diff --git a/backend/users/service.py b/backend/users/service.py
index a1e426c..e76df8d 100644
--- a/backend/users/service.py
+++ b/backend/users/service.py
@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 
 from google.cloud.firestore_v1 import DocumentReference, ArrayUnion, ArrayRemove
 
@@ -116,30 +116,45 @@ def get_user(user_doc_id: str) -> UserInDB:
 
 def create_user(data: UserCreate) -> UserInDB:
     """Create a new user: a Firebase Auth account (so they can log into the app
-    immediately) plus the matching Firestore profile document."""
+    immediately) plus the matching Firestore profile document.
+
+    The profile doc ID MUST be the Firebase Auth uid. The FlutterFlow app looks
+    up `users/{uid}` on login and creates a bare doc there if it's missing, and
+    MQTT auth (mqtt/app_users.py) + device `user_list` paths assume the same.
+    """
     if not data.password or len(data.password) < 6:
         raise ValidationError("Password must be at least 6 characters.")
-    if not data.email:
+    # Firebase Auth lowercases emails; store the same form so the profile
+    # matches what the app sees for the signed-in user.
+    email = (data.email or "").strip().lower()
+    if not email:
         raise ValidationError("Email is required.")
 
     db = get_db()
     doc_data = data.model_dump(exclude={"password"})
+    doc_data["email"] = email
     doc_data["friendsList"] = []
     doc_data["friendsInvited"] = []
+    doc_data["created_time"] = datetime.now(timezone.utc)
 
     try:
         firebase_user = firebase_auth.create_user(
-            email=data.email,
+            email=email,
             password=data.password,
             display_name=data.display_name or None,
         )
     except firebase_auth.EmailAlreadyExistsError:
-        raise ValidationError(f"A user with email {data.email} already exists.")
+        raise ValidationError(f"A user with email {email} already exists.")
     doc_data["uid"] = firebase_user.uid
 
-    _, doc_ref = db.collection(COLLECTION).add(doc_data)
+    try:
+        db.collection(COLLECTION).document(firebase_user.uid).set(doc_data)
+    except Exception:
+        # Don't leave an Auth account behind with no profile doc.
+        firebase_auth.delete_user(firebase_user.uid)
+        raise
 
-    return UserInDB(id=doc_ref.id, **doc_data)
+    return UserInDB(id=firebase_user.uid, **_sanitize_dict(doc_data))
 
 
 def update_user(user_doc_id: str, data: UserUpdate) -> UserInDB:
diff --git a/docs/mqtt-app-user-auth.md b/docs/mqtt-app-user-auth.md
index 874ac67..296cee5 100644
--- a/docs/mqtt-app-user-auth.md
+++ b/docs/mqtt-app-user-auth.md
@@ -64,7 +64,9 @@ flashed into NVS and the firmware uses it as its MQTT id. Legacy docs only have
   - `PUT /api/devices/{id}` when the body contains `user_list`
     (`devices.service.update_device`)
 - Users are resolved by the **`uid` field** (a query), **never by doc ID**. Console-
-  created users get random doc IDs (`.add()`); FlutterFlow uses the uid as the doc ID.
+  created users used to get random doc IDs (`.add()`) — fixed 2026-09-30, the Console
+  now writes `users/{uid}` like FlutterFlow does — but older docs may still have a
+  random ID, so keep resolving by the `uid` field.
 - A user is refused when `status == "blocked"`.
 - Lookups are cached in-process for 60 s (`mqtt/app_users.py`). Assign/unassign,
   update, block/unblock and delete call `invalidate(uid)`. If you add a new code path