b2KIT

YAML Validator

Validate YAML syntax with detailed error messages, line numbers, and formatted output display.

Tested tool guide Tested browser tools Checked August 16, 2026

What YAML Validator does, with a checked example

A single mis-indented line can invalidate an entire YAML document, and this tool finds it for you: give it the text and it parses the document in the browser - nothing is uploaded. Valid YAML is confirmed and rendered in the output pane as its resolved structure, with scalars converted to their types. Invalid YAML is rejected with the line number where parsing stopped and an explanation of what the parser expected there, so a stray tab or missing colon is located in seconds. What surprises most users: a green result proves only syntax. Indentation must be spaces - the spec forbids tabs - and the keys and types your application expects are never checked.

Worked example

A concrete input and expected output from the current implementation.

Input

name: validator
version: 2
tags:
  - yaml
  - tools
settings:
  strict: true

Expected output

{
  "name": "validator",
  "version": 2,
  "tags": ["yaml", "tools"],
  "settings": {
    "strict": true
  }
}

The document is syntactically valid, so the tool confirms it and displays the parsed structure. Plain scalars are resolved by their text: 2 becomes a number, true a boolean, and the remaining words stay strings.

How the result is produced

1

Parsing and error location

The tool reads the whole document and parses it from start to finish. At the first syntax failure it stops and reports the line where parsing broke down and what the parser expected to find there. Each run surfaces one error at a time, so you fix it and re-run. A document that parses cleanly is marked valid and its resolved structure is displayed.

2

Indentation and scalar typing

Block structure is defined by indentation, and the specification forbids tabs in it, so a tab where spaces are expected is a syntax error, reported with its line number. Unquoted plain scalars are typed by their text: 2 resolves to a number, true to a boolean, and everything else to a string. Quoting a value forces it to remain a string.

Good uses

  • A docker-compose.yml, GitHub Actions workflow, or Kubernetes manifest that a tool or CI pipeline refuses to load - paste the file in and read off the offending line instead of scanning hundreds of lines by eye.
  • Before committing a hand-edited or generated .yaml file, confirm it still parses and preview the resolved structure, so surprises about which values became numbers or booleans surface before they reach the application.
  • When writing YAML by hand and unsure whether a construct is legal - block versus flow style, when quotes are needed, whether an indented sequence sits under the right key - test the fragment here first.

Limits and checks

  • A pass is a syntax check only. Missing required keys, unexpected fields, and wrong value types are not detected, so a document can validate here and still fail to load in your application; keep your real loader as the final judge.
  • Type resolution depends on the YAML version. Under YAML 1.1, yes, no, on, and off are booleans; under the 1.2 core schema they are plain strings, and merge keys behave differently. Other tools may therefore parse the same document differently.
  • Unquoted scalars that look like numbers are converted to numbers: 1.0 becomes a float, and date-like values can be resolved to timestamps by parsers that support them. Read the formatted output to see the types your application will receive, and quote anything meant to stay text.

Common questions

The document validates here, but my program still rejects it. Is the validator wrong?

Not necessarily. The tool checks YAML syntax only: it cannot know which keys your application requires, what types it expects, or its other semantic rules. A version mismatch can also matter, since parsers targeting YAML 1.1 and 1.2 resolve some scalars differently. Treat a pass here as a first filter and always run your own loader.

Does it flag duplicate keys in a mapping?

Not necessarily. The YAML 1.2 specification declares duplicate keys an error, but not every parser enforces that, so the tool may accept the document with the last occurrence winning. If it does not flag a duplicate, do not assume the file is safe - other consumers of the file may reject it.

References and verification

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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