What is a pseudo-version, and when does the go command generate one?

A pseudo-version stands for an untagged commit. It has the form vX.Y.Z-yyyymmddhhmmss-abcdefabcdef, made of a base version, the UTC commit time, and a 12-character commit hash prefix. The go command generates one when you ask for a branch or a commit:

go get github.com/acme/lib@main
go get github.com/acme/lib@4f2a1c9
# go.mod: github.com/acme/lib v1.4.1-0.20240912083011-4f2a1c9e7b3d

There are three forms:

• v0.0.0-...: no earlier tag exists.
• vX.Y.(Z+1)-0.2024...: the commit comes after release tag vX.Y.Z.
• vX.Y.Z-pre.0.2024...: the commit comes after a pre-release tag.

Pseudo-versions sort below the next real release, so MVS upgrades to v1.4.1 once it is tagged.

Gotchas: never write one by hand. The go command validates that the timestamp and base tag match the commit. Depending on pseudo-versions in a published library forces them on your users. Interviewers like to hear "pin to a tag before release".