This is completely valid YAML. YAML does not allow tabs in indentation, but the tabs in your example are not indentation according to the YAML spec productions

This is completely valid YAML. YAML does not allow tabs in indentation, but the tabs in your example are not indentation according to the YAML spec productions. You can see it clearly here against many YAML parsers: https://play.yaml.io/main/parser?input=CXsKCQkibGlzdCI6IFsKCQkJe30sCgkJCXt9CgkJXQoJfQo= https://play.yaml.io/main/parser?input=CXsKCQkibGlzdCI6IFsKC... As tinita points out, sadly PyYAML and libyaml implement this wrong. See https://matrix.yaml.info/ https://matrix.yaml.info/