Conciseness

Most content is genuinely non-inferable project knowledge (file layout, import allowlist rules, echo builtin quirks, gate protocol), but there is noticeable padding: the gating instructions repeat the same TaskList/checkpoint boilerplate across every step, and the STOP section reiterates points made later. This fits 'Mostly efficient but includes some unnecessary explanation or could be tightened' — not 4, since the repetition is systematic rather than minor.

Actionability

The body provides copy-paste-ready Go test helpers (runScript, cmdRun, cmdRunCtxFuzz), exact function signatures, a concrete YAML scenario template, exact bash commands, and specific test matrices (integer edge cases, path edge cases, GNU-compat table). Fully executable guidance covering the common cases — matches the top anchor.

Workflow Clarity

A 10-step workflow is explicitly sequenced with a stated execution order, per-step gate checks (TaskList verification before advancing), parallelization rules, and validation feedback loops (run tests, fix, re-run, plus a second independent review pass). This matches 'Clear sequence with explicit validation steps; feedback loops for error recovery; checklists for complex processes'.

Progressive Disclosure

There are no bundle files (references/, scripts/, assets/ are absent) and the entire ~600-line body is one monolithic file: stable test-helper code, the GNU-compat coverage table, the pentest checklist, and the fuzz seed-corporus guidance are all inlined where they belong in separate reference files. Section headers keep it navigable, which lifts it above the 'minimal structure' level 2, but content that should be separate is inline with no references — anchor 3.

Specificity

The description names a concrete action ('Implement a new POSIX command as a builtin') and a specific target ('the safe shell interpreter'), but covers only that single action without enumerating what the implementation entails (tests, review, hardening). This matches the anchor 'Names domain and 1-2 concrete actions, but not comprehensive' — not 4, which requires several specific actions, and not 2, since the action is concrete rather than generic.

Completeness

It clearly answers 'what' (implement a POSIX command as a builtin) but has no 'when to use' clause whatsoever, which caps completeness at 3 per the judging guidelines. Not 4 because the 'when' is entirely absent rather than just imprecise; not 2 because the 'what' is clear, not vague.

Trigger Term Quality

Terms like 'POSIX command', 'builtin', and 'safe shell interpreter' are relevant and natural for this codebase, but there are only a few of them with no synonyms or common variations. This fits 'Some relevant keywords but missing common variations or synonyms' — better than the generic one-or-two-keyword level 2, short of the good-coverage level 4.

Distinctiveness Conflict Risk

'The safe shell interpreter' pins the skill to a specific project, making it mostly distinct with only minor overlap risk against other command-implementation skills. Not 5 because without trigger phrasing the fairly broad 'implement a new POSIX command' could still collide with sibling implementation skills.