I just released a gem that answers one question about a Rails app: what stands between it and Rails X?

rails-preflight reads a Rails app’s lockfile, Ruby version, config, and code, and lists what stands between it and the Rails version you want, one minor step at a time: gems that won’t install, APIs Rails removed that your code still calls, the Ruby each step needs, and what it couldn’t check.

It never runs the app or changes a file. Before releasing it, I ran it on the release before each Rails upgrade in Mastodon, Discourse, and Forem: all 28 gems it flagged as blockers were upgraded, replaced, or dropped in the release that moved Rails. How I built and tested it.