diff --git a/e2e/work-orders/wo-number-sh-placeholder.spec.ts b/e2e/work-orders/wo-number-sh-placeholder.spec.ts
new file mode 100644
index 00000000..6d14e801
--- /dev/null
+++ b/e2e/work-orders/wo-number-sh-placeholder.spec.ts
@@ -0,0 +1,109 @@
+import { expect, test, type Page, type Route } from "@playwright/test";
+
+const FROZEN_NOW = "2026-08-19T15:00:00.000Z";
+
+const TEST_USER = {
+  token: "wo-number-sh-placeholder-e2e-token",
+  expiration: "2030-01-01T00:00:00.000Z",
+  email: "admin@seahavenind.com",
+  userRoles: "Admin",
+  phoneNumber: "5551234567",
+  fullname: "Work Order E2E Admin",
+  id: "1",
+};
+
+async function fulfillJson(route: Route, body: unknown, status = 200) {
+  await route.fulfill({ status, contentType: "application/json", body: JSON.stringify(body) });
+}
+
+const REAL_ROW = {
+  id: 1000,
+  woNumber: "12345678901",
+  workOrderType: 2,
+  siteCode: "DAL1",
+  lifecycleStatusLabel: "Scheduled",
+  scheduledDate: "2026-08-19",
+  dispatcherId: "1",
+  dispatcherName: "Pat Dispatcher",
+  pm: "HVAC",
+};
+
+async function mockBoard(page: Page, patches: unknown[]) {
+  await page.route(
+    (url) => url.pathname.startsWith("/api/"),
+    async (route) => {
+      const request = route.request();
+      const url = request.url();
+      if (request.method() !== "GET") {
+        patches.push({ url, body: request.postDataJSON() });
+        await fulfillJson(route, {}, 500);
+        return;
+      }
+      if (url.includes("workorders/board/search")) {
+        await fulfillJson(route, { items: [REAL_ROW], totalCount: 1, page: 0, pageSize: 50 });
+        return;
+      }
+      if (url.includes("workorders/board")) {
+        await fulfillJson(route, {
+          weekStart: "2026-08-17",
+          weekEnd: "2026-08-21",
+          counts: { returned: 1, total: 1 },
+          unscheduled: [],
+          scheduled: [REAL_ROW],
+        });
+        return;
+      }
+      await fulfillJson(route, { data: [], items: [] });
+    },
+  );
+}
+
+test.describe("inline WO number SH placeholder (SH-320)", () => {
+  test.setTimeout(60_000);
+
+  test.beforeEach(async ({ page }) => {
+    await page.clock.install({ time: new Date(FROZEN_NOW) });
+    await page.clock.resume();
+    await page.addInitScript((user) => {
+      localStorage.setItem("auth", JSON.stringify(user));
+      const style = document.createElement("style");
+      style.textContent = ".tsqd-parent-container { display: none !important; }";
+      document.documentElement.appendChild(style);
+    }, TEST_USER);
+  });
+
+  test("keeps a typed SH number on a new inline row and blocks downgrading a saved real number", async ({
+    page,
+  }) => {
+    const mutations: unknown[] = [];
+    await mockBoard(page, mutations);
+    await page.goto("/workorders");
+    await expect(page.getByRole("heading", { name: "Work Orders" })).toBeVisible({
+      timeout: 20_000,
+    });
+    await expect(page.getByText("12345678901").first()).toBeVisible();
+
+    await page
+      .getByRole("button", { name: /Add WO/ })
+      .first()
+      .click();
+    const inlineRow = page.locator("tr").filter({ has: page.getByText("-", { exact: true }) });
+    await inlineRow.getByText("-", { exact: true }).first().click();
+    const draftInput = page.getByLabel("Work order number");
+    await draftInput.fill("SH00001");
+    await draftInput.press("Enter");
+    await expect(draftInput).toHaveCount(0);
+    await expect(page.getByText("SH00001", { exact: true })).toBeVisible();
+    await expect(page.getByText(/must be 11 digits/)).toHaveCount(0);
+
+    await page.getByText("12345678901", { exact: true }).first().click();
+    const savedInput = page.getByLabel("Work order number");
+    await savedInput.fill("SH00009");
+    await savedInput.press("Enter");
+    await expect(
+      page.getByText("Cannot replace an APM work order number with an internal number"),
+    ).toBeVisible();
+    await expect(page.getByText("12345678901", { exact: true }).first()).toBeVisible();
+    expect(mutations).toEqual([]);
+  });
+});