The ExitPlanMode tool description now explicitly states it's only for plan mode: "present plan for approval and start coding (plan mode only)". An error message was also added: "You are not in plan mode. This tool is only for exiting plan mode after writing a plan."
Evidence
ExitPlanMode update (search for "present plan for approval and start coding (plan mode only)")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.