diff --git a/src/agent/claude/stream-json.ts b/src/agent/claude/stream-json.ts
index b30db89a..a4ab19d0 100644
--- a/src/agent/claude/stream-json.ts
+++ b/src/agent/claude/stream-json.ts
@@ -15,6 +15,8 @@ interface ContentBlock {
 interface ClaudeRawEvent {
   type?: string;
   subtype?: string;
+  /** Parent tool ID for nested messages; null on top-level messages. */
+  parent_tool_use_id?: string | null;
   session_id?: string;
   cwd?: string;
   model?: string;
@@ -31,6 +33,9 @@ export function* translateEvent(raw: unknown): Generator<AgentEvent> {
   if (!raw || typeof raw !== 'object') return;
   const evt = raw as ClaudeRawEvent;
 
+  // Nested assistant/user messages are internal and must not reach Lark.
+  if (isNestedMessage(evt)) return;
+
   if (evt.type === 'system' && evt.subtype === 'init') {
     yield {
       type: 'system',
@@ -83,3 +88,10 @@ export function* translateEvent(raw: unknown): Generator<AgentEvent> {
     yield { type: 'done', sessionId: evt.session_id, terminationReason: 'normal' };
   }
 }
+
+function isNestedMessage(evt: ClaudeRawEvent): boolean {
+  return (
+    (evt.type === 'assistant' || evt.type === 'user') &&
+    typeof evt.parent_tool_use_id === 'string'
+  );
+}
diff --git a/tests/unit/agent/claude-stream-json.test.ts b/tests/unit/agent/claude-stream-json.test.ts
index fc340087..7b384b62 100644
--- a/tests/unit/agent/claude-stream-json.test.ts
+++ b/tests/unit/agent/claude-stream-json.test.ts
@@ -41,6 +41,70 @@ describe('Claude stream-json translator', () => {
     ]);
   });
 
+  it('filters nested assistant/user messages by their explicit parent tool-use ID', () => {
+    const nestedAssistant = {
+      type: 'assistant',
+      parent_tool_use_id: 'tool-agent-1',
+      message: {
+        content: [
+          { type: 'text', text: 'private research report' },
+          { type: 'thinking', thinking: 'private reasoning' },
+          { type: 'tool_use', id: 'tool-child-1', name: 'WebSearch', input: { query: 'x' } },
+        ],
+      },
+    };
+    const nestedToolResult = {
+      type: 'user',
+      parent_tool_use_id: 'tool-agent-1',
+      message: {
+        content: [
+          { type: 'tool_result', tool_use_id: 'tool-child-1', content: 'private result' },
+        ],
+      },
+    };
+
+    expect([...translateEvent(nestedAssistant)]).toEqual([]);
+    expect([...translateEvent(nestedToolResult)]).toEqual([]);
+  });
+
+  it('preserves top-level messages and lifecycle events', () => {
+    expect([
+      ...translateEvent({
+        type: 'assistant',
+        parent_tool_use_id: null,
+        message: { content: [{ type: 'text', text: 'public synthesis' }] },
+      }),
+    ]).toEqual([{ type: 'text', delta: 'public synthesis' }]);
+    expect([
+      ...translateEvent({
+        type: 'user',
+        parent_tool_use_id: null,
+        message: {
+          content: [
+            { type: 'tool_result', tool_use_id: 'tool-agent-1', content: 'agent report' },
+          ],
+        },
+      }),
+    ]).toEqual([
+      { type: 'tool_result', id: 'tool-agent-1', output: 'agent report', isError: false },
+    ]);
+    expect([
+      ...translateEvent({
+        type: 'system',
+        subtype: 'init',
+        parent_tool_use_id: 'unexpected',
+        session_id: 'sess-1',
+      }),
+    ]).toEqual([{ type: 'system', sessionId: 'sess-1', cwd: undefined, model: undefined }]);
+    expect([
+      ...translateEvent({
+        type: 'result',
+        parent_tool_use_id: 'unexpected',
+        session_id: 'sess-1',
+      }),
+    ]).toEqual([{ type: 'done', sessionId: 'sess-1', terminationReason: 'normal' }]);
+  });
+
   it('translates user tool_result blocks including structured output and errors', () => {
     expect([
       ...translateEvent({