As of yesterday subagents were often getting the entire session copied to them. Happened to me when 2 turns with Claude spawned a subagent, caused 2 compactions, and burned 15% of my 5-hour limit (Max 5x).

how long they stay around after the cache miss is irrelevant if I am burning all the prior tokens again. also, how much context they have depends entirely on the task and your workflow. I you have a subagent implement a feature and use the compile + test loop to ensure it is implemented correctly before a supervisor agent reviews what was implemented vs asked then yes, subagents do have a lot of context.