Conciseness

The ~45-line body contains no filler and no explanation of concepts Claude already knows; every sentence carries skill-specific information (intake command, symptom-splitting policy, sub-agent delegation warning, output contract). It matches the score-5 anchor 'every token earns its place' — the score-4 anchor would require identifiable over-explanation to trim, and none is present.

Actionability

The intake command is copy-paste ready with all flags and value forms (`--issue <N|url> | --jira <KEY|url> | --file <path> | --text "<report>"`), the trace steps name concrete tools with arguments (system-history `analyze.ts --focus` with every path file, `pr.ts --pr N --file <path>` in the reference), and the output section specifies exact enumerated fields and an evidence-level enum (`reproduced`, `traced`, `inferred`). This covers the common cases fully per the score-5 anchor.

Workflow Clarity

A clear numbered sequence (intake → split symptoms → trace → output) with explicit validation checkpoints baked into the method: "Check every precondition against today's code", "Hold at least two explanations, and run the check that separates them", and "Reproduce in the branch browser" are feedback loops, and the evidence-level gate plus the output checklist close the loop. This matches the score-5 anchor; the tracing work is read-only, so the destructive/batch validation cap does not apply.

Progressive Disclosure

Verified against the actual bundle: `references/bug-investigation.md` exists and is well-signaled ("Follow `references/bug-investigation.md`") one level deep with no nested reference chains, and `scripts/bug-intake.ts` exists and is invoked by exact path. The split is appropriate — overview and step summary in SKILL.md, the full investigation brief in the reference, the executable script in scripts/ — matching the score-5 anchor's clear overview with well-signaled one-level-deep references.

Specificity

The description lists four concrete, distinct actions — "take in the report (GitHub issue, Jira ticket, file, or pasted text)", "trace the failing path hop by hop with each hop's runtime precondition", "find the recent change that broke it", and "rule out rival explanations" — which comprehensively covers the bug-tracing workflow with no gaps, matching the top anchor. It exceeds the score-4 anchor because there is no missing slice of coverage for this task domain.

Completeness

It explicitly answers both questions: the 'what' is the four concrete actions, and the 'when' is the explicit trigger clause "Use for any bug investigation where the cause is not yet known", which even scopes the trigger (cause unknown). This clearly matches the score-5 anchor with concrete trigger phrases rather than the score-4 anchor's 'when could be more explicit'.

Trigger Term Quality

Natural user phrasings present include "bug", "root cause", "GitHub issue", "Jira ticket", and "bug investigation", giving good keyword coverage. It falls short of the score-5 anchor because common synonyms users would naturally say — "debug", "regression", "error", "crash" — are absent, and only the 'regression' idea is indirectly implied via "the recent change that broke it".

Distinctiveness Conflict Risk

The niche is clear (root-cause tracing of a reported bug, cause not yet known) and the "cause is not yet known" qualifier separates it from fix-oriented sibling skills. It sits at the score-4 anchor rather than 5 because "bug investigation" and "find the root cause" remain broad triggers that a general debugging or bug-fixing skill could also claim, leaving minor overlap risk with closely related skills.