YAML Validator

Frequently asked questions

What kind of errors does the validator catch?

Indentation mismatches, duplicate keys, unterminated strings, invalid escape sequences, and any structural issue that prevents the document from parsing.

Does the validator check semantic rules like Kubernetes schemas?

The validator confirms YAML syntax only. Schema-level rules for Kubernetes, OpenAPI, or other formats require a separate schema validator.

Why does the validator say my file is invalid when it looks fine?

The most common cause is a tab used for indentation: YAML allows only spaces there, and the parser stops with 'tab characters must not be used in indentation'. Other frequent causes are a key repeated in the same mapping ('duplicated mapping key'), a quote that is never closed, and a line indented one space more or less than its siblings ('bad indentation of a mapping entry'). The first line of the message ends with the line and column, for example (2:1), and the lines below it point to the spot.

Why does a CloudFormation template fail with an unknown tag error?

The validator parses with js-yaml's JSON schema, which knows mappings, lists, strings, numbers, booleans and null and their standard tags such as !!str. Custom tags such as CloudFormation's !Ref, !Sub and !GetAtt, and YAML 1.1 types such as !!binary, !!set and !!timestamp, stop the parse with 'unknown tag' even though CloudFormation accepts the short forms. Rewrite them in long form (Ref: or Fn::GetAtt:) to check the rest of the file.

Does the validator check every document in a multi-document file?

Yes. Every document separated by --- is parsed. A valid file with three documents reports 'Valid YAML. All 3 documents parsed successfully.', and an error gives the line and column counted from the start of the whole file.