diff --git a/backend/src/__tests__/cors.test.ts b/backend/src/__tests__/cors.test.ts
index 57bc5b16..d05eb0cb 100644
--- a/backend/src/__tests__/cors.test.ts
+++ b/backend/src/__tests__/cors.test.ts
@@ -65,6 +65,9 @@ describe("CORS middleware", () => {
       "https://frontend.example.com",
     );
     expect(response.headers["access-control-allow-credentials"]).toBe("true");
+    expect(response.headers["access-control-expose-headers"]).toBe(
+      "X-Idempotent-Replayed,X-Idempotency-Cache",
+    );
   });
 
   it("rejects unknown origins in production", async () => {
diff --git a/backend/src/app.ts b/backend/src/app.ts
index 5b33a565..7aafdbef 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -105,6 +105,7 @@ const corsOptions: cors.CorsOptions = {
     "x-request-id",
     "Idempotency-Key",
   ],
+  exposedHeaders: ["X-Idempotent-Replayed", "X-Idempotency-Cache"],
   credentials: true,
 };
 
diff --git a/backend/src/middleware/idempotency.ts b/backend/src/middleware/idempotency.ts
index 86b314dc..b4c87012 100644
--- a/backend/src/middleware/idempotency.ts
+++ b/backend/src/middleware/idempotency.ts
@@ -38,10 +38,13 @@ export const idempotencyMiddleware = async (
       res
         .status(cached.status)
         .set("X-Idempotency-Cache", "HIT")
+        .set("X-Idempotent-Replayed", "true")
         .json(cached.body);
       return;
     }
 
+    res.set("X-Idempotent-Replayed", "false");
+
     // Capture the original methods to intercept the response body
     const originalJson = res.json;
     const originalSend = res.send;
diff --git a/backend/src/tests/idempotency.test.ts b/backend/src/tests/idempotency.test.ts
index 96b0d5e3..d8e1a8c7 100644
--- a/backend/src/tests/idempotency.test.ts
+++ b/backend/src/tests/idempotency.test.ts
@@ -60,6 +60,7 @@ describe("Idempotency Middleware", () => {
     expect(cacheService.get).toHaveBeenCalledWith(`idemp:${key}`);
     expect(res.status).toHaveBeenCalledWith(201);
     expect(res.set).toHaveBeenCalledWith("X-Idempotency-Cache", "HIT");
+    expect(res.set).toHaveBeenCalledWith("X-Idempotent-Replayed", "true");
     expect(res.json).toHaveBeenCalledWith(cachedResponse.body);
     expect(next).not.toHaveBeenCalled();
   });
@@ -72,6 +73,70 @@ describe("Idempotency Middleware", () => {
     await idempotencyMiddleware(req as Request, res as Response, next);
 
     expect(next).toHaveBeenCalled();
+    expect(res.set).toHaveBeenCalledWith("X-Idempotent-Replayed", "false");
     expect(res.on).toHaveBeenCalledWith("finish", expect.any(Function));
   });
+
+  it("should mark first execution and sequential replay with header values", async () => {
+    const key = "sequential-key";
+    const cache = new Map<string, unknown>();
+    const firstFinishHandlers: Array<() => Promise<void>> = [];
+
+    asMock(req.header).mockReturnValue(key);
+    (cacheService.get as jest.Mock<(cacheKey: string) => Promise<unknown>>)
+      .mockImplementation(async (cacheKey: string) => cache.get(cacheKey))
+      .mockName("cacheService.get");
+    (
+      cacheService.set as jest.Mock<
+        (cacheKey: string, value: unknown) => Promise<void>
+      >
+    )
+      .mockImplementation(async (cacheKey: string, value: unknown) => {
+        cache.set(cacheKey, value);
+      })
+      .mockName("cacheService.set");
+    res.on.mockImplementation((event: string, handler: () => Promise<void>) => {
+      if (event === "finish") {
+        firstFinishHandlers.push(handler);
+      }
+      return res;
+    });
+
+    await idempotencyMiddleware(req as Request, res as Response, next);
+
+    expect(next).toHaveBeenCalledTimes(1);
+    expect(res.set).toHaveBeenCalledWith("X-Idempotent-Replayed", "false");
+
+    res.statusCode = 202;
+    res.json({ accepted: true });
+    const finishHandler = firstFinishHandlers[0];
+    if (!finishHandler) {
+      throw new Error(
+        "Expected idempotency middleware to register finish hook",
+      );
+    }
+    await finishHandler();
+
+    const replayRes = {
+      status: jest.fn().mockReturnThis(),
+      set: jest.fn().mockReturnThis(),
+      json: jest.fn().mockReturnThis(),
+      send: jest.fn().mockReturnThis(),
+      on: jest.fn(),
+      statusCode: 200,
+    };
+    const replayNext = jest.fn();
+
+    await idempotencyMiddleware(
+      req as Request,
+      replayRes as unknown as Response,
+      replayNext,
+    );
+
+    expect(replayRes.status).toHaveBeenCalledWith(202);
+    expect(replayRes.set).toHaveBeenCalledWith("X-Idempotency-Cache", "HIT");
+    expect(replayRes.set).toHaveBeenCalledWith("X-Idempotent-Replayed", "true");
+    expect(replayRes.json).toHaveBeenCalledWith({ accepted: true });
+    expect(replayNext).not.toHaveBeenCalled();
+  });
 });
diff --git a/docs/wiki/README.md b/docs/wiki/README.md
index ac1f208f..fc4e793f 100644
--- a/docs/wiki/README.md
+++ b/docs/wiki/README.md
@@ -4,7 +4,7 @@ This folder is a GitHub Wiki-style set of documents that live in the repo so the
 
