diff --git a/src/pages/TaskDetailPage.tsx b/src/pages/TaskDetailPage.tsx
index 72a4264..3469d6a 100644
--- a/src/pages/TaskDetailPage.tsx
+++ b/src/pages/TaskDetailPage.tsx
@@ -88,13 +88,27 @@ export default function TaskDetailPage() {
     () => members.find((m) => m.user_id === user?.id)?.role,
     [members, user?.id]
   )
-  const canEditTask = Boolean(task && project && user && (
+  const isAdmin = Boolean(user && (
     user.is_admin ||
-    user.id === project.owner_id ||
-    user.id === task.created_by_id ||
-    user.id === milestone?.created_by_id ||
+    (project && user.id === project.owner_id) ||
     currentMemberRole === 'admin'
   ))
+  // P5.7/P9.3: assignee-aware edit permission
+  const canEditTask = Boolean(task && project && user && (() => {
+    const st = task.status
+    // undergoing/completed/closed: no body edits
+    if (st === 'undergoing' || st === 'completed' || st === 'closed') return false
+    // open + assignee set: only assignee or admin
+    if (st === 'open' && task.assignee_id != null) {
+      return user.id === task.assignee_id || isAdmin
+    }
+    // open + no assignee, or pending: general permission
+    return (
+      isAdmin ||
+      user.id === task.created_by_id ||
+      user.id === milestone?.created_by_id
+    )
+  })())
 
   if (!task) return <div className="loading">Loading...</div>
 
@@ -113,7 +127,7 @@ export default function TaskDetailPage() {
           <span className="badge">{task.task_type}</span>{task.task_subtype && <span className="badge">{task.task_subtype}</span>}
           {task.tags && <span className="tags">{task.tags}</span>}
         </div>
-        {canEditTask && !isTerminal && task.status !== 'undergoing' && (
+        {canEditTask && (
           <button className="btn-transition" style={{ marginTop: 8 }} onClick={() => setShowEditTask(true)}>Edit Task</button>
         )}
       </div>