YAML Formatter & Validator

How to use this tool

1. Paste YAML or JSON into the input panel, or open/drop a local .yaml or .yml file.
2. Choose "YAML to format" or "JSON to YAML"; switching modes keeps your input in place.
3. Choose 2- or 4-space indentation and block or compact flow output; key sorting is optional.
4. Review the result or switch to JSON to inspect the parsed structure.
5. Copy the displayed text or download the formatted YAML file.

What is YAML?

YAML (YAML Ain't Markup Language) is a human-readable data serialisation format used extensively for configuration files: Docker Compose, Kubernetes manifests, GitHub Actions workflows, Ansible playbooks, and many more. Its whitespace-based syntax is easy to read and write, but a misplaced indentation or stray character will break the file entirely.

Why use a YAML formatter?

A formatter catches syntax errors before they reach production, normalises indentation so that YAML from different sources looks consistent, and makes deeply nested structures easier to read. This tool also lets you switch to the JSON view to confirm exactly what your YAML parser will see after loading the file.

JSON to YAML conversion

Many APIs return JSON. When you need to put that data into a YAML configuration file, manual conversion is tedious and error-prone. Select JSON to YAML, paste your JSON, and get a clean YAML representation instantly. Changing the input mode leaves the current text intact; use Load sample when you want to replace it with the built-in example.

Common YAML pitfalls

YAML's implicit typing is the source of many subtle bugs:

• Boolean ambiguity: in YAML 1.1 (used by PyYAML and many older parsers), bare words like yes, no, on, off, true, and false are parsed as booleans. In YAML 1.2 (stricter, used by newer parsers), only true and false are booleans.
• The Norwegian problem: NO (the ISO 3166-1 alpha-2 country code for Norway) parses as false in YAML 1.1. Similarly, country: NO in a config file will silently become false in PyYAML. Always quote strings that happen to match YAML keywords: country: "NO".
• Numeric confusion: bare values like 1.0, 1e3, and 0x1F are parsed as numbers. Phone numbers and version strings should be quoted.

Multiline string syntax

YAML provides two block scalar styles for multiline strings, each with three chomping options:

• Literal block (|): preserves newlines exactly as written. Useful for shell scripts, SQL, or any text where line breaks are meaningful.
• Folded block (>): folds newlines into spaces (like reflowing a paragraph). Single blank lines become newlines in the output.

Chomping indicators control trailing newlines:

• |- or >- - strip: remove all trailing newlines.
• | or > - clip (default): keep exactly one trailing newline.
• |+ or >+ - keep: preserve all trailing newlines.

YAML anchors and aliases

YAML anchors and aliases let a document reuse content, reducing repetition in complex configurations. This section is a syntax primer: formatting parses and re-emits data with js-yaml, so it does not retain the original anchor, alias, or merge-key syntax.

• &anchor-name - define an anchor on a value or mapping.
• *anchor-name - reference (alias) the anchored value.
• <<: *anchor-name - merge key: merge all key-value pairs from the anchored mapping into the current mapping (useful for shared defaults).

Example using merge keys in a Docker Compose-style config:

      defaults: &defaults
  restart: unless-stopped
  logging:
    driver: json-file

web:
  <<: *defaults
  image: nginx:alpine
  ports:
    - "80:80"

api:
  <<: *defaults
  image: node:20
  ports:
    - "3000:3000"
    

Formatting also removes comments because comments are not part of the parsed data model. If comments or the exact anchor structure matter, keep the original in a text editor and avoid replacing it with a formatter round-trip.

Open and save YAML files

Open a local .yaml or .yml file with the file picker, or drop one into the input panel. Files up to 2 MB are read and parsed on your device. After reviewing the result, choose Download YAML to save a formatted .yaml copy. The download uses the opened file's name when available; pasted text downloads as formatted.yaml.

Privacy

All processing runs in your browser using the open-source js-yaml library. Pasted values and opened files stay on your device; nothing is uploaded to a server. Downloaded output is created locally by your browser.