Law 02 · Context & Reliability
Compounding Error Law
Reliability multiplies, it doesn't add.
The principle
A step that works 95% of the time, run ten times in a row, gives you the right final answer only about 60% of the time. The failures don't announce themselves. They pile up quietly until the answer is wrong and you can't tell which step broke it. Every link you add lowers the ceiling for the whole chain.
Why it happens
In a serial agent run, each step feeds the next. A small mistake becomes a premise, then the next step builds on it. Ten steps at 95% reliability is about 60% end-to-end in the simple worst case, before recovery or checkpoints. Long runs add another problem: each turn leaves behind assumptions, partial decisions, and failed attempts that can pollute the next turn. The fix is to shorten the chain, improve the weakest steps, and checkpoint after pivotal work so one bad output cannot quietly poison the rest.
Watch for
- End-to-end success is far worse than the per-step accuracy you measured in isolation.
- Final outputs are wrong but no single step looks obviously broken when you inspect it.
- Adding more pipeline stages keeps lowering overall reliability even as each stage tests fine.
In practice
A six-step invoice pipeline (OCR, extract line items, match vendor, validate totals, post to ledger, notify) tests at 95% per step and you ship it, then watch roughly a third of invoices come out subtly wrong with no obvious culprit. The errors are multiplicative, not additive: 0.95 to the sixth is about 0.74. Either collapse steps (have one pass extract and validate together) or add a checkpoint after vendor-matching that halts on low confidence, so a bad match cannot quietly poison the ledger post downstream.
Apply it
- Count the sequential steps and multiply their reliabilities to get the real end-to-end ceiling.
- Collapse independent steps into one pass, or raise per-step reliability, before adding new stages.
- Insert a validation checkpoint after pivotal steps that halts or restarts from the last good state on low confidence.
The takeaway
Count your steps. Make the chain shorter, push up per-step reliability, and add checkpoints between stages so one bad step can't quietly poison everything after it.