From Excel Model to Governed API

Rob Tyrie · Field Note · Source date March 24, 2025 · This page published October 18, 2026

This page republishes the current Ironstone version of a note based on the linked March 24, 2025 source. The complete text remains in the Field Notes & Essays collection.

An insurance spreadsheet can contain valuable business logic. Making it available through an API should preserve that logic's inspectability while adding ownership, tests, version control and traceability. Connecting AI comes after those controls, not instead of them.

An insurance spreadsheet is often dismissed as something waiting to be replaced. In reality, it may be the most inspectable version of the business logic the company has.

Turning that model into an API can make pricing, eligibility or explanation available inside digital journeys and AI conversations. It can reduce re-keying and allow the same approved logic to operate consistently across channels. The spreadsheet becomes a kind of digital twin of the calculation that business experts already know.

But an API can scale a mistake as efficiently as it scales a good rule. Hidden links, manual overrides, stale assumptions and undocumented cells do not become governed simply because they are online.

Modernize In This Order

First name the business owner. Then define inputs, outputs, assumptions and permitted overrides. Create reference cases whose answers are already trusted. Version the workbook and API together. Reconcile results on every change. Log which version produced each consequential output.

Only then connect the logic to an AI interface. Let the AI collect information and explain the result; keep the approved calculation deterministic where determinism matters.

The useful transformation is not Excel to AI. It is understandable business logic becoming reusable, testable and governed.

Original source on LinkedIn

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