One bad step, N bad steps: how agent failures cascade
Originally published on Loop & Retry — field notes on building LLM agents that survive production. Here's the failure mode that surprises people who've only reasoned about agents statistically. You measure a per-step error rate — say 10% of steps produce something wrong — and you assume errors are independent, so a wrong step is a wrong step and the rest of the run is fine. Then you watch a real trajectory and see something else: step 4 gets a fact slightly wrong, step 5 reasons on top of that wrong fact and commits harder, step 6 takes an action premised on both, and by step 8 the agent is confidently executing a plan that was doomed at step 4. One mistake became five. The errors weren't independent — they were coupled through the context , and coupling is what turns a 10% step-error rate into a run that's wrong far more than 10% of the time. This is the cascade : a single fault amplifying down a single trajectory. It's distinct from the failure I wrote about in distributed retry patterns , where the problem is one bad condition hitting many workers at once — that's a blast radius, a horizontal spread. The cascade is vertical: it spreads through time within one run, because an agent's own past output is its future input. This post is about the vertical kind, why it's structural rather than bad luck, and where you can cut it. Why coupling is the default, not the exception A stateless function that fails just returns an error. An agent that fails does something worse: it writes the failure down where it can read it again. The mechanism is the same one that makes agents work at all — the transcript accumulates, and every step conditions on everything before it. That's a feature for carrying intent forward. It's also the exact channel a mistake travels down. Three ways a single fault propagates through the context: Poisoned premise. The agent derives or retrieves a wrong fact — a misparsed tool result, a hallucinated ID, a stale value — and it lands in the transcript a