Fix JSON serialization for D-Bus advertisement data

- Add recursive unwrap_variant() to handle nested Variant objects
- Convert manufacturer_data/service_data bytes to hex strings in DeviceInfo
- Remove redundant bytes-to-hex conversion from resources.py

BlueZ returns ManufacturerData as {int: Variant(bytes)} which wasn't
being fully unwrapped, causing JSON serialization failures during scan.

.gitignore

@@ -44,3 +44,4 @@ uv.lock
 
 # Project specific
 *.log
+.mcp.json

src/mcbluetooth/dbus_client.py

@@ -39,6 +39,21 @@ DBUS_PROPS_IFACE = "org.freedesktop.DBus.Properties"
 DBUS_OBJMANAGER_IFACE = "org.freedesktop.DBus.ObjectManager"
 
 
+def unwrap_variant(value: Any) -> Any:
+    """Recursively unwrap dbus-fast Variant objects to plain Python types.
+
+    BlueZ returns nested structures like ManufacturerData = {int: Variant(bytes)}.
+    This function ensures all Variant wrappers are removed at every level.
+    """
+    if isinstance(value, Variant):
+        return unwrap_variant(value.value)
+    elif isinstance(value, dict):
+        return {k: unwrap_variant(v) for k, v in value.items()}
+    elif isinstance(value, list):
+        return [unwrap_variant(v) for v in value]
+    return value
+
+
 def address_to_path(adapter: str, address: str) -> str:
     """Convert a Bluetooth address to D-Bus object path."""
     addr_path = address.upper().replace(":", "_")
@@ -94,8 +109,8 @@ class DeviceInfo:
     appearance: int = 0
     icon: str = ""
     services_resolved: bool = False
-    manufacturer_data: dict[int, bytes] = field(default_factory=dict)
-    service_data: dict[str, bytes] = field(default_factory=dict)
+    manufacturer_data: dict[int, str] = field(default_factory=dict)  # hex strings
+    service_data: dict[str, str] = field(default_factory=dict)  # hex strings
 
 
 @dataclass
@@ -163,7 +178,7 @@ class BlueZClient:
             result[path] = {}
             for iface, props in interfaces.items():
                 result[path][iface] = {
-                    k: v.value if isinstance(v, Variant) else v for k, v in props.items()
+                    k: unwrap_variant(v) for k, v in props.items()
                 }
         return result
 
@@ -200,7 +215,7 @@ class BlueZClient:
         """Get all properties from an object."""
         props_iface = await self._get_interface(path, DBUS_PROPS_IFACE)
         props = await props_iface.call_get_all(interface)
-        return {k: v.value if isinstance(v, Variant) else v for k, v in props.items()}
+        return {k: unwrap_variant(v) for k, v in props.items()}
 
     # ==================== Adapter Operations ====================
 
@@ -358,17 +373,19 @@ class BlueZClient:
             # Extract adapter name from path
             adapter_name = path.split("/")[3] if len(path.split("/")) > 3 else ""
 
-            # Parse manufacturer data
+            # Parse manufacturer data (convert bytes to hex strings for JSON)
             mfr_data = {}
             if "ManufacturerData" in props:
                 for k, v in props["ManufacturerData"].items():
-                    mfr_data[k] = bytes(v) if isinstance(v, (list, bytearray)) else v
+                    raw = bytes(v) if isinstance(v, (list, bytearray)) else v
+                    mfr_data[k] = raw.hex() if isinstance(raw, bytes) else str(raw)
 
-            # Parse service data
+            # Parse service data (convert bytes to hex strings for JSON)
             svc_data = {}
             if "ServiceData" in props:
                 for k, v in props["ServiceData"].items():
-                    svc_data[k] = bytes(v) if isinstance(v, (list, bytearray)) else v
+                    raw = bytes(v) if isinstance(v, (list, bytearray)) else v
+                    svc_data[k] = raw.hex() if isinstance(raw, bytes) else str(raw)
 
             devices.append(
                 DeviceInfo(

src/mcbluetooth/resources.py

@@ -193,17 +193,6 @@ def register_resources(mcp: FastMCP) -> None:
         devices = await client.list_devices()
         for d in devices:
             if d.address.upper() == address.upper():
-                data = asdict(d)
-                # Convert bytes to hex strings for JSON serialization
-                if data.get("manufacturer_data"):
-                    data["manufacturer_data"] = {
-                        k: v.hex() if isinstance(v, bytes) else str(v)
-                        for k, v in data["manufacturer_data"].items()
-                    }
-                if data.get("service_data"):
-                    data["service_data"] = {
-                        k: v.hex() if isinstance(v, bytes) else str(v)
-                        for k, v in data["service_data"].items()
-                    }
-                return json.dumps(data, indent=2)
+                # DeviceInfo already has manufacturer_data/service_data as hex strings
+                return json.dumps(asdict(d), indent=2)
         return json.dumps({"error": f"Device '{address}' not found"})