diff --git a/nanobot/channels/slack.py b/nanobot/channels/slack.py
index af03d4973..c68020ce7 100644
--- a/nanobot/channels/slack.py
+++ b/nanobot/channels/slack.py
@@ -120,8 +120,13 @@ class SlackChannel(BaseChannel):
             slack_meta = msg.metadata.get("slack", {}) if msg.metadata else {}
             thread_ts = slack_meta.get("thread_ts")
             channel_type = slack_meta.get("channel_type")
+            origin_chat_id = str((slack_meta.get("event", {}) or {}).get("channel") or msg.chat_id)
             # Slack DMs don't use threads; channel/group replies may keep thread_ts.
-            thread_ts_param = thread_ts if thread_ts and channel_type != "im" else None
+            thread_ts_param = (
+                thread_ts
+                if thread_ts and channel_type != "im" and target_chat_id == origin_chat_id
+                else None
+            )
 
             # Slack rejects empty text payloads. Keep media-only messages media-only,
             # but send a single blank message when the bot has no text or files to send.
@@ -145,7 +150,7 @@ class SlackChannel(BaseChannel):
             # Update reaction emoji when the final (non-progress) response is sent
             if not (msg.metadata or {}).get("_progress"):
                 event = slack_meta.get("event", {})
-                await self._update_react_emoji(event.get("channel") or msg.chat_id, event.get("ts"))
+                await self._update_react_emoji(origin_chat_id, event.get("ts"))
 
         except Exception as e:
             logger.error("Error sending Slack message: {}", e)
diff --git a/tests/channels/test_slack_channel.py b/tests/channels/test_slack_channel.py
index 6fb05a912..2e72c4e61 100644
--- a/tests/channels/test_slack_channel.py
+++ b/tests/channels/test_slack_channel.py
@@ -270,6 +270,38 @@ async def test_send_updates_reaction_on_origin_channel_for_cross_channel_send()
     ]
 
 
+@pytest.mark.asyncio
+async def test_send_does_not_reuse_origin_thread_ts_for_cross_channel_send() -> None:
+    channel = SlackChannel(SlackConfig(enabled=True), MessageBus())
+    fake_web = _FakeAsyncWebClient()
+    fake_web._conversations_pages = [
+        {
+            "channels": [{"id": "C999", "name": "channel_x"}],
+            "response_metadata": {"next_cursor": ""},
+        }
+    ]
+    channel._web_client = fake_web
+
+    await channel.send(
+        OutboundMessage(
+            channel="slack",
+            chat_id="channel_x",
+            content="done",
+            metadata={
+                "slack": {
+                    "event": {"ts": "1700000000.000100", "channel": "C_ORIGIN"},
+                    "thread_ts": "1700000000.000200",
+                    "channel_type": "channel",
+                },
+            },
+        )
+    )
+
+    assert fake_web.chat_post_calls == [
+        {"channel": "C999", "text": "done\n", "thread_ts": None}
+    ]
+
+
 @pytest.mark.asyncio
 async def test_send_raises_when_named_target_cannot_be_resolved() -> None:
     channel = SlackChannel(SlackConfig(enabled=True), MessageBus())