> I don't bother giving it guidelines or guardrails or anything of the sort

Where do you give these guardrails? In the chat or CLAUDE.md?

Basic level information like how to build and test the project belong in CLAUDE.md, it knows to re-check that now and then.

Yeah, CLAUDE.md. Sometimes it just ignores what was in there after the context window gets big enough (as it tends to with planning mode).