YAML Validator

YAML makes structured data comfortable to edit by allowing indentation, comments, plain scalars, anchors, aliases, and document separators. That flexibility also means two questions must be kept apart: whether the text is valid YAML, and what native values a particular loader creates from it.

A scalar such as true, null, or a date-like value may load as a boolean, null, timestamp, or string depending on the selected schema. Mappings and sequences add another risk because indentation decides which values belong together. A file can be visually tidy yet place a key at the wrong depth or resolve a plain value differently from the destination application.

The three basic node kinds provide a useful mental model:

Scalar

One value such as text, a number, a boolean, null, a timestamp, or tagged data.

Sequence

An ordered list whose items may be scalars or nested containers.

Mapping

A set of key-value pairs. Keys are required to be unique in the YAML representation.

Multi-document streams use --- to place several documents in one source. They are common in deployment files and generated configuration, but a successful parse only proves that each document can be loaded under the selected schema. Kubernetes, CI systems, cloud services, and applications still impose their own fields, types, and policy rules.

Duplicate mapping keys deserve explicit treatment. Strict review rejects them because keeping only the later value can hide an earlier setting. A permissive loader may deliberately accept the file and overwrite the earlier value, but that behavior should be matched to the destination rather than assumed.

Structure counts and paths help review deeply nested data after syntax succeeds. They are evidence about the loaded representation, not a schema check. A path ledger can show where a value landed and which type was loaded, while the destination validator remains responsible for whether that value is allowed.

How to Use This Tool:

Choose the loader profile that most closely matches the system that will consume the YAML.

1. Paste or load the YAML source. A leading byte-order mark and Windows or classic Mac line endings are normalized before parsing.
2. Select Validation profile: Default for general configuration, Core for YAML 1.2-style scalar resolution, JSON for JSON-compatible scalar types, or Failsafe when plain leaves should remain strings.
3. Keep Duplicate-key handling on strict rejection unless the destination deliberately keeps the later value. Permissive comparison changes loaded data and adds a warning.
4. Fix the first parser error, then inspect Validation evidence and the Path ledger. Change the path-row limit or include container paths only to adjust visible evidence; neither setting changes parser validity.

Interpreting Results:

Valid YAML means every document loaded under the selected schema and duplicate-key policy. It does not mean the source satisfies a platform schema. Compare the loaded types and important paths with what the destination expects before using the parsed JSON as a handoff artifact.

An invalid result reports the first parser location and message. Later counts and paths are not trustworthy until that error is corrected. Warnings identify permissive duplicate handling or evidence limits; they do not silently change an invalid parse into a valid one.

Path-row limits affect only the displayed ledger. Complete mapping, sequence, scalar, depth, and path counts are computed from the bounded traversal independently of the visible-row choice.

Technical Details:

YAML loading turns source text into a representation made of mappings, sequences, and scalars. The selected schema controls how untagged scalars resolve to native types, while the duplicate-key option controls whether repeated mapping keys stop parsing or let a later value replace an earlier one.

Mechanism Core

The validation path is normalization, loading, bounded traversal, then evidence projection.

Rule Core

Single-document parsed JSON contains that document directly. A multi-document stream becomes a JSON array. Loaded timestamps are serialized as ISO text, binary values become a small byte-count object, and a repeated alias reference is represented by a circular-alias marker when serialization would revisit the same container.

The path ledger starts at $, uses dot notation for simple mapping keys, bracket notation for other keys, and numeric brackets for sequence positions. Container paths can be included alongside the default leaf-value paths when the nesting itself needs review.

Accuracy and Privacy Notes:

Pasted text and selected files are parsed in the browser and are not submitted to a validation endpoint. Source size is limited to 2 MiB.

• The result reflects the selected browser parser and schema profile. Another YAML version or loader may resolve scalars and duplicate keys differently.
• Paths and parsed JSON describe the loaded representation, not original comments, anchors, quoting, formatting, or key order.
• Traversal and stored-path ceilings can truncate evidence for very large structures while leaving the parser result intact.
• Run the destination's own schema, configuration, or policy validator before deployment.

Worked Examples:

References:

• YAML Ain't Markup Language, Revision 1.2.2, YAML Language Development Team, October 2021.
• RFC 8259: The JavaScript Object Notation Data Interchange Format, RFC Editor, December 2017.