Boolean Handling Differences in JSON and YAML

Boolean handling is one of the most notorious sources of bugs when converting between JSON and YAML. JSON has exactly two boolean values: true and false. YAML, however, recognizes a much wider set of boolean-like values.

JSON booleans (strict):

{
  "enabled": true,
  "debug": false
}

YAML equivalent:

enabled: true
debug: false

This is straightforward. But YAML (version 1.1) also treats these as booleans:

Truthy	Falsy
true	false
True	False
TRUE	FALSE
yes	no
Yes	No
YES	NO
on	off
On	Off
ON	OFF

The Norway Problem: This is a famous YAML gotcha. Consider a mapping of country codes:

countries:
  - code: GB
  - code: FR
  - code: NO

In YAML 1.1, NO is interpreted as boolean false, not the string "NO". Converting to JSON yields:

{ "countries": [{"code": "GB"}, {"code": "FR"}, {"code": false}] }

The fix -- quote the value:

  - code: "NO"

YAML 1.2 (the latest spec) restricts booleans to only true and false (lowercase), which significantly reduces these issues. However, many parsers still default to YAML 1.1 behavior.

When converting JSON to YAML, tools should quote strings like "yes", "no", "on", "off" to prevent them from being misinterpreted as booleans on the round trip.