Test Gap from Diff Skill

Analyze a code diff to identify exactly which tests need to be written, using the test requirement matrix (rule 123) to determine required test types.

Trigger Conditions

• Source code changes without corresponding test changes
• PR is created for review
• Coverage drops below threshold
• User invokes with "test gaps" or "test-gap-from-diff"

Input Contract

• Required: Git diff or list of changed files
• Optional: Current coverage report

Output Contract

• Per-function list of missing tests with required test type
• Priority ranking (critical path functions first)
• Test skeleton code for top-priority gaps
• Coverage delta estimate if tests are added

Tool Permissions

• Read: Source files, test files, coverage reports
• Write: None (analysis only; test-engineer writes the tests)
• Search: Grep for function signatures, test file patterns

Execution Steps

1. Parse diff: Extract list of changed functions/methods with file paths
2. Classify changes: Categorize each change per rule 123 matrix (model, service, handler, etc.)
3. Determine required tests: Map each change type to required test types (unit, integration, property, concurrency, fuzz)
4. Check existing coverage: For each changed function, check if required test types already exist
5. Identify gaps: List functions where required tests are missing
6. Prioritize: Rank gaps by: financial operations first, then public API, then internal logic
7. Report: Produce gap report with test type, priority, and estimated effort

Success Criteria

• All changed functions mapped to required test types
• All existing test coverage identified
• Clear gap report with actionable items

References

• .cursor/rules/123-test-requirement-matrix.mdc