Multi-Agent Systems in Production: When One Agent Isn't Enough and How We Coordinate Them

We built our first "multi-agent system" by accident. What started as a single agent that could research a topic, draft a report, check it against source data, and send a summary email had grown into a 2,000-token system prompt and a function list so long that the model kept forgetting tools existed. It wasn't a system — it was a monolith pretending to be intelligent. Breaking it apart into coordinated agents fixed most of the problems. It also introduced a new category of problems we hadn't thought about. Here's what we actually learned. When One Agent Is Enough (and When It Isn't) The temptation to add more agents is real, but the overhead isn't free. Every agent boundary you add is a place where context can get lost, latency increases, and errors compound. One agent is the right call when: The task fits in a single LLM context window without crowding The steps are sequential and each depends heavily on the prior output You need tight reasoning across all the information (summarising a document, for example) You need multiple agents when: A single agent's context window is being maxed out with tool definitions, history, or data Different steps require genuinely different "personas" or instruction sets (research vs. writing vs. fact-checking) Steps can run in parallel and the latency saving matters You want to isolate failure — if the data extraction agent fails, the report-writing agent shouldn't be affected The key question we ask: Is this one job or a pipeline of jobs? If you'd describe it to a human as "first do X, then Y takes that and does Z", you probably have a pipeline, not a single task. The Three Patterns We Actually Use 1. Supervisor-Worker A thin orchestrator agent decides what needs doing, dispatches to specialised worker agents, and stitches the results together. The workers are narrow — they do one thing and don't need to know about the rest of the workflow. This is our most common pattern. The supervisor's system prompt stays small because it's rout