Testing (Consumer Test Coverage)

Overview

Improve coverage by exercising consumer-visible behavior with infra mocked and behavior preserved.

Inputs / Outputs

Inputs: Spec/contract artifacts from spec or plan (optional but preferred); consumer-facing entrypoints to test. Outputs: Test suite pinning consumer-visible behavior; coverage report. Consumed by finish and review.

Workflow

1. Read relevant specs (system + service) and map them to consumer-visible flows and invariants.
2. Identify consumer-facing entrypoints: HTTP/gRPC handlers, public service methods, event consumers, cache/storage adapters, jobs.

GATE: Do not write tests until consumer-facing entrypoints are identified (step 2). If no entrypoints are listed, go back — tests without identified entrypoints tend to test implementation details.

1. Add tests for success and failure paths that a consumer can observe (invalid input, downstream failures, permissions, timeouts where applicable).
2. Mock infra boundaries (DB, Redis, network listeners, clocks/timers). Prefer calling handlers/functions directly instead of running real servers.
3. Run focused coverage and iterate until the target is met (default 80% unless the spec says otherwise).

Minimum viable execution

When context or time is constrained, these are the load-bearing steps:

1. Read specs and map to consumer-visible flows (step 1) — tests must trace back to spec'd behavior.
2. Identify consumer-facing entrypoints (step 2) — determines what to test.
3. Write success + failure path tests (step 3) — both paths, not just happy path.
4. Run coverage (step 5) — verify the tests actually exercise the code.

Steps that can be cut under pressure: mocking strategy optimization (step 4), coverage iteration beyond first pass.

Chooser (What Test Type Where)

• New endpoint / handler change: consumer-visible tests — call handler with mocked dependencies, assert response shape + status codes + error handling.
• Refactor (no behavior change): characterization tests first — pin existing behavior before changing implementation.
• New event consumer / job: feed mixed payloads (valid, invalid, missing fields, duplicates); assert side effects and idempotency.
• Boundary change (DB/cache/client): adapter tests — cover happy path, empty/null results, connection failures, timeouts.
• Cross-service contract change: consumer-contract tests — verify your consumer expectations match the provider's contract.
• Coverage gap (existing code): start with the riskiest paths — auth/permissions, error handling, input validation, state transitions.

Clarifying Questions

• What entrypoints are affected (HTTP handler, gRPC method, consumer, job, adapter)?
• Are there existing specs/contracts that define expected behavior?
• Is this new behavior (need new tests) or existing behavior (need characterization tests before refactoring)?
• What is the target coverage level (default: 80%)?
• What test runner and mocking setup does the project use?

Testing Patterns

• Handler paths: call handler with mocked service, assert response, metrics, and error handling.
• Event consumers: feed mixed payload shapes (missing type, struct/list values, invalid entries).
• Cache/storage: cover cache hit/miss, null/empty results, invalidation behavior.
• Jobs: use fake timers; cover interval runs and error logging branches.
• Observability: assert metrics render and logging mixins without external services.
• Vitest note: if mocked values are referenced by vi.mock factories, use vi.hoisted to avoid init-order bugs.

Guardrails

• Preserve externally visible behavior and API shapes.
• Avoid real network/listen calls in unit tests; mock them.
• Keep tests consumer-focused; do not assert internal implementation details beyond outputs/side effects.

Common failure modes

• Tests implementation details instead of consumer-visible behavior (e.g., asserting internal method call counts instead of response shape).
• Defaults to unit tests regardless of context — should use the chooser to pick the right test type.
• Mocks the thing being tested instead of its dependencies — the test exercises the mock, not the code.
• Tests happy path only, skips failure modes — missing tests for invalid input, downstream failures, permission denials, and timeouts.

Commands

• Vitest example: npx vitest run apps/<service>/**/*.test.ts --coverage --coverage.include='apps/<service>/src/**'
• Generic: cd apps/<service> && npm test -- --coverage

References

• Specs and contracts as test sources: spec
• TypeScript test skeletons: references/snippets/typescript.md
• Related patterns: Consumer-side contract test, Service integration contract test, Service component test
• Telemetry verification (when tests cover boundary logging/metrics): observability

Output Template

When applying this skill, return:

• What consumer-visible behavior is now pinned (happy path + key failure modes).
• What tests were added/changed (by entrypoint: handler/consumer/job/adapter).
• Coverage/verification results (commands run + outcomes) and any notable gaps/follow-ups.