Config

Kamal Configuration

You are an expert in deploying applications with Kamal. Your goal is to help structure a clear, correct config/deploy.yml — the single file that drives every Kamal deployment.

Kamal reads its configuration from config/deploy.yml. This skill walks through the required keys, the full set of top-level options and their defaults, per-environment destinations (-d), extensions (x-), and how to keep the file DRY with YAML anchors.

Before You Start

Inspect what already exists before asking the user questions:

• Read config/deploy.yml if it exists — it tells you the service name, servers, registry, and which options are already tuned.
• Look for config/deploy.<destination>.yml files (e.g. config/deploy.staging.yml) — these signal the project already uses destinations.
• Check .kamal/secrets for referenced secret names so you don't duplicate or contradict them.

If there is no config/deploy.yml, generate one with kamal init, which creates the configuration file plus a .kamal/secrets file and sample hooks in .kamal/hooks.

How the Config File Works

After any edit, run kamal config to print the resolved configuration and confirm Kamal parses it the way you expect.

Note: Kamal rejects unrecognized keys. A typo'd or invented key is an error, not a silent no-op — see Extensions for the one supported escape hatch.

Step-by-Step Walk-Through

Step 1: Set the required key

service is the only required value. It is used as the container name prefix, so keep it short and stable.

service: myapp

Step 2: Point at an image

image is the Docker image name. The image will be pushed to the configured registry.

image: my-image

Step 3: Add the deploy targets

These three blocks describe where and how the app runs. Each is a topic of its own — set the keys here and follow the cross-references for detail:

registry:
  ...   # Docker registry — see servers / official Registry docs

servers:
  - 192.168.0.1   # a bare list is implicitly the `web` role

env:
  clear:
    DATABASE_HOST: mysql-db1
  secret:
    - DATABASE_PASSWORD

• For server lists, roles, and tags, see the servers skill.
• For clear/secret env, tags, and .kamal/secrets, see the env skill.

Step 4: Tune the top-level options

Set only the options you need to change from their defaults. The most commonly adjusted ones:

primary_role: workers
retain_containers: 3
require_destination: true

For the complete list of every top-level key and default, see references/config-keys.md.

Step 5: Validate

Run kamal config to render the merged result and catch parse errors or unexpected defaults before deploying.

Destinations (-d)

Destinations let one project target multiple environments from one base config plus a per-destination overlay.

When you pass a destination with the -d flag, e.g. kamal deploy -d staging, Kamal also reads config/deploy.staging.yml and merges it with the base config/deploy.yml.

# config/deploy.yml (base)
service: myapp
servers:
  - 192.168.0.1

# config/deploy.staging.yml (overlay, merged over the base)
servers:
  - 10.0.0.5

Secrets follow the destination too. With destinations, Kamal looks for <secrets_path>-common first and then <secrets_path>.<destination> (e.g. .kamal/secrets-common, then .kamal/secrets.staging), with later files overriding earlier ones.

To make a destination mandatory so nobody deploys to the wrong place by accident, set require_destination: true.

Extensions (x-)

Kamal will not accept unrecognized keys in the configuration file. To declare a reusable block without triggering an error, prefix the section with x-. Kamal ignores any x--prefixed key and treats it as an extension.

This is what makes YAML anchors usable in deploy.yml: define the anchor under an x- key so Kamal skips it, then alias it where it counts.

DRY Config with YAML Anchors

You can re-use parts of your configuration by defining them as anchors and referencing them with aliases. Anchors begin with x- and are defined at the root level of deploy.yml.

Define the anchor once:

x-worker-healthcheck: &worker-healthcheck
  health-cmd: bin/worker-healthcheck
  health-start-period: 5s
  health-retries: 5
  health-interval: 5s

Then reference it via the alias, merging it in with <<:

servers:
  worker:
    hosts:
      - 867.53.0.9
    cmd: bin/jobs
    options:
      <<: *worker-healthcheck

This keeps a single source of truth for shared blocks (like a healthcheck reused across multiple worker roles) without repeating yourself.

Reusable Command Shortcuts

The top-level aliases key defines shortcuts for Kamal commands — for example a console alias or a staging_deploy: deploy -d staging. These are configured in the same file. For the full pattern, see the aliases skill.

Related Skills

• setup: For a first-time Kamal setup and your first deployment.
• servers: For the servers block — host lists, roles, tags, and per-role options.
• env: For the env block — clear/secret values, tags, and .kamal/secrets.
• aliases: For defining command shortcuts under the aliases key.