The body is lean and code-forward — short intro, tight 'When to use' bullets, numbered steps each with an executable block plus a one-to-two sentence note, then an anti-patterns table and limitations — assuming Claude's competence without explaining basic concepts; not at 2 despite a few repeated 'Per the [testbook docs]' citations and a one-line docs-quote intro, because the dominant character is efficient and the padding is minor.

Every step provides fully executable, copy-paste-ready Python — @testbook decorator with execute=True, tb.ref(), tb.inject(), selective execute=['imports','data-load'], the pytest fixture pattern, and tb.patch() — with concrete assertions; not at 2 because the examples are complete executable code rather than pseudocode or vague direction.

Steps are clearly sequenced (Install → Object patching) and reinforced by an anti-patterns 'why it fails / fix' table and a limitations section that together provide error-recovery guidance; not at 2 because this is a non-destructive library skill so the validation-checkpoint cap does not bind, and the sequencing plus recovery material matches the clear-sequence anchor.

A single self-contained, well-organized reference with clearly labeled sections (When to use, Steps 1–7, Anti-patterns, Limitations, References) and one clearly signaled one-level-deep link to the external testbook docs; no bundle files exist and there is no nested reference nesting, so the under-50-lines / well-organized-sections allowance applies. Not at 2 because there is no monolithic wall of text or content that should be split out.

Names multiple concrete actions — 'write conventional pytest unit tests against functions defined in Jupyter notebooks' plus 'Covers tb.ref() (notebook object access) and tb.inject() (insert code into the kernel) for hermetic per-test setup' — matching the 'lists multiple specific concrete actions' anchor; it is not at 2 because it goes beyond naming only the domain and some actions.

Explicitly answers what (use the @testbook decorator to unit-test notebook functions via tb.ref/tb.inject) and when via the explicit 'Use when unit-testing functions that live in a Jupyter notebook' trigger; not at 2 because the when-clause is present rather than implied, satisfying the guideline that a missing 'Use when...' would cap at 2.

Includes natural terms a user would say — 'pytest unit tests', 'Jupyter notebooks', 'unit-testing functions that live in a Jupyter notebook' — giving good coverage of common phrasings; it is not at 2 because it is not limited to technical jargon or a single generic phrase.

Occupies a clear niche — pytest unit tests of functions living inside Jupyter notebooks — with distinct triggers unlikely to fire for unrelated skills; not at 2 because the trigger is specific enough that it would not readily overlap with sibling skills like full-notebook nbval regression testing.