Conciseness

Dense and fact-heavy with essentially no filler — version gates (PG 11 constant default, MySQL 8.0.12 INSTANT, the 256-byte VARCHAR boundary) and engine divergences earn their tokens. The over-explanation is confined to verbatim manual quotes for commonly known behavior (plain CREATE INDEX blocking writes, ALTER TABLE's ACCESS EXCLUSIVE conflicting with all modes), which could be trimmed to their operative clauses — more than trivial, less than padded sections.

Actionability

Fully executable guidance throughout: an 8-category table with SQL examples and default severities, per-check verdict rules ('Classify a plain CREATE INDEX on PostgreSQL as category 3: Warning with no size signal, Critical on a large table'), concrete boundary cases ('varchar(100) to varchar(500) crosses the boundary and copies; varchar(300) to varchar(900) does not'), and an exact output table schema with two filled example rows plus a count block.

Workflow Clarity

Steps 1-4 are clearly sequenced with genuine checkpoints: an input gate ('say "unknown" rather than guessing'), 'unverified' size flagging, worst-case multi-engine reporting when the engine is unknown, and an output reconciliation block ('Unclassifiable statements: 0'). It lacks an explicit error-recovery loop — nothing says what to do when a statement fits no category or counts don't reconcile — so it sits below the feedback-loop anchor.

Progressive Disclosure

The single bundle file (references/engine-quick-reference.md) exists and its contents match the body's description of it (consolidated lock/rewrite tables, the three severity-flipping divergences, a full worked classification for PostgreSQL 16 across users/orders/products). It is clearly signaled in a dedicated 'Per-engine quick reference' section, exactly one level deep, with the decision procedure correctly kept inline.

Specificity

Names concrete operations ('rename or drop a column, add a default, index or foreign key, change a type') and a concrete action with its outputs ('Classifies each statement (additive, locking, breaking, data-loss) by lock and rewrite'). It stops short of the top anchor because the skill performs a single core verb (classify) rather than multiple distinct actions like the anchor-5 example.

Completeness

Explicitly answers 'what' ('Classifies each statement (additive, locking, breaking, data-loss) by lock and rewrite') and 'when' ('Use for any question about whether a database migration is safe to run in production, including quick ones ... and after a migration locked a table') with concrete trigger scenarios, matching the anchor-5 pattern of both halves with explicit trigger phrases.

Trigger Term Quality

Includes natural phrases users would actually say ('safe to run in production', 'after a migration locked a table', the verbatim example question 'is adding a NOT NULL column to a big Postgres table safe?') plus both 'Postgres' and 'PostgreSQL' and 'MySQL'. A few common variations are missing — 'schema change', 'downtime', 'online DDL' — which keeps it below the comprehensive-synonyms anchor.

Distinctiveness Conflict Risk

Occupies a clear niche — migration-statement safety classification for 'a large PostgreSQL or MySQL table' — with distinct triggers ('after a migration locked a table') and minimal conflict risk; the enumerated operations are framed as classification inputs, not as generic authoring or execution tasks that would overlap other skills.