GitHub has refused to implement fast-forward merges for years, insisting that you can only have rebases or merge commits (including squash merge commits). I have no idea why. However, on projects where you have enough control over the merge flow, you can have your fast forward cake and eat it too:

1. Check out the branch locally (including, if you like with gh pr checkout using the gh CLI tool).

2. Fast forward the trunk branch to the commit that is the tip of that PR.

   The “correct” way: 1. Check out your trunk branch again. 2. Do a fast-forward-only merge.

   git checkout main
   git merge --ff-only target-branch
   git branch --delete target-branch
   git push
   

   The “why isn’t this easier” way: forcibly move your trunk branch to the tip of the checked out branch (which is what a fast-forward merge is anyway).¹

   git branch --force main target-branch
   git branch --delete target-branch
   git push
   

On GitHub, this will mark the PR for target-branch as merged. Neat!

Caveats:

• This only works if you don’t protect your default branch (main/trunk/master/etc.) from pushes, because you won’t be able to push to it if you do.
• Nothing will stop you from merging something which did not pass your continuous integration this way.

Net, you may not want to do this in certain repos. (It sure would be nice if GitHub supported it nicely, wouldn’t it?) However, it is totally the kind of thing you could automate if you wanted to: a GitHub Action “bot” could do this. On small repos where you find PRs useful but also would like a linear history where “merges” aren’t relevant, I highly recommend it.