diff --git a/nanobot/channels/feishu.py b/nanobot/channels/feishu.py
index 1442c3637..9f2d13975 100644
--- a/nanobot/channels/feishu.py
+++ b/nanobot/channels/feishu.py
@@ -1624,6 +1624,15 @@ class FeishuChannel(BaseChannel):
             if not content and not media_paths:
                 return
 
+            # Build topic-scoped session key for conversation isolation.
+            # Group chat: thread replies (root_id != message_id) get a scoped
+            # session so each Feishu thread has its own conversation context.
+            # Private chat: no override — same behavior as Telegram/Slack.
+            if chat_type == "group" and root_id and root_id != message_id:
+                session_key = f"feishu:{chat_id}:{root_id}"
+            else:
+                session_key = None
+
             # Forward to message bus
             reply_to = chat_id if chat_type == "group" else sender_id
             await self._handle_message(
@@ -1640,6 +1649,7 @@ class FeishuChannel(BaseChannel):
                     "root_id": root_id,
                     "thread_id": thread_id,
                 },
+                session_key=session_key,
             )
 
         except Exception as e:
diff --git a/tests/channels/test_feishu_reply.py b/tests/channels/test_feishu_reply.py
index 0753653a7..15495fcd1 100644
--- a/tests/channels/test_feishu_reply.py
+++ b/tests/channels/test_feishu_reply.py
@@ -3,7 +3,7 @@ import asyncio
 import json
 from pathlib import Path
 from types import SimpleNamespace
-from unittest.mock import MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
@@ -26,13 +26,14 @@ from nanobot.channels.feishu import FeishuChannel, FeishuConfig
 # Helpers
 # ---------------------------------------------------------------------------
 
-def _make_feishu_channel(reply_to_message: bool = False) -> FeishuChannel:
+def _make_feishu_channel(reply_to_message: bool = False, group_policy: str = "mention") -> FeishuChannel:
     config = FeishuConfig(
         enabled=True,
         app_id="cli_test",
         app_secret="secret",
         allow_from=["*"],
         reply_to_message=reply_to_message,
+        group_policy=group_policy,
     )
     channel = FeishuChannel(config, MessageBus())
     channel._client = MagicMock()
@@ -443,3 +444,93 @@ async def test_on_message_no_extra_api_call_when_no_parent_id() -> None:
 
     channel._client.im.v1.message.get.assert_not_called()
     assert len(captured) == 1
+
+
+# ---------------------------------------------------------------------------
+# Session key derivation tests
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_session_key_group_with_root_id_is_thread_scoped() -> None:
+    """Group message with root_id gets a thread-scoped session key."""
+    channel = _make_feishu_channel(group_policy="open")
+    bus_spy = []
+    original_publish = channel.bus.publish_inbound
+
+    async def capture(msg):
+        bus_spy.append(msg)
+        await original_publish(msg)
+
+    channel.bus.publish_inbound = capture
+    channel._download_and_save_media = AsyncMock(return_value=(None, ""))
+    channel.transcribe_audio = AsyncMock(return_value="")
+    channel._add_reaction = AsyncMock(return_value=None)
+
+    event = _make_feishu_event(
+        chat_type="group",
+        content='{"text": "hello"}',
+        root_id="om_root123",
+        message_id="om_child456",
+    )
+    await channel._on_message(event)
+
+    assert len(bus_spy) == 1
+    assert bus_spy[0].session_key == "feishu:oc_abc:om_root123"
+
+
+@pytest.mark.asyncio
+async def test_session_key_group_no_root_id_uses_default() -> None:
+    """Group message without root_id uses default session key (no override)."""
+    channel = _make_feishu_channel(group_policy="open")
+    bus_spy = []
+    original_publish = channel.bus.publish_inbound
+
+    async def capture(msg):
+        bus_spy.append(msg)
+        await original_publish(msg)
+
+    channel.bus.publish_inbound = capture
+    channel._download_and_save_media = AsyncMock(return_value=(None, ""))
+    channel.transcribe_audio = AsyncMock(return_value="")
+    channel._add_reaction = AsyncMock(return_value=None)
+
+    event = _make_feishu_event(
+        chat_type="group",
+        content='{"text": "hello"}',
+        root_id=None,
+        message_id="om_001",
+    )
+    await channel._on_message(event)
+
+    assert len(bus_spy) == 1
+    assert bus_spy[0].session_key_override is None
+    assert bus_spy[0].session_key == "feishu:oc_abc"
+
+
+@pytest.mark.asyncio
+async def test_session_key_private_chat_no_override() -> None:
+    """Private chat never overrides session key (consistent with Telegram/Slack)."""
+    channel = _make_feishu_channel()
+    bus_spy = []
+    original_publish = channel.bus.publish_inbound
+
+    async def capture(msg):
+        bus_spy.append(msg)
+        await original_publish(msg)
+
+    channel.bus.publish_inbound = capture
+    channel._download_and_save_media = AsyncMock(return_value=(None, ""))
+    channel.transcribe_audio = AsyncMock(return_value="")
+    channel._add_reaction = AsyncMock(return_value=None)
+
+    event = _make_feishu_event(
+        chat_type="p2p",
+        content='{"text": "hello"}',
+        root_id=None,
+        message_id="om_001",
+    )
+    await channel._on_message(event)
+
+    assert len(bus_spy) == 1
+    assert bus_spy[0].session_key_override is None