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+# Elisp Testing Rules
+
+Applies to: `**/tests/*.el`
+
+## Framework: ERT
+
+Use `ert-deftest` for all tests. One test = one scenario.
+
+## File Layout
+
+- `tests/test-<module>.el` — tests for `modules/<module>.el`
+- `tests/test-<module>--<helper>.el` — tests for a specific private helper (matches `<module>--<helper>` function naming)
+- `tests/testutil-<module>.el` — fixtures and mocks for one module
+- `tests/testutil-general.el`, `testutil-filesystem.el`, `testutil-org.el` — cross-module helpers
+
+Tests must `(require 'module-name)` before the testutil file that stubs its internals, unless documented otherwise. Order matters — a testutil that defines a stub can be shadowed by a later `require` of the real module.
+
+## Test Naming
+
+```elisp
+(ert-deftest test-<module>-<function>-<scenario> ()
+  "Normal/Boundary/Error: brief description."
+  ...)
+```
+
+Put the category (Normal, Boundary, Error) in the docstring so the category is grep-able.
+
+## Required Coverage
+
+Every non-trivial function needs at least:
+- One **Normal** case (happy path)
+- One **Boundary** case (empty, nil, min, max, unicode, long string)
+- One **Error** case (invalid input, missing resource, failure mode)
+
+Missing a category is a test gap. If three cases look near-identical, parametrize with a loop or `dolist` rather than copy-pasting.
+
+## TDD Workflow
+
+Write the failing test first. A failing test proves you understand the change. Assume the bug is in production code until the test proves otherwise — never fix the test before proving the test is wrong.
+
+For untested code, write a **characterization test** that captures current behavior before you change anything. It becomes the safety net for the refactor.
+
+## Mocking
+
+Mock at boundaries:
+- Shell: `cl-letf` on `shell-command`, `shell-command-to-string`, `call-process`
+- File I/O when tests shouldn't touch disk
+- Network: URL retrievers, HTTP clients
+- Time: `cl-letf` on `current-time`, `format-time-string`
+
+Never mock:
+- The code under test
+- Core Emacs primitives (buffer ops, string ops, lists)
+- Your own domain logic — restructure it to be testable instead
+
+## Idioms
+
+- `cl-letf` for scoped overrides (self-cleaning)
+- `with-temp-buffer` for buffer manipulation tests
+- `make-temp-file` with `.el` suffix for on-disk fixtures
+- Tests must run in any order; no shared mutable state
+
+## Running Tests
+
+```bash
+make test                               # All
+make test-file FILE=tests/test-foo.el   # One file
+make test-name TEST=pattern             # Match by test name pattern
+```
+
+A PostToolUse hook runs matching tests automatically after edits to a module, when the match count is small enough to be fast.
+
+## Anti-Patterns
+
+- Hardcoded timestamps — generate relative to `current-time` or mock
+- Testing implementation details (private storage structure) instead of behavior
+- Mocking the thing you're testing
+- Skipping a failing test without an issue to track it