Root block scalar with column-0 content: libyaml family rejects it, go-yaml v3 returns ""
At document level a block scalar's content may start at column 0 (--- >\nline1\nline3\n, yaml-test-suite DK3J/M7A3, spec example 9.5), because the root node is at indentation -1. ruamel, js-yaml 5, yaml (npm), go.yaml.in/yaml/v4, yaml-rust2 and saphyr read it. PyYAML (pure and C), serde_yaml/serde_yaml_ng/serde_norway (unsafe-libyaml) and js-yaml 4 raise an error. gopkg.in/yaml.v3 and go.yaml.in/yaml/v3 3.0.5 silently return "": they read an empty scalar, treat the rest as a second document, and Unmarshal never looks at it. Emitters that target 1.2 (yaml npm stringify('a\nb') -> |-\na\nb\n, js-yaml 5 |3- headers at root) therefore produce text these readers lose. If your output has to work with any reader, don't emit a root-level block scalar: wrap the string in a mapping or force a quoted style.
Confirmed on
--- >\nline1\nline3\n. PyYAML 6.0.3 pure: ParserError "expected '<document start>', but found '<scalar>'". CSafeLoader: "did not find expected <document start>". ruamel.yaml 0.19.1: 'line1 line3\n'. Go (go1.27.3): go.yaml.in/yaml/v3 v3.0.5 and gopkg.in/yaml.v3 v3.0.1 Unmarshal into a string gives "" with err=nil; go.yaml.in/yaml/v4 v4.0.0-rc.7 gives "line1 line3\n". A detail that helps for detection: the rest isn't a valid second document. Loopingv3.NewDecoder(r).Decodereturns "" for doc 1, then the second Decode fails withyaml: line 1: did not find expected <document start>. So the loss is only silent with Unmarshal. A reader that drains the decoder until io.EOF and treats any other error as fatal does catch it with v3.