diff --git a/packages/ui/src/components/chat/work-status/subagentCost.test.ts b/packages/ui/src/components/chat/work-status/subagentCost.test.ts
new file mode 100644
index 00000000..be6bf4e1
--- /dev/null
+++ b/packages/ui/src/components/chat/work-status/subagentCost.test.ts
@@ -0,0 +1,17 @@
+import { describe, expect, test } from 'bun:test';
+import type { Session } from '@opencode-ai/sdk/v2';
+import { buildChildrenIndex } from './subagentCost';
+
+function makeSession(id: string, cost: number, parentID?: string): Session {
+  return { id, cost, parentID } as unknown as Session;
+}
+
+describe('buildChildrenIndex', () => {
+  test('groups sessions by parentID', () => {
+    const root = makeSession('root', 1);
+    const childA = makeSession('a', 2, 'root');
+    const childB = makeSession('b', 3, 'root');
+    const index = buildChildrenIndex([root, childA, childB]);
+    expect(index.get('root')).toEqual([childA, childB]);
+  });
+});