diff --git a/src/hooks/useDrawerUrlState.test.ts b/src/hooks/useDrawerUrlState.test.ts
new file mode 100644
index 0000000..0699e37
--- /dev/null
+++ b/src/hooks/useDrawerUrlState.test.ts
@@ -0,0 +1,198 @@
+// @vitest-environment happy-dom
+// Mirror the IS_REACT_ACT_ENVIRONMENT setup from the other hook tests so
+// React doesn't log act() warnings about the createRoot render/unmount
+// and the popstate-driven state updates.
+(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+
+import React, { act } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { describe, expect, it, vi, beforeEach, afterEach } from "vitest";
+import { useDrawerUrlState } from "./useDrawerUrlState";
+
+/**
+ * Minimal renderHook shim — same pattern as `useClaimDetail.test.ts`.
+ * The project doesn't ship `@testing-library/react`, so we wire a Probe
+ * component into a real `createRoot` and read its return value through
+ * a shared `result` object. `act` flushes React's state updates between
+ * micro-tasks so assertions after a popstate dispatch see the new value.
+ */
+function renderHook<TResult>(setup: () => TResult): {
+  result: { current: TResult | undefined };
+  unmount: () => void;
+} {
+  const result: { current: TResult | undefined } = { current: undefined };
+  const container = document.createElement("div");
+  document.body.appendChild(container);
+
+  function Probe() {
+    result.current = setup();
+    return null;
+  }
+
+  const root: Root = createRoot(container);
+  act(() => {
+    root.render(React.createElement(Probe));
+  });
+
+  return {
+    result,
+    unmount: () => {
+      act(() => root.unmount());
+      container.remove();
+    },
+  };
+}
+
+/**
+ * Point happy-dom's URL at a known value. happy-dom v20 doesn't expose a
+ * writable `window.location.search`, but `window.happyDOM.setURL` updates
+ * the URL the window reports without triggering a navigation — exactly
+ * what we want for mounting the hook at `/claims?claim=CLM-1` etc.
+ */
+function setLocation(url: string): void {
+  // happy-dom attaches the setURL helper under `window.happyDOM`. Cast
+  // through `unknown` to keep the test file free of happy-dom imports.
+  (window as unknown as { happyDOM: { setURL: (u: string) => void } }).happyDOM.setURL(url);
+}
+
+describe("useDrawerUrlState", () => {
+  // `vi.fn()` with no args in vitest 4 is typed `Mock<Procedure |
+  // Constructable>`, which carries a constructor signature and won't
+  // assign to `() => void`. Constraining the generic to `vi.fn<() => void>()`
+  // gives a plain function-typed mock that flows directly into `pushState`
+  // / `replaceState` slots.
+  let pushStateMock: ReturnType<typeof vi.fn>;
+  let replaceStateMock: ReturnType<typeof vi.fn>;
+
+  beforeEach(() => {
+    pushStateMock = vi.fn();
+    replaceStateMock = vi.fn();
+
+    // Replace `history` with a barebones stub. We only need pushState /
+    // replaceState (and a state slot for the History spec). `vi.stubGlobal`
+    // also affects `window.history` because happy-dom's window is the
+    // global object.
+    vi.stubGlobal("history", {
+      pushState: pushStateMock,
+      replaceState: replaceStateMock,
+      state: null,
+    });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    // Reset happy-dom's URL between tests so one test's `?claim=…` doesn't
+    // leak into the next test's "no param" assertion.
+    setLocation("http://localhost/");
+  });
+
+  it("reads the ?claim= param from window.location.search on mount", () => {
+    setLocation("http://localhost/claims?claim=CLM-1");
+
+    const { result, unmount } = renderHook(() => useDrawerUrlState());
+
+    expect(result.current?.claimId).toBe("CLM-1");
+    expect(typeof result.current?.open).toBe("function");
+    expect(typeof result.current?.close).toBe("function");
+    expect(typeof result.current?.setClaimId).toBe("function");
+
+    unmount();
+  });
+
+  it("returns null claimId when no ?claim= param is set", () => {
+    // setLocation to a URL with no ?claim= (default afterLocation is
+    // bare, but afterEach resets it explicitly — be defensive).
