Free · Open source · Client-side

YAML Validator

Validate YAML syntax and find errors

How to Validate YAML

Paste your YAML content into the editor and validation runs instantly. The validator checks syntax, catches duplicate keys at any nesting level, and flags tabs used for indentation. Valid YAML is parsed and displayed as formatted JSON in the output panel. Errors include line numbers, column positions, and descriptive messages.

What the validator checks

The YAML Validator checks for syntax errors (malformed key-value pairs, incorrect indentation), duplicate keys at any nesting level (using the yaml package with uniqueKeys mode), and tabs in indentation (YAML requires spaces, not tabs). Valid input is parsed and displayed as formatted JSON so you can verify the parsed structure matches your intent.

Duplicate key detection

Duplicate keys in YAML silently overwrite earlier values, which causes hard-to-find bugs in configuration files. This validator uses strict mode to flag duplicate keys as errors instead of silently merging them. Keys with the same name under different parent objects are allowed — only true duplicates at the same level are flagged.

Common YAML mistakes

The most common YAML errors are: tabs instead of spaces for indentation, missing colons after keys, incorrect nesting depth, unquoted strings that look like booleans (yes/no become true/false), and duplicate keys that silently overwrite values. This validator catches all of these with clear error messages.

Frequently Asked Questions

Is the YAML Validator free and open source?

Yes. The YAML Validator is free and open source under the Apache 2.0 license. The full source code is on GitHub at github.com/jamdesk/utilities, and there are no ads, accounts, or usage limits.

Does it catch duplicate keys?

Yes. The validator uses strict mode with duplicate key detection. If the same key appears twice at the same nesting level, it is flagged as an error with the exact line number.

What is the difference between YAML and JSON?

YAML uses indentation for structure while JSON uses braces and brackets. YAML supports comments, multi-line strings, and anchors. JSON is stricter but more widely supported by APIs. Both represent the same data structures — you can convert between them losslessly.

Why does YAML not allow tabs?

The YAML specification requires spaces for indentation because tab width varies between editors. A file that looks correct in one editor may have wrong indentation in another. Spaces ensure consistent parsing regardless of editor settings.

About this tool

The YAML Validator is free and open source, and runs entirely in your browser. Your input is never uploaded, stored, or logged — you can verify this by opening the network tab in your browser developer tools while the tool runs. The full source code is on GitHub under the Apache 2.0 license, so you can read the code, file issues, or fork the project. There are no ads, no accounts, and no usage limits. Built and maintained by Jamdesk.

Maintained by Jamdesk · Last reviewed 2026-04-29

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