Docker Compose Validator

What is checked

• YAML syntax with the failing line highlighted.
• Top-level structure: a services mapping must exist; unknown top-level keys are flagged unless they start with x-.
• Every service must be a mapping with either image or build.
• Ports must be valid HOST:CONTAINER mappings, optionally with an IP, a range, or /udp.
• depends_on targets must be defined services, and conditions must be one of the three allowed values.
• Named volumes and networks used by a service must be declared at the top level.
• environment must be a list of KEY=value strings or a mapping of scalars; boolean values are flagged so you can quote them.
• restart must be no, always, on-failure, or unless-stopped.
• container_name duplicates and healthcheck blocks without a test are reported.
• Unknown service keys (usually typos like enviroment) are reported as warnings.

Example

services:
  web:
    build: .
    ports:
      - "8000:8000"
    depends_on:
      - db
    volumes:
      - app-data:/data     # not declared below
  db:
    image: postgres:16
    restart: no            # quote it: YAML 1.1 parsers read this as false
volumes:
  db-data:

The validator reports the undeclared app-data volume. Click Load sample above for a longer example with more issues.

Common Docker Compose YAML mistakes

• Indentation under a service. If the keys under web: are not indented, web is parsed as null and the validator reports that it must be a mapping.
• Unquoted port ranges. Values like 8080:80 are safe in YAML 1.2 parsers but quoting them avoids surprises with tools that still use YAML 1.1.
• Mixing list and map syntax. environment accepts either - KEY=value items or KEY: value pairs, not both in one block.
• Boolean strings. DEBUG: false passes the boolean to your app; if it expects the text "false", quote it.

Once the file is clean, keep it that way with the YAML formatter, and compare environments with the YAML diff tool.