Phoenix Auth Customization

Use this skill when extending phx.gen.auth with custom fields.

Canonical FP bar: docs/fcis-engineering-rules.md — Functional Core, Imperative Shell: pure domain modules; side effects at edges. Authorization decisions should be pure checks on scope/user data; persist only at the edge.

RULES — Follow these with no exceptions

1. Never modify generated auth migrations — create separate migrations for custom fields 2. Update registration_changeset to cast and validate new fields — don't create a separate changeset 3. Update test fixtures when adding required fields — missing fixture fields cause cryptic test failures 4. Confirm users in test fixtures for password-based auth — set confirmed_at or tests will fail 5. Update both the registration form AND the save/2 handler — the form must send the field 6. Use unique_constraint + database unique index for uniqueness — never validate in application code alone

Running phx.gen.auth