Create Engine

Use this skill when the task is to create, scaffold, or refactor a Rails engine, Rails plugin, or engine gem.

Keep this skill focused on structure and design. Use adjacent skills for installer details, deep test coverage, release workflow, or documentation work.

Quick Reference

HARD-GATE

Before engine work is complete, confirm all of the following:

STRUCTURE & CONTRACT:
1. Root file requires only version, configuration, and engine.
2. Public engines use isolate_namespace; configuration exposes .configure block.
3. Host model references are configurable strings (e.g., "User"), never hard-coded ::User.
4. Host-app contract is documented (see Host App Contract section).

SAFETY CHECKS:
5. Engine code never auto-applies migrations at boot (no db:migrate, ActiveRecord::Migrator, or config.paths['db/migrate'] in initializers).
6. Initializers are idempotent and safe in development reloads.
7. Assets and generators are namespaced and idempotent.

VERIFICATION COMMANDS:
8. Dummy app exists: `ls spec/dummy` or `ls test/dummy` should return the app directory.
9. Integration tests pass: `bundle exec rspec` or `bundle exec rake test` exits 0.
10. Routes load correctly: `bundle exec rails routes` inside dummy app shows engine routes.
11. No hard-coded host constants: `grep -r "::User\|::Employee" lib/ app/` returns nothing.
12. No migration auto-apply patterns: `grep -r "db:migrate\|ActiveRecord::Migrator\|config.paths\['db/migrate'\]" lib/` returns nothing.

Core Process

1. Identify the engine type before writing code. Scaffold with the correct generator:

   rails plugin new my_engine --mountable   # mountable engine
   rails plugin new my_engine --full        # full engine (non-isolated)
   rails plugin new my_engine               # plain Railtie/gem

2. Define the host-app contract (what the host must provide, what the engine exposes, which extension points are supported). See reference.md for the full contract template.
3. Create the minimal engine structure. Checkpoint: bundle exec rake inside the engine must pass.
4. Implement features behind the namespace. Checkpoint: mount engine in dummy app routes and verify with bundle exec rails routes.
5. Write minimum integration coverage through the dummy app. See TESTING.md for coverage requirements.
6. Document the host-app contract clearly enough for follow-on work.

If the user does not specify the engine type, infer it from the requested behavior and say which type you chose.

Extended Resources

• reference.md — full host-app contract template, recommended file structure, and code scaffolding examples
• EXAMPLES.md — extended engine examples
• TESTING.md — coverage requirements and dummy app setup
• assets/examples.md
• assets/release-checklist.md

Output Style

When asked to create or scaffold a Rails engine, your output answer.md MUST follow this style:

1. Concrete Artifact Files: Display the full generated .gemspec and Rakefile contents, correctly namespaced under the engine namespace.
2. Verification & Rake Status: Explicitly state that bundle exec rake inside the engine passes (exits 0), with a simulated passing output block.
3. Integration Test Coverage: Write integration specs covering configuration, routing, HTTP request flow, domain services, and host-integration hooks. See TESTING.md for the required spec categories and paths. Keep unit tests for models/services separate from request/integration specs.
4. Language: Must be in English unless explicitly requested otherwise.

Integration