diff --git a/src/pages/Acks.test.tsx b/src/pages/Acks.test.tsx
index 3a3d076..8029b57 100644
--- a/src/pages/Acks.test.tsx
+++ b/src/pages/Acks.test.tsx
@@ -9,13 +9,17 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { Acks } from "./Acks";
 import { api } from "@/lib/api";
 
-vi.mock("@/lib/api", () => ({
-  api: {
-    isConfigured: true,
-    listAcks: vi.fn(),
-    getAck: vi.fn(),
-  },
-}));
+vi.mock("@/lib/api", async (importOriginal) => {
+  const actual = await importOriginal();
+  return {
+    ...actual,
+    api: {
+      isConfigured: true,
+      listAcks: vi.fn(),
+      getAck: vi.fn(),
+    },
+  };
+});
 
 function renderIntoContainer(element: React.ReactElement): {
   container: HTMLDivElement;
@@ -105,6 +109,10 @@ function hasExactlyOneSelectedRow(): boolean {
 describe("Acks", () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    // Reset URL state between tests so a previous `?ack=` doesn't leak.
+    (window as unknown as { happyDOM: { setURL: (u: string) => void } }).happyDOM.setURL(
+      "http://localhost/acks"
+    );
   });
 
   it("renders a single ack row with counts and ack code", async () => {
@@ -448,4 +456,115 @@ describe("Acks", () => {
 
