diff --git a/composeApp/src/commonTest/kotlin/com/tt/honeyDue/data/DataManagerTaskCacheTest.kt b/composeApp/src/commonTest/kotlin/com/tt/honeyDue/data/DataManagerTaskCacheTest.kt
index 98722eb..b050dd7 100644
--- a/composeApp/src/commonTest/kotlin/com/tt/honeyDue/data/DataManagerTaskCacheTest.kt
+++ b/composeApp/src/commonTest/kotlin/com/tt/honeyDue/data/DataManagerTaskCacheTest.kt
@@ -88,6 +88,21 @@ class DataManagerTaskCacheTest {
         assertEquals("v2", inProgress.tasks.first { it.id == 5 }.title)
     }
 
+    /// Lockdown: updateTask must NOT touch `_tasksByResidence`. That cache
+    /// is being deleted in Phase 3; until then, updateTask must leave it
+    /// alone. If a future commit re-introduces the conditional write
+    /// branch this test catches it.
+    @Test
+    fun updateTask_doesNotMutate_tasksByResidence() {
+        val before = DataManager.tasksByResidence.value
+        DataManager.updateTask(sampleTask(id = 1, residenceId = 100, column = "upcoming_tasks"))
+        assertEquals(
+            before,
+            DataManager.tasksByResidence.value,
+            "updateTask must not write to _tasksByResidence — that cache is deprecated"
+        )
+    }
+
     private fun sampleTask(
         id: Int,
         residenceId: Int,