YAML Formatter and Validator

Where you meet YAML

YAML is the configuration language of Kubernetes manifests, Helm values, GitHub Actions and GitLab CI pipelines, Docker Compose files, Ansible playbooks and OpenAPI specs. It is meant to be easy to read, but its reliance on indentation means one misplaced space can turn a list item into a map key, and the resulting error often points somewhere other than the real mistake.

This formatter parses the whole document with the yaml library (YAML 1.2 by default, 1.1 on request), reports every error with a line number and a hint, and re-prints the file with consistent indentation. Comments, blank-line grouping, anchors (&base), aliases (*base), merge keys (<<), tags and multi-document streams separated by --- survive the round trip.

Quick start

Paste a manifest or drop a .yaml / .yml file. Format with Ctrl/Cmd+Enter, copy with Ctrl/Cmd+Shift+C, and use Ctrl/Cmd+K for the command palette (switch to the Tree or Table view, convert to JSON, query with JSONPath). Ctrl/Cmd+Shift+M produces a one-line flow-style version with comments removed, handy for embedding YAML in an environment variable.

Secrets in Helm values or CI files are a real concern with online tools. Here parsing happens inside your browser and no request carries your text.

Formatting options

• Collections: As written keeps each map and list in the style it already has. Block (indented) expands inline { } and [ ] collections into indented lines, which is what most style guides want for Kubernetes. Flow ({ } and [ ]) does the reverse and wraps at the line width.
• String quotes: As written leaves quoting alone. Plain where possible removes quotes that are not needed; it keeps them where removing would change the type, so "8080" stays a string. That judgement follows the selected YAML version: under 1.2, "NO" loses its quotes, so pick 1.1 if a 1.1 parser will read the file. Double quotes and Single quotes quote every string value. Block scalars (| and >) are never touched, and keys stay plain.
• Indent lists under keys: on gives containers:\n - name: web; off gives the compact containers:\n- name: web style that kubectl output uses.
• YAML version: see the next section.

Indent can be 2 or 4 spaces; if you choose tabs, the output uses 2 spaces and says why, because the YAML spec forbids tab indentation. Sort keys orders map keys alphabetically, which is useful before diffing two exported manifests.

YAML 1.1, YAML 1.2 and the Norway problem

In YAML 1.1, the plain words yes, no, on, off, y and n are booleans. YAML 1.2 dropped that rule, so they are ordinary strings. Many widely used libraries still follow 1.1, including PyYAML (and therefore Ansible) and the go-yaml v2 library behind older Kubernetes tooling. The classic failure is a list of country codes where NO (Norway) silently becomes false.

With YAML version set to YAML 1.2, the formatter adds an info note on every such value. Switch to YAML 1.1 to see how a 1.1 parser will read the file; those notes become warnings. Quoting the value ("NO") is correct under both versions.

Anchors, aliases and alias bombs

Formatting never expands aliases; *defaults stays *defaults. The Tree view and conversions such as YAML to JSON do have to expand them, and a malicious file can nest aliases so that a few lines expand to billions of nodes (the YAML version of “billion laughs”). Expansion stops at 1,000 aliases with a clear message instead of freezing the tab.

Examples

apiVersion: apps/v1
kind: Deployment
metadata:
    name: checkout
    labels: {app: checkout, tier: backend}
spec:
  replicas: 3
  selector:
    matchLabels: {app: checkout}
  template:
    metadata:
      labels: {app: checkout}
    spec:
      containers:
      - name: api
        image: "registry.example.com/checkout:2.4.0"
        ports:
        - containerPort: 8080
        resources:
            limits: {cpu: 500m, memory: 512Mi}

apiVersion: apps/v1
kind: Deployment
metadata:
  name: checkout
  labels:
    app: checkout
    tier: backend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: checkout
  template:
    metadata:
      labels:
        app: checkout
    spec:
      containers:
        - name: api
          image: "registry.example.com/checkout:2.4.0"
          ports:
            - containerPort: 8080
          resources:
            limits:
              cpu: 500m
              memory: 512Mi

name: CI
on:
  push: {branches: [main]}
  pull_request:
jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix: {node: [20, 22, 24]}
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: {node-version: "${{ matrix.node }}", cache: npm}
      - run: npm ci
      - run: npm test -- --coverage

name: CI
on:
  push: { branches: [ main ] }
  pull_request:
jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix: { node: [ 20, 22, 24 ] }
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: { node-version: "${{ matrix.node }}", cache: npm }
      - run: npm ci
      - run: npm test -- --coverage

x-service-defaults: &defaults
  restart: unless-stopped
  logging: {driver: json-file, options: {max-size: "10m"}}
services:
  api:
    <<: *defaults
    image: shop/api:1.8
    environment: [DATABASE_URL=postgres://db:5432/shop, LOG_LEVEL=info]
  worker:
    <<: *defaults
    image: shop/worker:1.8
    depends_on: [api]

x-service-defaults: &defaults
  restart: unless-stopped
  logging: { driver: json-file, options: { max-size: "10m" } }
services:
  api:
    <<: *defaults
    image: shop/api:1.8
    environment: [ DATABASE_URL=postgres://db:5432/shop, LOG_LEVEL=info ]
  worker:
    <<: *defaults
    image: shop/worker:1.8
    depends_on: [ api ]

ntp_enabled: yes
firewall: off
allowed_countries: [SE, DK, NO, FI]
admin_users:
  - name: deploy
    sudo: true

ntp_enabled: yes
firewall: off
allowed_countries: [ SE, DK, NO, FI ]
admin_users:
  - name: deploy
    sudo: true

Common errors and how to fix them

Related tools

• yaml validator
• yaml minifier
• YAML to JSON
• JSON to YAML
• YAML to TOML
• json vs yaml
• yaml multiline strings
• yaml norway problem