diff --git a/libs/ag-ui/src/lib/reducer.subagent.spec.ts b/libs/ag-ui/src/lib/reducer.subagent.spec.ts
index f35004eaf..ee44ed45e 100644
--- a/libs/ag-ui/src/lib/reducer.subagent.spec.ts
+++ b/libs/ag-ui/src/lib/reducer.subagent.spec.ts
@@ -94,6 +94,47 @@ describe('reduceEvent SUBAGENT_* lifecycle', () => {
     expect(store.toolCalls()).toHaveLength(0);
   });
 
+  it('attributed TOOL_CALL_START links the toolCallId onto the child message via parentMessageId', () => {
+    const store = makeStore();
+    reduceEvent(ev({ type: 'SUBAGENT_STARTED', subagentRunId: 'sa-1', name: 'researcher' }), store);
+    reduceEvent(ev({ type: 'TEXT_MESSAGE_START', messageId: 'sa-1-m1', role: 'assistant', subagentRunId: 'sa-1' }), store);
+    reduceEvent(ev({ type: 'TOOL_CALL_START', toolCallId: 't-1', toolCallName: 'web_search', subagentRunId: 'sa-1', parentMessageId: 'sa-1-m1' }), store);
+    const content = store.activities().get('sa-1')!.content();
+    const msgs = content['messages'] as Array<Record<string, unknown>>;
+    const calls = content['toolCalls'] as Array<Record<string, unknown>>;
+    expect(msgs.find((m) => m['id'] === 'sa-1-m1')).toMatchObject({ toolCallIds: ['t-1'] });
+    expect(calls).toHaveLength(1);
+    expect(calls[0]).toMatchObject({ id: 't-1', name: 'web_search' });
+  });
+
+  it('attributed TOOL_CALL_START with a parentMessageId for an unseen message creates a child message slot', () => {
+    const store = makeStore();
+    reduceEvent(ev({ type: 'SUBAGENT_STARTED', subagentRunId: 'sa-1', name: 'researcher' }), store);
+    reduceEvent(ev({ type: 'TOOL_CALL_START', toolCallId: 't-1', toolCallName: 'web_search', subagentRunId: 'sa-1', parentMessageId: 'sa-1-m1' }), store);
+    const msgs = store.activities().get('sa-1')!.content()['messages'] as Array<Record<string, unknown>>;
+    expect(msgs).toHaveLength(1);
+    expect(msgs[0]).toMatchObject({ id: 'sa-1-m1', role: 'assistant', content: '', toolCallIds: ['t-1'] });
+  });
+
+  it('attributed TOOL_CALL_START without a parentMessageId attaches to the most recently opened child message', () => {
+    const store = makeStore();
+    reduceEvent(ev({ type: 'SUBAGENT_STARTED', subagentRunId: 'sa-1', name: 'researcher' }), store);
+    reduceEvent(ev({ type: 'TEXT_MESSAGE_START', messageId: 'sa-1-m1', role: 'assistant', subagentRunId: 'sa-1' }), store);
+    reduceEvent(ev({ type: 'TOOL_CALL_START', toolCallId: 't-1', toolCallName: 'web_search', subagentRunId: 'sa-1' }), store);
+    const msgs = store.activities().get('sa-1')!.content()['messages'] as Array<Record<string, unknown>>;
+    expect(msgs.find((m) => m['id'] === 'sa-1-m1')).toMatchObject({ toolCallIds: ['t-1'] });
+  });
+
+  it('attributed TOOL_CALL_START without a parentMessageId and no open child message leaves messages untouched', () => {
+    const store = makeStore();
+    reduceEvent(ev({ type: 'SUBAGENT_STARTED', subagentRunId: 'sa-1', name: 'researcher' }), store);
+    reduceEvent(ev({ type: 'TOOL_CALL_START', toolCallId: 't-1', toolCallName: 'web_search', subagentRunId: 'sa-1' }), store);
+    const content = store.activities().get('sa-1')!.content();
+    expect(content['messages']).toEqual([]);
+    const calls = content['toolCalls'] as Array<Record<string, unknown>>;
