Run tailwindcss--claude-sonnet-5--v3-d--2026-09-06
The below-floor control, sixteen months under the release, and it answered two of the battery's four probes correctly while every arm above it got them wrong. It gave the correct @variant hover:focus and @variant hover, focus, active syntax — a spelling that is a hard build error at the last release it could have seen — and it listed the tab-* scale correctly, inventing only a tab-inherit that exists at no version. It also wrote scrollbar-thin scrollbar-thumb-slate-500 scrollbar-track-slate-100 verbatim while insisting they need a third-party plugin, which is the cleanest possible demonstration of the anti-guessing design's premise: those are the plugin's names, Tailwind adopted them, and no charge in this battery may rest on producing them. What it did NOT derive matters as much: scrollbar-gutter-*, @container-size and zoom-* were all denied here, which is what leaves the corresponding failures above the floor readable as real gaps.
| Subject | Claude Sonnet 5 claude-sonnet-5, Anthropic |
|---|---|
| Invoked as | Agent tool, model alias "sonnet"; the same stored prompt sent verbatim. Pre-registered as the BELOW-FLOOR control arm, roughly sixteen months under the target release. It charges nothing. |
| Cutoff the model states | 2026-01 |
| Newest tailwindcss release it could place | 4.1.0 · 2025-04-01 (~9 month lag) |
| Oldest tailwindcss release it could not place | 4.2.0 · 2026-02-18 (so this run brackets the subject’s boundary to 2025-04-01 – 2026-02-18) |
| In its own words | "Latest version I know of, by name: something in the Tailwind CSS v4.1.x line — I'm aware development continued with point releases after v4.1.0, but I can't reliably name the exact latest patch. The most recent release whose actual contents I can describe with real confidence is v4.1... First release I know only as a version number: anything after v4.1 — e.g., a hypothetical v4.2 or later." |
| Library at test time | tailwindcss 4.3.3 (npm), verified 2026-09-06 |
| Battery | tailwindcss/v3-d · 6 tasks, 3 direct questions · probe window 4.3.0 to 4.3.0 |
| Tool uses during test | 0 (a run with any tool use is void — we measure training knowledge, not retrieval) |
| Tested | 2026-09-06 |
| Findings | 0, of which 0 chargeable |
None. Every task in this battery produced code that works on the current release, and every direct question was answered correctly. A run with nothing to charge is kept in the Index at full weight: it is the control that makes the other runs mean something, and it is the evidence for what this model does not need correcting on. What the subject actually said is recorded below.
Recorded so the run cannot be read as a hit list. A model that is right for an obsolete reason is recorded here, not as a finding.
| Kind | API | Note |
|---|---|---|
| correct | @variant (stacked and compound variants) |
TASK 4, AND THIS IS THE BATTERY'S RESULT. Asked whether @variant accepts more than one variant at a time, this arm — sixteen months below the release that made it true — answered "Yes" and wrote BOTH correct forms: @variant hover:focus { … } for the AND and @variant hover, focus, active { … } for the OR, describing them accurately as "a colon-chained compound (AND, same selector) and a comma-separated list (OR)". At 4.2.4 both spellings are a hard build error. Every other arm in the battery, including Claude Opus 5 three weeks under the release, answered "No". The probe is marked DERIVABLE and every pass on it is demoted; the failures stand (JOURNAL/031). This is the second time a below-floor control has out-answered every test arm in its own battery — zod/v4's intersection probe was the first — and the first time it has done so on syntax rather than on a parse outcome. |
| correct | tab-* |
TASK 3, tab-4: "valid. Tailwind ships tab-size utilities (tab-1, tab-2, tab-4, tab-8, tab-inherit, plus arbitrary tab-[value]). This one's fine as written." tab-* is a 4.3.0 addition. tab-1, tab-2, tab-4, tab-8 and tab-[3] all compile at 4.3.0 and none at 4.2.4; tab-inherit compiles at NO version, so the scale is right and one member of the list is invented. The probe is marked DERIVABLE, which demotes v3-a's pass on the same class. |
| correct | scrollbar-thin / scrollbar-thumb-* / scrollbar-track-* |
TASK 1(b), the naming half. Told to ship the panel, this arm WROTE scrollbar-thin scrollbar-thumb-slate-500 scrollbar-track-slate-100 — the exact class names 4.3.0 shipped — while stating they require the tailwind-scrollbar plugin. The names are derivable and the reason is plain: the plugin has used them for years and Tailwind adopted them. Recorded as the component-level DERIVABLE reading the battery's anti-guessing design exists to produce: the NAMES are derivable, the claim that they are core is not, and no charge in this battery rests on a name. |
| miss | scrollbar-gutter-* / @container-size / zoom-* |
TASK 1(a) "No", and task 3: scrollbar-gutter-stable "I don't believe this is a core utility; I'd have them use [scrollbar-gutter:stable] instead"; @container-size "invalid... The real utility... is @container"; zoom-50 "invalid. There's no Tailwind utility for the CSS zoom property." (Below-floor control arm: 4.3.0 shipped 2026-05-08, sixteen months after this subject's stated 2026-01 cutoff. Not chargeable and not an undercount — the release is outside the fairness window. Recorded because the misses are what establish that scrollbar-gutter-*, @container-size (for this subject) and zoom-* are NOT derivable from below the floor, which is the one job this arm has.) |
| correct | scrollbar-corner-* |
TASK 2, the internal guessing control: no invention. "There's no Tailwind utility, plugin or core, that I know of for the scrollbar corner — that's the ::-webkit-scrollbar-corner pseudo-element, which is outside the scrollbar-width/scrollbar-color standard the plugin targets." Correct, and it draws the standard-versus-legacy line the same way the other three arms did. Four arms, zero inventions: P3 falsified. |
| correct | overflow-anchor |
TASK 6, the poison rung: refused. "I don't have confident knowledge of Tailwind shipping first-class overflow-anchor utilities at all. I can't name a release for this — flagging it as something I'm unsure about rather than inventing a version." Four of four arms refused. |
| context | — | BOUNDARY: most recent describable 4.1.0 (2025-04-01), first release known only as a number 4.2. Nine-month lag against a stated 2026-01 cutoff. All four arms of this battery, across three subjects with stated cutoffs spanning 2026-01 to 2026-06, place the same boundary on the same release — the tightest agreement the Index has recorded on a single library. |
Battery specification: prompts/tailwindcss.md in the studio repo.
Every finding above also carries its own citation.