Excel-to-web migration with proof the numbers still match
Causal Labs migrates fragile, single-user Excel workbooks into fast, multi-user web applications — TypeScript, Vite, SheetJS, Vitest, IndexedDB — with tests proving the new system reproduces the spreadsheet's exact numbers. A prior migration was verified by 21 cell-for-cell golden tests proving exact parity with the original workbook.
Causal Labs migrates fragile, single-user Excel workbooks into fast, multi-user web applications — TypeScript, Vite, SheetJS, Vitest, IndexedDB — with tests proving the new system reproduces the spreadsheet's exact numbers. A prior migration was verified by 21 cell-for-cell golden tests proving exact parity with the original workbook.
Why 'just rebuild it as a web app' is the wrong first sentence
A business-critical spreadsheet usually encodes years of undocumented logic — edge-case handling, manual overrides, formulas nobody fully remembers the reasoning for. Rebuilding it as a web app without first proving parity means shipping a system that quietly produces different numbers than the spreadsheet it replaced, which is worse than not migrating at all.
How we approach an Excel-to-web migration
Requirements gathering
We reverse-engineer the spreadsheet's actual logic, including undocumented edge cases, before writing any web application code.
Build
A multi-user web application (TypeScript/Vite) that reproduces the spreadsheet's calculation logic exactly.
QA & review
Cell-for-cell golden tests compare the new system's output against the original spreadsheet across representative scenarios.
What's included
Logic reverse-engineering
Full extraction of the spreadsheet's calculation logic, including undocumented edge cases and manual overrides.
Cell-for-cell parity testing
Golden tests proving the new system reproduces the spreadsheet's exact numbers across real scenarios.
Multi-user access
Concurrent, permissioned access replacing the single-file, single-user spreadsheet bottleneck.
Offline-capable data layer
IndexedDB-backed local storage where the workflow needs to function without a constant connection.
- Production web application
- Cell-for-cell golden test suite
- Logic documentation extracted from the spreadsheet
- Multi-user access controls
Type-safe reimplementation of the spreadsheet's calculation logic.
Fast build tooling for the migrated application.
Used for spreadsheet import/export compatibility during and after migration.
Runs the cell-for-cell golden test suite that proves parity.
Local data persistence where offline capability is required.
Replaced an Excel workbook with a web app verified by 21 cell-for-cell golden tests proving exact parity.
cell-for-cell golden tests proving exact parity in a past migration
Questions about this service
How do you prove the new system matches the old spreadsheet exactly?
Golden tests: the same input scenarios are run through both the original spreadsheet and the new application, and the outputs are compared cell-for-cell. The migration isn't considered complete until they match.
Still running critical logic through a spreadsheet only one person understands?
Tell us what the spreadsheet does and who relies on it. We'll scope a migration with proof, not just promises, of parity.
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