Five subagents died on me in two weeks. Three on one day, two a couple of weeks before. The cause was the same every time: they read too much.
A subagent death looks like this. It gets a task, opens a large file with a full read to "understand the context," fills its window, keeps exploring past the point where it already had enough to act, then times out or runs out of room before it writes a single line to disk. Hours of a capable model, zero output. The plan was fine. The agent never executed it because it spent its whole budget looking.
Once I saw the same failure five times, it stopped being bad luck and became a class. So I wrote a contract every spawn has to follow. Five sections: objective, output path, write scope, verification, and the one that carries the weight, a context budget.
The context budget is a short list of rules that sound obvious and get ignored under pressure. Write the skeleton of your output file first, before reading anything, so partial progress survives. Read narrowly and on purpose, never a whole large file when a targeted slice answers the question. Route the genuinely heavy files to a supervised executor instead of a chat subagent, since that's the tool built to chew through them. Stop exploring the moment you have enough to decide. Don't re-read what you already read.
The one that saved the most agents is the first. Skeleton file first. An agent that writes its output structure before it starts reading will, at worst, leave a half-finished file I can inspect and salvage. An agent that reads first and writes last leaves nothing behind when it dies. I would rather have a rough draft on disk than a perfect plan inside a process that just ran out of memory.
There's a lesson in here about my own instincts. I kept blaming the model for the deaths and shopping for a smarter one. A smarter model dies the same way if you point it at a huge file and tell it to understand everything before it acts. The problem was discipline, and discipline lives in the brief.
So the contract is the rule now. No spawn without those five sections, and the context budget is not the optional one, because the expensive failures on this stack have all been the same quiet death.