+    setLocation("http://localhost/claims");
+
+    const { result, unmount } = renderHook(() => useDrawerUrlState());
+
+    expect(result.current?.claimId).toBeNull();
+
+    unmount();
+  });
+
+  it("open(id) pushes a new history entry containing ?claim=id", () => {
+    setLocation("http://localhost/claims");
+
+    const { result, unmount } = renderHook(() => useDrawerUrlState());
+
+    act(() => {
+      result.current?.open("CLM-2");
+    });
+
+    expect(pushStateMock).toHaveBeenCalledTimes(1);
+    expect(replaceStateMock).not.toHaveBeenCalled();
+    // The third arg to pushState is the URL. The plan calls for a relative
+    // URL built from pathname+search+hash, so it should contain the
+    // ?claim= param.
+    const urlArg = pushStateMock.mock.calls[0][2] as string;
+    expect(urlArg).toContain("?claim=CLM-2");
+
+    unmount();
+  });
+
+  it("setClaimId(id) replaces the current history entry (no new entry) and does NOT pushState", () => {
+    setLocation("http://localhost/claims?claim=CLM-1");
+
+    const { result, unmount } = renderHook(() => useDrawerUrlState());
+
+    act(() => {
+      result.current?.setClaimId("CLM-3");
+    });
+
+    expect(replaceStateMock).toHaveBeenCalledTimes(1);
+    expect(pushStateMock).not.toHaveBeenCalled();
+    const urlArg = replaceStateMock.mock.calls[0][2] as string;
+    expect(urlArg).toContain("?claim=CLM-3");
+
+    unmount();
+  });
+
+  it("close() pushes a new history entry with the ?claim= param stripped", () => {
+    setLocation("http://localhost/claims?claim=CLM-1");
+
+    const { result, unmount } = renderHook(() => useDrawerUrlState());
+
+    act(() => {
+      result.current?.close();
+    });
+
+    expect(pushStateMock).toHaveBeenCalledTimes(1);
+    expect(replaceStateMock).not.toHaveBeenCalled();
+    const urlArg = pushStateMock.mock.calls[0][2] as string;
+    // The plan's spec: close strips the param. After close, the URL should
+    // not contain ?claim= (or contain it as empty — either way the param
+    // is effectively gone). Our implementation deletes the param entirely.
+    expect(urlArg).not.toContain("?claim=");
+    // claimId must also flip back to null synchronously after close.
+    expect(result.current?.claimId).toBeNull();
+
+    unmount();
+  });
+
+  it("updates claimId in response to popstate (browser back/forward)", async () => {
+    // Start on CLM-1; simulate the user navigating forward to CLM-2 via
+    // browser history (back/forward fires popstate). The hook must pick up
+    // the new URL even though it didn't trigger the navigation itself.
+    setLocation("http://localhost/claims?claim=CLM-1");
+
+    const { result, unmount } = renderHook(() => useDrawerUrlState());
+
+    expect(result.current?.claimId).toBe("CLM-1");
+
+    // Simulate browser back/forward: the URL changes and popstate fires.
+    // happy-dom does not dispatch popstate automatically when setURL is
+    // called, so we change the URL and dispatch the event ourselves — the
+    // same shape a real browser would produce.
+    setLocation("http://localhost/claims?claim=CLM-2");
+    await act(async () => {
+      window.dispatchEvent(new PopStateEvent("popstate"));
+      // Yield once so React's setState inside the listener flushes.
+      await Promise.resolve();
+    });
+
+    expect(result.current?.claimId).toBe("CLM-2");
+
+    unmount();
+  });
+});
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diff --git a/src/hooks/useDrawerUrlState.ts b/src/hooks/useDrawerUrlState.ts
new file mode 100644
index 0000000..ed109f0
--- /dev/null
+++ b/src/hooks/useDrawerUrlState.ts
@@ -0,0 +1,104 @@
+import { useCallback, useEffect, useState } from "react";
+
+/**
+ * Read the current `?claim=…` query param off `window.location.search`.
