Why can't YAML use tabs for indentation?

The YAML specification explicitly prohibits tab characters for indentation to avoid ambiguity: different editors render tabs as different widths (2, 4, or 8 spaces), which would make the structural indentation unpredictable. YAML requires spaces for indentation, with consistent spacing throughout the document.

My string "no" is being parsed as a boolean false — how do I fix it?

In YAML 1.1, the strings yes, no, on, off, true, false are automatically coerced to booleans. To force a string, quote the value: "no" (with quotes in your YAML). In YAML 1.2, only true and false are booleans; yes/no/on/off remain strings.

What is the difference between | and > for multi-line strings?

The | (literal block scalar) preserves newlines as actual newline characters in the string. The > (folded block scalar) converts single newlines to spaces but preserves paragraph breaks (double newlines). Use | for content where line breaks matter (code, poems) and > for prose text that should flow.

YAML Validator | DevStackTools

YAML Validator — Check YAML Syntax & Structure Online

Understanding the Basics

YAML (YAML Ain't Markup Language) is a human-friendly data serialization format widely used for configuration files in DevOps, cloud infrastructure, and application development. Unlike JSON's strict syntax, YAML's indentation-based structure and minimal punctuation make it highly readable — but also more prone to subtle errors that are difficult to spot visually. A single incorrect indentation level can completely change the document's meaning; a tab character where spaces are expected (YAML prohibits tab indentation) causes a parse failure; special characters in unquoted strings trigger unexpected type coercions (the string "on" becomes boolean true in YAML 1.1); and multi-line string blocks (using | or > indicators) have nuanced whitespace handling rules. YAML validation is critical for Kubernetes manifests, Helm charts, GitHub Actions workflows, Docker Compose files, Ansible playbooks, and CI/CD pipeline definitions where a YAML syntax error silently breaks deployments or causes cryptic runtime failures. Our validator uses the js-yaml library in YAML 1.2 strict mode, which reports precise line numbers and character positions for every syntax error.

How to Use the Tool

Paste your YAML document into the editor.
The validator parses the document automatically using js-yaml in strict mode.
Valid YAML shows a green success indicator and displays the parsed structure as JSON for verification.
Invalid YAML shows the exact error message with line number and column position.
Common errors (tab indentation, missing quotes around special values) are highlighted in the editor.

Key Features & Specifications

Validates YAML 1.2 syntax using the js-yaml library in strict mode
Detects tab indentation errors (YAML requires spaces, not tabs)
Reports precise line number and column position for every error
Shows the parsed result as JSON so you can verify the parsed structure matches expectations
Warns about YAML 1.1 boolean pitfalls (yes/no/on/off coerced to true/false)

Frequently Asked Questions (FAQ)

Q:Why can't YAML use tabs for indentation?: The YAML specification explicitly prohibits tab characters for indentation to avoid ambiguity: different editors render tabs as different widths (2, 4, or 8 spaces), which would make the structural indentation unpredictable. YAML requires spaces for indentation, with consistent spacing throughout the document.
Q:My string "no" is being parsed as a boolean false — how do I fix it?: In YAML 1.1, the strings yes, no, on, off, true, false are automatically coerced to booleans. To force a string, quote the value: "no" (with quotes in your YAML). In YAML 1.2, only true and false are booleans; yes/no/on/off remain strings.
Q:What is the difference between | and > for multi-line strings?: The | (literal block scalar) preserves newlines as actual newline characters in the string. The > (folded block scalar) converts single newlines to spaces but preserves paragraph breaks (double newlines). Use | for content where line breaks matter (code, poems) and > for prose text that should flow.