feat: add buildChildrenIndex and shared formatCost helper

packages/ui/src/components/chat/work-status/subagentCost.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from 'bun:test';
 import type { Session } from '@opencode-ai/sdk/v2';
-import { buildChildrenIndex } from './subagentCost';
+import { buildChildrenIndex, formatCost } from './subagentCost';
 
 function makeSession(id: string, cost: number, parentID?: string): Session {
   return { id, cost, parentID } as unknown as Session;
@@ -15,3 +15,11 @@ describe('buildChildrenIndex', () => {
     expect(index.get('root')).toEqual([childA, childB]);
   });
 });
+
+describe('formatCost', () => {
+  test('prefixes with $ and trims trailing zeros', () => {
+    expect(formatCost(1.5)).toBe('$1.5');
+    expect(formatCost(0.0001)).toBe('$0.0001');
+    expect(formatCost(2)).toBe('$2');
+  });
+});

packages/ui/src/components/chat/work-status/subagentCost.ts

@@ -0,0 +1,57 @@
+import type { Session } from '@opencode-ai/sdk/v2';
+
+// Spend is read against a budget, so it keeps its real precision instead of
+// collapsing to two decimals. Trailing zeros are dropped so exact values stay
+// short. Relocated from WorkStatusPrimaryGroup.tsx so both that component and
+// WorkStatusSubagentsSection share one implementation.
+const trimZeros = (value: string): string =>
+  (value.includes('.') ? value.replace(/0+$/, '').replace(/\.$/, '') : value);
+
+export const formatCost = (cost: number): string => `$${trimZeros(cost.toFixed(4))}`;
+
+/**
+ * Groups a flat live-session list by parentID. One pass, O(n). Sessions
+ * without a parentID (roots) are simply absent as keys — callers look up a
+ * specific id's children via `.get(id) ?? []`.
+ */
+export function buildChildrenIndex(sessions: Session[]): Map<string, Session[]> {
+  const index = new Map<string, Session[]>();
+  for (const session of sessions) {
+    const parentID = (session as unknown as { parentID?: string }).parentID;
+    if (!parentID) continue;
+    const existing = index.get(parentID);
+    if (existing) {
+      existing.push(session);
+    } else {
+      index.set(parentID, [session]);
+    }
+  }
+  return index;
+}
+
+function sessionCost(session: Session | undefined): number {
+  const cost = (session as unknown as { cost?: number } | undefined)?.cost;
+  return typeof cost === 'number' ? cost : 0;
+}
+
+/**
+ * Own cost plus every descendant's cost, recursively. Cycle-guarded with a
+ * visited set: parentID should form a tree, but this does not trust that
+ * invariant blindly (mirrors opencode-session-cost's src/cost.ts).
+ */
+export function computeSubtreeCost(
+  id: string,
+  sessionsById: Map<string, Session>,
+  childrenByParent: Map<string, Session[]>,
+  visited: Set<string> = new Set(),
+): number {
+  if (visited.has(id)) return 0;
+  visited.add(id);
+
+  let total = sessionCost(sessionsById.get(id));
+  const children = childrenByParent.get(id) ?? [];
+  for (const child of children) {
+    total += computeSubtreeCost(child.id, sessionsById, childrenByParent, visited);
+  }
+  return total;
+}