diff --git a/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-actions.tsx b/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-actions.tsx
index 51c262be..2853c205 100644
--- a/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-actions.tsx
+++ b/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-actions.tsx
@@ -8,13 +8,29 @@ type SlideOverCompletionActionsProps = {
   docStatus: CompletionDocStatus;
   rowVersion?: string | number | null;
   completionPending: boolean;
+  /** Completion documents require a selected primary service. */
+  hasService: boolean;
   hasUploadHandler: boolean;
   pdfInputRef: RefObject<HTMLInputElement | null>;
   onPdfSelected: (files: FileList | null) => void;
   onOpenCompletionDoc: () => void;
 };
 
-function DocStatusButtonContent({ docStatus }: { docStatus: CompletionDocStatus }) {
+function DocStatusButtonContent({
+  docStatus,
+  hasService,
+}: {
+  docStatus: CompletionDocStatus;
+  hasService: boolean;
+}) {
+  if (!hasService) {
+    return (
+      <>
+        <FileText size={13} />
+        Select service
+      </>
+    );
+  }
   if (docStatus === "Yes") {
     return (
       <>
@@ -43,6 +59,7 @@ export function SlideOverCompletionActions({
   docStatus,
   rowVersion,
   completionPending,
+  hasService,
   hasUploadHandler,
   pdfInputRef,
   onPdfSelected,
@@ -83,10 +100,17 @@ export function SlideOverCompletionActions({
       <Button
         variant="outline"
         className="w-full gap-1.5 text-[12px]"
+        disabled={!hasService}
+        title={hasService ? undefined : "Select a service before opening the completion document"}
         onClick={onOpenCompletionDoc}
       >
-        <DocStatusButtonContent docStatus={docStatus} />
+        <DocStatusButtonContent docStatus={docStatus} hasService={hasService} />
       </Button>
+      {!hasService && (
+        <Text as="p" style={{ fontSize: 11, color: "var(--color-text-muted)" }}>
+          Select a service before opening the completion document.
+        </Text>
+      )}
     </div>
   );
 }
diff --git a/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-tab.tsx b/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-tab.tsx
index c44465b3..cf89d2bf 100644
--- a/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-tab.tsx
+++ b/src/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-tab.tsx
@@ -6,6 +6,7 @@ import type { UploadableMediaCategory } from "@/domain/work-orders/mappers/work-
 import type { WorkOrderBoardCompletion } from "@/domain/work-orders/types/work-order-board-detail";
 import type { MediaFile } from "@/domain/work-orders/types/work-order-media";
 import type { WorkOrderTableRow } from "@/domain/work-orders/types/work-order-table-row";
+import { hasCompletionDocService } from "@/domain/work-orders/utils/completion-doc-service-gate";
 import { hasCompletionMediaGallery } from "@/domain/work-orders/utils/partition-slide-over-media";
 import {
   formatMediaSizeLimit,
@@ -126,6 +127,7 @@ export function SlideOverCompletionTab({
           docStatus={docStatus}
           rowVersion={row.rowVersion}
           completionPending={completionPending}
+          hasService={hasCompletionDocService(row.pm)}
           hasUploadHandler={Boolean(onUploadCompletionPdf)}
           pdfInputRef={pdfInputRef}
           onPdfSelected={handlePdfSelected}
diff --git a/src/test/app/(protected)/workorders/slide-over-completion-service-gate.test.tsx b/src/test/app/(protected)/workorders/slide-over-completion-service-gate.test.tsx
new file mode 100644
index 00000000..d87162fe
--- /dev/null
+++ b/src/test/app/(protected)/workorders/slide-over-completion-service-gate.test.tsx
@@ -0,0 +1,46 @@
+import { fireEvent, render, screen } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+import { SlideOverCompletionTab } from "@/app/(protected)/workorders/_components/detail/tabs/slide-over-completion-tab";
+import type { WorkOrderTableRow } from "@/domain/work-orders/types/work-order-table-row";
+
+function renderTab(pm: string, onOpenCompletionDoc = vi.fn()) {
+  const row = { id: "1", type: "PM", pm, docStatus: "No", rowVersion: "1" } as WorkOrderTableRow;
+  render(
+    <SlideOverCompletionTab
+      row={row}
+      isReadOnly={false}
+      media={[]}
+      onOpenCompletionDoc={onOpenCompletionDoc}
+    />,
+  );
+  return onOpenCompletionDoc;
+}
+
+describe("SlideOverCompletionTab service gate", () => {
+  it("disables the completion document button until a service is selected", () => {
+    const onOpen = renderTab("  ");
+
+    const button = screen.getByRole("button", { name: /select service/i });
+    expect(button).toBeDisabled();
+    expect(
+      screen.getByText("Select a service before opening the completion document."),
+    ).toBeInTheDocument();
+    expect(
+      screen.queryByRole("button", { name: /open completion document/i }),
+    ).not.toBeInTheDocument();
+
+    fireEvent.click(button);
+    expect(onOpen).not.toHaveBeenCalled();
+  });
+
+  it("opens the completion document when a service is selected", () => {
+    const onOpen = renderTab("HVAC Inspection");
+
+    const button = screen.getByRole("button", { name: /open completion document/i });
+    expect(button).toBeEnabled();
+    expect(screen.queryByText(/select service/i)).not.toBeInTheDocument();
+
+    fireEvent.click(button);
+    expect(onOpen).toHaveBeenCalledTimes(1);
+  });
+});