Partly: in 2 runs, Claude Fable 5's tailwindcss attribution reaches 4.1.0 (2025-04-01), inside tailwindcss 4, and stops there — tailwindcss was at 4.3.3 when this index last verified against it (2026-09-06). That is ~9 months below the training cutoff the subject stated in those runs (2026-01). No finding is currently charged against Claude Fable 5 on tailwindcss: every probe in the 2 runs published here passed, which is a result rather than an absence of testing. Read the boundary precisely: it is the newest release whose contents the model can correctly attribute to that release, not the newest tailwindcss feature it can use. Past it a model often writes working code with a newer API while naming the wrong release for it.
Answer class inside, decided by one rule applied to every page
of this kind: every measured boundary is at or above the first release of the major named in the question. Every figure below is read from
the dataset at build time; nothing on this page is written by hand.
2 measurements of this pair, all giving the same boundary. No two of them were drawn from an identical prompt, so the spread mixes prompt differences with draw variance.
tailwindcss published 2 minor or major releases in the twelve months before this model’s stated cutoff, which is the scale a spread should be read against.
Nothing is charged. Every probe in the runs below passed, which this index publishes the same way it publishes a failure.
The tailwindcss correction pack states what is true now for each corrected fact, with a primary-source citation, as markdown you can paste into a rules file (raw). What a correction pack measurably changed when one was tested — a pre-registered run on zod — is on the benchmark page, including where it changed nothing.
All of them at once: Which Claude model knows tailwindcss 4 best?
Other subjects on tailwindcss:
Claude Fable 5 on the other libraries: