A claims-triage agent runs a four-step loop on every incoming claim: extract loss details (97% accuracy), classify loss type (96%), verify coverage (94%), assign adjuster (95%). Each step has been individually validated and meets the 95% accuracy bar set by ops.
The team ships it. After six weeks in production, the audit team flags that 17% of fully-routed claims have at least one error somewhere in the chain.
The ops team double-checks the per-step numbers. They’re correct: each step is at or above 94%.
Why is the end-to-end error rate 17% when no individual step is below 6%?
Answer: Errors compound. The probability that all four steps run without error is 0.97 × 0.96 × 0.94 × 0.95 ≈ 0.831 — meaning ~17% of claims have at least one error somewhere in the chain. The audit and the per-step numbers are both right.
This is the Cascade. Per-step accuracy tells you almost nothing about a multi-step agent; what counts is end-to-end task success, measured at the boundary the customer or the auditor actually sees. A 95%-per-step agent is 81% reliable over four steps, 60% over ten, and 36% over twenty. So either shorten the chain, or pick problems where you can catch the error and recover.