YAML (YAML Ain't Markup Language) is a human-readable data serialization format commonly used for configuration files, deployment manifests, and data exchange. It uses indentation to represent structure, making it cleaner and more readable than JSON or XML for configuration purposes.

Why does my YAML fail validation?

Common YAML errors include using tabs instead of spaces for indentation (YAML only allows spaces), inconsistent indentation levels, duplicate keys at the same nesting level, missing colons after keys, and incorrect list syntax. This validator reports the specific error and line number to help you fix the issue.

Can I use tabs in YAML?

No. The YAML specification explicitly forbids tab characters for indentation. You must use spaces only. Most code editors can be configured to insert spaces when you press the Tab key. This is the most common source of YAML validation errors.

What indentation should I use in YAML?

YAML allows any number of spaces for indentation, but you must be consistent within each level. The most common convention is 2 spaces per indent level. Some projects use 4 spaces. Whatever you choose, be consistent throughout the entire file.

Is my YAML data safe with this tool?

Yes. All processing happens entirely in your browser using JavaScript. Your YAML data is never sent to any server.

YAML Validator

What is YAML?

YAML (YAML Ain’t Markup Language) is a human-readable data serialization language that has become the standard for configuration files in modern DevOps and software development. Originally released in 2001, YAML was designed to be more readable than XML and simpler than JSON for configuration purposes.

YAML is used extensively in tools like Kubernetes, Docker Compose, Ansible, GitHub Actions, GitLab CI, and many other platforms. Its reliance on indentation rather than brackets or tags makes it visually clean, but also makes it sensitive to formatting errors.

How to Use the YAML Validator

Paste your YAML content into the input area
Click “Validate” or press Ctrl+Enter to check syntax
Review any errors reported with line numbers
Fix the issues and re-validate until your YAML is clean

The validator checks for the most common YAML issues: tab characters, inconsistent indentation, duplicate keys, malformed key-value pairs, and incorrect list syntax.

Common YAML Errors and How to Fix Them

Tab Characters

YAML strictly forbids tab characters for indentation. This is the single most common YAML error:

# Wrong (uses tabs)
server:
	port: 8080

# Correct (uses spaces)
server:
  port: 8080

Configure your editor to use spaces instead of tabs for YAML files. In VS Code, set "editor.insertSpaces": true and "editor.tabSize": 2 for .yml and .yaml files.

Inconsistent Indentation

All items at the same level must use the same number of spaces:

# Wrong
database:
  host: localhost
   port: 5432

# Correct
database:
  host: localhost
  port: 5432

Duplicate Keys

YAML doesn’t allow duplicate keys at the same level. The second value silently overwrites the first, which can cause hard-to-debug issues:

# Wrong - 'port' appears twice
server:
  port: 8080
  host: localhost
  port: 3000

Missing Colon in Key-Value Pairs

Every key must be followed by a colon and a space:

# Wrong
name John

# Correct
name: John

YAML vs JSON

Feature	YAML	JSON
Readability	More readable	More verbose
Comments	Supported (`#`)	Not supported
Indentation	Significant	Not significant
Data types	Auto-detected	Explicit
Trailing commas	Not applicable	Not allowed
Multi-line strings	Native support	Requires `\n`

YAML is generally preferred for configuration files that humans edit frequently, while JSON is better suited for data exchange between systems and APIs. Many tools accept both formats.

YAML Best Practices

Use Consistent Indentation

Pick 2 spaces per level (the most common convention) and stick with it throughout your project. Configure your editor and linter to enforce this.

Quote Strings When Ambiguous

YAML auto-detects types, which can cause surprises. Values like yes, no, true, false, null, 1.0, and 3:00 are interpreted as booleans, null, floats, or sexagesimal numbers:

# Dangerous - interpreted as boolean true
country: Norway
uses_yaml: yes

# Safe - explicitly a string
country: "Norway"
uses_yaml: "yes"

Use Linting in CI/CD

Add YAML linting to your CI/CD pipeline using tools like yamllint. This catches formatting issues before they cause deployment failures. A misconfigured YAML file in Kubernetes or Docker Compose can bring down your entire infrastructure.

Validate Before Deploying

Always validate YAML configuration files before applying them to production systems. This online validator provides instant feedback, but for automated workflows, integrate a YAML linting step into your build pipeline.