Two tests can pass while applyDiscount(100) returns 100 instead of the ticket's 90. One replaces the function with a mock. The other copies its boundary bug into the expected value.

Agents love to create a thousand unit tests. All green. Then review smashes the whole thing.

The agent already implemented a trash solution, and only then wrote generalized unit tests for it. Those tests describe the code it just wrote. So they all pass, bugs included.

When an agent writes both the code and its tests, don't count the green checks. Look at where each expected value comes from. If it comes from the agent's code, the test can repeat the agent's mistake instead of catching it. If it comes from the ticket, the spec or a real example, and the test runs the real code and failed because the behavior was missing, it checks something.

Two patterns that pass with the bug

Here are both in TypeScript with Vitest. Suppose an agent writes this for a discount ticket:

// pricing.ts.…