YAML Validator Free Online Tool 2026

Last updated March 22, 2026

YAML Validator Examples

The YAML Validator checks your YAML documents for syntax errors, indentation problems, duplicate keys, and type issues. Below are examples of common YAML mistakes and what valid YAML looks like.

Valid YAML — Basic Structure

A well-formed YAML configuration file:

server:
  host: localhost
  port: 8080
  debug: false

database:
  url: jdbc:postgresql://localhost:5432/mydb
  username: admin
  pool:
    min: 2
    max: 10

features:
  - authentication
  - logging
  - caching

Validator result: Valid YAML. No errors found.

Indentation Error

Inconsistent indentation breaks YAML parsing:

server:
  host: localhost
    port: 8080   # ERROR: over-indented

Validator output:

Error at line 3, column 5:
  Mapping values are not allowed here.
  Expected indentation of 2 spaces, found 4.

Fixed version:

server:
  host: localhost
  port: 8080

Tab Character Error

YAML requires spaces for indentation — tabs are not allowed:

server:
	host: localhost   # ERROR: tab character used

Validator output:

Error at line 2, column 1:
  Tab characters are not allowed for indentation.
  Replace the tab with spaces.

Fixed version (using 2 spaces):

server:
  host: localhost

Duplicate Key Detection

Duplicate keys in a mapping are invalid:

database:
  host: localhost
  port: 5432
  host: remoteserver   # ERROR: duplicate key

Validator output:

Warning at line 4:
  Duplicate key "host" found in mapping.
  First occurrence at line 2. Last value will be used.

Missing Colon

A missing colon after a key is a common typo:

app
  name: my-service   # ERROR: missing colon after "app"

Validator output:

Error at line 1:
  Could not find expected ':' after key "app".

Kubernetes Manifest Validation

A valid Kubernetes Deployment manifest:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-app
  labels:
    app: web
spec:
  replicas: 2
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
        - name: web
          image: nginx:1.25
          ports:
            - containerPort: 80

Validator result: Valid YAML. Parsed structure shown in the output panel.

Multi-line String Validation

Both block scalar styles are valid:

# Literal block (preserves newlines)
description: |
  This is line one.
  This is line two.

# Folded block (newlines become spaces)
summary: >
  This long text will be
  folded into a single line.

Anchor and Alias Validation

Anchors and aliases must reference defined anchors:

defaults: &defaults
  retries: 3
  timeout: 30

service_a:
  <<: *defaults
  name: service-a

service_b:
  <<: *defaults
  name: service-b

Validator result: Valid. Anchors resolved correctly.

Invalid alias (referencing undefined anchor):

service:
  <<: *undefined_anchor   # ERROR

Validator output:

Error at line 2:
  Alias "*undefined_anchor" references an undefined anchor.

Docker Compose Validation

Valid Docker Compose file:

version: "3.8"
services:
  web:
    image: nginx:latest
    ports:
      - "80:80"
    depends_on:
      - db
  db:
    image: postgres:15
    environment:
      POSTGRES_DB: appdb
      POSTGRES_USER: admin
      POSTGRES_PASSWORD: secret
    volumes:
      - db_data:/var/lib/postgresql/data
volumes:
  db_data:

Common Validation Errors Summary

• Indentation inconsistency — mixing 2-space and 4-space indentation
• Tab characters used instead of spaces
• Missing colon after a mapping key
• Duplicate keys in the same mapping block
• Unclosed quotes around string values
• Invalid boolean values (use true/false, not yes/no in strict mode)
• Alias referencing an undefined anchor
• Incorrect list item indentation (dash must align with siblings)

Common Use Cases

• Validating Kubernetes manifests before running kubectl apply
• Checking Docker Compose files before docker-compose up
• Verifying CI/CD pipeline YAML (GitHub Actions, GitLab CI, CircleCI)
• Validating Helm chart values files
• Checking application configuration files before deployment
• Pre-commit hook validation to catch errors early
• Normalizing YAML formatting with the built-in formatter

Paste your YAML into the validator to get instant feedback with precise line and column error locations, making it easy to find and fix issues before they cause deployment failures.