diff --git a/clients/desktop/src-tauri/src/lib.rs b/clients/desktop/src-tauri/src/lib.rs
index bf9d030..f110cee 100644
--- a/clients/desktop/src-tauri/src/lib.rs
+++ b/clients/desktop/src-tauri/src/lib.rs
@@ -14,6 +14,16 @@ use sync::config::AppConfig;
 use sync::engine::{SyncEngine, SyncMode, SyncPath};
 use sync::watcher::{FileWatcher, ChangeKind};
 
+use std::collections::HashMap;
+
+/// Tracks a file opened from a .cloud placeholder
+#[derive(Clone, Debug)]
+struct OpenedFile {
+    file_id: i64,
+    real_path: PathBuf,
+    cloud_name: String, // original .cloud filename
+}
+
 struct AppState {
     api: Mutex<Option<MiniCloudApi>>,
     sync_engine: Mutex<Option<SyncEngine>>,
@@ -21,6 +31,7 @@ struct AppState {
     watchers: Mutex<Vec<FileWatcher>>,
     sync_running: Arc<Mutex<bool>>,
     locked_files: Mutex<Vec<i64>>,
+    opened_files: Mutex<HashMap<i64, OpenedFile>>, // file_id -> opened file info
     sync_paths: Mutex<Vec<SyncPath>>,
 }
 
@@ -225,10 +236,17 @@ async fn open_cloud_file(state: State<'_, AppState>, cloud_path: String) -> Resu
     // Remove placeholder
     std::fs::remove_file(&path).ok();
 
-    // Lock
+    // Lock on server
     let _ = engine.lock_file(file_id, "Desktop Sync Client").await;
     state.locked_files.lock().unwrap().push(file_id);
 
+    // Track opened file for auto-close detection
+    state.opened_files.lock().unwrap().insert(file_id, OpenedFile {
+        file_id,
+        real_path: real_path.clone(),
+        cloud_name: path.file_name().unwrap().to_string_lossy().to_string(),
+    });
+
     // Open with default application
     let _ = open::that(&real_path);
 
@@ -463,17 +481,71 @@ fn start_background_sync(
         }
     });
 
-    // Heartbeat every 60 seconds
+    // Heartbeat every 60s + check if opened files are still in use
     let app_hb = app.clone();
     let api_hb = api.clone();
     std::thread::spawn(move || {
         let rt = tokio::runtime::Runtime::new().unwrap();
         loop {
-            std::thread::sleep(Duration::from_secs(60));
+            std::thread::sleep(Duration::from_secs(10));
             let state = app_hb.state::<AppState>();
+
+            // Heartbeat for locked files
             let locked = state.locked_files.lock().unwrap().clone();
-            for file_id in locked {
-                let _ = rt.block_on(api_hb.heartbeat(file_id));
+            for file_id in &locked {
+                let _ = rt.block_on(api_hb.heartbeat(*file_id));
+            }
+
+            // Check if opened files are still in use by another process
+            let opened = state.opened_files.lock().unwrap().clone();
+            for (file_id, info) in &opened {
+                if !info.real_path.exists() {
+                    // File was deleted/moved - clean up
+                    state.opened_files.lock().unwrap().remove(file_id);
+                    state.locked_files.lock().unwrap().retain(|&id| id != *file_id);
+                    let _ = rt.block_on(api_hb.unlock_file(*file_id));
+                    let _ = app_hb.emit("file-change",
+                        format!("Geschlossen + entsperrt: {}", info.cloud_name));
+                    continue;
+                }
+
+                // Check if file is still locked by another process
+                let still_open = is_file_in_use(&info.real_path);
+                if !still_open {
+                    // File closed! Sync back, recreate .cloud, unlock
+                    let _ = app_hb.emit("file-change",
+                        format!("Datei geschlossen, synchronisiere: {}", info.real_path.file_name().unwrap().to_string_lossy()));
+
+                    // Upload changes
+                    let _ = rt.block_on(api_hb.upload_file(&info.real_path, None));
+
+                    // Unlock on server
+                    let _ = rt.block_on(api_hb.unlock_file(*file_id));
+
+                    // Recreate .cloud placeholder
+                    let cloud_path = info.real_path.parent().unwrap().join(&info.cloud_name);
+                    let size = std::fs::metadata(&info.real_path).map(|m| m.len() as i64).unwrap_or(0);
+                    let checksum = sync::engine::compute_file_hash(&info.real_path);
+                    let placeholder = serde_json::json!({
+                        "id": file_id,
+                        "name": info.real_path.file_name().unwrap().to_string_lossy(),
+                        "size": size,
+                        "checksum": checksum,
+                        "updated_at": chrono::Utc::now().to_rfc3339(),
+                        "server_path": "",
+                    });
+                    std::fs::write(&cloud_path, serde_json::to_string_pretty(&placeholder).unwrap()).ok();
+
+                    // Remove local copy
+                    std::fs::remove_file(&info.real_path).ok();
+
+                    // Clean up tracking
+                    state.opened_files.lock().unwrap().remove(file_id);
+                    state.locked_files.lock().unwrap().retain(|&id| id != *file_id);
+
+                    let _ = app_hb.emit("file-change",
+                        format!("Entsperrt + .cloud: {}", info.cloud_name));
+                }
             }
         }
     });
@@ -526,6 +598,32 @@ fn start_background_sync(
 // --- App Setup ---
 
 /// Check if another instance is running. If yes, pass the .cloud file to it and exit.
+/// Check if a file is still being used by another process
+fn is_file_in_use(path: &Path) -> bool {
+    // Try to open the file with exclusive access
+    // If it fails, another process has it open
+    #[cfg(target_os = "windows")]
+    {
+        use std::fs::OpenOptions;
+        // On Windows, try to open with write access - fails if file is locked by Office etc.
+        match OpenOptions::new().write(true).open(path) {
+            Ok(_) => false, // We could open it -> not in use
+            Err(_) => true, // Can't open -> still in use
+        }
+    }
+    #[cfg(not(target_os = "windows"))]
+    {
+        // On Linux/Mac, check /proc or lsof
+        let output = std::process::Command::new("lsof")
+            .arg(path.to_string_lossy().as_ref())
+            .output();
+        match output {
+            Ok(o) => !o.stdout.is_empty(), // lsof found processes -> in use
+            Err(_) => false, // lsof not available -> assume not in use
+        }
+    }
+}
+
 /// Single instance per user. On terminal servers each user gets their own
 /// instance because the lock file is in %APPDATA% (user-specific).
 fn handle_single_instance() {
@@ -594,6 +692,7 @@ pub fn run() {
             watchers: Mutex::new(Vec::new()),
             sync_running: Arc::new(Mutex::new(false)),
             locked_files: Mutex::new(Vec::new()),
+            opened_files: Mutex::new(HashMap::new()),
             sync_paths: Mutex::new(Vec::new()),
         })
         .on_window_event(|window, event| {