YAML Input

What is YAML Validator?

YAML (YAML Ain't Markup Language) is a human-readable data serialization format widely used for configuration files in tools like Kubernetes, Docker Compose, GitLab CI, Ansible, and GitHub Actions. Despite its readability, YAML is notoriously strict about indentation — a single tab character or misaligned space can cause a parser to fail with cryptic error messages. This validator performs client-side static analysis to catch the most common YAML mistakes before you deploy: tab characters (which YAML forbids entirely), inconsistent indentation, unclosed quotes, missing spaces after colons, duplicate keys in the same mapping, and unquoted values containing special characters. It does not require a full YAML parser — instead, it inspects the raw text line by line to surface structural issues that would otherwise only appear at runtime.

How to Use

Paste your YAML content into the input area, or click "Load Example" to try a sample configuration.
Click "Validate YAML" to run the syntax checker — errors appear with line numbers.
Review each error message: it identifies the line number and the specific issue (tabs, indentation, quotes, etc.).
Fix the reported issues in your YAML and re-validate until no errors remain.
Use the "Clear" button to reset the input and start fresh with new content.

Why Use This Tool?

Catch YAML syntax errors before deploying to Kubernetes, GitLab CI, or Ansible

Line-numbered error messages make it easy to locate and fix problems

Detects tabs — the single most common YAML mistake that causes silent failures

Identifies duplicate keys that would silently overwrite values at runtime

All validation runs in your browser — your configuration data never leaves your device

No installation required — works on any device with a modern browser

Tips & Best Practices

YAML requires spaces for indentation — never use tabs. Configure your editor to insert spaces when you press Tab.
Keep indentation consistent: 2 spaces per level is the most common convention. Mixing 2 and 4 spaces causes errors.
Wrap values containing colons (": ") in quotes to prevent parsing ambiguity.
Use a linter like yamllint in your CI pipeline to catch issues that manual validation might miss.

Frequently Asked Questions

What errors does this YAML validator detect?

It detects tab characters, indentation mismatches (where indent levels don't align with any parent), unclosed single and double quotes, missing spaces after colons in key-value pairs, duplicate keys in the same mapping, and unquoted values containing ": " sequences. These are the most common YAML errors that cause parser failures.

When should I NOT use this YAML validator?

This validator performs structural checks, not full parsing. It cannot validate YAML anchors and aliases, multi-document markers (---/...), type coercion (e.g., "true" becoming boolean), or schema-specific rules. For those, use a full YAML parser like PyYAML or js-yaml. Also, very large YAML files (thousands of lines) may be better validated locally with command-line tools.

Is my YAML data safe?

Yes. All validation happens entirely in your browser using JavaScript. Your YAML content is never sent to any server, stored, or transmitted. You can safely validate configurations containing passwords, API keys, or other sensitive data.

Why are tabs not allowed in YAML?

The YAML specification (RFC 1.2) explicitly forbids tab characters for indentation because they create ambiguity — a tab may represent different numbers of spaces depending on the editor. YAML parsers universally reject tabs, producing errors like "found character that cannot start any token". Always use spaces.

Does this validator support YAML with multiple documents?

It can detect basic issues in multi-document YAML (separated by ---), but it does not validate document boundaries or the ... end marker. For complex multi-document files, use a dedicated YAML parser that fully implements the specification.

Why does my YAML work in some parsers but not others?

Different YAML parsers implement different versions of the specification (1.1 vs 1.2) and have varying strictness levels. For example, YAML 1.1 treats "yes/no/on/off" as booleans, while YAML 1.2 does not. Some parsers are more lenient with indentation. Always validate against the parser used in your target environment.

Real-world Examples

Detecting tabs in a Kubernetes Deployment

A common mistake when editing Kubernetes manifests: pressing Tab instead of Space creates invisible tab characters that cause kubectl to reject the YAML.

Input

apiVersion: apps/v1
kind: Deployment
metadata:
	name: my-app
	namespace: production
spec:
	replicas: 3

Output

Line 4: Tabs are not allowed in YAML — use spaces for indentation
Line 5: Tabs are not allowed in YAML — use spaces for indentation
Line 7: Tabs are not allowed in YAML — use spaces for indentation

Duplicate key in a Docker Compose file

Having the same key twice in a mapping is technically allowed by some parsers but usually overwrites the first value silently — a source of subtle bugs.