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Claude with Skills

Exercise 02: Restrict a Skill by Path

Create a skill that auto-activates only when the user is working with database migration files. This exercise focuses on the paths field and writing a skill body that enforces structural conventions.

Goal

The skill should:

  1. Auto-activate when the user opens, edits, or references a file matching:

    • migrations/** (any file under a migrations directory)
    • *.sql (any SQL file in the project root)
    • db/migrate/** (Rails-style migration directory)

  2. Check migration file naming conventions:

    • Timestamp-prefixed format: YYYYMMDDHHMMSS_description.sql (e.g., 20260513143207_add_user_roles.sql)
    • Sequential format: NNNN_description.sql (e.g., 0042_add_user_roles.sql)
    • Report non-conforming names as a warning

  3. Warn about potentially dangerous SQL operations:

    • DROP TABLE without a preceding check (like DROP TABLE IF EXISTS)
    • DELETE without a WHERE clause
    • UPDATE without a WHERE clause
    • TRUNCATE on any table
    • Column removal (DROP COLUMN) — flags for attention, not necessarily wrong

  4. Look for a rollback or a comment marking the migration as irreversible.

What to write

Create SKILL.md in a new directory. Think through:

  • Which paths patterns cover all three migration directory conventions above?
  • Should this be user-invocable: false or left as default?
  • Does this skill need any allowed-tools?
  • Should disable-model-invocation be set?

Output format

The skill should produce a checklist:

Migration review: 20260513143207_add_user_roles.sql


[ ] Naming convention: PASS / FAIL: <reason>
[ ] Rollback present: YES / NO / MARKED IRREVERSIBLE
[ ] Dangerous operations: NONE / WARNING: <list each one with line number>

Validation

  1. Create a test migration file with at least one dangerous operation and a non-conforming name.
  2. Open that file in Claude Code.
  3. Confirm the skill activates automatically (without you typing /skill-name).
  4. Confirm the output matches the expected checklist format.
  5. Also confirm the skill does NOT activate when you open a regular TypeScript or Python file.

Solution

A worked solution is in solutions/02-migration-guard/SKILL.md.