For me, it was the agent that decided about plan mode, when that happened. Most of the time I didn't invoke it but kept discussing and tweaking and if the context was close to full, make it dump it out to disk and start a new session. I felt like that worked more flexibly for me because I could keep steering and adjusting until it was what I wanted. At that point I made it write a detailed spec. Like my homegrown plan mode.

The "standard" plan mode felt too stifling.