Payload Validator

Payload Validator / YAML

Validate YAML

Checks well-formedness, then reports the values that mean different things to different loaders — found by resolving each unquoted scalar under both spec versions and comparing, not by matching a list of suspicious words.

Up to 6,000 characters here, because this form puts the payload in the URL. For anything larger, POST it to the API — the limit there is 1 MB. Nothing is stored either way.

Valid YAML

0 errors, 1 warning

bytes:
13
documents:
1
keys:
1
anchors:
0
aliases:
0
max depth:
2

1 finding

What goes wrong in YAML

The Norway problem: a country list loses Norway

countries:
  - se
  - no
  - dk

In YAML 1.1 the bare words no, yes, on, off, y and n are booleans. PyYAML implements 1.1, so that list is ["se", false, "dk"]. Go's yaml.v3 and the yaml npm package implement 1.2, where they are strings. Quoting is the fix, and quoting is exactly how YAML says "this is a string".

yaml.version_divergence · run this example

Every GitHub Actions workflow has a key that is not "on"

on: push
jobs:
  build:
    runs-on: ubuntu-latest

Under YAML 1.1 the key `on` resolves to the boolean true, so the mapping has a key `true` and no key `on`. GitHub's own parser handles it, but a script reading the workflow with PyYAML and looking for "on" finds nothing. A divergent key is worse than a divergent value: the field is simply not there.

yaml.ambiguous_key · run this example

A file mode is the wrong number, and both readings are numbers

mode: 0755

YAML 1.1 reads a leading zero as octal, giving 493. YAML 1.2 reads it as decimal, giving 755. Both are integers, so nothing about the value looks wrong — the permissions are just not what was written. Quote it, or use 0o755, which only 1.2 understands.

yaml.version_divergence · run this example

YAML 1.1 has base-60 integers

timeout: 1:30

Under YAML 1.1 that is the sexagesimal integer 90. It was intended for durations and is a reliable surprise: a value that looks like a time becomes a number nobody wrote. Under 1.2 it is the string "1:30".

yaml.version_divergence · run this example

Tabs are forbidden as indentation and look identical to spaces

parent:
	child: 1

YAML forbids tabs for indentation outright. Because a tab and four spaces are indistinguishable on screen, the error message points at a line that looks perfectly aligned. Configure your editor to insert spaces for .yaml and .yml.

yaml.tab_indentation · run this example

A non-breaking space is content, not indentation

parent:
  child: 1

Copying YAML out of a rendered web page or a PDF frequently brings U+00A0 with it. YAML treats it as part of the value, so the line is structurally somewhere other than where it appears. Completely invisible in every editor.

yaml.invisible_whitespace · run this example

Duplicate keys, again, with a different set of behaviours

port: 8080
host: a
port: 9090

YAML requires unique mapping keys, but loaders disagree about enforcement: PyYAML keeps the last silently, others error. In a Kubernetes manifest or a CI config assembled from templates, this is how two sources of truth both appear to win.

yaml.duplicate_key · run this example

Call it instead of pasting

curl --data-binary @your-file \
  'https://payload-validator.gumballtools.com/api/v1/validate?format=yaml'

API and MCP setup — the MCP tool is validate_yaml.

Other formats