You are designing the observability layer for a multi-agent system where a planner, three specialized workers, and a synthesizer exchange approximately 50 messages per user request. An incident last week took 3 hours to diagnose because there was no way to replay the exact message sequence that caused a failure. What is the minimum observability architecture that would have prevented this diagnostic delay?

D is correct because the described 3-hour diagnostic delay was caused by the inability to replay the exact message sequence. Distributed tracing with a propagated root trace ID allows every message in a 50-turn agent conversation to be correlated to a single request. Full input/output capture enables exact replay. Tool call logs with parameters and results identify where tool failures introduced bad state. Durable storage ensures traces survive system restarts. This provides the complete observability needed to reproduce any incident. A is wrong because logging only final outputs loses all intermediate reasoning steps, tool calls, and inter-agent messages — precisely the data needed to diagnose multi-agent failures. C is wrong because API response codes only surface transport-level errors; logic errors, hallucinated tool parameters, and incorrect reasoning that produce 200 responses are the more common multi-agent failure modes. B is wrong because test suites cover known patterns; the value of incident replay is handling unknown failure modes that did not appear in test design.