diff --git a/apps/web/src/app/api/usage/topup/__tests__/route.test.ts b/apps/web/src/app/api/usage/topup/__tests__/route.test.ts
new file mode 100644
index 0000000..5d4f2fb
--- /dev/null
+++ b/apps/web/src/app/api/usage/topup/__tests__/route.test.ts
@@ -0,0 +1,73 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+// ─── Mocks ───
+
+const mockGetUser = vi.fn();
+const mockInsert = vi.fn();
+const mockCreatePayment = vi.fn();
+
+vi.mock("@/lib/supabase", () => ({
+  getSupabaseClient: () => ({
+    auth: { getUser: mockGetUser },
+  }),
+  getSupabaseAdmin: () => ({
+    from: () => ({ insert: mockInsert }),
+  }),
+}));
+
+vi.mock("@/lib/coinpay-client", () => ({
+  createCoinpayPayment: mockCreatePayment,
+}));
+
+import { POST } from "@/app/api/usage/topup/route";
+
+function makeRequest(body: unknown) {
+  return new Request("http://localhost/api/usage/topup", {
+    method: "POST",
+    headers: { "Content-Type": "application/json", Authorization: "Bearer tok-abc" },
+    body: JSON.stringify(body),
+  }) as unknown as import("next/server").NextRequest;
+}
+
+describe("POST /api/usage/topup", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockGetUser.mockResolvedValue({
+      data: { user: { id: "user-123", email: "test@example.com" } },
+      error: null,
+    });
+    mockCreatePayment.mockResolvedValue({
+      payment: { id: "cp-1", payment_address: "0xabc", amount_crypto: 50 },
+      checkout_url: "https://pay/cp-1",
+    });
+    mockInsert.mockResolvedValue({ error: null });
+  });
+
+  it("rejects a non-numeric amount_usd instead of forwarding a garbage amount", async () => {
+    const res = await POST(makeRequest({ amount_usd: "abc", currency: "usdc_sol" }));
+    const body = await res.json();
+    expect(res.status).toBe(400);
+    expect(body.error).toContain("between $1 and $10,000");
+    expect(mockCreatePayment).not.toHaveBeenCalled();
+    expect(mockInsert).not.toHaveBeenCalled();
+  });
+
+  it("rejects out-of-range amounts", async () => {
+    expect((await POST(makeRequest({ amount_usd: 0, currency: "usdc_sol" }))).status).toBe(400);
+    expect((await POST(makeRequest({ amount_usd: 20000, currency: "usdc_sol" }))).status).toBe(400);
+    expect(mockCreatePayment).not.toHaveBeenCalled();
+  });
+
+  it("accepts a valid amount and passes a NUMBER to CoinPay and the deposit row", async () => {
+    const res = await POST(makeRequest({ amount_usd: 50, currency: "usdc_sol" }));
+    expect(res.status).toBe(200);
+    expect(mockCreatePayment).toHaveBeenCalledWith(expect.objectContaining({ amount_usd: 50 }));
+    expect(mockInsert).toHaveBeenCalledWith(expect.objectContaining({ amount_usd: 50 }));
+  });
+
+  it("coerces a numeric string to a number", async () => {
+    const res = await POST(makeRequest({ amount_usd: "50", currency: "usdc_sol" }));
+    expect(res.status).toBe(200);
+    expect(mockCreatePayment).toHaveBeenCalledWith(expect.objectContaining({ amount_usd: 50 }));
+  });
+});
diff --git a/apps/web/src/app/api/usage/topup/route.ts b/apps/web/src/app/api/usage/topup/route.ts
index 6a51fe6..929ff90 100644
--- a/apps/web/src/app/api/usage/topup/route.ts
+++ b/apps/web/src/app/api/usage/topup/route.ts
@@ -18,13 +18,15 @@ export async function POST(request: NextRequest) {
     }
 
     const body = await request.json();
-    const { amount_usd, currency } = body;
+    const { currency } = body;
 
-    if (!amount_usd) {
-      return NextResponse.json({ error: "amount_usd required" }, { status: 400 });
-    }
-
-    if (amount_usd < 1 || amount_usd > 10000) {
+    // Coerce and validate the amount (mirrors funding/create-invoice/route.ts).
+    // Without Number()/isFinite, a non-numeric amount_usd like "abc" is truthy
+    // (so it passes the presence check) and both "abc" < 1 and "abc" > 10000 are
+    // false, so it slipped through and was sent to CoinPay / stored on the
+    // credit_deposits row as a garbage amount.
+    const amount_usd = Number(body.amount_usd);
+    if (!Number.isFinite(amount_usd) || amount_usd < 1 || amount_usd > 10000) {
       return NextResponse.json({ error: "Amount must be between $1 and $10,000" }, { status: 400 });
     }