diff --git a/app/src/main/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroup.kt b/app/src/main/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroup.kt
index 386f4926..a09bc467 100644
--- a/app/src/main/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroup.kt
+++ b/app/src/main/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroup.kt
@@ -82,10 +82,19 @@ fun groupConversationTimeline(messages: List<ChatMessage>): List<TimelineEntry>
     var turnStartAt: Long? = null
     var turnCompletedAt: Long? = null
     var turnLastId: String? = null
+    // Claude Code reuses tool_use call ids (toolu_…) across retries, so two distinct runs can both
+    // begin with a part of the same id. Tag each group with how many runs have already started with
+    // that first part id so LazyColumn keys stay unique — the first group keeps the bare id so a
+    // growing run's identity stays stable while its steps stream in.
+    val firstIdOccurrences = mutableMapOf<String, Int>()
 
     fun flush() {
         if (pending.isEmpty()) return
-        entries += TimelineEntry.Activity("activity:${pending.first().id}", pending.toList())
+        val firstId = pending.first().id
+        val occurrence = firstIdOccurrences[firstId] ?: 0
+        firstIdOccurrences[firstId] = occurrence + 1
+        val id = if (occurrence == 0) "activity:$firstId" else "activity:$firstId:$occurrence"
+        entries += TimelineEntry.Activity(id, pending.toList())
         pending.clear()
     }
 
diff --git a/app/src/test/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroupTest.kt b/app/src/test/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroupTest.kt
index 1c86f44f..14e553df 100644
--- a/app/src/test/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroupTest.kt
+++ b/app/src/test/java/com/yugahashimoto/andcode/feature/chat/AssistantActivityGroupTest.kt
@@ -316,4 +316,54 @@ class AssistantActivityGroupTest {
 
         assertEquals(parts.size, keys.toSet().size)
     }
+
+    @Test
+    fun `activity group ids stay unique when separate runs reuse the same first part id`() {
+        // Claude Code reuses tool_use call ids (toolu_…) across retries, so two independent runs can
+        // each begin with the very same part id. That must not produce duplicate LazyColumn keys.
+        val entries =
+            groupConversationTimeline(
+                listOf(
+                    assistant("m1", tool("toolu_retry", "bash")),
+                    ChatMessage(id = "m2", isUser = true, parts = listOf(ChatPart.Text("u1", "try again"))),
+                    assistant("m3", tool("toolu_retry", "bash")),
+                ),
+            )
+
+        val activityIds = entries.filterIsInstance<TimelineEntry.Activity>().map { it.id }
+        assertEquals(listOf("activity:toolu_retry", "activity:toolu_retry:1"), activityIds)
+        assertEquals(activityIds.size, activityIds.toSet().size)
+    }
+
+    @Test
+    fun `activity group id stays bare for the first run sharing a part id`() {
+        val entries =
+            groupConversationTimeline(
+                listOf(
+                    assistant("m1", tool("toolu_same", "bash")),
+                    ChatMessage(id = "m2", isUser = true, parts = listOf(ChatPart.Text("u1", "again"))),
+                    assistant("m3", tool("toolu_same", "bash")),
+                ),
+            )
+
+        assertEquals("activity:toolu_same", (entries[0] as TimelineEntry.Activity).id)
+    }
+
+    @Test
+    fun `activity group ids stay stable as a growing run streams in`() {
+        // The first occurrence keeps the bare id, so an already-flushed group's identity survives a
+        // sibling run that later reuses the same first part id.
+        val first = listOf(assistant("m1", tool("toolu_stream", "bash")))
+        val firstId = (groupConversationTimeline(first).single() as TimelineEntry.Activity).id
+        assertEquals("activity:toolu_stream", firstId)
+
+        val grown =
+            first +
+                listOf(
+                    ChatMessage(id = "m2", isUser = true, parts = listOf(ChatPart.Text("u1", "retry"))),
+                    assistant("m3", tool("toolu_stream", "bash")),
+                )
+        val grownIds = groupConversationTimeline(grown).filterIsInstance<TimelineEntry.Activity>().map { it.id }
+        assertEquals(listOf("activity:toolu_stream", "activity:toolu_stream:1"), grownIds)
+    }
 }