+    expect(calls).toMatchObject([{ id: 't-1', name: 'web_search' }]);
+  });
+
   it('an attributed event before SUBAGENT_STARTED creates the entry instead of dropping (buffer-not-drop)', () => {
     const store = makeStore();
     reduceEvent(ev({ type: 'TEXT_MESSAGE_START', messageId: 'm-1', role: 'assistant', subagentRunId: 'sa-late' }), store);
diff --git a/libs/ag-ui/src/lib/reducer.ts b/libs/ag-ui/src/lib/reducer.ts
index 1c96a9281..d64567a40 100644
--- a/libs/ag-ui/src/lib/reducer.ts
+++ b/libs/ag-ui/src/lib/reducer.ts
@@ -690,9 +690,33 @@ function routeSubagentContentEvent(subagentRunId: string, event: BaseEvent, stor
       }
       case 'TEXT_MESSAGE_END':
         return c;
-      case 'TOOL_CALL_START':
+      case 'TOOL_CALL_START': {
         toolCalls.push({ id: e['toolCallId'], name: e['toolCallName'], args: {}, status: 'running' });
-        return { ...c, toolCalls };
+        // Mirror the parent TOOL_CALL_START handler: link the call to its
+        // parent child message so chat-subagent-card can draw it from
+        // message.toolCallIds, same as the top-level transcript does.
+        const parentId = e['parentMessageId'] as string | undefined;
+        if (parentId) {
+          const idx = messages.findIndex((m) => m['id'] === parentId);
+          if (idx < 0) {
+            messages.push({ id: parentId, role: 'assistant', content: '', toolCallIds: [e['toolCallId']] });
+          } else {
+            messages[idx] = {
+              ...messages[idx],
+              toolCallIds: [...((messages[idx]['toolCallIds'] as unknown[]) ?? []), e['toolCallId']],
+            };
+          }
+        } else if (messages.length > 0) {
+          // No parentMessageId on the wire: attach to the most recently
+          // opened child message, mirroring TEXT_MESSAGE_START's append order.
+          const lastIdx = messages.length - 1;
+          messages[lastIdx] = {
+            ...messages[lastIdx],
+            toolCallIds: [...((messages[lastIdx]['toolCallIds'] as unknown[]) ?? []), e['toolCallId']],
+          };
+        }
+        return { ...c, messages, toolCalls };
+      }
       case 'TOOL_CALL_ARGS':
         return parsedArgs === undefined
           ? c
diff --git a/libs/ag-ui/src/lib/to-agent.spec.ts b/libs/ag-ui/src/lib/to-agent.spec.ts
index e35bd7058..7d2873b8b 100644
--- a/libs/ag-ui/src/lib/to-agent.spec.ts
+++ b/libs/ag-ui/src/lib/to-agent.spec.ts
@@ -1271,6 +1271,12 @@ describe('SUBAGENT_* lifecycle projection', () => {
     expect(sa?.messages()).toEqual([
       { id: 'm-1', role: 'assistant', content: 'Checking flights', delivery: expect.objectContaining({ phase: 'streaming' }) },
     ]);
+
+    // Attributed TOOL_CALL_START with parentMessageId must link the call
+    // onto the child message's toolCallIds so chat-subagent-card can draw
+    // it — not just append to the entry's toolCalls[].
+    source.emit({ type: 'TOOL_CALL_START', toolCallId: 't-1', toolCallName: 'web_search', subagentRunId: 'sa-1', parentMessageId: 'm-1' } as never);
+    expect(agent.subagents!().get('sa-1')?.messages()[0].toolCallIds).toEqual(['t-1']);
   });
 
   it('a wrapper read before SUBAGENT_STARTED reflects the real identity once STARTED arrives (no stale name/toolCallId)', () => {