Partly: in 4 runs, Claude Fable 5.1's next.js attribution reaches 16.1.0 (2025-12-18), inside next.js 16, and stops there — next.js was at 16.3.4 when this index last verified against it (2026-09-05). That is ~6 months below the training cutoff the subject stated in those runs (2026-06). No finding is currently charged against Claude Fable 5.1 on next.js: every probe in the 4 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 next.js 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 |
|---|---|---|---|
| v4-a 2026-09-05 | 16.1.0 2025-12-18 |
16.2.0 2026-03-18 |
~6 months |
| v4-b 2026-09-05 | 16.1.0 2025-12-18 |
16.2.0 2026-03-18 |
~6 months |
| v4-c 2026-09-05 | 16.1.0 2025-12-18 |
16.2.0 2026-03-18 |
~6 months |
| v4-d 2026-09-05 | 16.1.0 2025-12-18 |
16.2.0 2026-03-18 |
~6 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.
next.js published 4 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 next.js 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 next.js 16 best?
Other subjects on next.js:
Claude Fable 5.1 on the other libraries: