diff --git a/.github/instructions/testing.instructions.md b/.github/instructions/testing.instructions.md
index 6477db9d7c..7df2b70b00 100644
--- a/.github/instructions/testing.instructions.md
+++ b/.github/instructions/testing.instructions.md
@@ -75,6 +75,18 @@ Before changing a test that fails, investigate whether the test is intentionally
 
 A failing test often reveals a production bug, not a test bug.
 
+## Prove a new test can fail
+
+A test that never fails asserts nothing, and reads exactly like one that works.
+Before calling a new test done, break what it covers — comment out the constraint, invert the condition — and confirm it goes red, then restore.
+
+Watch for assertions that depend on the problem having a unique answer.
+An optimisation test over devices with no incentive to move has the same optimum with or without the constraint, so it passes either way.
+In `test_highspy_equivalence.py`, where two backends are compared, also disable the new code path on one side: a scenario that survives that is comparing two no-ops.
+Both traps have been hit — a balance-group scenario there passed with the constraint it was named after entirely disabled.
+
+Say in the PR description what you broke to prove it — a reviewer cannot tell a binding test from a vacuous one by reading it.
+
 ## Module-scoped fixture state
 
 Module-scoped fixtures are shared across tests. When modifying shared objects (e.g. `asset.sensors_to_show`), reset them to the column default — not to `None` — in teardown:
diff --git a/CLAUDE.md b/CLAUDE.md
index 33fd4af42d..0781af5a5f 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -16,6 +16,9 @@ Most-missed rules, called out so they are not forgotten:
   commit again. See [`pre-commit-hooks.instructions.md`](.github/instructions/pre-commit-hooks.instructions.md).
 - **Add a changelog entry** for user-facing changes, in the right section, with a PR link. See
   [`changelog.instructions.md`](.github/instructions/changelog.instructions.md).
+- **Prove a new test can fail** — break what it covers, confirm it goes red, restore. A test that passes
+  with the feature disabled asserts nothing. See
+  [`testing.instructions.md`](.github/instructions/testing.instructions.md).
 - **One logical change per commit** ([`atomic-commits.instructions.md`](.github/instructions/atomic-commits.instructions.md)),
   **timezone-aware datetimes always** ([`timezone-awareness.instructions.md`](.github/instructions/timezone-awareness.instructions.md)),
   **catch specific exceptions** ([`error-handling.instructions.md`](.github/instructions/error-handling.instructions.md)),