Conciseness

The body is lean and opinionated — every bullet is a terse, non-obvious rule ('`async def` — use ONLY with non-blocking `await` calls; blocks event loop otherwise', 'Table names: `lower_case_snake`, singular (`post`, `user`, `post_like`)') with no explanation of concepts Claude already knows. The only near-redundancy is the 'How to Use' section restating the per-section pointers, which is minor navigation rather than padding, so the anchor-5 fit (every token earns its place) beats anchor 4.

Actionability

Concrete, directly applicable directives appear throughout (`Annotated[T, Depends(fn)]`, `run_in_threadpool()` from Starlette, `openapi_url=None`, `_at`/`_date` column suffixes), and the directory layout is explicit. It is not a 5 because no copy-paste-ready code blocks live in the body — e.g., 'Set up an async test client (httpx + ASGITransport) from day one' names the tools but defers the actual fixture code to a reference file, leaving minor gaps for the common cases.

Workflow Clarity

This is a conventions-reference skill with no multi-step process to sequence; the usage flow (category table → quick-reference rules → 'See references/…' for details) is unambiguous and the 'How to Use' section states it explicitly. It sits at 4 rather than 5 because organization is clear but there is no explicit sequencing or checkpoints — acceptable since no destructive or batch operations are involved, which would otherwise cap the score at 3.

Progressive Disclosure

The body is a clean overview with a categories table mapping each topic to its reference file, per-category summaries, and clearly signaled one-level-deep pointers ('See `references/async-patterns.md` for decision matrix, threadpool caveats, and examples'). All four referenced files (async-patterns.md, dependencies.md, project-conventions.md, pydantic-patterns.md) exist in the bundle, and detail content is appropriately split out — matching the anchor-5 example structure.

Specificity

It names the domain ('FastAPI best practices and conventions') and enumerates multiple concrete action areas — 'writing, reviewing, or refactoring FastAPI applications — route handlers, Pydantic schemas, dependency injection, project structure, async patterns, testing, or API documentation' — giving comprehensive coverage of the domain, matching the anchor-5 pattern of multiple specific concrete actions rather than the 'minor gaps' of anchor 4.

Completeness

It explicitly answers both questions: the 'what' ('FastAPI best practices and conventions' with enumerated scope) and the 'when' via two explicit trigger clauses — 'Use when writing, reviewing, or refactoring FastAPI applications…' and 'Triggers on tasks involving FastAPI routers, endpoints, request validation, response models, or application configuration.' This matches the anchor-5 example structure exactly.

Trigger Term Quality

Good natural keyword coverage: users would genuinely say 'FastAPI routers, endpoints, request validation, response models', 'Pydantic schemas', 'dependency injection'. It falls short of anchor 5's 'comprehensive coverage including synonyms' because common variations like 'REST API', 'backend', 'uvicorn', or 'Starlette' are absent, leaving a few natural terms missing per anchor 4.

Distinctiveness Conflict Risk

Clear niche (FastAPI-specific) with distinct triggers, and it actively defuses the main overlap risk: 'Does not cover general Python syntax or typing — see modern-python-development for that.' A user mentioning FastAPI endpoints or Pydantic schemas would unambiguously land here, matching anchor 5's minimal conflict risk.