diff --git a/server/database.js b/server/database.js
index d1fe4d8..065f7ca 100644
--- a/server/database.js
+++ b/server/database.js
@@ -375,12 +375,22 @@ export function getNextTask() {
 }
 
 export function assignTask(taskId, turtleId) {
-  const stmt = db.prepare(`
-    UPDATE task_queue 
-    SET assigned_turtle_id = ?, status = 'assigned', updated_at = ?
-    WHERE id = ?
-  `);
-  stmt.run(turtleId, Date.now(), taskId);
+  if (turtleId === null || turtleId === undefined) {
+    // Un-assign: clear turtle and revert to pending
+    const stmt = db.prepare(`
+      UPDATE task_queue
+      SET assigned_turtle_id = NULL, status = 'pending', updated_at = ?
+      WHERE id = ?
+    `);
+    stmt.run(Date.now(), taskId);
+  } else {
+    const stmt = db.prepare(`
+      UPDATE task_queue 
+      SET assigned_turtle_id = ?, status = 'assigned', updated_at = ?
+      WHERE id = ?
+    `);
+    stmt.run(turtleId, Date.now(), taskId);
+  }
 }
 
 export function updateTaskStatus(taskId, status, result = null) {