diff --git a/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-cell.tsx b/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-cell.tsx
index 369c1101..db0db0f9 100644
--- a/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-cell.tsx
+++ b/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-cell.tsx
@@ -3,6 +3,12 @@ import { toast } from "react-toastify";
 import { workOrdersApi } from "@/domain/work-orders/api/work-orders-api";
 import type { WorkOrderTableRow } from "@/domain/work-orders/types/work-order-table-row";
 import { findDuplicateWoNumberRow } from "@/domain/work-orders/utils/find-duplicate-wo-number-row";
+import {
+  WO_NUMBER_DOWNGRADE_MESSAGE,
+  WO_NUMBER_FORMAT_MESSAGE,
+  isValidBoardWoNumber,
+  isWoNumberDowngrade,
+} from "@/domain/work-orders/utils/wo-number-input";
 import { EditableWONumberInput } from "./editable-wo-number-input";
 import { HighlightText } from "./highlight-text";
 import { AddOnPill, CarriedOverCounterPill, RescheduleCounterPill } from "./wo-counter-pills";
@@ -24,6 +30,9 @@ type EditableWONumberCellProps = {
   locked?: boolean;
 };
 
+/** Inline-create rows pass this id; their number is draft state, never a saved WO number. */
+const UNSAVED_DRAFT_ID = "draft";
+
 async function resolveDuplicateWoNumber(
   draft: string,
   allOrders: WorkOrderTableRow[],
@@ -63,8 +72,14 @@ export function EditableWONumberCell({
       setEditing(false);
       return;
     }
-    if (draft.length !== 11) {
-      toast("Work order number must be exactly 11 digits");
+    if (!isValidBoardWoNumber(draft)) {
+      toast(WO_NUMBER_FORMAT_MESSAGE);
+      return;
+    }
+    if (String(currentId) !== UNSAVED_DRAFT_ID && isWoNumberDowngrade(value, draft)) {
+      toast.error(WO_NUMBER_DOWNGRADE_MESSAGE);
+      setDraft(value);
+      setEditing(false);
       return;
     }
     setIsChecking(true);
@@ -97,14 +112,14 @@ export function EditableWONumberCell({
 
   const handleBlur = () => {
     if (isChecking) return;
-    if (draft !== value && draft.length === 11) {
+    if (draft !== value && isValidBoardWoNumber(draft)) {
       commit();
       return;
     }
     setEditing(false);
   };
 
-  const invalid = editing && draft.length > 0 && draft.length !== 11;
+  const invalid = editing && draft.length > 0 && !isValidBoardWoNumber(draft);
 
