Output whether using CPP or Python so test runs are NOT ambiguous

migration/tests/test_fragmentation_backend_shim.py

@@ -136,6 +136,41 @@ def test_cpp_backend_request_fails_clearly_when_unavailable():
     assert "C++ BLE fragmentation backend is not available" in completed.stderr
 
 
+def test_bleinterface_does_not_silently_fallback_when_cpp_requested_but_unavailable():
+    env = os.environ.copy()
+    env["BLE_RETICULUM_FRAGMENTATION_BACKEND"] = "cpp"
+    env["PYTHONPATH"] = SRC_DIR
+
+    completed = subprocess.run(
+        [
+            sys.executable,
+            "-c",
+            """
+import sys, types
+RNS = types.ModuleType('RNS')
+sys.modules['RNS'] = RNS
+transport = types.ModuleType('RNS.Transport')
+sys.modules['RNS.Transport'] = transport
+interfaces = types.ModuleType('RNS.Interfaces')
+sys.modules['RNS.Interfaces'] = interfaces
+interface_mod = types.ModuleType('RNS.Interfaces.Interface')
+class Interface:
+    MODE_FULL = 0
+interface_mod.Interface = Interface
+sys.modules['RNS.Interfaces.Interface'] = interface_mod
+import ble_reticulum.BLEInterface
+""",
+        ],
+        cwd=REPO_ROOT,
+        env=env,
+        text=True,
+        capture_output=True,
+    )
+
+    assert completed.returncode != 0
+    assert "C++ BLE fragmentation backend is not available" in completed.stderr
+
+
 def test_selected_backend_exposes_methods_expected_by_current_python_code():
     result = run_probe("cpp", include_cpp=True)
 

src/ble_reticulum/BLEInterface.py

@@ -79,15 +79,23 @@ except ImportError:
 # Import fragmentation backend selector.
 # Note: When loaded as external interface, use absolute imports.
 try:
+    from BLEFragmentationBackend import BACKEND as BLE_FRAGMENTATION_BACKEND
     from BLEFragmentationBackend import BLEFragmenter, BLEReassembler
-except ImportError:
+except ImportError as e:
+    if "C++ BLE fragmentation backend is not available" in str(e):
+        raise
     try:
+        from ble_reticulum.BLEFragmentationBackend import BACKEND as BLE_FRAGMENTATION_BACKEND
         from ble_reticulum.BLEFragmentationBackend import BLEFragmenter, BLEReassembler
-    except ImportError:
+    except ImportError as e:
+        if "C++ BLE fragmentation backend is not available" in str(e):
+            raise
         # Compatibility fallback if the backend shim is not installed.
         try:
+            BLE_FRAGMENTATION_BACKEND = "python-direct"
             from BLEFragmentation import BLEFragmenter, BLEReassembler
         except ImportError:
+            BLE_FRAGMENTATION_BACKEND = "python-direct"
             from ble_reticulum.BLEFragmentation import BLEFragmenter, BLEReassembler
 
 # Import GATT server for peripheral mode
@@ -463,6 +471,12 @@ class BLEInterface(Interface):
 
 
         RNS.log(f"{self} initializing with service UUID {self.service_uuid}", RNS.LOG_INFO)
+        RNS.log(
+            f"{self} fragmentation backend: {BLE_FRAGMENTATION_BACKEND} "
+            f"(fragmenter={BLEFragmenter.__module__}.{BLEFragmenter.__name__}, "
+            f"reassembler={BLEReassembler.__module__}.{BLEReassembler.__name__})",
+            RNS.LOG_NOTICE
+        )
         RNS.log(f"{self} power mode: {self.power_mode}, max peers: {self.max_peers}", RNS.LOG_DEBUG)
         RNS.log(f"{self} central mode: {'ENABLED' if self.enable_central else 'DISABLED'}", RNS.LOG_INFO)
         RNS.log(f"{self} peripheral mode: {'ENABLED' if self.enable_peripheral else 'DISABLED'}", RNS.LOG_INFO)