Six topics on splitting an agent, and on the far more common case for not splitting it: the two circumstances where a second agent earns its keep, why a subagent's own context window is the one argument that survives scrutiny, what belongs in a task envelope and what has to come back, the two channels a handoff travels on, the difference between parallel tool calls that are nearly free and parallel agents that cost 3.2 times the tokens, and the split Sundry actually shipped — reported with the number it made worse.
6 topics
The Sundry agent now carries nine tools, a policy retriever, a five-state returns machine and a system prompt that has grown three sections. It is the biggest single thing in the codebase, and the obvious next move — the one every architecture diagram on the internet agrees on — is to split it into a team. A researcher, a writer, a critic. Boxes with arrows between them.
This chapter is about how often that is the wrong move. A second agent is not a refactor: it is a new failure surface, and the failures it adds are specific rather than theoretical. Information crosses the boundary as a summary and the evidence behind it does not. A subagent that ran out of turns reports a tidy conclusion anyway. Two agents write to the same order. None of that shows up in the diagram, and all of it shows up in the eval report — which is why the decision belongs to Chapter 9's instrument rather than to anybody's taste.
Two cases do win, and they are worth learning to recognize: a context that must not contain something, and work that is genuinely independent with a deadline attached. Sundry's own split is built on the first. It is a cheap triage agent that answers the simple majority and routes the rest to two specialists, and it moved handle time from 34 seconds to 19 while raising cost per ticket by 11%. The resolution number went up by two points, which on a 120-case set means nothing at all. Topic 58 reports all three of those honestly, along with the agents that were measured and rejected and the conditions under which Sundry would merge the whole thing back into one loop.
What each shape keeps, and what the second one gives away at every boundary
One agent, nine toolsthe default answer
Keeps: every fact the run gathered is visible to the decision that uses it, in its original wording. One message list, so a bug reproduces by replaying it. One place counting turns, one measuring spend, one dispatcher deciding what may execute. Loses: nothing to a handoff — but the context fills up, and tool selection falls as the list grows.
Orchestrator plus specialistsa new failure surface
Keeps: contexts small, tool lists narrow, and the simple majority never loads the machinery that moves money. Sundry's split took handle time from 34 seconds to 19. Loses: the evidence behind every crossing — the receiver gets a summary, a child that ran out of turns still reports a tidy conclusion, two agents can write the same order, and the bill goes up 11% per ticket.