Honestly, skip planning mode and tell it you simply want to discuss and to write up a doc with your discussions. Planning mode has a whole system encouraging it to finish the plan and start coding. It's easier to just make it clear you're in a discussion and write a doc phase and it works way better.

That's a good suggestion. I'll try it next time. That said, it's really easy to start small things in planning mode and it's still an annoyance for them. This feels like a workflow that should be native.