diff --git a/backend/devices/service.py b/backend/devices/service.py
index f8de946..5b12203 100644
--- a/backend/devices/service.py
+++ b/backend/devices/service.py
@@ -1,6 +1,8 @@
 import secrets
 import string
 
+from google.cloud.firestore_v1 import GeoPoint
+
 from shared.firebase import get_db
 from shared.exceptions import NotFoundError
 from devices.models import DeviceCreate, DeviceUpdate, DeviceInDB
@@ -36,9 +38,38 @@ def _ensure_unique_serial(db) -> str:
     raise RuntimeError("Could not generate a unique serial number after 100 attempts")
 
 
+def _convert_firestore_value(val):
+    """Convert Firestore-specific types (Timestamp, GeoPoint) to strings."""
+    if hasattr(val, "seconds") and hasattr(val, "nanos"):
+        # Firestore Timestamp → ISO string
+        from datetime import datetime, timezone
+        dt = datetime.fromtimestamp(val.seconds + val.nanos / 1e9, tz=timezone.utc)
+        return dt.strftime("%d %B %Y at %H:%M:%S UTC")
+    if isinstance(val, GeoPoint):
+        return f"{val.latitude}° N, {val.longitude}° E"
+    return val
+
+
+def _sanitize_dict(d: dict) -> dict:
+    """Recursively convert Firestore-native types in a dict to plain strings."""
+    result = {}
+    for k, v in d.items():
+        if isinstance(v, dict):
+            result[k] = _sanitize_dict(v)
+        elif isinstance(v, list):
+            result[k] = [
+                _sanitize_dict(item) if isinstance(item, dict)
+                else _convert_firestore_value(item)
+                for item in v
+            ]
+        else:
+            result[k] = _convert_firestore_value(v)
+    return result
+
+
 def _doc_to_device(doc) -> DeviceInDB:
     """Convert a Firestore document snapshot to a DeviceInDB model."""
-    data = doc.to_dict()
+    data = _sanitize_dict(doc.to_dict())
     return DeviceInDB(id=doc.id, **data)