1/64, actually, because RFC-compliant (variant 1) UUIDv4 requires fixed values for both the version nibble and two bits of the variant nibble.

The fact that we're discussing this at all is a reasonable argument for using a library function.

While it might be invalid, will most libraries choke if you give them a pseudo UUIDv4?

What do you mean? Are you talking about validation of UUIDs?

If you generate random bytes, which are unlikely to conform to the UUIDv4 spec, my guess is that most libraries will silently accept the id. That is, generating random bytes, will probably work just work.

But what libraries are you talking about? What is their purpose?

Nice, thanks and I agree.