AI Regression Testing

AI Regression Testing

Add tests that catch the failures AI agents commonly miss when the same model writes code and reviews its own assumptions.

Contract

Inputs:

• Bug report, code diff, API change, route, component, feature flag, or sandbox path
• Existing test commands and project conventions
• Optional known production incident or regression id

Outputs:

• Regression test plan
• New or updated tests when implementation is requested
• Parity checklist for sandbox, mock, feature-flag, and production paths
• Verification commands and results

Creates/Modifies:

• Test files, fixtures, mocks, and minimal helpers when asked to implement
• Application code only when the user also asks to fix the failing behavior

External Side Effects:

• None by default
• Do not hit production services; use sandbox, fixtures, local test databases, or mocked providers

Confirmation Required:

• Before changing production data, external accounts, CI settings, or destructive test fixtures

Delegates To:

• testing-expert for general testing strategy
• react-testing-library for React component tests
• playwright-e2e-init for browser setup or test-runner for existing browser tests
• debug when the root cause is still unknown

When to Use

• AI modified API routes, backend logic, serializers, or response schemas.
• A bug was fixed and must not reappear.
• A feature has sandbox/mock and production paths.
• A model-generated review says the change is correct but no test proves it.
• Build and lint pass, but the user-facing contract may still be wrong.
• A feature flag, mock mode, fallback path, or error path was changed.

Core Principle

Test the contract that failed, not the implementation the agent wrote. Assume the reviewing model shares the same blind spot as the writing model.

Common AI blind spots:

• production path fixed but sandbox/mock path still wrong
• response includes a field in one path but not another
• SELECT, serializer, DTO, fixture, and frontend type drift apart
• optimistic UI path works but rollback/error path is untested
• feature flag changes one branch but not the fallback branch
• tests assert status codes but not response shape or state transition

Workflow

1. Identify the Contract

State the behavior as a user or caller contract:

• endpoint returns these fields
• component shows this state after this interaction
• webhook processes duplicate events idempotently
• sandbox and production return the same shape
• failed operation rolls back visible and persisted state

Create a required-field or required-state list when useful.

2. Find All Paths

Trace every path that should satisfy the contract:

• production data path
• sandbox/mock/demo path
• feature-flag on/off path
• authenticated/unauthenticated path
• success, validation error, provider error, and retry path
• server response, client type, and UI rendering path

3. Write the Regression First

The first test should fail on the unfixed bug or on a representative broken fixture. If the bug is already fixed, make the test specific enough that the previous bug would have failed.

Prefer deterministic checks:

• unit or route handler tests for response shape
• integration tests for database, serializer, and service contracts
• component tests for state transitions
• E2E tests only for critical user journeys or cross-system behavior

4. Test Parity

For API and data contracts, compare path shapes:

production fields == sandbox fields == mock fixture fields
frontend type accepts exactly the returned shape
empty/error states still include documented envelope fields

Use fixtures with realistic nulls, missing optional values, and date formats.

5. Verify

Run the narrow test first, then the relevant suite:

bun test path/to/regression.test.ts
bun test
bun run typecheck

Use the repo's actual package manager and test commands. Do not invent new tooling when the project already has a pattern.

Test Design Checklist

• Does the test fail for the old bug?
• Does it cover every branch that returns or mutates the contract?
• Does it assert the actual response/body/state, not only 200 OK?
• Does it include sandbox/mock parity when those modes exist?
• Does it cover null, empty, duplicate, retry, and provider-failure cases when relevant?
• Is the test small enough to run in a bug-check loop?
• Is the regression named after the bug or contract it protects?

Output Format

Regression target: `GET /api/profile` must include `notification_settings` in production and sandbox responses.

Tests added:
- `tests/api/profile.test.ts`: response-shape contract
- `tests/api/profile.test.ts`: sandbox/production field parity

Verification:
- `bun test tests/api/profile.test.ts` passed
- `bun run typecheck` passed

Residual risk:
- No E2E coverage for the settings page rendering this field.

Anti-Patterns

• Letting an AI review replace a mechanical test.
• Testing only the path the fix touched.
• Asserting vague success instead of the exact contract.
• Using production systems for regression tests.
• Adding broad flaky E2E coverage when a route or unit test catches the bug.
• Forgetting fixtures and generated types after changing response shape.