ANSWER
What is a divergence in a dual-run?
Reviewed · Reviewed by Paulo Pierrondi, founder.
During dual-run, every variance needs a class before any aggregate match rate is considered: blocker, material difference, cosmetic difference, confirmed legacy defect or approved exception. The record should include the affected rule, evidence, impact, owner, decision and expiry. Without this, a high agreement rate can hide the one difference that prevents cutover. It is not a generic accuracy percentage: a small difference can be material while a large formatting difference may be irrelevant. An exception without an owner stays a blocker. An approved exception enters the Evidence Pack with an owner, a rationale and an expiry. The Evidence Pack is the dossier that supports the decision to shut the process down. Aggregate numbers without a period, an unclassified variance and an approval without a name are not evidence. During the dual-run the legacy system keeps operating. The replacement produces a comparable output with no production effect. Cutover comes later, with human approval, a tested rollback and an Evidence Pack that records diffs, owners and the way back. How to classify is at /en/answers/classify-variances-during-dual-run/. What a dual-run is is at /en/answers/what-is-a-dual-run/.
Step by step
- Link the variance to a rule and fields in the equivalence contract.
- Use blocker when a difference violates a control, critical state, obligation or zero-tolerance limit.
- Separate material differences from accepted format, ordering or rounding variations.
- When the legacy system is wrong, record evidence and explicitly choose not to reproduce the defect.
- Require an owner, rationale, compensating action, expiry and exit condition for exceptions.
How an agent does this
AgenticosCore Parallel produces rule-level diffs and routes blockers and exceptions for approval. House Rules stop progression outside the contract, while Trail preserves the decision for the Evidence Pack. How to classify is at /en/answers/classify-variances-during-dual-run/.
Frequently asked questions
Can one blocker stop cutover?
Yes, when the contract classifies it as a blocker. The point is to prevent averages from hiding a material violation.
Can an exception enter the contract without an owner?
No. Every exception needs an owner, rationale and expiry, recorded in the Evidence Pack. Without that, the difference stays a blocker.
Primary sources
- AI RMF Core — NIST
- Retiring service components securely — UK Government Security