Whilst a thorough test suite is invaluable, I'm tempted to just hack on the code until they pass, instead of understanding the root issue.
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I love how the CommonMark Spec has a test suite that's just a JSON array. It's really easy to test a library for compliance, and I've seen developers nerd-sniped into full compliance.
https://spec.commonmark.org/0.31.2/spec.json
LLMs seem to handle dependency upgrades really well.
The task is well-specified, there's usually a build/test suite to check correctness of the modifications, and there's often a changelog they can consume too.
Today I learnt that Racket *intentionally* doesn't have a traditional REPL workflow. The authors were concerned about students not understanding the state between the current session and the code on disk.
(Arguably Jupyter has some of these features now.)
https://blog.racket-lang.org/2009/03/the-drscheme-repl-isnt-the-one-in-emacs.html