+ * Returns `null` when the param is absent or empty.
+ *
+ * `URLSearchParams` is the standard, locale-free way to parse query
+ * strings in the browser. Using it (rather than hand-rolled string
+ * slicing) means we correctly handle multiple params and percent-encoded
+ * characters in claim IDs without surprises.
+ */
+function readClaimId(): string | null {
+  const params = new URLSearchParams(window.location.search);
+  return params.get("claim");
+}
+
+/**
+ * Build the URL we want to push/replace into history.
+ *
+ * - `claimId === null`  →  drop the `?claim=` param, preserving any
+ *   other params (e.g. `?foo=bar` stays).
+ * - `claimId !== null`  →  set the param to the new id, also preserving
+ *   any other params.
+ *
+ * We return `pathname + search + hash` (a relative URL) rather than the
+ * full href — `history.pushState` accepts a relative URL and rewriting
+ * only the relative form keeps the document's origin stable. `hash` is
+ * appended so a `#section` anchor survives navigation, matching what
+ * the browser would do natively.
+ */
+function buildUrl(claimId: string | null): string {
+  const url = new URL(window.location.href);
+  if (claimId === null) {
+    url.searchParams.delete("claim");
+  } else {
+    url.searchParams.set("claim", claimId);
+  }
+  return url.pathname + url.search + url.hash;
+}
+
+/**
+ * Per-claim detail drawer URL state (SP4).
+ *
+ * Reads `?claim=` from the URL on mount and keeps the value in sync with
+ * history as the drawer is opened, navigated (j/k), and closed. The
+ * drawer parent uses this hook as the single source of truth for "is a
+ * claim open and which one" — the URL is the durable record so that
+ * deep links (`/claims?claim=CLM-1`) restore the drawer state on reload.
+ *
+ *   - `claimId`: the id parsed from the URL (or `null` when the param
+ *     is absent). React state so consumers re-render on changes.
+ *   - `open(id)`: pushes a NEW history entry with `?claim={id}` — so the
+ *     browser Back button returns to the previous page (e.g. the claims
+ *     list) and not just to the previously-open claim.
+ *   - `setClaimId(id)`: REPLACES the current history entry — used by the
+ *     j/k nav handler so j/k moves through the list without polluting
+ *     history with one entry per keystroke.
+ *   - `close()`: pushes a NEW entry that strips the param, so Back from
+ *     the closed drawer returns to whatever page the user was on before
+ *     opening the drawer.
+ *
+ * The hook subscribes to `popstate` so that browser Back/Forward
+ * (which fire popstate rather than our own pushState) propagate into
+ * the React state. Without this, hitting Back would change the URL but
+ * leave the drawer open on the stale id.
+ *
+ * Implementation note: this is a single-operator local-only tool, so we
+ * call `history.pushState` / `replaceState` directly without try/catch
+ * wrappers — the only way they throw in modern browsers is when the URL
+ * is malformed, which we control.
+ */
+export function useDrawerUrlState(): {
+  claimId: string | null;
+  open: (id: string) => void;
+  close: () => void;
+  setClaimId: (id: string) => void;
+} {
+  const [claimId, setClaimIdState] = useState<string | null>(() => readClaimId());
+
+  const open = useCallback((id: string) => {
+    window.history.pushState(null, "", buildUrl(id));
+    setClaimIdState(id);
+  }, []);
+
+  const setClaimId = useCallback((id: string) => {
+    window.history.replaceState(null, "", buildUrl(id));
+    setClaimIdState(id);
+  }, []);
+
+  const close = useCallback(() => {
+    window.history.pushState(null, "", buildUrl(null));
+    setClaimIdState(null);
+  }, []);
+
+  useEffect(() => {
+    const onPopState = () => {
+      setClaimIdState(readClaimId());
+    };
+    window.addEventListener("popstate", onPopState);
+    return () => window.removeEventListener("popstate", onPopState);
+  }, []);
+
+  return { claimId, open, close, setClaimId };
+}
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