diff --git a/rest/nodejs/package-lock.json b/rest/nodejs/package-lock.json
index e0fab66..33b0d3b 100644
--- a/rest/nodejs/package-lock.json
+++ b/rest/nodejs/package-lock.json
@@ -8,7 +8,7 @@
       "dependencies": {
         "@hono/node-server": "^2.0.10",
         "@hono/zod-validator": "^0.7.6",
-        "@ucp-js/sdk": "0.4.3",
+        "@ucp-js/sdk": "0.4.5",
         "better-sqlite3": "^12.11.1",
         "hono": "^4.13.0",
         "pino": "^10.1.0",
@@ -526,9 +526,9 @@
       "license": "MIT"
     },
     "node_modules/@ucp-js/sdk": {
-      "version": "0.4.3",
-      "resolved": "https://registry.npmjs.org/@ucp-js/sdk/-/sdk-0.4.3.tgz",
-      "integrity": "sha512-A8Gwl5s4nFHDCESUhxR9SPBAS1hIg67LawMmEH46dGw19uuSXsMQkUtgdJkuewlBCGW9PIP6Inq2AxU3qBEWeg==",
+      "version": "0.4.5",
+      "resolved": "https://registry.npmjs.org/@ucp-js/sdk/-/sdk-0.4.5.tgz",
+      "integrity": "sha512-KS80aNxoOUpOrIqhvpT7tKXATch8yWJgGLAzLGACUFpQeAy3UljFi3TEgKEdJNJbEMQ+aF/a7F+J6c+VB2RRLw==",
       "license": "Apache-2.0",
       "dependencies": {
         "zod": "^3.23.8"
diff --git a/rest/nodejs/package.json b/rest/nodejs/package.json
index eed92ce..900a18e 100644
--- a/rest/nodejs/package.json
+++ b/rest/nodejs/package.json
@@ -13,7 +13,7 @@
   "dependencies": {
     "@hono/node-server": "^2.0.10",
     "@hono/zod-validator": "^0.7.6",
-    "@ucp-js/sdk": "0.4.3",
+    "@ucp-js/sdk": "0.4.5",
     "better-sqlite3": "^12.11.1",
     "hono": "^4.13.0",
     "pino": "^10.1.0",
diff --git a/rest/nodejs/src/api/checkout.ts b/rest/nodejs/src/api/checkout.ts
index dc7fda1..553140a 100644
--- a/rest/nodejs/src/api/checkout.ts
+++ b/rest/nodejs/src/api/checkout.ts
@@ -1267,7 +1267,7 @@ export class CheckoutService {
     order.fulfillment.events.push({
       id: `evt_${uuidv4()}`,
       type: "shipped",
-      occurred_at: new Date(),
+      occurred_at: new Date().toISOString(),
       line_items: order.line_items.map((lineItem) => ({
         id: lineItem.id,
         quantity: lineItem.quantity.total,
diff --git a/rest/nodejs/test/quantity_bounds.test.ts b/rest/nodejs/test/quantity_bounds.test.ts
new file mode 100644
index 0000000..36d44d7
--- /dev/null
+++ b/rest/nodejs/test/quantity_bounds.test.ts
@@ -0,0 +1,121 @@
+// Copyright 2026 UCP Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// line_item.json declares quantity as an integer with minimum 1, and
+// checkout.md requires additive well known totals types to have non negative
+// amounts. The schema boundary is where that holds: a quantity below 1 must
+// be rejected at validation, never priced.
+
+import assert from "node:assert/strict";
+import { before, test } from "node:test";
+
+import { zValidator } from "@hono/zod-validator";
+import { Hono } from "hono";
+
+import { CheckoutService } from "../src/api/checkout";
+import { getProductsDb, getTransactionsDb, initDbs } from "../src/data/db";
+import {
+  ExtendedCheckoutCreateRequestSchema,
+  ExtendedCheckoutUpdateRequestSchema,
+} from "../src/models";
+import { IdParamSchema, prettyValidation } from "../src/utils/validation";
+
+const JSON_HEADERS = { "Content-Type": "application/json" };
+
+function buildApp() {
+  const service = new CheckoutService();
+  const app = new Hono<{ Variables: { logger: typeof console } }>();
+  app.use(async (c, next) => {
+    c.set("logger", console);
+    await next();
+  });
+  app.post(
+    "/checkout-sessions",
+    zValidator("json", ExtendedCheckoutCreateRequestSchema, prettyValidation),
+    service.createCheckout
+  );
+  app.put(
+    "/checkout-sessions/:id",
+    zValidator("param", IdParamSchema, prettyValidation),
+    zValidator("json", ExtendedCheckoutUpdateRequestSchema, prettyValidation),
+    service.updateCheckout
+  );
+  return app;
+}
+
+before(() => {
+  initDbs(":memory:", ":memory:");
+  getProductsDb()
+    .prepare(
+      "INSERT INTO products (id, title, price, image_url) VALUES (?, ?, ?, ?)"
+    )
+    .run("bouquet_roses", "Red Rose", 3500, "");
+  getTransactionsDb()
+    .prepare("INSERT INTO inventory (product_id, quantity) VALUES (?, ?)")
+    .run("bouquet_roses", 100);
+});
+
+async function create(app: ReturnType<typeof buildApp>, quantity: number) {
+  return app.request("/checkout-sessions", {
+    method: "POST",
+    headers: JSON_HEADERS,
+    body: JSON.stringify({
+      line_items: [{ item: { id: "bouquet_roses" }, quantity }],
+    }),
+  });
+}
+
+test("create rejects quantity 0 and negative quantity at validation", async () => {
+  const app = buildApp();
+  for (const quantity of [0, -1]) {
+    const res = await create(app, quantity);
+    assert.equal(
+      res.status,
+      422,
+      `quantity ${quantity} must be rejected, not priced`
+    );
+  }
+});
+
+test("create rejects a fractional quantity", async () => {
+  const app = buildApp();
+  const res = await create(app, 1.5);
+  assert.equal(res.status, 422);
+});
+
+test("update rejects quantity below 1 on an existing session", async () => {
+  const app = buildApp();
+  const created = await create(app, 2);
+  assert.equal(created.status, 201);
+  const { id } = await created.json();
+  const res = await app.request(`/checkout-sessions/${id}`, {
+    method: "PUT",
+    headers: JSON_HEADERS,
+    body: JSON.stringify({
+      line_items: [{ item: { id: "bouquet_roses" }, quantity: -1 }],
+    }),
+  });
+  assert.equal(res.status, 422);
+});
+
+test("a positive integer quantity still succeeds and prices normally", async () => {
+  const app = buildApp();
+  const res = await create(app, 2);
+  assert.equal(res.status, 201);
+  const body = await res.json();
+  const total = (body.totals ?? []).find(
+    (t: { type?: string }) => t.type === "total"
+  );
+  assert.equal(total?.amount, 7000);
+});