Unquoted Booleans In YAML 1.1 Vs 1

The key difference between YAML 1.1 and YAML 1.2 is that YAML 1.2 significantly restricts which values are interpreted as Booleans, making it more predictable and less ambiguous. Only true, True, TRUE, false, False, and FALSE remain as Boolean values in YAML 1.2.

YAML 1.1 defined several unquoted words as booleans. In particular on and off (any casing) are treated as true/false:

# YAML 1.1 parser interpretation
feature_enabled: on # → boolean true
feature_disabled: off # → boolean false

YAML 1.2 tightened this to match JSON’s strict true/false, so only unquoted true and false (all lowercase) are booleans. Others become plain strings:

# YAML 1.2 parser interpretation
feature_enabled: on # → string "on"
feature_disabled: off # → string "off"
feature_flag: true # → boolean true
feature_flag: false # → boolean false

Example Comparison:

Key	YAML 1.1 Parse	YAML 1.2 Parse
toggle: on	toggle: true	toggle: "on"
toggle: OFF	toggle: false	toggle: "OFF"
toggle: true	toggle: true	toggle: true
toggle: FALSE	toggle: false	Error (unquoted)

Complete Comparison Table

YAML 1.1 Value	YAML 1.1 Interpretation	Value Type	YAML 1.2 Interpretation
y	True	Single Char	String
Y	True	Single Char	String
yes	True	Yes/No	String
Yes	True	Yes/No	String
YES	True	Yes/No	String
true	True	True/False	Boolean
True	True	True/False	Boolean
TRUE	True	True/False	Boolean
on	True	On/Off	String
On	True	On/Off	String
ON	True	On/Off	String
n	False	Single Char	String
N	False	Single Char	String
no	False	Yes/No	String
No	False	Yes/No	String
NO	False	Yes/No	String
false	False	True/False	Boolean
False	False	True/False	Boolean
FALSE	False	True/False	Boolean
off	False	On/Off	String
Off	False	On/Off	String
OFF	False	On/Off	String

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