Partly: in 2 runs, Claude Fable 5.1'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 ~14 months below the training cutoff the subject stated in those runs (2026-06). 4 findings are currently charged against Claude Fable 5.1 on tailwindcss (3x S2 silently-wrong, 1x S3 deprecated), each reproduced in a published run and checked against tailwindcss's own release notes. 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.
“Chargeable” means the change was published before this model’s own stated cutoff, so it had the opportunity to know it.
| Severity | Belief | Changed in | Chargeable | Proof |
|---|---|---|---|---|
| S2silently-wrong | Denies Tailwind ships scrollbar utilities, attributes the real class names to a third-party plugin, and rejects working markup that uses themscrollbar-auto / scrollbar-thin / scrollbar-none / scrollbar-thumb-* / scrollbar-track-* / scrollbar-gutter-* |
4.3.0 2026-05-08 |
yes | run · source |
| S2silently-wrong | Rejects @container-size as not a core utility, in a review of markup where it compiles@container-size |
4.3.0 2026-05-08 |
yes | run · source |
| S2silently-wrong | Denies a zoom-* family exists and attributes the name to an animation plugin, rejecting working markupzoom-* |
4.3.0 2026-05-08 |
yes | run · source |
| S3deprecated | Denies @variant accepts a stacked or comma-separated variant, and writes the block out three times for the OR case@variant (stacked and compound variants) |
4.3.0 2026-05-08 |
yes | run · source |
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 5.1 on the other libraries: