# Simulation Memory Fallback Hardening — 2026-08-24

## Scope

Close one local-memory safety gap without changing the migration decision:

- `MEMORY_BACKEND=zep` remains the default compatibility path.
- `MEMORY_BACKEND=local` must use an explicitly injected, request-scoped entity-reader factory.
- Local mode must not silently construct `ZepEntityReader` when the factory is absent.

## TDD evidence

1. Added `test_local_backend_refuses_unscoped_zep_reader_fallback` to
   `backend/tests/test_simulation_memory_injection.py`.
2. The test failed against the pre-change fallback behavior.
3. `SimulationManager.create_entity_reader()` now raises the stable internal
   sentinel `local_entity_reader_factory_required` in local mode when no
   factory is injected.
4. Explicit factory injection remains unchanged.

## Verification

- Local-memory regression slice: **11 passed**.
- Backend full suite: **73 passed**.
- Frontend contract tests: **10 passed**.
- Frontend production build: **PASS**; existing chunk-size/dynamic-import warnings remain.
- Python `compileall`: **PASS**.
- `git diff --check`: **PASS**.
- Target-slice added-lines security scan: **0 findings**.
- Full-diff scan matched two password patterns in test-only fixtures; no production credential was identified by that scan.
- `ruff` is unavailable in the current backend environment; no lint pass is claimed.

## Independent review gate

- Fresh read-only reviewer: `deleg_029828b6`.
- Scope was limited to `SimulationManager.create_entity_reader()` and
  `test_local_backend_refuses_unscoped_zep_reader_fallback()`.
- Required five-key verdict: `passed=true`, `security_concerns=[]`,
  `logic_errors=[]`.
- Reviewer-reported runtime spy: local mode made **0** Zep constructor calls;
  the default path made **1** Zep constructor call.
- Reviewer suggestions are non-blocking: add explicit constructor-spy
  regression coverage for the local no-call path and the legacy/default path.
  They are deferred to a later test-hardening pass; no production behavior gap
  was found in this bounded slice.
- This closes the review gate for this slice only. It is not approval of the
  full migration, semantic parity, or production readiness.

## Remaining migration boundary

This is a fail-closed guard, not Zep removal or semantic parity proof. Remaining
work includes complete local consumer cutover, parity fixtures, durable resource
persistence, production workers/object storage, and the full tenant E2E matrix.