diff --git a/src/pages/stock/hooks/usePortfolio.js b/src/pages/stock/hooks/usePortfolio.js
index 2bb8d78..31bb8d2 100644
--- a/src/pages/stock/hooks/usePortfolio.js
+++ b/src/pages/stock/hooks/usePortfolio.js
@@ -3,7 +3,7 @@ import {
     getPortfolio, addPortfolio, updatePortfolio, deletePortfolio,
     upsertCash, deleteCash,
 } from '../../../api';
-import { emptyPortfolioForm } from '../stockUtils';
+import { emptyPortfolioForm, computeBrokerSummary } from '../stockUtils';
 
 export default function usePortfolio() {
     const [portfolio, setPortfolio] = useState(null);
@@ -38,7 +38,12 @@ export default function usePortfolio() {
 
     /* derived */
     const portfolioHoldings = portfolio?.holdings ?? [];
-    const portfolioSummary = portfolio?.summary ?? {};
+    // 총 매입은 "각 종목 매입가의 단순 합(수량 미곱산)"으로 표시 (박재오 정의).
+    // 백엔드 summary.total_buy(매입가×수량)는 무시하고 프론트에서 재계산해
+    // 요약카드·증권사별·AI 프롬프트가 모두 같은 값을 쓰도록 통일.
+    const portfolioSummary = portfolioHoldings.length
+        ? { ...(portfolio?.summary ?? {}), total_buy: computeBrokerSummary(portfolioHoldings).totalBuy }
+        : (portfolio?.summary ?? {});
     const cashList = portfolio?.cash ?? [];
     const totalCash = portfolioSummary.total_cash ?? null;
     const totalAssets = portfolioSummary.total_assets ?? null;
@@ -69,23 +74,7 @@ export default function usePortfolio() {
         return map;
     }, [brokerGroups]);
 
-    const getBrokerSummary = (items) => {
-        // totalBuy: 요약 표시용 (매입가 purchase_price 기준)
-        // totalCostBasis: 손익 계산용 (평균단가 avg_price 기준)
-        let totalBuy = 0, totalCostBasis = 0, totalEvalAmt = 0, hasNullPrice = false;
-        for (const item of items) {
-            const qty = item.quantity ?? 0;
-            const purchase = item.purchase_price ?? item.avg_price ?? 0;
-            // 총 매입 = 종목별 매입가의 단순 합 (수량 미곱산)
-            totalBuy += purchase;
-            totalCostBasis += (item.avg_price ?? 0) * qty;
-            if (item.eval_amount != null) totalEvalAmt += item.eval_amount;
-            else hasNullPrice = true;
-        }
-        const totalProfit = totalEvalAmt - totalCostBasis;
-        const totalProfitRate = totalCostBasis > 0 ? (totalProfit / totalCostBasis) * 100 : 0;
-        return { totalBuy, totalEval: totalEvalAmt, totalProfit, totalProfitRate, hasNullPrice };
-    };
+    const getBrokerSummary = computeBrokerSummary;
 
     /* loaders */
     const loadPortfolio = useCallback(async () => {
diff --git a/src/pages/stock/stockUtils.js b/src/pages/stock/stockUtils.js
index 8b0533f..f58b2f2 100644
--- a/src/pages/stock/stockUtils.js
+++ b/src/pages/stock/stockUtils.js
@@ -125,6 +125,25 @@ export const emptySellForm = () => ({
     sold_at: toLocalDatetimeValue(new Date().toISOString()),
 });
 
+/* ── 증권사별 요약 집계 ──────────────────────────────────────────── */
+// totalBuy: 총 매입 = 각 종목 매입가(purchase_price)의 단순 합 (수량 미곱산, 박재오 정의).
+//           매입가 미설정 시 avg_price 폴백. 백엔드 total_buy(×수량)는 표시에 쓰지 않음.
+// totalCostBasis: 손익 계산용 매입원가 = SUM(avg_price × quantity) — 손익은 수량 곱산 유지.
+export const computeBrokerSummary = (items) => {
+    let totalBuy = 0, totalCostBasis = 0, totalEval = 0, hasNullPrice = false;
+    for (const item of items) {
+        const qty = item.quantity ?? 0;
+        const purchase = item.purchase_price ?? item.avg_price ?? 0;
+        totalBuy += purchase;
+        totalCostBasis += (item.avg_price ?? 0) * qty;
+        if (item.eval_amount != null) totalEval += item.eval_amount;
+        else hasNullPrice = true;
+    }
+    const totalProfit = totalEval - totalCostBasis;
+    const totalProfitRate = totalCostBasis > 0 ? (totalProfit / totalCostBasis) * 100 : 0;
+    return { totalBuy, totalEval, totalProfit, totalProfitRate, hasNullPrice };
+};
+
 /* ── TAB IDs ─────────────────────────────────────────────────────── */
 
 export const TAB_PORTFOLIO = 'portfolio';
diff --git a/src/pages/stock/stockUtils.test.js b/src/pages/stock/stockUtils.test.js
new file mode 100644
index 0000000..5b4bf7f
--- /dev/null
+++ b/src/pages/stock/stockUtils.test.js
@@ -0,0 +1,48 @@
+import { describe, it, expect } from 'vitest';
+import { computeBrokerSummary } from './stockUtils.js';
+
+describe('computeBrokerSummary - 총 매입(total_buy) 계산', () => {
+    it('총 매입 = 각 종목 매입가(purchase_price)의 단순 합 (수량 미곱산)', () => {
+        const items = [
+            { quantity: 100, avg_price: 72000, purchase_price: 70000, eval_amount: 7450000 },
+        ];
+        // 매입가 70000 (수량 곱하지 않음)
+        expect(computeBrokerSummary(items).totalBuy).toBe(70_000);
+    });
+
+    it('purchase_price 미설정 시 avg_price로 폴백 (단순 합)', () => {
+        const items = [
+            { quantity: 100, avg_price: 72000, purchase_price: null, eval_amount: 7450000 },
+        ];
+        // 매입가 미입력 → 평균단가 72000 폴백
+        expect(computeBrokerSummary(items).totalBuy).toBe(72_000);
+    });
+
+    it('여러 종목 합산: 각 매입가의 단순 합', () => {
+        const items = [
+            { quantity: 100, avg_price: 70000, purchase_price: 70000, eval_amount: 7500000 },
+            { quantity: 50, avg_price: 130000, purchase_price: 130000, eval_amount: 6800000 },
+        ];
+        // 70000 + 130000 = 200,000 (수량 미곱산)
+        expect(computeBrokerSummary(items).totalBuy).toBe(200_000);
+    });
+
+    it('손익 = 총 평가 - 매입원가(avg_price × qty) — 손익은 수량 곱산 유지', () => {
+        const items = [
+            { quantity: 10, avg_price: 100000, purchase_price: 90000, eval_amount: 1_200_000 },
+        ];
+        const s = computeBrokerSummary(items);
+        // cost_basis = 100000 × 10 = 1,000,000; profit = 1,200,000 - 1,000,000 = 200,000
+        expect(s.totalEval).toBe(1_200_000);
+        expect(s.totalProfit).toBe(200_000);
+        expect(s.totalProfitRate).toBeCloseTo(20, 5);
+        expect(s.hasNullPrice).toBe(false);
+    });
+
+    it('eval_amount가 null인 종목이 있으면 hasNullPrice=true', () => {
+        const items = [
+            { quantity: 10, avg_price: 100000, purchase_price: 100000, eval_amount: null },
+        ];
+        expect(computeBrokerSummary(items).hasNullPrice).toBe(true);
+    });
+});