YAML Validator

1. Paste or type your YAML in the input area.
2. Click "Validate" or use an example to test.
3. If the YAML is valid, the formatted output appears on the right.
4. If invalid, an error message shows what went wrong and the approximate location.
5. Click "Copy" to copy the formatted result.

What is YAML?

YAML (YAML Ain't Markup Language) is a human-friendly data serialization format designed for configuration files and data exchange. It uses indentation with spaces to represent hierarchy, making it more readable than JSON or XML for configuration purposes. YAML is the format of choice for Docker Compose, Kubernetes manifests, GitHub Actions, GitLab CI/CD, Ansible playbooks, and many other DevOps tools. Unlike JSON, YAML supports comments (prefixed with #), multi-line strings, and anchors/aliases for reusing values within a document.

How YAML Validation Works

This tool parses your YAML input using a JavaScript YAML parser and checks for syntax errors including incorrect indentation, invalid use of tabs (YAML requires spaces only), duplicate keys, improper nesting, unclosed quotes, and malformed data types. If the YAML is valid, it is re-formatted and displayed cleanly. If invalid, an error message identifies what went wrong and the approximate line where the problem occurs. The parser follows the YAML 1.2 specification, which is the version used by most modern tools. All validation happens entirely in your browser with no data sent to any server.

Common Use Cases

• Validating Docker Compose files before running docker compose up
• Checking Kubernetes manifests for syntax errors before applying them
• Debugging CI/CD pipeline configurations (GitHub Actions, GitLab CI)
• Verifying Ansible playbooks and inventory files
• Validating Helm chart values and templates
• Checking application configuration files (Spring Boot, Rails, etc.)

Common YAML Pitfalls

The most frequent YAML error is incorrect indentation, since YAML uses whitespace to define structure. Mixing tabs and spaces causes parsing failures because the YAML specification explicitly forbids tabs for indentation. Another common issue is the "Norway problem": unquoted values like no, yes, on, and off are interpreted as booleans rather than strings. Country codes like NO (Norway) become false unless quoted. Similarly, unquoted version numbers like 3.10 may be interpreted as the float 3.1, losing the trailing zero. Always quote strings that could be ambiguous. Duplicate keys are silently overwritten in most parsers, which can lead to hard-to-find configuration bugs.

For validating Docker Compose files specifically, use the Docker Compose Validator. For converting between YAML and JSON, try the JSON to YAML Converter. For validating JSON data, see the JSON Validator.