 113 bug-fixing tasks, pairing 7 models with 5 harnesses each. Another model, GLM 5.3 Flash, showed a similar gap: 59% in its best harness and 24% in its worst (Chamath Palihapitiya, 2026-10-12; 8090, 2026-10-12). 8090's post says each combination ran only twice, so gaps within 9 percentage points may be noise. Fable 5.1's gap is 20 percentage points, larger than that range. This is a single test published by the people who commissioned it, and no third party has rerun it. Chamath has a business relationship with 8090. ⚠️ Disclosure: our analysis is produced with help from Anthropic's models, and Claude Code and Fable 5.1 are both Anthropic products. How we checked →

Concept illustration of two open laptops side by side on a beige tabletop in front of a large pale pink circle; each screen's top bezel is lettered "FABLE 5.1", and each screen shows a brick-orange speech bubble holding three white lines in the centre, with a right-hand column of grey horizontal lines. The left laptop's window has a dark frame with three small dots at the top left and a dark sidebar, and its base is lettered "CODEX"; the right laptop's window has a grey top bar with three tabs, and its base is lettered "CLAUDE CODE".

Source and checks

This item ran in the brief of Oct 13, 2026, alongside 13 other items. How we checked it, the quote checks and what was still missing that day are on that issue's How we checked page.

Cite this item

SecondSource (2026). "Codex tops Claude Code as Fable 5.1's harness; it's one test." https://secondsource.io/en/research/2026-10-13-1/. Accessed access date.