Multi-agent system producing confidently wrong results? Diagnosing handoff and propagation failures

A single agent that hallucinates a fact gives you one wrong answer. An orchestrator that passes that fact to three specialised subagents gives you three wrong answers — each one reasoned out with apparent coherence, none raising an exception. Multi-agent systems don't just have more failures; they amplify the ones they have. And the damage rarely shows up where it started.

Why orchestration amplifies failures

The core problem is that one agent's output becomes another's input. When that first output is wrong — a hallucinated value, a dropped constraint, a mis-parsed result — the error doesn't stop. It propagates, and every downstream agent reasons confidently on top of it:

• Information loss at the handoff. The upstream agent knew a constraint; the handoff didn't carry it; the downstream agent never knew to apply it.
• A bad output trusted as input. The downstream agent treats the previous agent's output as ground truth and builds on a mistake.
• Coherent, exception-free wrongness. Each agent produces something plausible, so nothing errors — the system quietly degrades instead of failing loudly.

That's what makes multi-agent failures hard: the symptom (a wrong final answer) is several hops from the cause (an early handoff), and no stack trace connects them.

Find where it actually broke

Debugging a multi-agent run means finding the earliest point where a good input became a bad output — the agent whose output first diverged from what it was handed. Snapback diagnoses the whole trace and identifies the failure and where it originated, so you fix the source rather than the subagent that merely inherited the error:

diagnose_trace({trace: { final_status: "failed", agents: ["planner","researcher","writer"], steps: [
  /* planner hands the researcher a mis-scoped task... */
  /* researcher returns confident but off-target data... */
  /* writer produces a coherent, wrong result */ ]}})

// → { failure_class: "information_loss_handoff" | "coordination_failure",
//      failure_agent, failed_at_step, root_cause, fix_suggestion }

The fix

1. Validate outputs at each handoff. Before an agent's output becomes the next agent's input, check it against the contract — schema, constraints, expected shape.
2. Carry the constraints forward. Make the handoff explicit about what the downstream agent must honour, so information isn't silently lost.
3. Fail loud at the boundary. A malformed handoff should stop there, not propagate — catch it before three subagents reason on it.
4. Diagnose to the source. When a multi-agent run goes wrong, trace it to the originating hop rather than patching the last agent in the chain.