Break This PR

Break This PR helps developers catch checkout pricing bugs before they merge. A change to discounts or vouchers can pass every existing test and still overcharge shoppers or cost the business money. The problem isn’t always a failing test. Sometimes it’s the test nobody thought to write. Meta’s Muse Spark 1.3 compares the written pricing rule with the proposed checkout code and looks for a basket that exposes a mistake. The developer confirms what the shopper should pay, then Break This PR runs the code to see what it actually charges. If the amounts differ, the developer has a bug they can reproduce, not just a comment to investigate. Muse can propose a targeted correction, and the runner checks that the same regression test fails on the proposed code and passes after the correction. For example, a £110 basket drops to £99 after a 10% discount, so it no longer qualifies for a voucher. All six existing checks passed, but the changed code applied the voucher anyway and charged £94.50. The developer can keep the new test so that mistake doesn’t slip through again.