I built a harness for my own use that allows the agent to fork its own history. So for example it can “compact from” a specific item, replacing a branch with a summary; or do an “excursion”, basically a temporary branch, which is like a subagent but inherits context.

Sounds cool and it does make sensible decisions optically but I haven’t been able to prove that it is meaningfully better than normal compaction. Building harnesses that do interesting things is a lot easier than building more effective harnesses, I guess.