Nil UUID

What the Nil UUID is

The Nil UUID has all 128 of its bits set to zero. It is not a version: RFC 9562 defines versions 1, 3, 4, 5, 6, 7 and 8, and defines Nil beside them as a special value. Written out it is thirty-six characters, of which thirty-two are zeroes.

When someone reaches for it

It is the conventional way to write “no identifier” in a field that has to hold a UUID anyway: a test fixture that needs a well-formed value with no meaning, a sentinel for a record that has not been assigned yet, a placeholder in documentation.

What to watch out for

Nothing enforces that meaning. Some systems treat the Nil UUID as equivalent to null, others treat it as an ordinary identifier that happens to be all zeroes, and a value passed between the two will be read differently on each side. If it matters, say which one you mean rather than assuming.