It does not write anything in plan mode, it's documented here it has only readonly tools available in plan mode: https://docs.anthropic.com/en/docs/claude-code/common-workfl...

But here are fine prints, it has "exit plan mode" tool, documented here: https://minusx.ai/blog/decoding-claude-code/#appendix

So it can exit plan mode on its own and you wouldn't know!

Ok, it's done it to me 3 times today, so I don't know what to tell you. I remind it that it's in plan mode and it goes "oh no I shouldn't have modified that file then!"