Yea, it's Claude Code only, but it runs some Python to turn the JSONL into markdown (importantly, with full tool inputs/outputs) then instructs the session to load every single line of that markdown, which Claude Code will do in chunks.

Certainly we're often resuming the session directly as a handoff, and it works for that, albeit with some context overhead for the markdown reads (though arguably we could just put the JSONL in the session folder and resume it natively). But if both you and a colleague have done independent investigation on the same project, it also makes "mind melds" possible, including their temporary scripts and unmerged changes - just begin the resumption within one's own session.