YAML 1.1 vs 1.2 implicit typing: the Norway problem, octal numbers and which parsers use which

What it is

Parser / spec	Boolean literals	Octal integers
YAML 1.1 type repository	y, Y, yes, Yes, YES, n, N, no, No, NO, true, True, TRUE, false, False, FALSE, on, On, ON, off, Off, OFF	leading 0 plus digits, e.g. 0755
YAML 1.2.2 Core Schema	true, True, TRUE, false, False, FALSE only	requires a 0o prefix, e.g. 0o755; a bare 0755 is just the decimal integer 755
PyYAML	1.1-style: yes/no/on/off in any listed case become booleans, but the single letters y/n stay strings; its documentation points readers to the YAML 1.1 specification	1.1-style: 0755 loads as 493, 0o755 stays a string; 1:30 loads as the base-60 integer 90
js-yaml 4.x	"Supports the YAML 1.2 and YAML 1.1 specifications"; the default schema loads only the six true/false spellings as booleans, yes/no/on/off stay strings	1.2-style: 0755 loads as 755, 0o755 as 493
gopkg.in/yaml.v3	1.1 words "are supported as long as they are being decoded into a typed bool value. Otherwise they behave as a string"	encodes/decodes 1.1-style 0777 by default; also accepts, but does not produce, 0o777

Why it matters

This is the "Norway problem": a country-code field containing the string NO silently becomes the boolean false under YAML 1.1 rules, because NO is one of the tokens the 1.1 boolean type accepts. YAML 1.2.2's Core Schema deliberately narrows this to six tokens, removing yes/no/on/off entirely, and tightens octal integers to require an explicit 0o prefix. Parsers did not move to 1.2 in lockstep: PyYAML's documentation still points readers to the YAML 1.1 specification, and yaml.safe_load turns NO into False and 0755 into 493 — yet even PyYAML leaves the 1.1 single letters y/n as strings. gopkg.in/yaml.v3's own README is explicit that it "supports most of YAML 1.2, but preserves some behavior from 1.1 for backwards compatibility" — 1.1 booleans are accepted only when the destination field is a typed bool; decoding the same YAML into a generic map or a string field leaves yes/no as plain strings. js-yaml advertises support for both specifications, but its 4.x default schema types scalars the 1.2 way: no stays a string and 0755 is decimal 755 — the same file loaded by PyYAML and js-yaml yields different types.

How to apply

• Quote any scalar value that could be misread as a boolean or number, especially codes like NO, ON, OFF, Y and N, and values like 0755 or 1:30.
• Do not assume a "clean" round-trip between two YAML libraries preserves a value's type.
• When decoding into a loosely typed structure with gopkg.in/yaml.v3, verify whether ambiguous words came back as booleans or strings — the destination field's declared type decides.
• Prefer the explicit 0o prefix for octal file-mode values meant for a 1.2-oriented parser.

Pitfalls

• Assuming every YAML parser treats on/off/yes/no the same way.
• Assuming a leading-zero number is always interpreted as octal.
• Assuming "supports YAML 1.2" means every 1.1 behavior has been dropped — several parsers keep it on purpose.