error: cannot lock ref 'refs/remotes/origin/main'

The error

$ git fetch origin
error: cannot lock ref 'refs/remotes/origin/main': is at 880dd2bc80cfabddec339973e92aee00044b8b11 but expected 9ff6770542ff7a568cd442af02d10db723dd00ba
From github.com:you/app
 ! 9ff6770..880dd2b  main       -> origin/main  (unable to update local ref)

git pull fetches first, so it fails the same way.

Why it happens

A fetch notes where origin/main points, downloads what's new, and then moves the ref only if it still points there. When another fetch moved it in the meantime, git refuses: the ref is at the new commit, but this fetch expected the old one.

Separate clones have their own refs, but every worktree of a clone shares them. So agents working in their own worktrees still race when they fetch at the same moment, and so does an editor that fetches in the background. In our test, two worktrees fetching together, each time just after a push, failed 28 times in 30.

The fix

Nothing is broken. The other fetch already moved origin/main to the new commit: in our test it matched the remote after every failed round. Fetch again, and it succeeds.

In a script, retry once:

git fetch origin || { sleep 2; git fetch origin; }

In PowerShell:

git fetch origin; if ($LASTEXITCODE) { Start-Sleep 2; git fetch origin }

core.filesRefLockTimeout doesn't help: it makes git wait for a lock another process holds, and here nothing holds it, the ref has simply moved. With a 3-second timeout, 25 of 30 rounds still failed.

To see it less often, fetch from one place at a time: let one agent fetch for the others, and turn off the editor's automatic fetch in a repository agents work in.