Conciseness

The ~20-line body explains nothing Claude already knows and wastes no tokens: it opens with the endpoints path, a numbered checklist of exact commands, and a two-column reference table. Every line is operational, matching the lean anchor 5; anchor 4's "minor instances of over-explanation" do not exist here.

Actionability

Guidance is fully executable: exact commands ("make openapi", "make lint-python"), the exact file to edit ("tests/integration/_helpers.py"), the exact list names (_COMMON_RESOURCE_ENDPOINTS, _ADMIN_ONLY_ENDPOINTS, _VIEWER_BLOCKED_WRITE_OPERATIONS), and path-parameter conventions ("fake-id-{}", "test-tag"). Copy-paste ready with no gaps, matching anchor 5.

Workflow Clarity

The section is explicitly framed as a checklist "run before committing any endpoint change" with a numbered 3-step sequence and an explicit validation gate with a fix loop ("make lint-python — fix any lint errors before committing"). Anchor 5 requires explicit validation steps, error-recovery feedback, and checklist form — all present; anchor 4's "minor validation gaps" do not apply since lint gatekeeping is stated.

Progressive Disclosure

The body is a pure overview routing to three real, one-level-deep bundle files via a Reference/When table ("references/endpoint-patterns.md", "references/openapi-codegen.md", "references/testing-patterns.md" — all verified present on disk with matching content). Combined with its under-50-line length and clean section organization, this matches anchor 5's clear overview with well-signaled references.

Specificity

"REST API development for Phoenix" names the domain and "adding, modifying, or reviewing endpoints" states actions, but all actions target one artifact and no further capabilities (codegen, testing) are enumerated — matching anchor 3 rather than anchor 4's several distinct actions. It is clearly above anchor 2, whose actions are minimal or generic with no concrete target.

Completeness

Both parts are present: a clear "what" ("REST API development for Phoenix") and an explicit "when" with concrete triggers ("Use when adding, modifying, or reviewing endpoints in src/phoenix/server/api/routers/v1/"). Not 5 because the "what" is a category label rather than enumerated concrete actions as in the 5-anchor example; not 3 because the "when" is explicit, not weakly implied.

Trigger Term Quality

"REST API", "endpoints", and "adding, modifying, or reviewing" are natural phrases users would say when requesting this skill, giving good keyword coverage; a few natural variations are missing ("routes", "router", "handlers", "OpenAPI/schema"). Falls between anchor 3 (missing common synonyms) and anchor 4 (a few natural terms missing), noticeably above the midpoint, so 4.

Distinctiveness Conflict Risk

The description is pinned to a specific framework (Phoenix) and an exact directory path ("src/phoenix/server/api/routers/v1/"), establishing a clear niche with distinct triggers and minimal conflict risk with other skills. No neighboring anchor fits better.