Conciseness

The body is dense and assumption-respecting: no padding, no explanation of concepts Claude already knows, and every section (comparison table, error handling, path buffers, common mistakes) carries project-specific information that cannot be inferred. Not score 4: there are no sections that read as over-explanation needing a trim.

Actionability

Examples are executable and cover the common cases: a complete copy-paste `write_file` function, the preferred `File::openat` pattern, errno branching with ENOENT, and pooled path-buffer usage. The `to_js` and path-join snippets use placeholder variables (`global`, `dir`, `name`) as illustrative patterns rather than gaps, matching the score-5 anchor.

Workflow Clarity

This is a single-purpose reference skill and the single action — use `bun_sys::File` instead of `std::fs` — is unambiguous, supported by a preferred path, explicit error-handling patterns, and a "Common Mistakes" pitfalls list. No destructive or batch operations exist that would require a validation cap, so the simple-skill exception applies.

Progressive Disclosure

Sections are well organized and the closing pointer ("See `src/CLAUDE.md` for the full bun_core/bun_sys reference") is clearly signaled, but it targets a file outside the skill bundle (no references/ directory exists), and the ~90-line body inlines a "Key Types and Functions" API listing that could live in a separate reference file. This matches score 4 (good structure, minor organization gaps) rather than 5, which expects well-signaled one-level-deep references within the bundle.

Specificity

"Guides using bun_sys for system calls and file I/O in Rust" names the domain and two capability areas, but relies on a single generic action verb rather than listing several concrete operations (open, read, write, stat, unlink). This matches the score-3 anchor (domain plus 1-2 concrete actions) better than score 4, which expects several specific listed actions.

Completeness

Both what ("using bun_sys for system calls and file I/O in Rust") and when ("Use when implementing file operations, opening fds, or any syscall path instead of std::fs or libc") are explicitly stated with concrete trigger phrases, matching the score-5 anchor. The when-clause is far more explicit than the score-4 anchor allows.

Trigger Term Quality

Natural trigger phrases are well covered — "file operations", "opening fds", "syscall path", "std::fs", "libc" — with synonyms (system calls / syscall, file I/O / file operations). Not score 5: some natural phrasings a user might say, such as "file descriptor" spelled out or "reading/writing files", are missing.

Distinctiveness Conflict Risk

This occupies a clear niche — the bun_sys API within Rust, explicitly framed as the alternative to "std::fs or libc" — so triggers are distinct and conflict risk with other skills is minimal.