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Thanks for flagging - filing a ticket! (@Brendan Ngo for mergeability?)

We currently don't support the

Require branches to be up to date before merging

branch rule which is probably why you're seeing this. There are some complexities on our side with knowing when the branch is not on the most up to date target branch but this is something we're aware of and hope to tackle at some point!

Just showing a message on merge attempt (I'm guessing GitHub API returns some sort of error to you) would be helpful. The first time I saw this I thought it was just Graphite being broken.

I agree we should have better error messaging here. Thanks for flagging!