YAML formatter, check it, tidy it, convert it.

A YAML formatter rewrites a YAML file with steady indentation and spacing, and tells you if the file is not valid YAML. This one keeps your comments, anchors, aliases, tags and quote marks, shows the line and column of every mistake, and can turn the same data into JSON, or JSON into YAML. It reads YAML 1.2 by default and has a switch for the older 1.1 rules.

How to use it

1. Paste YAML into Input, press Open file, drop a file on the box, or press Example.
2. Read the line under the boxes. It says Valid YAML with the number of documents, keys and levels, or Not valid YAML with a line and column.
3. Choose Indent (2 or 4 spaces) and Line width, and tick or untick the options on the right.
4. Pick the view above the result: YAML, JSON or Tree. In the YAML view, Flow (minified) puts every list and mapping in [ ] and { } on one line.
5. Press Download or Copy.

What stays and what changes

Wrapping changes how the text is laid out, not the value: YAML reads the wrapped line back as one line. Choose No wrapping if you would rather it never wraps.

Worked examples

A number that must not change. In a Kubernetes file, value: 0755 is printed as 0755. The JSON view reads it by YAML 1.2, where it is the number 755. With YAML version 1.1 it is an octal number, 7 × 64 + 5 × 8 + 5 = 493.

The key called on. In a GitHub Actions workflow, on: is the key that holds the triggers. Under YAML 1.2 it is the text on, and the JSON view shows "on". Older YAML 1.1 readers, such as PyYAML, turn it into true.

Four real files. A Docker Compose file, a GitHub Actions workflow, a Kubernetes manifest with two documents and an OpenAPI file were formatted with every combination of indent 2 or 4, line width 80, 120 or none, lists indented or not, brackets padded or not, and as written, block or flow style. That is 72 results for each file, and reading each result back gave the same data as the original every time. The JSON view of all four matched what ruamel.yaml's YAML 1.2 reader gives. PyYAML differed on two of them, on and 0755.

Flow style. The 874 byte compose file becomes one line of 688 bytes, the 946 byte OpenAPI file one line of 764 bytes. Comments are dropped in flow style, since a comment cannot sit inside a single line.

Matching a list style. The Kubernetes sample writes - name: at the same column as its key. With Indent lists under their key ticked, 17 of its 43 lines change. Untick it and none do. The compose file and the OpenAPI file indent their lists, so the tick matches them.

Where the rules come from. The YAML 1.2 rules for true, false, null and numbers are in the YAML 1.2.2 specification, section 10.3 (core schema): only true and false are booleans, and an octal number is written 0o17. The YAML 1.1 rules are on its boolean and integer pages: y, yes, on and their opposites are booleans, and a leading 0 means octal. All were read on 19 Oct 2026.

Mistakes it finds

• Tabs. YAML forbids a tab in the indentation. The error offers Replace tabs with spaces.
• A repeated key. Two keys with the same name in one mapping are an error here, with the line of the second one.
• A wrong indent, or a missing ] or }. The line and column point at the first place the reader got lost, which is sometimes the line after the real slip.
• An alias with no anchor above it. *name must come after &name.

Sorting keys

Sort keys A to Z orders every mapping, with numbers in natural order (item2 before item10) and the merge key first. An anchor must stay above the aliases that use it, so a mapping that holds an anchor is left in its own order when the file also has aliases, and the line under the boxes says how many were left.

Limits

Custom tags such as !Ref and !Sub are kept in the YAML and left out of the JSON, since JSON has no tags. A key that is itself a list is written as a string in JSON. JSON with comments or trailing commas is not valid YAML or JSON, so use the JSON formatter, which can repair it. To format XML, see the XML formatter, and to compare two versions of a file, use the diff checker.