Testing Patterns

Testing Patterns

Use this skill for deciding what to test, where tests belong, and how broad verification should be. Use the bun-test skill for Bun test runner mechanics.

Do not use this skill for web-only packages/icarus-web testing unless the user explicitly asks to cover web.

Placement

• Put tests near the package that owns the behavior.
• Prefer package-local src/**/*.test.ts or src/**/*.test.tsx patterns already used by the package.
• Keep reusable test helpers under a package-local test utility module. Do not leak helpers into public exports unless they are intended public test utilities.
• Cross-package behavior should be tested at the lowest package that can observe it. Use server-level tests only for real wiring, persistence, transport, or runtime assembly.

What Needs Tests

• New runtime behavior needs focused tests.
• Runtime behavior owners need isolated tests by default. Effect DI should be

used to cut the graph and replace dependencies with fake layers/services.

• Services with behavior need direct tests: registries, stores, parsers, dispatch loops, satellite rings, tool execution boundaries, persistence adapters, and protocol serializers.
• Systems should test command/control/event/lifecycle behavior.
• Projections should test derivation from stored events.
• Sinks should test curation and side effects.
• Tool/model catalog modules should test selection and hydration.
• Codecs should test _tag round trips and unknown boundary handling.
• Constructors for exported protocol variants need tests when they apply defaults, normalize input, or enforce invariants.
• Boundary adapters that translate external/provider data into internal data need focused tests for the translation shape.
• Pure type surfaces and thin barrels do not need runtime tests unless they contain constructors or behavior.
• Refactors of working behavior should start with characterization tests when the current behavior is not already pinned down.

Effect Tests

• Keep Effect at the boundary of the test. Build the program, provide test layers, then run with Effect.runPromise or the repo's existing test helper.
• Prefer test layers and fake services over global mutation.
• Keep typed expected failures in the error channel when that is the contract being tested.
• Test defect paths separately from expected domain failures.
• When a service requires time, randomness, IDs, storage, or a language model, inject a deterministic test service instead of relying on ambient behavior.

Bun Test Style

• Import test APIs from bun:test.
• Prefer direct assertions for small units and table tests for closed protocol variants.
• Use mocks and spies only at process or dependency boundaries. Prefer fake services for domain logic.
• Keep snapshots for stable rendered output or protocol artifacts. Avoid snapshots for logic that would be clearer as explicit assertions.
• Avoid broad root test runs while iterating when a package-local test reproduces the issue.

Fixtures

• Name fixtures by the behavior they support, not by incidental data shape.
• Keep fixtures small and local until multiple tests need the same object.
• Use constructors or builders for protocol variants with required defaults. Do not duplicate _tag literals and default fields across tests.
• Do not make a shared fixture so configurable that each test has to mentally execute a framework.

Verification Scope

• Run the narrowest useful test during iteration.
• Run bun run typecheck when changing public types, package exports, Effect environments, schemas, or cross-package signatures.
• Run broader tests when behavior crosses package boundaries or when a refactor moves ownership.
• Narrow package integration tests are allowed when they prove local assembly of

a few services.

• Do not add broad runtime/server/web E2E tests without explicit user

confirmation or an explicit wiring-proof request.

• In reviews, missing tests are findings. Either add the test, defer it explicitly, or explain why the changed surface is type-only.

Anti-Patterns

• Do not add tests only around mocks while leaving the real behavior unexercised.
• Do not hide flaky timing with sleeps; inject clocks or await deterministic signals.
• Do not test through the server when the owning primitive can be tested directly.
• Do not create one giant integration test for several unrelated behaviors.
• Do not use broad E2E as the first proof for behavior that has a clear owner.
• Do not leave TODO comments for obvious missing tests; either write them or call out the gap.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: theseus-run
• Source: theseus-run/theseus
• License: MIT
• Homepage: https://www.theseus.run

Install and usage instructions live in the source repository linked above.