migrate

name	migrate
description	Lint a database migration for safety before it runs — classify lock level, table rewrite, and reversibility; flag destructive/blocking operations; and propose an expand-contract rollout. Read-only analysis by default; any data-loss step requires an explicit token handshake before the writer is involved. Use when the user asks to review, lint, check, or de-risk a migration, an ALTER/DROP, an enum change, a backfill, or a schema change before deploying.
argument-hint	<migration-file|migration-dir> | <from-schema> <to-schema> [--engine postgres|mysql|...] [--apply]
allowed-tools	Read, Grep, Glob, Bash, Task

/claude-db:migrate

Migration-safety lint, owned by module db-migration-safety (M22, axis performance) via the db-migration-safety / MSA checks. Default mode is read-only: it analyzes and explains, it does not run the migration.

$ARGUMENTS has two forms:

• single-file lint — <migration-file|migration-dir> [flags]: parse and classify an existing migration (below).
• diff mode — <from-schema> <to-schema> [flags]: two schema paths. Run node scripts/schema-diff.mjs --from <from-schema> --to <to-schema> to generate the up/down migration, then classify the generated migration exactly as in single-file mode.

If no argument is given, ask for one (or point at the project's migration directory).

Diff mode (<from-schema> <to-schema>)

1. Run node scripts/schema-diff.mjs --from <A> --to <B> to produce the up and down migration; schema-diff.mjs flags destructive steps.
2. Present the generated up/down migration to the user, with the plain-language summary of each step.
3. Route every step schema-diff.mjs flagged destructive (DROP TABLE/COLUMN, type narrowing) through the data-loss handshake below before anything is written.
4. Then classify the generated migration through the same lock/rewrite/reversibility analysis as single-file mode.

What to do (single-file lint)

1. Parse the migration (raw SQL or generated ORM migration). Detect the engine; load engine-specific lock/rewrite rules.
2. For each statement, classify:
   • Lock level — e.g. Postgres ACCESS EXCLUSIVE (blocks reads+writes) vs SHARE UPDATE EXCLUSIVE; whether it blocks the table for the duration.
   • Table rewrite — does the change rewrite every row (e.g. adding a NOT NULL column with a volatile default on an old engine, changing a column type)? Rewrites scale with table size.
   • Reversibility — is there a clean down-migration, or is it one-way (data already dropped/coerced)?
   • Destructive / data-loss — DROP COLUMN/TABLE, TRUNCATE, narrowing types, enum-value removal, irreversible coercions.
3. Emit findings per schema/finding.schema.json (status/severity/evidence.observed quoting the offending statement with secrets redacted/verification.reproduce/expected_impact.axis=performance+confidence+magnitude). A destructive migration without reversibility/expand-contract is severity-5 (caps Performance). Never fabricate a row-count or lock-duration — magnitude is banded high|medium|low and scales with table size described, not invented.
4. Propose an expand-contract rollout for risky changes: expand (add the new nullable column / new table / dual-write), migrate (backfill in batches, online), contract (flip constraints, drop the old shape) — each as a separate, individually-reversible step with the lock level per step.

Safety / handshake

• This command never applies a migration on its own. --apply only requests application.
• Any step classified data-loss / destructive requires an explicit token handshake: present the exact statement(s) and the irreversible consequence, then require the user to type back the affected object's name verbatim — not a yes/no. Example: to confirm DROP TABLE orders, the user must type orders; to drop a column email, they type email. A reply that does not exactly match the object name aborts the step. See references/migration-safety.md for the handshake rules.
• Actual writes are delegated only to the db-migration-writer subagent (the one agent with Write/Edit), and only after the handshake. Read-only by default.