It's not a responsibly of the format, so, at most, it's a mere side effect. In any practical process which could result with truncated data, even if it handles data with such property, it should be wrapped in a container which includes length. At the very least, it would allow to trace the source of truncation, e.g. was the data originally truncated, or was it truncated in the process, and change the format without shooting in oneselves foot. And the generating side should always provide success flag which should be properly handled, since it may produce syntactically correct, but semantically invalid data. Such as checking exit code of process which generates json/yaml in question

You just check the exit code, no? The other process may fail while generating syntactically correct data too, regardless of format.