diff --git a/.claude/commands/review-backend.md b/.claude/commands/review-backend.md
index 141c475..057c27d 100644
--- a/.claude/commands/review-backend.md
+++ b/.claude/commands/review-backend.md
@@ -102,6 +102,17 @@ Check new code against the mandatory patterns from `CLAUDE.md`:
 - **No raw collections as public API** — functions returning `dict`/`list` for structured data should return a typed model instead.
 - **Model with the right paradigm for clarity** — choose the tool that makes the domain intent obvious at the call site: pure functions for stateless transforms, magic methods for natural domain operations (`current + delta` via `__add__` instead of manual field construction), OOP for encapsulating state and invariants. Flag code that uses a weaker paradigm when a stronger one would eliminate boilerplate and make the intent self-evident.
 
+**14. Guards that defend against developer errors instead of runtime input**
+Classify every `if/else` and `try/except` by **what it guards against**. A guard is only legitimate when the value it checks can be wrong for reasons **outside our code's control**. This is the sibling of item #12 (`assert` forbidden outside tests) — same philosophy: internal invariants belong in unit tests, not hot-path branches.
+- **Decision heuristic:** "Can this value only be wrong if another part of OUR code is wrong? → delete the guard, add a unit test, let it raise. Can it be wrong from input or environment outside our control? → keep the guard and handle it gracefully."
+- **Keep** guards on real runtime/external conditions: user/API request bodies, network/HTTP responses, filesystem, DB availability, third-party payloads, parsing external/untrusted data, concurrency races. Validating a request field from a caller SHOULD be guarded and return a clear error.
+- **Flag** defensive branches on trusted internal values — an internal constant, an `Enum` member, a dict key that exists by construction, a field our own code sets, a registry entry. These add dead branches to the hot path and hide the bug. Prefer letting the operation raise naturally (`KeyError`/`AttributeError`/`TypeError`) per Python **EAFP** (Easier to Ask Forgiveness than Permission), and cover the invariant with a unit test — unit tests are the sanctioned place to assert internal invariants. A swallowed or re-wrapped developer error violates PEP 20 (Zen of Python): "Errors should never pass silently" and "Explicit is better than implicit" — it slips past the test suite.
+- **Real examples from this repo:**
+  - An internal table-name registry lookup on an internal constant should be `table = _TABLE[name]` (raises `KeyError`; add a test asserting every collection constant is registered) — NOT `if name not in _TABLE: raise ValueError("register it")`.
+  - `db_adapter.py` normalizes query rows with `if "id" not in r and "_id" in r: r["id"] = r["_id"]` (repeated at ~L136/L147/L187). The `IDatabase.query()` contract (`interface.py`) guarantees every row has `_id`, and both backends honor it (`firestore.py` L191, `memory.py` L350), so the `and "_id" in r` re-check defends against a backend breaking its own contract — a developer error. Prefer `r["id"] = r["_id"]` (raises `KeyError` if the contract is broken) and cover the "`query()` returns `_id`" invariant with a per-backend unit test.
+  - `memory.py` `_matches_filter` (L394–L419) is a `match operator:` with no `case _:` default, so an unsupported operator silently matches nothing — while `firestore.py` `_apply_filter` (L298) raises `ValueError` for the same input. Operators are always hardcoded literals in our own query code (e.g. `[("status", "==", "running")]`), so a bad operator is a developer error; the memory backend lets it pass silently (violating PEP 20) and diverges from Firestore. Add a `case _: raise ValueError(...)` so both backends fail the same way, and unit-test the unsupported-operator path.
+- Flag "just in case" guards on trusted internal inputs as **CHANGES REQUIRED** (or a nit if trivial).
+
 ## What NOT to flag
 
 - Minor inefficiencies (extra dict copy, redundant log line)