When do multi-agent architectures actually earn their complexity over one well-tooled agent, and what fails in them?

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Accepted Answer

My default is one agent with well-designed tools, and I hold that default until a measured wall says otherwise, because the single agent is dramatically easier to debug, eval and run. The legitimate triggers for splitting are structural, not metaphorical. Context isolation is the strongest: one task genuinely needs large context the other would pollute, a research sub-agent reading 50 documents and returning a 500-token brief keeps the main agent context clean, and that pattern, sub-agents as context firewalls, is most of what multi-agent is good for. Genuine parallelism: independent workstreams, searching 5 sources concurrently, where fan-out and join beats sequence. Privilege separation as a security boundary: the agent that reads untrusted web content holds no tools with side effects, so injection lands in a sandbox, that is a real boundary. And occasionally measured specialisation, where distinct prompts or models per stage demonstrably beat one generalist on the eval. What multi-agent adds in failure modes is the part the diagrams omit. Hand-offs are lossy: agent A summarises for agent B, and the detail B needed did not survive the summary, the whole system degrades into a game of telephone. Shared state is unsolved by default: two agents believing different things about progress. Errors cascade: A retrieves something subtly wrong, B builds on it confidently, and by the output it is unattributable. Cost multiplies, 5 agents deliberating is 5x the tokens, And debugging becomes distributed-systems archaeology across interleaved traces. So my bar is empirical: show me the single-agent eval plateauing for a reason that maps to one of those triggers, split along that one seam, keep the orchestration as flat as possible, and re-run the same eval to prove the split paid for itself, quality or cost. Most systems that impress in production are one good agent, good tools, and one or two sub-agents used as context firewalls.

Answer

Design the hand-off so it is not lossy.

Structured artifacts, not prose summaries: a schema for findings with sources and confidence, plus access to the underlying material so the receiver can drill down rather than trusting the summary blindly.

Answer

Reviewer and generator as separate agents, worth it?

A verification pass is often worth it, but it does not require a separate stateful agent, a fresh single call with a critique rubric gets the benefit without the orchestration. Fresh context is the value, not the second identity.

When do multi-agent architectures actually earn their complexity over one well-tooled agent, and what fails in them?

When do multi-agent architectures actually earn their complexity over one well-tooled agent, and what fails in them?

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