tdd-red-phase

TDD RED Phase

Version: {{VERSION}}

When to Use

• Starting implementation of a new feature or behavior
• User issues "Start-Story [ID]" command in IDPF-Agile framework
• Beginning a new TDD iteration

RED Phase Objective

Write a test that fails for the right reason.

Right Reason

• Test fails because feature doesn't exist yet
• Test fails because behavior not implemented
• Failure message clearly indicates what's missing

Wrong Reason

• Test fails due to syntax error
• Test fails due to missing imports
• Test passes unexpectedly

Workflow

Step 1: Identify Testable Behavior

Good:

• "Function returns sum of two numbers"
• "GET /users returns 200 status"
• "Invalid email shows validation error"

Poor:

• "User management works" (too broad)
• "Fix the bug" (not a behavior)

Step 2: Write the Failing Test

Test structure:

1. ARRANGE: Set up test data and preconditions
2. ACT: Execute the behavior being tested
3. ASSERT: Verify the expected outcome

Provide single code block:

TASK: [Brief description]

STEP 1: [Create or open test file]
STEP 2: [Add necessary imports]
STEP 3: [Write complete test function]
STEP 4: [Save file]
STEP 5: [Run test command]
STEP 6: [Verify test FAILS with expected message]
STEP 7: [Report back: Did test fail as expected?]

Step 3: Execute and Verify Failure

Checklist:

• Test executed without syntax errors
• Test failed (not passed)
• Failure message indicates missing implementation

Step 4: Analyze and Proceed

If fails as expected: Proceed to GREEN phase If passes unexpectedly: Revise test If errors instead of fails: Fix test code

Best Practices

Write Minimal Tests

• Test verifies one specific behavior
• Uses simplest possible test data
• Single assertion (or closely related)

Use Clear Test Names

test_[feature]_[scenario]_[expected_result]

Examples:
- test_add_function_with_positive_numbers_returns_sum
- test_get_users_when_authenticated_returns_200

Common Mistakes

1. Test Passes Immediately - Feature already exists or test wrong
2. Test Has Syntax Errors - Fix before proceeding
3. Test Too Broad - Split into multiple tests
4. Unclear Failure Message - Add descriptive assertions

Anti-Patterns

1. Writing Implementation First - Test first, then code
2. Skipping Failure Verification - Always run and verify
3. Tolerating Test Errors - Fix so test fails cleanly

Checklist Before GREEN Phase

• Test code is complete and syntactically correct
• Test executes without errors
• Test FAILS (does not pass)
• Failure message indicates missing implementation
• Test is focused on single behavior

Resources

• resources/red-phase-checklist.md
• resources/test-structure-patterns.md
• resources/failure-verification-guide.md

End of TDD RED Phase Skill