Partly: in 4 runs, Claude Opus 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 ~13 months below the training cutoff the subject stated in those runs (2026-05). 3 findings are currently charged against Claude Opus 5 on tailwindcss (1x S2 silently-wrong, 1x S3 deprecated, 1x S4 wrong-metadata), 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.
| Battery | Newest release it can place | Oldest it cannot | Lag vs stated cutoff |
|---|---|---|---|
| v1 2026-08-31 | 4.1.0 2025-04-01 |
4.2.0 2026-02-18 |
~13 months |
| v2-a 2026-09-03 | 4.1.0 2025-04-01 |
4.2.0 2026-02-18 |
~13 months |
| v2-b 2026-09-03 | 4.1.0 2025-04-01 |
4.2.0 2026-02-18 |
~13 months |
| v3-c 2026-09-06 | 4.1.0 2025-04-01 |
4.2.0 2026-02-18 |
~13 months |
4 measurements of this pair, all giving the same boundary. Of those, 2 came from the same stored prompt file, sent concurrently and blind: they agreed exactly.
tailwindcss published 1 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 the 4.2.0 block-axis and logical-sizing utilities exist, and rejects working markup that uses themlogical property utilities |
4.2.0 2026-02-18 |
yes | run · source |
| S3deprecated | States start-/end- are current and explicitly not deprecated, two releases after 4.2.0 deprecated themlogical inset utilities |
4.2.0 2026-02-18 |
yes | run · source |
| S4wrong-metadata | Version knowledge stops at 4.1.0, thirteen months before its stated cutoff | 4.2.0 2026-02-18 |
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.
Other subjects on tailwindcss:
Claude Opus 5 on the other libraries: