Live demo

Edit the JSON and ask Z3. Calls the real public route.

The public route runs the engine deployed before 2026-11-02 until the next gateway deploy. The first three examples behave the same before and after. The fourth shows the fix: until the deploy, the old engine returns count = 0 as an ordinary violation without saying that Python raises there; after it, the answer names the failure: function_raises. (Checked against the live route on 2026-11-02.)

A real bug

Expected: contract_holds: false, counterexample price = 0 (discount(0) is -5).

A proof

Expected: contract_holds: true: for every integer.

Python's modulo, not the solver's

Expected: contract_holds: true: in Python x % -3 is 0, -1 or -2.

Division by zero

Expected: after the deploy: contract_holds: false, failure: function_raises, count = 0.

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