diff --git a/src/pages/BatchDiff.test.tsx b/src/pages/BatchDiff.test.tsx
index 37cb37b..c58c612 100644
--- a/src/pages/BatchDiff.test.tsx
+++ b/src/pages/BatchDiff.test.tsx
@@ -29,6 +29,7 @@ vi.mock("@/lib/api", () => ({
     isConfigured: true,
     listBatches: vi.fn(),
     getBatchDiff: vi.fn(),
+    getRemittance: vi.fn(),
   },
   ApiError: class ApiError extends Error {
     constructor(public status: number, message: string) {
@@ -180,6 +181,13 @@ function makeDiffPayload(): BatchDiffResponse {
 describe("BatchDiff page", () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    // Default for the per-remit detail fetch — the drawer fetches
+    // this whenever `?remit=` is in the URL. Return a never-resolving
+    // promise so the drawer stays in the loading state; the smoke
+    // test only asserts the drawer mounts, not the loaded data.
+    (
+      api.getRemittance as unknown as ReturnType<typeof vi.fn>
+    ).mockReturnValue(new Promise(() => {}));
   });
 
   afterEach(() => {
@@ -189,6 +197,42 @@ describe("BatchDiff page", () => {
       .happyDOM.setURL("http://localhost/batch-diff");
   });
 
+  it("SP21 Task 4.5: deep-link ?remit=ID opens the RemitDrawer on mount", async () => {
+    // Pre-set the URL with `?remit=`. The page doesn't surface any
+    // per-remit rows (the diff payload is claim-level), but the
+    // drawer must still mount so the URL contract is honored.
+    //
+    // `useRemitDrawerUrlState` reads `window.location.search`
+    // (NOT React Router's search params) so we have to set the
+    // global window URL via happyDOM AND give MemoryRouter an
+    // initial entry — both stay in lockstep.
+    (window as unknown as { happyDOM: { setURL: (u: string) => void } })
+      .happyDOM.setURL("http://localhost/batch-diff?remit=REM-7");
+    (
+      api.listBatches as unknown as ReturnType<typeof vi.fn>
+    ).mockResolvedValue([BATCH_A, BATCH_B]);
+
+    const { unmount } = renderIntoContainer(
+      <BatchDiff />,
+      ["/batch-diff?remit=REM-7"],
+    );
+
+    // Page header must be present + drawer must be open.
+    await waitFor(
+      () => !!document.querySelector('[data-testid="batch-diff-page"]'),
+      "page header mounted",
+    );
+    await waitFor(
+      () => !!document.querySelector('[data-testid="remit-drawer"]'),
+      "remit drawer mounted via deep link",
+    );
+    expect(
+      document.querySelector('[data-testid="remit-drawer"]'),
+    ).not.toBeNull();
+
+    unmount();
+  });
+
   it("renders the awaiting-picks empty state when no batches are selected", async () => {
     (api.listBatches as unknown as ReturnType<typeof vi.fn>).mockResolvedValue([
       BATCH_A, BATCH_B,
diff --git a/src/pages/BatchDiff.tsx b/src/pages/BatchDiff.tsx
index 14607ea..b4762cf 100644
--- a/src/pages/BatchDiff.tsx
+++ b/src/pages/BatchDiff.tsx
@@ -21,8 +21,10 @@ import {
   BatchDiffView,
   BatchDiffViewSkeleton,
 } from "@/components/BatchDiffView";
+import { RemitDrawer } from "@/components/RemitDrawer";
 import { useBatches } from "@/hooks/useBatches";
 import { useBatchDiff } from "@/hooks/useBatchDiff";
+import { useRemitDrawerUrlState } from "@/hooks/useRemitDrawerUrlState";
 import type { BatchSummary as ApiBatchSummary } from "@/lib/api";
 import { cn } from "@/lib/utils";
 
@@ -269,6 +271,14 @@ export function BatchDiff() {
   // BrowserRouter (production) symmetric: both update the
   // useSearchParams hook on every navigation.
   const [searchParams, setSearchParams] = useSearchParams();
+  // SP21 Phase 4 Task 4.5: mount the RemitDrawer so a deep link to
+  // /batch-diff?remit=REM-1 opens the drawer in-place. The BatchDiff
+  // data model (BatchClaimDiffSummary) is a claim-level projection —
+  // there are no per-remit IDs in the diff payload to wrap, so no
+  // click-to-drill is wired here. The drawer still mounts so the
+  // `?remit=` URL contract is honored when the user lands on this
+  // page with the param set (e.g. from an external link).
+  const { remitId, open, close } = useRemitDrawerUrlState();
   const { a, b } = useMemo(
     () => readIdsFromParams(searchParams),
     [searchParams],
@@ -835,6 +845,24 @@ export function BatchDiff() {
           <span>{ready ? "A vs B" : "awaiting picks"}</span>
         </div>
       </div>
+
+      {/* SP21 Phase 4 Task 4.5: RemitDrawer mount. There are no
+          per-remit rows in the diff payload (the page is a
+          claim-level projection), so this drawer is for deep-link
+          support only — clicking a claim row does not open it.
+          When the user lands on /batch-diff?remit=REM-1, the drawer
+          opens in-place. The `remits` list is empty so j/k is a
+          no-op while the drawer is open. */}
+      <RemitDrawer
+        remitId={remitId}
+        remits={[]}
+        onClose={close}
+        onNavigate={open}
+        onToggleHelp={() => {
+          // BatchDiff has no cheatsheet surface; `?` is a no-op
+          // here, but the prop is required by the drawer's contract.
+        }}
+      />
     </div>
   );
 }