YAML Validator
DataValidate YAML syntax instantly. Parses the full document, reports line and column for every error and warning. Browser-only — your data is never uploaded.
What is a YAML?
The YAML Validator parses your YAML content using a full standards-compliant parser and reports every syntax error and warning with its exact line and column position. Unlike a simple format checker, this tool actually builds the document structure from your input — catching indentation errors, tab characters, duplicate keys, unclosed quotes, and structural inconsistencies that simpler validators miss.
YAML (YAML Ain't Markup Language) is the configuration language behind Kubernetes manifests, Docker Compose files, GitHub Actions workflows, Ansible playbooks, Helm charts, and countless other infrastructure tools. A single misplaced space or stray tab character in a YAML file causes the entire document to fail to parse — and the error message from the tool consuming the file is often unhelpful ("unexpected token at line 42" with no indication of what the actual problem is).
This validator uses the yaml npm package, which implements the YAML 1.2 specification. It surfaces errors and warnings separately — errors mean the document is invalid; warnings flag non-fatal issues such as deprecated syntax or values that may be interpreted differently across YAML versions.
For related format validation, the JSON Validator and XML Validator cover the other common structured data formats.
How to use this YAML calculator
- Paste your YAML content into the text area. The default example shows a simple mapping with a nested list.
- The result updates automatically as you type or paste.
- Check the Valid or Invalid badge.
- If Invalid, read the error details — each error includes its line and column number for direct navigation.
- If Valid, check the structure summary in the details section to confirm the document parsed as expected.
- Clear the field and paste new content to validate a different document.
Formula & Methodology
This validator uses the `yaml` npm package (v2), which implements the YAML 1.2 specification. Internally, it callsparseDocument()which builds the full document AST and collects errors rather than throwing on the first issue, allowing all errors in the document to be reported at once. Common YAML rules checked: - Indentation must use spaces only — tab characters are never valid for indentation - All keys in a mapping must be unique at the same level - Block sequences use-(dash space) at consistent indentation - Quoted strings must have matching close quotes - Colons in unquoted scalars must be followed by a space (or end of line) to be interpreted as key-value separators - Multi-document files are separated by---(document start) and optionally...(document end) Valid example:yaml server: host: localhost port: 8080 tags: - production - v2Invalid examples: - Using a tab character beforehost:— tab is not a valid indentation character in YAML - Writingport:8080without a space after the colon — parsed as the stringport:8080, not a key-value pair - Duplicate keyhost:appearing twice in the same mapping block
Frequently Asked Questions
What is YAML?
YAML (YAML Ain't Markup Language) is a human-readable data serialisation format commonly used for configuration files, infrastructure-as-code definitions, CI/CD pipeline specifications, and data exchange between services. It uses indentation to express structure, making it more readable than JSON or XML for complex nested data. YAML is used extensively in tools like Kubernetes, Docker Compose, Ansible, GitHub Actions, and Helm charts.
What kinds of YAML errors does this validator catch?
The validator uses a full YAML parser and catches all syntax errors, including: incorrect indentation (tabs instead of spaces, inconsistent indent levels), unclosed quotes, invalid escape sequences, duplicate keys in the same mapping, tab characters used for indentation (YAML requires spaces), invalid scalar values, and structural issues like mismatched block sequences and mappings. It also reports warnings for non-fatal issues such as deprecated syntax.
What is the difference between a YAML error and a YAML warning?
An error means the YAML is syntactically invalid and cannot be parsed — the document structure cannot be determined. A warning means the YAML is technically parseable but uses syntax that is deprecated, ambiguous, or non-standard. For example, using an unquoted string that could be interpreted as a different type (like 'yes' being parsed as a boolean in YAML 1.1) may produce a warning in strict parsers. This validator surfaces both errors and warnings with their line and column positions.
Why does YAML use spaces and not tabs for indentation?
The YAML specification explicitly prohibits tab characters for indentation because tabs are rendered differently across editors and environments, making it impossible to determine consistent indentation depth. Tabs must be used only where a literal tab character is part of a value (inside a quoted string). Using a tab for indentation in YAML always results in a parse error. This is one of the most common YAML mistakes, especially for users coming from languages like Python that also enforce indentation but allow tabs.
Is YAML a superset of JSON?
YAML 1.2 (the current specification) defines itself as a superset of JSON, meaning any valid JSON document is also valid YAML. This is not true for YAML 1.1 (the older version still used by many tools), which has subtle differences in how it handles certain values (for example, 'yes' and 'no' are booleans in YAML 1.1 but strings in YAML 1.2 and JSON). In practice, most YAML files use YAML-specific syntax (block sequences, unquoted strings, comments) that is not valid JSON.
What is the difference between block style and flow style in YAML?
Block style uses newlines and indentation to express structure — it is the standard human-readable format. Flow style uses braces and brackets, similar to JSON syntax, and keeps everything on one line. For example, a block-style list uses '- item' notation, while the flow-style equivalent is '[item1, item2]'. Both are valid YAML and can be mixed within the same document. This validator accepts both styles.
Is my YAML content stored or transmitted when I use this tool?
No. The YAML parser runs entirely in your browser. Your content is never sent to any server, stored in a database, or shared with any third party. This makes the tool safe to use with configuration files that contain environment variables, API keys, or other sensitive information — though you should always exercise caution when pasting sensitive data into any web tool.
Why do I get a YAML parse error for content that works in my application?
Different YAML parsers implement different versions of the YAML specification or have their own extensions. A file that parses successfully in Python's PyYAML (which uses YAML 1.1) may fail in a strict YAML 1.2 parser, or produce different values for the same input. This validator uses the 'yaml' npm package, which implements YAML 1.2 with optional 1.1 compatibility. If your content works in your application but fails here, check whether the issue is a YAML 1.1 vs 1.2 compatibility difference.
Can I validate multi-document YAML files (with ---)?
Yes. YAML supports multiple documents within a single file, separated by '---' (document start marker). This validator parses the full input including multiple documents and reports errors with their specific line and column positions within the file.
What is the difference between YAML and JSON, and when should I use each?
YAML is more human-readable — it supports comments, uses indentation instead of braces, and allows unquoted strings. It is the preferred format for configuration files that humans edit directly (Kubernetes manifests, CI/CD configs, Ansible playbooks). JSON is stricter, more compact, and universally supported by APIs and data exchange protocols. Use YAML when humans need to read and edit the file; use JSON for machine-to-machine data exchange. The [JSON Validator](/json-validator/) is available for JSON-specific validation.