diff --git a/mobile/src/main/java/net/activitywatch/android/RustInterface.kt b/mobile/src/main/java/net/activitywatch/android/RustInterface.kt
index cac41c9a..18196bb6 100644
--- a/mobile/src/main/java/net/activitywatch/android/RustInterface.kt
+++ b/mobile/src/main/java/net/activitywatch/android/RustInterface.kt
@@ -172,8 +172,16 @@ class RustInterface(context: Context? = null) {
     }
 
     fun getDeviceName(context: Context): String {
-        return Settings.Global.getString(context.contentResolver, Settings.Global.DEVICE_NAME)
-            ?: android.os.Build.MODEL ?: "Unknown"
+        val raw = Settings.Global.getString(context.contentResolver, Settings.Global.DEVICE_NAME)
+            ?.trim()
+            ?.takeIf { it.isNotEmpty() }
+            ?: android.os.Build.DEVICE
+            ?: "unknown"
+        return raw.trim()
+            .lowercase(java.util.Locale.ROOT)
+            .replace(Regex("[^a-z0-9_-]+"), "_")
+            .trim('_')
+            .ifEmpty { "unknown" }
     }
 
 }
diff --git a/mobile/src/main/java/net/activitywatch/android/SyncInterface.kt b/mobile/src/main/java/net/activitywatch/android/SyncInterface.kt
index a7d9372d..481ae8ab 100644
--- a/mobile/src/main/java/net/activitywatch/android/SyncInterface.kt
+++ b/mobile/src/main/java/net/activitywatch/android/SyncInterface.kt
@@ -60,10 +60,16 @@ class SyncInterface(context: Context) {
     external fun getSyncDir(): String
     
     private fun getDeviceName(): String {
-        return android.provider.Settings.Global.getString(
-            appContext.contentResolver, 
+        val raw = android.provider.Settings.Global.getString(
+            appContext.contentResolver,
             android.provider.Settings.Global.DEVICE_NAME
-        ) ?: android.os.Build.MODEL ?: "Unknown"
+        )?.trim()?.takeIf { it.isNotEmpty() }
+            ?: android.os.Build.DEVICE ?: "unknown"
+        return raw.trim()
+            .lowercase(java.util.Locale.ROOT)
+            .replace(Regex("[^a-z0-9_-]+"), "_")
+            .trim('_')
+            .ifEmpty { "unknown" }
     }
     
     // Async wrapper for syncPullAll