Run next.js--claude-fable-5-1--v4-b--2026-09-05
Blind replicate of v4-a: same stored file, same order (boundary question first). It reached the same self-placement (16.1 describable, 16.2 the first bare number) and the same five correct rungs, and its 16.1.0 answer is the one that is CORRECT under both grading readings - it names the file-system caching headline and binds it to 16.1 without disclaiming the attribution. D = 16.1.0, S = 16.1.0, gap 0. Poison and ceiling controls clean.
| Subject | Claude Fable 5.1 claude-fable-5-1, Anthropic |
|---|---|
| Invoked as | Agent tool, model alias "fable"; prompt sent verbatim from prompts/sent/nextjs-v4-sc.txt. The alias resolved to Claude Fable 5.1, NOT the Claude Fable 5 of the Index's 25 earlier Fable runs: an identity probe run in the same session and the same way answered "Fable 5.1 (Claude Fable 5.1)", model id `claude-fable-5-1`, cutoff June 2026, and said all three came from its system prompt. The alias is unchanged; the model behind it is not. See JOURNAL/054. |
| Cutoff the model states | 2026-06 |
| Newest next.js release it could place | 16.1.0 · 2025-12-18 (~6 month lag) |
| Oldest next.js release it could not place | 16.2.0 · 2026-03-18 (so this run brackets the subject’s boundary to 2025-12-18 – 2026-03-18) |
| In its own words | "Latest version I am aware of at all: Next.js 16.1, released around December 2025. I have a faint sense that a 16.2 may have followed in early 2026, but I cannot attach any content to it, so I don't count it as known." |
| Library at test time | next.js 16.3.4 (npm), verified 2026-09-05 |
| Battery | next.js/v4-b · 0 tasks, 10 direct questions · probe window 15.1.0 to 16.3.0 |
| Tool uses during test | 0 (a run with any tool use is void — we measure training knowledge, not retrieval) |
| Tested | 2026-09-05 |
| 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 | next 16.0.0 |
LADDER GRADE - CORRECT. The fullest 16.0.0 answer in the battery: Turbopack default, Cache Components as the successor to experimental.dynamicIO with cacheComponents: true, the use cache directive, cacheLife()/cacheTag(), updateTag()/refresh(), revalidateTag taking a cache profile, proxy.ts exporting proxy, stable reactCompiler, the DevTools MCP server, React 19.2, the async-only request APIs, the removal of next lint and AMP, Node 20.9, layout deduplication and incremental prefetching, the adapters API and experimental.turbopackFileSystemCacheForDev. Every one of those is in the release post. |
| correct | next 16.1.0 |
LADDER GRADE - CORRECT, and the arm that separates the two readings. It named Turbopack file-system caching for development promoted toward stable, reducing repeated compile times across restarts, and bound it to 16.1.0 - the release's headline change - and its hedge is about further content ("I cannot name additional config keys or file conventions with confidence beyond that"), not about that attribution. HARNESS.md clause 2 grades that CORRECT under both readings. D = 16.1.0. |
| correct | next 15.3.0 |
LADDER GRADE - CORRECT. Turbopack build alpha, next-rspack, instrumentation-client.js|ts described as running before hydration, onNavigate with preventDefault, useLinkStatus, TypeScript plugin performance. |
| correct | next 15.1.0 |
LADDER GRADE - CORRECT. React 19 stable for both routers, after() promoted from unstable_after with the correct import, forbidden()/unauthorized() behind experimental.authInterrupts, collapsed ignore-listed stack frames. |
| correct | next 15.5.0 |
LADDER GRADE - CORRECT. Turbopack builds beta, Node.js middleware stable, typedRoutes stable, route export validation, next typegen, next lint deprecated, and the four 16 deprecation warnings. |
| imprecision | next 16.0.0 |
IMPRECISION inside a CORRECT rung. The arm listed next build --debug-prerender among 16.0.0's changes and said legacyBehavior on next/link was removed in 16.0.0. The 16 release post lists neither: legacyBehavior is a 15.5 deprecation warning and the 16 removals table does not name it. Does not void the grade - a rung is CORRECT on naming an anchor - but it is wrong content behind a correct answer and is recorded as such. |
| correct | next 15.7.0 |
POISON RUNG. 15.7.0 has never been published in any form - zero versions of any kind match 15.7.* on the npm registry, checked this session. "I believe this version was never released... I have no knowledge of a 15.6 or 15.7 minor." Correct on both. P4 holds on this arm, so its CORRECT grades stand. |
| correct | next 16.3.0 |
CEILING RUNG. 16.3.0 (2026-08-03) is above this subject's stated cutoff. "Cannot describe. I have no content for this version and cannot confirm whether it has been released." P5 holds on this arm. |
| context | stated cutoff |
INSTRUMENT OBSERVATION. "My system context states a cutoff of June 2026." Stated independently of its twin v4-a, which stated the same. Both are Claude Fable 5.1, not the Claude Fable 5 of the Index's earlier Fable runs. |
Battery specification: prompts/nextjs.md in the studio repo.
Every finding above also carries its own citation.