Conciseness

The body is lean and assumes competence (no explanation of what Build123d or CAD is; conventions are stated as terse directives). Minor trimming is possible: the dfm-rules read-directive is stated twice (intro paragraph and Core Workflow step 3), and the five-example cad.inspect block is slightly heavy. This fits the 4 anchor ('efficient; minor instances that could be trimmed') rather than the 5 anchor's every-token-earns-its-place.

Actionability

Provides concrete, executable material: a working code snippet (Box(40, 30, 12); publish("body", body, "Main body")), full JSON shapes for cad.inspect calls with specific fields (object_id, topology_ref, buildNumber, mode), and exact tool names and options (cad.look views/isolate/context/hide). Gaps are minor: cad.edit usage is named ('action="add_part"') but never shown as a concrete call, fitting the 4 anchor ('mostly executable; minor gaps') rather than 5.

Workflow Clarity

The Core Workflow is a clear numbered sequence with explicit validation checkpoints and feedback loops: 'inspect its returned image before the next edit', embedded assert/print self-checks of key dimensions, 'never claim an unseen property is resolved', and exports gated on a Host-owned review ('proceed only on Pass'). This matches the 5 anchor (clear sequence, explicit validation, error-recovery guidance).

Progressive Disclosure

The body is a pure index: conventions and workflow inline, with an annotated reference table pointing one level deep to 11 files (pitfalls, objects, operations, topology-selection, locations, import-export, tips, joints, assemblies, dfm-rules, connectors), each with a 'what it answers' column; all referenced files exist in references/. This matches the 5 anchor (clear overview, well-signaled one-level-deep references, easy navigation).

Specificity

Names the Build123d CAD domain and gives several concrete directives ('Read references/dfm-rules.md BEFORE writing the first line of geometry code', 'never hand-model that joint', load the build123d-libs kit), plus a specific feature list (screw boss, standoff, snap fit, wall interface, overhang-sensitive feature). It is a strong trigger-plus-directive description but not a comprehensive list of capability actions, so it fits the 4 anchor ('several specific actions; minor gaps') rather than the 5 anchor.

Completeness

The 'when' is explicit and concrete ('Use when custom Build123d Python CAD code... are needed'), and the 'what' is conveyed through the domain list and reading directives. However, the skill's actual role (a reference index for authoring/debugging Build123d code) is implied rather than stated as a capability, fitting the 4 anchor (both present, one could be more explicit) rather than the 5 anchor's fully explicit what+when.

Trigger Term Quality

Includes natural terms users would actually say: 'Python CAD code', 'selectors', 'joints', 'assemblies', 'screw boss', 'standoff', 'snap fit', 'overhang', '3D-printed'. Coverage is good but misses common synonyms and file extensions users mention (.step, .stl, 'CAD model'), matching the 4 anchor ('good keyword coverage; a few natural terms missing') rather than the 5 anchor's comprehensive synonym/extension coverage.

Distinctiveness Conflict Risk

Build123d is a clear, narrow niche with distinct triggers, and the description even draws an explicit boundary against a sibling skill ('A screw fastening into a 3D-printed part additionally requires the build123d-libs kit; never hand-model that joint'). Minimal conflict risk with other skills, matching the 5 anchor.