Format, validate, and lint YAML files with proper indentation and error highlighting.
YAML is forgiving about whitespace, which is why config files drift into a mess: sequences at odd indent levels, double spaces after colons, tabs where spaces belong. This tool parses the document you paste, re-emits it with a consistent two-space layout, and flags anything that does not parse, highlighting the offending line and column. The surprise most people hit: validation passing does not mean the document means what you think. Unquoted values become numbers, booleans, or timestamps, duplicate keys are often accepted with the last one winning, and processing happens entirely in the browser, which matters because config files routinely contain credentials. Read the lint output, not just the check mark.
A concrete input and expected output from the current implementation.
Input
name: demo version: "1.0" enabled: true servers: - host: web1 port: 80 - host: web2 port: 8080
Expected output
name: demo
version: "1.0"
enabled: true
servers:
- host: web1
port: 80
- host: web2
port: 8080
The input parses as written, because YAML permits a block sequence at the same column as a top-level key, so validation passes before formatting runs; the formatter then re-emits the same structure with two-space indentation, dashes under their key and nested keys two spaces deeper, while the quotes around 1.0 are untouched because quoting controls the value's type.
The formatter does not edit the text line by line. It parses the whole document first, so malformed input stops with a line-and-column error rather than half-formatted output. The parsed structure is then written back with fixed rules: two spaces per indentation level, one space after each colon, dashes indented under their parent key. Scalar quoting is preserved as written, because rewriting quotes can change a value's type.
Results come from two separate passes. Validation checks the grammar: tabs used for indentation, mismatched flow-collection brackets, unterminated quotes, and inconsistent indentation inside one block all fail here. Lint runs on top for constructs that parse but look wrong, notably duplicate keys, which many parsers accept silently, last value winning. Read them together: green validation with lint warnings means the file will load, but possibly not as you wrote it.
Validation covers YAML syntax only; applications add their own requirements, like mandatory keys, allowed value types, or a different YAML version, and 1.1 versus 1.2 changes how values such as yes and 07 are read. Duplicate keys are another gap: this tool flags them, while your app's parser may accept them with the last value winning. Compare the app's exact error message with the line the validator highlights.
Not for ordinary documents: the formatter re-serializes the same parsed structure, and it never rewrites quotes because quoting controls type. The caveats are edge-case documents that implementations parse differently, and comments, which formatters attach to the re-emitted structure in different ways, so check the diff if your file is comment-heavy.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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