Conciseness

The body is lean and information-dense: a compact function table, exact counts verified by a script, and pointed notes like 'there is no m.insert call site to add to, and hand-editing either is impossible' — every token earns its place and nothing explains concepts Claude already knows. It is not 4 because there is no over-explanation anywhere to trim.

Actionability

Guidance is concrete and mostly executable: exact function signatures, file paths ('crates/xberg/src/core/mime.rs', 'extractors/mod.rs::register_default_extractors()'), and commands ('scripts/sync_supported_counts.py sync' then 'verify'). It is not 5 because the registry-selection Rust snippet is illustrative usage rather than copy-paste ready code (no imports/error handling context).

Workflow Clarity

'Adding a New Format' is a clearly numbered 4-step sequence with an explicit validation checkpoint (run sync, then run the verify command). It is not 5 because there is no feedback loop telling Claude what to do when verify fails, and no final post-registration check; it is not 3 because a validation checkpoint is explicitly present rather than implicit.

Progressive Disclosure

The single-file body is well organized into focused sections (Detection Flow, Key Functions, registry, selection, wildcards, add-a-format, Critical Rules) with all detail appropriately inline; references that do appear ('plugin-architecture-patterns', scripts/sync_supported_counts.py) are one level deep. It is not 5 because those cross-references are not clearly signaled as navigable pointers (no path or link format), making discovery rely on inference.

Specificity

The description lists multiple concrete capabilities — 'the FORMATS registry that EXT_TO_MIME and SUPPORTED_MIME_TYPES are derived from, the path-based and bytes-based detection functions, priority-based registry selection, wildcard MIME families, and the real procedure for adding a format' — giving comprehensive coverage of the skill's scope. It is not 4 because there are no meaningful gaps in capability coverage.

Completeness

It explicitly answers both 'what' (detection and routing in core/mime.rs, the registry, detection functions, selection, wildcards, the add-a-format procedure) and 'when' via the explicit clause 'Load when adding a format, wiring an extractor to a MIME type, or debugging why a file routes to the wrong (or no) extractor.' It is not 4 because the 'when' is fully explicit with concrete trigger phrases, not just present.

Trigger Term Quality

Triggers like 'adding a format', 'wiring an extractor to a MIME type', and 'debugging why a file routes to the wrong (or no) extractor' are natural phrases a developer would say, with good keyword coverage (MIME type, extractor, format). It is not 5 because synonym and extension-level variations (e.g. 'file type detection', 'content sniffing', specific extensions) are sparse.

Distinctiveness Conflict Risk

The description is pinned to a precise niche ('MIME type detection and extractor routing in core/mime.rs') with triggers scoped to format addition, extractor wiring, and routing debugging — minimal overlap with any other skill. It is not 4 because the scoping to this specific subsystem leaves essentially no ambiguity about which skill should trigger.