YAML Formatter & Validator - Format YAML Online

What a YAML Formatter Should Do

YAML is the one common config format where indentation is the structure. Two spaces instead of four is not a style preference — it changes which block a key belongs to. That makes formatting YAML a different job from formatting Java or SQL: a tool that guesses wrong does not produce ugly output, it produces a different document.

This formatter reads the file first. It recovers each line's nesting depth from the original indentation, then re-emits the whole document at one consistent width. A file indented three spaces in one place and five in another comes out uniform, with the same meaning it went in with.

# Service config
name:    my-service
server:
      host: localhost
      tags:
      - web
      - api

# Service config
name: my-service
server:
  host: localhost
  tags:
    - web
    - api

Your Comments Survive

Most online YAML formatters parse the document into an object and print it back out. That produces tidy output and silently deletes every comment you wrote — which, in a file that is half explanation, is usually the worst possible outcome.

This one keeps them. Full-line comments are aligned with whatever they introduce, and trailing comments stay on their line. Block scalars (| and >) are copied through byte for byte, including the indentation inside the block, because there the whitespace is part of your value rather than part of the structure.

It Validates Before It Writes

Because the document is parsed before anything is emitted, unreadable YAML is rejected with the line number rather than reformatted into something that means something else:

• Tabs used for indentation — YAML forbids them, and most editors hide the difference
• A dedent that lines up with no open block — genuinely ambiguous structure
• The same key twice in one mapping — invalid per the spec, and a common merge artefact
• An unterminated quote
• An unclosed [ or {
• A mapping entry with no key

It reads structure, not meaning. It will not tell you that a port should have been a number rather than a string, and unrecognised tags or anchors are passed through untouched.

Where YAML Bites

Sequences Get Nested

YAML lets you write a list at the same indent as the key that owns it, or indented under it. Both are valid and mean exactly the same thing, which is why the same repository often contains both. The formatter settles on the indented form:

tags:              tags:
- web        →       - web
- api                - api

YAML ↔ JSON Conversion

The two conversion buttons run entirely in your browser — no round trip. They are useful for moving a config into an API payload, or for turning a JSON response into something readable. One caveat worth stating plainly: JSON has nowhere to put comments, so converting to JSON drops them. If you want your comments kept, use Format rather than convert. For working the other way, the JSON formatter handles payloads directly.

Formatting Is Idempotent

Running the formatter on its own output produces exactly the same text. That means the tool has one fixed idea of correct layout, so repeated use never drifts and formatted YAML committed to version control will not produce a spurious diff the next time somebody formats it.

Privacy and Limits

Formatting and validation run on our servers: your document is sent over HTTPS, processed, and returned. It is never shared with third parties or used for advertising. Even so, treat it as an upload — Kubernetes Secrets, API keys, database passwords and certificates do not belong in any online tool, this one included. Redact them first, or format a representative sample. Input is capped at 2 MB. The YAML ↔ JSON conversion is the exception: it never leaves your browser.

Working with other formats too? The SQL formatter and Java formatter take the same conservative approach to code, and XML formatter covers the other config format you probably still have lying around.