Thanks for sharing!
> When asking Claude Code to write tests, I find they are inevitably coupled to implementation details, mockist, brittle, and missing coverage.
Interestingly, I haven't noticed any of that so far, using Claude Code on a new-ish project (couple 10k loc). However, I also went out of my way in my CLAUDE.md to instruct it to write functional code, avoid side effects / push side effects to the shell (functional core, imperative shell), avoid mocks in tests, etc. etc.
That's the kicker - you taught it conventions that it follows. Even better (in my view) is to ensure you're getting higher coverage and tests focused on behaviour by getting it to write the tests first.
Even moreso by ensuring it writes "feature complete" tests for each feature first.
Even moreso by running mutation testing to backfill tests for logic it didn't cover.