 ## Contents
 
+- [API Idempotency](./api-idempotency.md)
 - [Soroban Contract State Machine](./contract-state-machine.md)
 - [Indexer ↔ Database Sync Flow](./indexer-sync-flow.md)
 - [Frontend “Standard Library” Patterns](./frontend-patterns.md)
-
diff --git a/docs/wiki/api-idempotency.md b/docs/wiki/api-idempotency.md
new file mode 100644
index 00000000..76671a63
--- /dev/null
+++ b/docs/wiki/api-idempotency.md
@@ -0,0 +1,23 @@
+# API Idempotency
+
+Write endpoints that opt into `idempotencyMiddleware` accept an
+`Idempotency-Key` request header. Clients should reuse the same key when
+retrying the same logical write after a timeout or transient network failure.
+
+## Replay Headers
+
+Responses include `X-Idempotent-Replayed` when an idempotency key is present:
+
+- `false` means the request ran normally and the response can be cached for
+  later retries.
+- `true` means the request matched a cached response for the same key and the
+  body was replayed without running the write handler again.
+
+Cached replays also keep the legacy `X-Idempotency-Cache: HIT` response header.
+The replay header is the stable value API consumers should use when deciding
+whether to suppress duplicate success toasts or transaction counters.
+Both idempotency response headers are exposed through CORS so browser clients
+can read them from `fetch()` responses.
+
+Clients should generate a fresh idempotency key for every new write attempt.
+Do not reuse a key for a different loan, pool, or transaction operation.
diff --git a/frontend/e2e/borrower-repay-flow.spec.ts b/frontend/e2e/borrower-repay-flow.spec.ts
index c87e7b0a..87ad5ca1 100644
--- a/frontend/e2e/borrower-repay-flow.spec.ts
+++ b/frontend/e2e/borrower-repay-flow.spec.ts
@@ -102,7 +102,11 @@ test.describe("Borrower Repayment Flow", () => {
     await expect(page.locator("text=4,500")).toBeVisible();
   });
 
-  test("rejects a repayment greater than the outstanding balance", async ({ page }: { page: Page }) => {
+  test("rejects a repayment greater than the outstanding balance", async ({
+    page,
+  }: {
+    page: Page;
+  }) => {
     await page.goto("/en");
 
     const repayBtn = page.getByRole("button", { name: /Repay/i }).first();
@@ -115,9 +119,11 @@ test.describe("Borrower Repayment Flow", () => {
 
     const review = page.getByRole("button", { name: /Review Repayment/i });
     // The flow should not allow proceeding to confirmation with an invalid amount.
-    await expect(review).toBeDisabled().catch(async () => {
-      await review.click();
-      await expect(page.locator("text=/exceeds|too (large|high)|maximum/i")).toBeVisible();
-    });
+    await expect(review)
+      .toBeDisabled()
+      .catch(async () => {
+        await review.click();
+        await expect(page.locator("text=/exceeds|too (large|high)|maximum/i")).toBeVisible();
+      });
   });
 });
diff --git a/frontend/e2e/lender-withdraw-flow.spec.ts b/frontend/e2e/lender-withdraw-flow.spec.ts
index 946578f8..ed04fabe 100644
--- a/frontend/e2e/lender-withdraw-flow.spec.ts
+++ b/frontend/e2e/lender-withdraw-flow.spec.ts
@@ -106,9 +106,14 @@ test.describe("Lender Withdraw Flow", () => {
     }, lenderWalletState("1500.00"));
 
     await page.click('button:has-text("Review Withdrawal"), button:has-text("Confirm Withdrawal")');
-    await page.getByRole("button", { name: /Confirm Withdrawal/i }).click().catch(() => {});
+    await page
+      .getByRole("button", { name: /Confirm Withdrawal/i })
+      .click()
+      .catch(() => {});
 
-    await expect(page.locator("text=/Withdrawal (Successful|Complete)/i")).toBeVisible({ timeout: 10000 });
+    await expect(page.locator("text=/Withdrawal (Successful|Complete)/i")).toBeVisible({
+      timeout: 10000,
+    });
 
     await page.reload();
     await expect(page.locator("text=1,500")).toBeVisible();
@@ -126,9 +131,11 @@ test.describe("Lender Withdraw Flow", () => {
     await page.fill('input[type="number"]', "999999");
 
     const confirm = page.getByRole("button", { name: /Confirm Withdrawal/i });
-    await expect(confirm).toBeDisabled().catch(async () => {
-      await confirm.click();
-      await expect(page.locator("text=/exceeds|insufficient|maximum/i")).toBeVisible();
-    });
+    await expect(confirm)
+      .toBeDisabled()
+      .catch(async () => {
+        await confirm.click();
+        await expect(page.locator("text=/exceeds|insufficient|maximum/i")).toBeVisible();
+      });
   });
 });