diff --git a/agent/webhooks/slack.py b/agent/webhooks/slack.py
index 842709d7..96b8e47f 100644
--- a/agent/webhooks/slack.py
+++ b/agent/webhooks/slack.py
@@ -290,6 +290,7 @@ async def process_slack_mention(event_data: dict[str, Any], repo_config: dict[st
                 thread_ts,
                 run_id,
                 message_ts=trace_message_ts,
+                trace_message_ts=trace_message_ts,
                 triggering_user_id=user_id,
             )
     else:
diff --git a/tests/test_slack_context.py b/tests/test_slack_context.py
index 6772414c..3d24c881 100644
--- a/tests/test_slack_context.py
+++ b/tests/test_slack_context.py
@@ -42,6 +42,22 @@ class _FakeClient:
         self.threads = threads_client
 
 
+class _FakeSlackMappingStore:
+    def __init__(self) -> None:
+        self.items: dict[tuple[tuple[str, ...], str], dict] = {}
+
+    async def put_item(self, namespace: tuple[str, ...], key: str, value: dict) -> None:
+        self.items[(namespace, key)] = {"value": value}
+
+    async def get_item(self, namespace: tuple[str, ...], key: str) -> dict | None:
+        return self.items.get((namespace, key))
+
+
+class _FakeSlackMappingClient:
+    def __init__(self) -> None:
+        self.store = _FakeSlackMappingStore()
+
+
 def test_generate_thread_id_from_slack_thread_is_deterministic() -> None:
     channel_id = "C12345"
     thread_ts = "1730900000.123456"
@@ -51,6 +67,63 @@ def test_generate_thread_id_from_slack_thread_is_deterministic() -> None:
     assert len(first) == 36
 
 
+@pytest.mark.asyncio
+async def test_slack_run_mapping_preserves_trace_message_ts() -> None:
+    client = _FakeSlackMappingClient()
+
+    await slack_utils.store_slack_run_mapping(
+        client,
+        "C123",
+        "1.0",
+        "run-1",
+        message_ts="1.1",
+        triggering_user_id="U123",
+        trace_message_ts="1.1",
+    )
+    await slack_utils.store_slack_message_run_mapping(client, "C123", "1.0", "1.2")
+
+    thread_mapping = await slack_utils.lookup_slack_thread_run_mapping(client, "C123", "1.0")
+    message_mapping = await slack_utils.lookup_slack_run_mapping(client, "C123", "1.2")
+    assert thread_mapping is not None
+    assert thread_mapping["trace_message_ts"] == "1.1"
+    assert thread_mapping["triggering_user_id"] == "U123"
+    assert message_mapping is not None
+    assert message_mapping["trace_message_ts"] == "1.1"
+    assert message_mapping["message_ts"] == "1.2"
+
+
+@pytest.mark.asyncio
+async def test_slack_run_mapping_preserves_trace_message_ts_on_followup_mention() -> None:
+    """A subsequent Slack mention without trace_message_ts must not clobber the stored timestamp."""
+    client = _FakeSlackMappingClient()
+
+    # First mention stores the trace message ts.
+    await slack_utils.store_slack_run_mapping(
+        client,
+        "C123",
+        "1.0",
+        "run-1",
+        message_ts="1.1",
+        triggering_user_id="U123",
+        trace_message_ts="1.1",
+    )
+
+    # Follow-up mention (non-first) stores a new run_id without trace_message_ts.
+    await slack_utils.store_slack_run_mapping(
+        client,
+        "C123",
+        "1.0",
+        "run-2",
+        triggering_user_id="U456",
+    )
+
+    thread_mapping = await slack_utils.lookup_slack_thread_run_mapping(client, "C123", "1.0")
+    assert thread_mapping is not None
+    assert thread_mapping["run_id"] == "run-2"
+    assert thread_mapping["trace_message_ts"] == "1.1"
+    assert thread_mapping["triggering_user_id"] == "U456"
+
+
 def test_select_slack_context_messages_uses_thread_start_when_no_prior_mention() -> None:
     bot_user_id = "UBOT"
     messages = [