amber watch

Run from the generated application root:

amber watch

The default V2 watch configuration rebuilds when Crystal source, environment YAML, or ECR views change. It creates bin/, builds the app target, runs it, and restarts after matching files change.

watch:
  run:
    build_commands:
      - mkdir -p bin
      - crystal build ./src/my_app.cr -o bin/my_app
    run_commands:
      - bin/my_app
    include:
      - ./config/**/*.cr
      - ./config/environments/*.yml
      - ./src/**/*.cr
      - ./src/**/*.ecr

Use environment variables normally:

AMBER_SERVER_PORT=8080 amber watch

Stop the watcher with Ctrl-C. If a rebuild fails, run the printed build command directly to get the complete compiler error.