Your assumptions don't quietly drift out of date

Tax thresholds change. Inflation shifts. Policy moves. If your model doesn't keep up, it becomes wrong—gradually, then materially. FIRElogic keeps everything aligned with the world as it changes.

No silent errors compounding for decades

Spreadsheets don't tell you when something breaks. A single bad reference. A hard-coded number. A missed update. The model still runs. It just stops being right. FIRElogic removes that entire class of risk. FIRElogic uses differential testing: the engine's output is checked against an independent test oracle that re-implements the UK tax logic from scratch.

Still works when you're not maintaining it

At some point, you stop checking every detail. That's normal. What matters is whether the plan still holds without constant attention. No re-learning your logic. No version control. No "what did I mean by this?" Open it, and it's still correct.