   if (locked) {
     return (
diff --git a/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-input.tsx b/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-input.tsx
index f4becca8..1ef6512e 100644
--- a/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-input.tsx
+++ b/src/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-input.tsx
@@ -1,3 +1,5 @@
+import { sanitizeBoardWoNumberInput } from "@/domain/work-orders/utils/wo-number-input";
+
 type EditableWONumberInputProps = {
   draft: string;
   invalid: boolean;
@@ -21,9 +23,8 @@ export function EditableWONumberInput({
     <input
       autoFocus
       disabled={isChecking}
-      inputMode="numeric"
       value={draft}
-      onChange={(event) => onChange(event.target.value.replace(/\D/g, "").slice(0, 11))}
+      onChange={(event) => onChange(sanitizeBoardWoNumberInput(event.target.value))}
       onFocus={(event) => event.target.select()}
       onClick={(event) => event.stopPropagation()}
       onKeyDown={(event) => {
@@ -36,7 +37,7 @@ export function EditableWONumberInput({
         }
       }}
       onBlur={onBlur ?? onCommit}
-      placeholder="12345678901"
+      placeholder="e.g. 12345678901 or SH00001"
       aria-label="Work order number"
       aria-invalid={invalid}
       className="font-mono focus:outline-none"
diff --git a/src/domain/work-orders/utils/wo-number-input.ts b/src/domain/work-orders/utils/wo-number-input.ts
new file mode 100644
index 00000000..39aec309
--- /dev/null
+++ b/src/domain/work-orders/utils/wo-number-input.ts
@@ -0,0 +1,33 @@
+import { isRealWoNumber } from "@/domain/work-orders/utils/wo-closability";
+
+/** Board WO numbers are an 11-digit APM number or an SH placeholder of at most 11 characters. */
+export const BOARD_WO_NUMBER_MAX_LENGTH = 11;
+
+export const WO_NUMBER_FORMAT_MESSAGE =
+  "Work order number must be 11 digits or an SH number (e.g. SH00001)";
+
+export const WO_NUMBER_DOWNGRADE_MESSAGE =
+  "Cannot replace an APM work order number with an internal number";
+
+export function sanitizeBoardWoNumberInput(raw: string): string {
+  return raw
+    .replace(/[^a-zA-Z0-9]/g, "")
+    .toUpperCase()
+    .slice(0, BOARD_WO_NUMBER_MAX_LENGTH);
+}
+
+export function isProvisionalWoNumber(woNumber: string): boolean {
+  return /^SH/i.test(woNumber.trim());
+}
+
+export function isValidBoardWoNumber(woNumber: string): boolean {
+  return /^\d{11}$/.test(woNumber) || /^SH\d{1,9}$/i.test(woNumber);
+}
+
+/**
+ * SH-320: only an already-saved real APM number is protected from being replaced by an
+ * SH placeholder. A brand-new WO has no saved number, so the rule never applies there.
+ */
+export function isWoNumberDowngrade(savedWoNumber: string, nextWoNumber: string): boolean {
+  return isRealWoNumber(savedWoNumber) && isProvisionalWoNumber(nextWoNumber);
+}
diff --git a/src/test/app/(protected)/workorders/editable-wo-number-cell-sh-placeholder.test.tsx b/src/test/app/(protected)/workorders/editable-wo-number-cell-sh-placeholder.test.tsx
new file mode 100644
index 00000000..5ae3cde3
--- /dev/null
+++ b/src/test/app/(protected)/workorders/editable-wo-number-cell-sh-placeholder.test.tsx
@@ -0,0 +1,105 @@
+import { fireEvent, screen, waitFor } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { toast } from "react-toastify";
+import { EditableWONumberCell } from "@/app/(protected)/workorders/_components/list/table/cells/editable-wo-number-cell";
+import { workOrdersApi } from "@/domain/work-orders/api/work-orders-api";
+import { renderWithProviders } from "@/test/test-utils";
+
+vi.mock("react-toastify", () => {
+  const toastFn = Object.assign(vi.fn(), { error: vi.fn(), success: vi.fn() });
+  return { toast: toastFn };
+});
+
+vi.mock("@/domain/work-orders/api/work-orders-api", () => ({
+  workOrdersApi: { findByWoNumber: vi.fn() },
+}));
+
+function renderCell(value: string, currentId: string | number) {
+  const onSave = vi.fn();
+  renderWithProviders(
+    <EditableWONumberCell
+      value={value}
+      rescheduleCount={0}
+      currentId={currentId}
+      allOrders={[]}
+      onSave={onSave}
+    />,
+  );
+  return onSave;
+}
+
+function typeAndCommit(nextValue: string) {
+  fireEvent.click(screen.getByText(/./, { selector: "span" }));
+  const input = screen.getByLabelText("Work order number");
+  fireEvent.change(input, { target: { value: nextValue } });
+  fireEvent.keyDown(input, { key: "Enter" });
+}
+
+describe("EditableWONumberCell SH placeholder numbers (SH-320)", () => {
+  beforeEach(() => {
+    vi.mocked(workOrdersApi.findByWoNumber).mockResolvedValue(null as never);
+    vi.mocked(toast).mockClear();
+    vi.mocked(toast.error).mockClear();
+  });
+
+  it("saves a freshly typed SH number on a brand-new inline work order", async () => {
+    const onSave = renderCell("", "draft");
+
+    typeAndCommit("SH00001");
+
+    await waitFor(() => expect(onSave).toHaveBeenCalledWith("SH00001"));
+    expect(toast).not.toHaveBeenCalled();
+    expect(toast.error).not.toHaveBeenCalled();
+  });
+
+  it("keeps a typed SH prefix instead of stripping it to digits", () => {
+    renderCell("", "draft");
+
+    fireEvent.click(screen.getByText("-"));
+    const input = screen.getByLabelText("Work order number");
+    fireEvent.change(input, { target: { value: "sh00001" } });
+
+    expect(input).toHaveValue("SH00001");
+  });
+
+  it("does not treat an unsaved inline draft number as a real number to protect", async () => {
+    const onSave = renderCell("12345678901", "draft");
+
+    typeAndCommit("SH00002");
+
+    await waitFor(() => expect(onSave).toHaveBeenCalledWith("SH00002"));
+  });
+
+  it("blocks replacing a saved real APM number with an SH number", async () => {
+    const onSave = renderCell("12345678901", 42);
+
+    typeAndCommit("SH00001");
+
+    await waitFor(() =>
+      expect(toast.error).toHaveBeenCalledWith(
+        "Cannot replace an APM work order number with an internal number",
+      ),
+    );
+    expect(onSave).not.toHaveBeenCalled();
+    expect(screen.getByText("12345678901")).toBeInTheDocument();
+  });
+
+  it("lets a saved SH number be replaced by a real 11-digit number", async () => {
+    const onSave = renderCell("SH00001", 42);
+
+    typeAndCommit("12345678901");
+
+    await waitFor(() => expect(onSave).toHaveBeenCalledWith("12345678901"));
+  });
+
+  it("still rejects a numeric number that is not 11 digits", () => {
+    const onSave = renderCell("", "draft");
+
+    typeAndCommit("12345");
+
+    expect(toast).toHaveBeenCalledWith(
+      "Work order number must be 11 digits or an SH number (e.g. SH00001)",
+    );
+    expect(onSave).not.toHaveBeenCalled();
+  });
+});