Silent Drift in Agent Decision Quality: Catching It Before Your Users Do

Book: Observability for LLM Applications — Tracing, Evals, and Shipping AI You Can Trust Also by me: Agents in Production — the companion book in The AI Engineer's Library (2-book series) My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub Your triage agent has been in production for three months. The traces look clean. Every span is green, every run terminates, the p95 latency is flat, the token bill is boring. Then support forwards you a screenshot: the agent routed a billing refund to the security queue. You pull the trajectory. Nothing is broken. The agent called a reasonable tool with reasonable arguments, got a reasonable result, and picked the wrong queue with total confidence. That is silent drift. The trace shows you what the agent did. It does not tell you whether what it did was any good. Between a model provider's minor version bump, a prompt tweak someone shipped on Tuesday, and the slow shift in your incoming traffic, the quality of your agent's decisions moves. It rarely announces itself with an error. It shows up as a support ticket, then a second one, then a churned account. You catch it the same way you catch a memory leak: with a baseline and an alarm, not by staring at dashboards. Decision quality is a distribution, not a number The failing traces in Chapter 12 of Agents in Production are the easy case. Twenty retries of the same empty search is visibly wrong. You see the loop count in the invoke_agent parent and you know. Most quality regressions are not visible in a single trace. They show up only when you look at the distribution of decisions across thousands of runs. So the first thing to instrument is the decision itself. If you followed the tracing chapter you already emit a small fixed vocabulary per chat span: span . set_attribute ( " gen_ai.agent.step " , 3 ) span . set_attribute ( " gen_ai.agent.decision " , " call_tool " ) span . set_attribute ( " gen_ai.