diff --git a/src/storage/mod.rs b/src/storage/mod.rs
index 493675c..ad9ae7b 100644
--- a/src/storage/mod.rs
+++ b/src/storage/mod.rs
@@ -1581,10 +1581,19 @@ impl SessionStore {
         }
     }
 
-    /// 获取指定话题的消息数量（动态计算，确保准确）
+    /// 获取指定话题的消息数量。
+    ///
+    /// 使用 `SELECT COUNT(*)` 在数据库侧计数，避免将所有消息
+    /// （含 content、tool_calls_json 等大字段反序列化）加载到内存。
+    /// 查询命中 `idx_messages_topic_seq(topic_id, seq)` 索引。
     pub fn get_topic_message_count(&self, topic_id: &str) -> Result<usize, StorageError> {
-        self.load_messages_for_topic(topic_id, None)
-            .map(|msgs| msgs.len())
+        let conn = self.pool.get()?;
+        let count: i64 = conn.query_row(
+            "SELECT COUNT(*) FROM messages WHERE topic_id = ?1",
+            params![topic_id],
+            |row| row.get(0),
+        )?;
+        Ok(count as usize)
     }
 
     pub fn load_all_messages(&self, session_id: &str) -> Result<Vec<ChatMessage>, StorageError> {
diff --git a/src/storage/tests.rs b/src/storage/tests.rs
index 1cefb3f..ef4553e 100644
--- a/src/storage/tests.rs
+++ b/src/storage/tests.rs
@@ -687,3 +687,41 @@ fn test_scheduler_job_roundtrip_and_runtime_update() {
     assert_eq!(fetched.run_count, 1);
     assert_eq!(fetched.completed_at, Some(1_700_000_000_100));
 }
+
+#[test]
+fn test_get_topic_message_count_uses_count_query() {
+    let store = SessionStore::in_memory().unwrap();
+    let session = store.create_cli_session(Some("topic-count")).unwrap();
+    let topic = store
+        .create_topic(&session.id, "topic-1", None)
+        .unwrap();
+
+    // 初始计数为 0
+    assert_eq!(store.get_topic_message_count(&topic.id).unwrap(), 0);
+
+    // 追加 3 条带 topic_id 的消息
+    for content in ["m1", "m2", "m3"] {
+        store
+            .append_message_with_topic(
+                &session.id,
+                Some(&topic.id),
+                &ChatMessage::user(content),
+            )
+            .unwrap();
+    }
+
+    // 计数应为 3，且不需要加载消息内容
+    assert_eq!(store.get_topic_message_count(&topic.id).unwrap(), 3);
+
+    // 另一个 topic 的计数应为 0（隔离验证）
+    let other_topic = store
+        .create_topic(&session.id, "topic-2", None)
+        .unwrap();
+    assert_eq!(store.get_topic_message_count(&other_topic.id).unwrap(), 0);
+
+    // 不存在的 topic_id 返回 0
+    assert_eq!(
+        store.get_topic_message_count("topic:nonexistent").unwrap(),
+        0
+    );
+}