test-writer

Write comprehensive tests for the code that was just added or changed.

Step 1: Discover What Changed

Change under test (injected at load — already in front of you, no tool call needed):

git diff --stat HEAD 2>/dev/null || echo "(no diff vs HEAD — inspect git status manually)"

git diff HEAD 2>/dev/null || true

• The diff above is the change under test. If it is empty (already committed or amended elsewhere), run git diff, git diff --cached, or git show HEAD to locate the change
• Read each changed file to understand the behavior being added
• Identify the project's existing test framework, patterns, and conventions by finding existing test files
• Place new test files next to the source files or in the project's established test directory. Match whatever the project already does

Step 2: Analyze Every Code Path

For each new or modified function/method/component, map out:

• Happy path. Normal input, expected output
• Edge cases. Empty input, single element, boundary values (0, 1, -1, MAX_INT)
• Null/undefined/nil. What happens with missing data
• Type boundaries. Wrong types, type coercion traps
• Error paths. Invalid input, network failures, timeouts, permission denied
• Concurrency. Race conditions, parallel calls with shared state
• State transitions. Initial state, intermediate states, final state
• Integration points. How this code interacts with its dependencies

Step 3: Write the Tests

For EACH scenario identified above, write a test. No skipping.

Structure

• One assertion per test. If a test name needs "and", split it into two tests
• Descriptive names. Test names read as sentences describing the behavior:
  • should return empty array when input is empty
  • should throw ValidationError when email format is invalid
  • should retry 3 times before failing on network timeout
• Arrange-Act-Assert. Set up, execute, verify. Clear separation.

What to Test

Pure functions / business logic:

• Every branch (if/else, switch, ternary)
• Every thrown error with exact error type and message
• Return value types and shapes
• Side effects (mutations, calls to external services)