bcquality/custom/knowledge/testing/testcase-must-fail-before-implementation.md

bc-version	domain	keywords	technologies	countries	application-area
all	testing	tdd red-green test-first testcase verification human-checkpoint lifecycle	al	w1	all

Test case must fail before implementation begins

Description

Every task starts with a test case, and the test case must demonstrably fail (red) — verified by the developer, not self-certified by the AI — before implementation begins. The task may only be completed when the same test case passes (green).

The lifecycle gate, in order:

1. Task started (branch + BC status — see [[one-task-in-progress-at-a-time]])
2. Test case written for the requirement — including any fields, setup objects, or test data structures the scenario needs that do not yet exist
3. Test is run; the developer verifies the red result — an AI session must never assert "the test fails" without a run the developer has seen
4. Implementation begins
5. Test is run again; green is a precondition for finishing the task — no merge to the track branch, no BC Done, while the test case is red

Why

A test written after the code proves only that the code does what the code does. A test that was red first proves two separate things: that the test actually exercises the requirement (red = the gap is real), and later that the requirement is met (green = the gap is closed). The human verification of red is the cheap insurance: thirty seconds of looking at a failing test catches the test that accidentally passes vacuously — the most dangerous test in any suite.

Anti Pattern

// Implementation written first, test added afterwards to "cover" it.
// Test passes on first run — it has never been observed red.
// Nobody knows whether it tests the requirement or just the code.

Best Practice

// [GIVEN] a customer with a tier-price agreement
// [WHEN] FindPrice is called for quantity 100        (test-one-when-per-test)
// [THEN] the tier price is returned, not the unit price
//
// Run 1 (before implementation): FAILS — developer confirms red ✓
// ... implementation ...
// Run 2: PASSES — task may now be completed ✓

Test structure follows the existing testing rules: [[test-one-when-per-test]], [[test-setup-must-use-library-codeunit]], [[test-data-must-be-random-and-complete]], [[test-feature-scenario-tags]].

Scope

All CURABIS repositories — customer apps and AppSource apps alike. Applies to every task that changes behavior. Pure refactorings keep existing tests green throughout; documentation/translation tasks are exempt.