Write the Smallest Failing Test

Reproduce it with the least input, not with a whole test suite

You are an engineer who is good at isolating edge cases. Write one minimal test for the bug below: 1) keep only the input and state required to trigger it, and delete everything else; 2) assert the exact expected versus actual value, not just 'should not throw'; 3) state that it must fail before the fix and pass after; 4) if it cannot be reproduced reliably, list the missing environmental conditions instead of inventing them. Put it in a single file using the existing test framework.

[bug report / relevant code / runtime]

How to use

The minimal case should survive as a regression test. A case that does not run, or passes only sometimes, is worse than none.

Good for

A customer reports that leaving the search box empty crashes the results page, and you want that behavior pinned before touching the code.
You are fixing an off-by-one in pagination and need a test that is guaranteed to fail on the current implementation.

Not for

You need regression coverage for the whole checkout module: this prompt produces one case, not a suite.
The cause is unknown and you cannot reproduce it at all: add instrumentation and logs first, then write the test.

Example

Input

function parseRange(s) {
  const [from, to] = s.split('-').map(Number);
  return { from, to };
}
// parseRange('10') returns { from: 10, to: NaN }; expected to = 10

What you get

Keep only `parseRange('10')` and assert `{ from: 10, to: 10 }`; leave out the UI and network layers entirely. Run it against the current code and paste the failure (`to: NaN`) so the test is proven to catch the bug. If the product rule is that a bare value is invalid, rename the test and assert a thrown error instead — do not leave both semantics ambiguous.

Common mistakes

Turning the full checkout flow into the 'minimal' test: when it fails, you cannot tell the target bug from environment flakiness.
Asserting only that nothing throws: an implementation that returns null still passes, and the bug is not pinned at all.

Try tweaking it like this

Parameterize it over three boundary values (empty string, zero, negative) and you cover the neighbors without a second file.
Replace 'must fail before the fix' with 'paste the current failure output' and you get evidence rather than a promise.