I feel like the state of the art is baked into the compaction logic, and I've had a lot of problems with compaction (absent other prompting) losing key bits of state.

https://github.com/toon-format/toon is another interesting one, and I feel like it takes on a much more achievable goal - reduce whitespace and verbosity of JSON, not overall context compression.

Personally, I find compaction to be unreliable, which forces me to rely heavily on session-specific planning documents and inter-agent handoff messages.