     unmount();
   });
+
+  it("test_clicking_a_row_opens_the_ack_drawer", async () => {
+    // SP21 Phase 5 Task 5.3: clicking an acks row drills into the
+    // matching ack via `?ack=ID` URL state. The AckDrawer mounts
+    // but the actual content depends on `useAckDetail` — we don't
+    // need to verify drawer internals here, just that the URL got
+    // pushed and the drawer portal opens.
+    (api.listAcks as unknown as ReturnType<typeof vi.fn>).mockResolvedValue({
+      items: [
+        {
+          id: 42,
+          sourceBatchId: "b-uuid-1",
+          acceptedCount: 3,
+          rejectedCount: 1,
+          receivedCount: 4,
+          ackCode: "P",
+          parsedAt: "2026-06-20T12:00:00Z",
+        },
+      ],
+      total: 1,
+      returned: 1,
+      has_more: false,
+    });
+    // Stub the per-ack fetch so `useAckDetail` resolves cleanly
+    // (avoids TanStack Query's "Query data cannot be undefined"
+    // warning). We only assert on the drawer's presence, so the
+    // shape doesn't need to be precise.
+    (api.getAck as unknown as ReturnType<typeof vi.fn>).mockResolvedValue({
+      id: 42,
+      sourceBatchId: "b-uuid-1",
+      acceptedCount: 3,
+      rejectedCount: 1,
+      receivedCount: 4,
+      ackCode: "P",
+      parsedAt: "2026-06-20T12:00:00Z",
+      raw_999_text: "ISA*~\n",
+    });
+
+    const { unmount } = renderIntoContainer(React.createElement(Acks));
+    await waitForText("b-uuid-1");
+
+    // No drawer yet.
+    expect(
+      document.body.querySelector('[data-testid="ack-drawer"]')
+    ).toBeNull();
+
+    // Click the row.
+    const row = rowAt(0);
+    expect(row).not.toBeNull();
+    await act(async () => {
+      row!.click();
+      await Promise.resolve();
+    });
+    await settle(
+      () => document.body.querySelector('[data-testid="ack-drawer"]') !== null
+    );
+
+    // The drawer is now in the DOM.
+    expect(
+      document.body.querySelector('[data-testid="ack-drawer"]')
+    ).not.toBeNull();
+
+    unmount();
+  });
+
+  it("test_deep_link_with_ack_param_opens_drawer_on_mount", async () => {
+    // /acks?ack=42 deep link → drawer opens on mount without a click.
+    (api.listAcks as unknown as ReturnType<typeof vi.fn>).mockResolvedValue({
+      items: [
+        {
+          id: 42,
+          sourceBatchId: "b-uuid-1",
+          acceptedCount: 3,
+          rejectedCount: 1,
+          receivedCount: 4,
+          ackCode: "P",
+          parsedAt: "2026-06-20T12:00:00Z",
+        },
+      ],
+      total: 1,
+      returned: 1,
+      has_more: false,
+    });
+    (api.getAck as unknown as ReturnType<typeof vi.fn>).mockResolvedValue({
+      id: 42,
+      sourceBatchId: "b-uuid-1",
+      acceptedCount: 3,
+      rejectedCount: 1,
+      receivedCount: 4,
+      ackCode: "P",
+      parsedAt: "2026-06-20T12:00:00Z",
+      raw_999_text: "ISA*~\n",
+    });
+
+    (window as unknown as { happyDOM: { setURL: (u: string) => void } }).happyDOM.setURL(
+      "http://localhost/acks?ack=42"
+    );
+
+    const { unmount } = renderIntoContainer(React.createElement(Acks));
+    await waitForText("b-uuid-1");
+    await settle(
+      () => document.body.querySelector('[data-testid="ack-drawer"]') !== null
+    );
+
+    // The drawer is in the DOM on first render.
+    expect(
+      document.body.querySelector('[data-testid="ack-drawer"]')
+    ).not.toBeNull();
+
+    unmount();
+  });
 });
\ No newline at end of file
diff --git a/src/pages/Acks.tsx b/src/pages/Acks.tsx
index d748abc..42663af 100644
--- a/src/pages/Acks.tsx
+++ b/src/pages/Acks.tsx
@@ -12,6 +12,8 @@ import { Skeleton } from "@/components/ui/skeleton";
 import { EmptyState } from "@/components/ui/empty-state";
 import { ErrorState } from "@/components/ui/error-state";
 import { KeyboardCheatsheet } from "@/components/KeyboardCheatsheet";
+import { AckDrawer } from "@/components/AckDrawer";
+import { useAckDrawerUrlState } from "@/hooks/useAckDrawerUrlState";
 import { useAcks } from "@/hooks/useAcks";
 import { useRowKeyboard } from "@/hooks/useRowKeyboard";
 import { api } from "@/lib/api";
@@ -91,7 +93,14 @@ function DownloadButton({ id, sourceBatchId }: { id: number; sourceBatchId: stri
   return (
     <button
       type="button"
-      onClick={onClick}
+      onClick={(e) => {
+        // The row's onClick drills into AckDrawer; this button is a
+        // nested control and we want the click to NOT bubble into the
+        // row's onClick handler (matches the precedent set by
+        // DrillableCell onClick in `src/components/drill/DrillableCell.tsx:39`).
+        e.stopPropagation();
+        onClick();
+      }}
       disabled={busy}
       className={cn(
         "inline-flex items-center gap-1.5 rounded-sm border px-2 py-1 mono text-[10.5px] uppercase tracking-[0.14em] font-semibold transition-colors",
@@ -113,6 +122,10 @@ function DownloadButton({ id, sourceBatchId }: { id: number; sourceBatchId: stri
 export function Acks() {
   const { data, isLoading, isError, error, refetch } = useAcks({ limit: 100 });
   const items = data?.items ?? [];
+  // SP21 Phase 5 Task 5.3: drill-down from an acks row into
+  // AckDrawer. The hook reads `?ack=` off `window.location.search`
+  // so deep links restore the open ack on reload.
+  const { ackId, open, close } = useAckDrawerUrlState();
 
   const [selectedIndex, setSelectedIndex] = useState<number | null>(null);
   const [helpOpen, setHelpOpen] = useState(false);
@@ -172,6 +185,12 @@ export function Acks() {
         open={helpOpen}
         onClose={() => setHelpOpen(false)}
       />
+      {/* SP21 Phase 5 Task 5.3: AckDrawer mount. Row click drills
+          into the matching ack; the drawer portals into document.body
+          (Radix Dialog), so the surrounding paper plane stays put
+          while the drawer is open. Deep links via /acks?ack=ID
+          restore the open ack on reload. */}
+      <AckDrawer ackId={ackId} onClose={close} />
       <div className="space-y-0">
         {/* =================================================================
             HERO — DARK EDITORIAL HEADER
@@ -650,7 +669,9 @@ export function Acks() {
                             data-row-index={idx}
                             data-state={isSelected ? "selected" : undefined}
                             aria-selected={isSelected}
+                            onClick={() => open(String(a.id))}
                             className={cn(
+                              "cursor-pointer",
                               isSelected && [
                                 "ring-1 ring-inset",
                               ],