A test is really two things: some code that runs, and one line that says what should have happened. Only the second can fail for the reason you care about.
Assertion
The line in a test that compares what happened with what should have happened, such as expect(total).toBe(550). Without one, a test only proves the code did not crash.
Agents reach for weak assertions, like "the result exists", because they pass quickly. The strength of the suite lives almost entirely in these lines, so these are the lines to read.
Say it to your agent
“These tests only check that the result is defined. Change each assertion to the exact expected value, such as expect(discount(200, 10)).toBe(180), and add a case at percent 0 and percent 100.”