diff --git a/apps/api/src/services/channel_hub.py b/apps/api/src/services/channel_hub.py
index facf0395..e6903986 100644
--- a/apps/api/src/services/channel_hub.py
+++ b/apps/api/src/services/channel_hub.py
@@ -489,6 +489,7 @@ async def record_outbound_message(
     source_envelope_json = json.dumps(envelope, ensure_ascii=False, default=str)
 
     actual_status = "shadow" if is_shadow else send_status
+    sent_at = datetime.now(UTC) if actual_status == "sent" else None
 
     await ensure_completed_shadow_run(
         db,
@@ -522,8 +523,7 @@ async def record_outbound_message(
                 :content_hash, :content_preview, :content_redacted,
                 :redaction_version, CAST(:source_envelope AS jsonb),
                 :provider_message_id,
-                :send_status, NOW(),
-                CASE WHEN CAST(:send_status AS text) = 'sent' THEN NOW() ELSE NULL END,
+                :send_status, NOW(), :sent_at,
                 :triggered_by_state, :waiting_since
             )
             RETURNING message_id
@@ -542,6 +542,7 @@ async def record_outbound_message(
             "source_envelope": source_envelope_json,
             "provider_message_id": provider_message_id,
             "send_status": actual_status,
+            "sent_at": sent_at,
             "triggered_by_state": triggered_by_state,
             "waiting_since": waiting_since,
         },
diff --git a/apps/api/tests/test_channel_hub_grouped_alert_events.py b/apps/api/tests/test_channel_hub_grouped_alert_events.py
index 44e23b4a..cd1714b6 100644
--- a/apps/api/tests/test_channel_hub_grouped_alert_events.py
+++ b/apps/api/tests/test_channel_hub_grouped_alert_events.py
@@ -132,5 +132,6 @@ async def test_record_outbound_message_sets_sent_at_for_sent_messages() -> None:
 
     insert_statement = session.statements[-1]
     assert "sent_at" in insert_statement
-    assert "CASE WHEN CAST(:send_status AS text) = 'sent' THEN NOW() ELSE NULL END" in insert_statement
+    assert ":sent_at" in insert_statement
     assert session.param_sets[-1]["send_status"] == "sent"
+    assert session.param_sets[-1]["sent_at"] is not None
diff --git a/docs/LOGBOOK.md b/docs/LOGBOOK.md
index 0a3d5209..20d24276 100644
--- a/docs/LOGBOOK.md
+++ b/docs/LOGBOOK.md
@@ -7307,6 +7307,6 @@ OK
 **production smoke 途中補修**：
 
 - rollback transaction smoke 抓到 asyncpg bind parameter 型別推論問題：`CASE WHEN :send_status = 'sent'` 會被推成 text/varchar ambiguous。
-- 已改成 `CASE WHEN CAST(:send_status AS text) = 'sent' THEN NOW() ELSE NULL END`，避免 outbound mirror 在 production 寫入時失敗。
+- 第一版 `CAST(:send_status AS text)` 仍會因同一 bind param 同時插入 varchar 與比較而 ambiguous；最終改成 Python 端計算 `sent_at` 參數，SQL 只插入 `:sent_at`，避免 outbound mirror 在 production 寫入時失敗。
 
 **目前整體進度**：約 69%。