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docs/testing.md

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+# Testing
+
+Verify behavior automatically so you can change code with confidence.
+
+## Tools by Language
+
+| Language | Recommended | Alternatives |
+|---|---|---|
+| Python | `pytest` | unittest, hypothesis |
+| JavaScript/TS | `vitest` | jest, mocha |
+| Go | `testing` (stdlib) | testify |
+| C++ | `googletest` | catch2, doctest |
+
+## Python — `pytest`
+
+```bash
+pytest                    # run all tests
+pytest -x                 # stop on first failure
+pytest -k "test_login"    # run matching tests
+pytest --cov=src          # with coverage
+```
+
+Minimal test:
+
+```python
+def add(a: int, b: int) -> int:
+    return a + b
+
+def test_add():
+    assert add(2, 3) == 5
+```
+
+Configure in `pyproject.toml`:
+
+```toml
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+addopts = "-ra -q"
+```
+
+## JavaScript — `vitest`
+
+```bash
+vitest run          # run once
+vitest              # watch mode
+vitest --coverage
+```
+
+## Go
+
+```bash
+go test ./...           # run all tests
+go test -race ./...     # with race detector
+go test -cover ./...
+```
+
+## Key Principles
+
+- Test behavior, not implementation — tests that break on refactors aren't useful
+- Fast tests get run; slow tests get skipped
+- Don't mock more than necessary — integration tests catch what unit tests miss
+- Add a test when you fix a bug; the bug was a missing test