diff --git a/backend/tests/test_secure_runtime_defaults.py b/backend/tests/test_secure_runtime_defaults.py
new file mode 100644
index 00000000..ed7b1c67
--- /dev/null
+++ b/backend/tests/test_secure_runtime_defaults.py
@@ -0,0 +1,86 @@
+from __future__ import annotations
+
+import pytest
+from pydantic import ValidationError
+
+from app.core.config import Settings
+
+
+def _settings(**overrides: object) -> Settings:
+    return Settings(_env_file=None, **overrides)
+
+
+def test_guest_access_is_opt_in_by_default() -> None:
+    settings = _settings()
+    assert settings.guest_access_enabled is False
+
+
+def test_yolo_validation_scope_is_enforced_by_default() -> None:
+    settings = _settings()
+    assert settings.yolo_enforce_validation_scope is True
+
+
+def test_production_yolo_rejects_disabled_validation_scope() -> None:
+    with pytest.raises(ValidationError, match="YOLO_ENFORCE_VALIDATION_SCOPE"):
+        _settings(
+            app_env="production",
+            yolo_enabled=True,
+            yolo_enforce_validation_scope=False,
+        )
+
+
+def test_production_yolo_requires_validation_scope_manifest() -> None:
+    with pytest.raises(ValidationError, match="YOLO_VALIDATION_SCOPE_MANIFEST_PATH"):
+        _settings(
+            app_env="production",
+            yolo_enabled=True,
+            yolo_require_cuda=True,
+            yolo_device="cuda:0",
+        )
+
+
+def test_production_yolo_requires_validation_scope_hash() -> None:
+    with pytest.raises(ValidationError, match="YOLO_VALIDATION_SCOPE_MANIFEST_SHA256"):
+        _settings(
+            app_env="production",
+            yolo_enabled=True,
+            yolo_validation_scope_manifest_path="/app/storage/model-scope.json",
+            yolo_require_cuda=True,
+            yolo_device="cuda:0",
+        )
+
+
+def test_production_yolo_requires_cuda() -> None:
+    with pytest.raises(ValidationError, match="YOLO_REQUIRE_CUDA"):
+        _settings(
+            app_env="production",
+            yolo_enabled=True,
+            yolo_validation_scope_manifest_path="/app/storage/model-scope.json",
+            yolo_validation_scope_manifest_sha256="a" * 64,
+            yolo_require_cuda=False,
+            yolo_device="cuda:0",
+        )
+
+
+def test_production_yolo_requires_cuda_device() -> None:
+    with pytest.raises(ValidationError, match="YOLO_DEVICE"):
+        _settings(
+            app_env="production",
+            yolo_enabled=True,
+            yolo_validation_scope_manifest_path="/app/storage/model-scope.json",
+            yolo_validation_scope_manifest_sha256="a" * 64,
+            yolo_require_cuda=True,
+            yolo_device="cpu",
+        )
+
+
+def test_production_yolo_accepts_checksum_bound_cuda_scope() -> None:
+    settings = _settings(
+        app_env="production",
+        yolo_enabled=True,
+        yolo_validation_scope_manifest_path="/app/storage/model-scope.json",
+        yolo_validation_scope_manifest_sha256="a" * 64,
+        yolo_require_cuda=True,
+        yolo_device="cuda:0",
+    )
+    assert settings.yolo_enforce_validation_scope is True