Test quality

Test Quality Nine Rules / 测试质量九规

Before writing any test, answer: "What bug does this catch that no other test catches?" No answer, no test. 写任何测试前先答:"它能抓到什么别的测试抓不到的 bug?"答不出就不写。

The discipline / 纪律

1. Test behavior, not implementation. Assert return values and observable side effects. Never assert that an internal helper was called with specific arguments — that breaks on every refactor and catches nothing. 测行为不测实现。 断言返回值和可观察副作用;永不断言"内部函数被以某参数调用过"——重构必碎,什么也抓不到。
2. Every mock must be justified. Mock only at system boundaries: network/HTTP, LLM APIs, databases, external file I/O, clock & randomness, third-party SDKs. Never mock internal classes to isolate a "unit". When you mock a boundary, assert what the caller does with the response. 每个 mock 要有正当性。 只在系统边界 mock;禁止 mock 内部类来隔离"单元"。mock 边界后,断言调用方拿响应做了什么。
3. One scenario per test; variants go data-driven. Same setup differing only in values → merge into parametrize / test.each / DataProvider. 一个场景一个测试,变体走参数化。 setup 相同只差值的合并成数据驱动。
4. Every test must justify its existence. Delete tests that only catch typos, verify dataclass defaults, or exercise trivial pass-throughs. 每个测试自证价值。 只能抓 typo 的、验证默认值的、测平凡透传的,删。
5. Name tests for the scenario. test_<scenario>_<expected_outcome> — reads like a requirement, not an echo of the function signature. 测试名说场景。 读起来像需求,不是函数签名回声。
6. Production regression tests are sacred. A test reproducing a real production bug is always justified; reference the incident and never delete it. Exempt from rule 4. 生产回归测试神圣不可删。 复现真实生产 bug 的测试永远正当,注明事故来源,豁免第 4 条。
7. Don't test framework guarantees. If the test would still pass with all your custom code deleted, it tests the framework, not the project. 不测框架保证。 把项目自定义代码全删了还能过的测试,测的是框架。
8. State and value objects are real, never mocked. Construct real DTOs/entities. Mocking state hides field-name typos and validation errors — exactly the bugs worth catching. Painful construction is design feedback: add a builder, don't mock. 状态与值对象永不 mock。 构造真实例;mock 状态恰好藏住字段 typo 和校验错误。构造太痛苦是设计反馈,加 builder 而不是 mock。
9. Infrastructure under test gets real infrastructure. When persistence is the subject, run a real test database with real migrations; mocking the session there tests nothing. Mocking is fine when persistence is only a side effect. 测基础设施就上真基础设施。 持久化是被测对象时跑真库真迁移;只是副作用时才可 mock。

Severity / 严重度: rules 1, 2, 8 = must-fix (they hide real bugs) · rules 3, 4, 5, 7 = should-fix (bloat) · rule 6 = sacred (never delete) · rule 9 = note, don't block. 分档: 1/2/8 必须修(藏真 bug) · 3/4/5/7 应该修(膨胀) · 6 神圣 · 9 记录不阻塞。

When this triggers / 触发时机

• About to write or generate tests (put the nine rules in a sub-agent's task brief too).
• Reviewing a diff or MR that contains test changes.
• QA / inspector reviewing a sub-agent's test output.
• Noticing unusually high mock density or near-duplicate test bodies.

Done criterion (verifiable) / 完成判据（可验证）

✅ Every mock in the diff sits on a system boundary you can name; no test asserts internal call arguments; no two tests differ only by values; state objects are constructed real. ⚠️ Should-fix violations (bloat, naming, framework tests) noted and queued — acceptable for small changes. ❌ A must-fix violation ships: an internal mock, an implementation-detail assertion, or a mocked DTO. Fix before merge.

Worked examples / 实战反例

Real before/after cases for this discipline live in EXAMPLES.md — read them before you act. 本纪律的真实 before/after 反例见 EXAMPLES.md —— 动手前先对照。

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Adapted from guard-skills' test-guard (MIT, Ahmed Nagdy), rebuilt for the agent-discipline verifiable-check format.