diff --git a/src/tools/task/runtime.rs b/src/tools/task/runtime.rs
index e111dac..8e3192f 100644
--- a/src/tools/task/runtime.rs
+++ b/src/tools/task/runtime.rs
@@ -342,7 +342,7 @@ impl DefaultSubAgentRuntime {
                 sender_id: None,
                 chat_id: Some(session.parent_chat_id.clone()),
                 session_id: Some(session.session_id.clone()),
-                topic_id: None,
+                topic_id: session.parent_topic_id.clone(),
                 message_id: None,
                 message_seq: None,
                 subagent_description: Some(session.description.clone()),
diff --git a/src/tools/todo_write.rs b/src/tools/todo_write.rs
index 1656be6..6195e48 100644
--- a/src/tools/todo_write.rs
+++ b/src/tools/todo_write.rs
@@ -328,11 +328,17 @@ impl Tool for TodoWriteTool {
 
 // ── 辅助函数 ──────────────────────────────────────────────
 
-/// 计算 scope_key：优先 topic_id，否则 session_id
+/// 计算 scope_key：
+/// - 主代理 (nesting_depth == 0)：优先 topic_id，否则 session_id
+/// - 子/孙代理 (nesting_depth > 0)：使用 session_id 隔离，避免污染父代理 todo
 pub(crate) fn scope_key_from_context(context: &ToolContext) -> Option<String> {
-    let tid = context.topic_id.as_deref().filter(|t| !t.is_empty());
-    let sid = context.session_id.as_deref().filter(|s| !s.is_empty());
-    tid.or(sid).map(str::to_string)
+    if context.nesting_depth > 0 {
+        context.session_id.clone().filter(|s| !s.is_empty())
+    } else {
+        let tid = context.topic_id.as_deref().filter(|t| !t.is_empty());
+        let sid = context.session_id.as_deref().filter(|s| !s.is_empty());
+        tid.or(sid).map(str::to_string)
+    }
 }
 
 /// 校验状态转换合法性