TL;DR

MonSuiviCA keeps its URSSAF contribution and income tax calculations in a single TypeScript module. URSSAF is the French agency that collects social contributions. The module's package has 199 tests, 99 of them in the file dedicated to this module. On November 1, 2026, they all pass.

Every figure in this article is the one in the code as of November 1, 2026. These figures describe software and do not constitute tax advice.

The product's origin story is in the MonSuiviCA launch article .

One module holds the numbers

The file packages/shared/src/calculations.ts serves both the app and the marketing site.…