diff --git a/Auto Run Docs/SpecKit-web-header-analyzer-Phase-07-Browser-Caching.md b/Auto Run Docs/SpecKit-web-header-analyzer-Phase-07-Browser-Caching.md
index a60e314..836e6a0 100644
--- a/Auto Run Docs/SpecKit-web-header-analyzer-Phase-07-Browser-Caching.md	
+++ b/Auto Run Docs/SpecKit-web-header-analyzer-Phase-07-Browser-Caching.md	
@@ -26,13 +26,13 @@ Use a consistent prefix to avoid collisions:
 
 ## Tasks
 
-- [x] T037 [US5] Write failing tests (TDD Red) in `frontend/src/__tests__/useAnalysisCache.test.ts` (save/load/clear with mocked localStorage, size awareness) and `frontend/src/__tests__/page.test.tsx` (cache restoration on mount, clear cache button)
-- [ ] T038 [P] [US5] Create `frontend/src/hooks/useAnalysisCache.ts` — hook managing localStorage with namespaced keys. Saves: headers text, analysis config, analysis result. Size-aware (warns near limits). Methods: `save()`, `load()`, `clear()`, `hasCachedData()`. Verify `useAnalysisCache.test.ts` passes (TDD Green)
+- [x] T037 [US5] Write failing tests (TDD Red) in `frontend/src/__tests__/useAnalysisCache.test.tsx` (save/load/clear with mocked localStorage, size awareness) and `frontend/src/__tests__/page.test.tsx` (cache restoration on mount, clear cache button)
+- [x] T038 [P] [US5] Create `frontend/src/hooks/useAnalysisCache.ts` — hook managing localStorage with namespaced keys. Saves: headers text, analysis config, analysis result. Size-aware (warns near limits). Methods: `save()`, `load()`, `clear()`, `hasCachedData()`. Verify `useAnalysisCache.test.tsx` passes (TDD Green)
 - [ ] T039 [US5] Integrate caching into `frontend/src/app/page.tsx` — on mount restore cached data, render cached report, "Clear Cache" button with FontAwesome trash icon (FR-13), subtle indicator when viewing cached results. Verify `page.test.tsx` passes (TDD Green)
 
 ## Completion
 
-- [ ] All vitest tests pass: `npx vitest run src/__tests__/useAnalysisCache.test.ts src/__tests__/page.test.tsx`
+- [ ] All vitest tests pass: `npx vitest run src/__tests__/useAnalysisCache.test.tsx src/__tests__/page.test.tsx`
 - [ ] After analysis, headers and results are saved to localStorage
 - [ ] Page refresh restores the previous analysis (headers in input, report rendered)
 - [ ] "Clear Cache" button removes all stored data and resets the view to empty state
diff --git a/frontend/src/__tests__/useAnalysisCache.test.ts b/frontend/src/__tests__/useAnalysisCache.test.tsx
similarity index 100%
rename from frontend/src/__tests__/useAnalysisCache.test.ts
rename to frontend/src/__tests__/useAnalysisCache.test.tsx
diff --git a/frontend/src/hooks/useAnalysisCache.ts b/frontend/src/hooks/useAnalysisCache.ts
new file mode 100644
index 0000000..3f53894
--- /dev/null
+++ b/frontend/src/hooks/useAnalysisCache.ts
@@ -0,0 +1,171 @@
+import { useCallback, useState } from "react";
+
+import type { AnalysisConfig, AnalysisReport } from "../types/analysis";
+
+export interface CachedAnalysisPayload {
+  headers: string;
+  config: AnalysisConfig;
+  result: AnalysisReport;
+}
+
+export interface CachedAnalysisData extends CachedAnalysisPayload {
+  timestamp: number;
+}
+
+export interface UseAnalysisCacheState {
+  save: (payload: CachedAnalysisPayload) => void;
+  load: () => CachedAnalysisData | null;
+  clear: () => void;
+  hasCachedData: () => boolean;
+  isNearLimit: boolean;
+}
+
+const STORAGE_LIMIT_BYTES = 5 * 1024 * 1024;
+const WARNING_THRESHOLD_RATIO = 0.9;
+
+const HEADER_KEY = "wha:headers";
+const CONFIG_KEY = "wha:config";
+const RESULT_KEY = "wha:result";
+const TIMESTAMP_KEY = "wha:timestamp";
+
+const canUseStorage = (): boolean => {
+  if (typeof window === "undefined") {
+    return false;
+  }
+  if (!window.localStorage) {
+    return false;
+  }
+
+  try {
+    const probeKey = "wha:probe";
+    window.localStorage.setItem(probeKey, probeKey);
+    window.localStorage.removeItem(probeKey);
+    return true;
+  } catch {
+    return false;
+  }
+};
+
+const estimateBytes = (value: string): number => {
+  if (typeof TextEncoder !== "undefined") {
+    return new TextEncoder().encode(value).length;
+  }
+
+  return value.length;
+};
+
+const estimateStorageSize = (): number => {
+  if (!canUseStorage()) {
+    return 0;
+  }
+
+  let total = 0;
+  for (let index = 0; index < localStorage.length; index += 1) {
+    const key = localStorage.key(index);
+    if (!key) {
+      continue;
+    }
+
+    const value = localStorage.getItem(key) ?? "";
+    total += estimateBytes(key);
+    total += estimateBytes(value);
+  }
+
+  return total;
+};
+
+const parseJson = <T>(value: string | null): T | null => {
+  if (!value) {
+    return null;
+  }
+
+  try {
+    return JSON.parse(value) as T;
+  } catch {
+    return null;
+  }
+};
+
+const useAnalysisCache = (): UseAnalysisCacheState => {
+  const [isNearLimit, setIsNearLimit] = useState(false);
+
+  const save = useCallback((payload: CachedAnalysisPayload) => {
+    if (!canUseStorage()) {
+      setIsNearLimit(false);
+      return;
+    }
+
+    const timestamp = Date.now();
+    localStorage.setItem(HEADER_KEY, payload.headers);
+    localStorage.setItem(CONFIG_KEY, JSON.stringify(payload.config));
+    localStorage.setItem(RESULT_KEY, JSON.stringify(payload.result));
+    localStorage.setItem(TIMESTAMP_KEY, String(timestamp));
+
+    const usedBytes = estimateStorageSize();
+    const warningThreshold = STORAGE_LIMIT_BYTES * WARNING_THRESHOLD_RATIO;
+    setIsNearLimit(usedBytes >= warningThreshold);
+  }, []);
+
+  const load = useCallback((): CachedAnalysisData | null => {
+    if (!canUseStorage()) {
+      return null;
+    }
+
+    const headers = localStorage.getItem(HEADER_KEY);
+    const config = parseJson<AnalysisConfig>(localStorage.getItem(CONFIG_KEY));
+    const result = parseJson<AnalysisReport>(localStorage.getItem(RESULT_KEY));
+    const timestampValue = localStorage.getItem(TIMESTAMP_KEY);
+
+    if (!headers || !config || !result || !timestampValue) {
+      return null;
+    }
+
+    const timestamp = Number(timestampValue);
+    if (Number.isNaN(timestamp)) {
+      return null;
+    }
+
+    return {
+      headers,
+      config,
+      result,
+      timestamp,
+    };
+  }, []);
+
+  const clear = useCallback(() => {
+    if (!canUseStorage()) {
+      setIsNearLimit(false);
+      return;
+    }
+
+    localStorage.removeItem(HEADER_KEY);
+    localStorage.removeItem(CONFIG_KEY);
+    localStorage.removeItem(RESULT_KEY);
+    localStorage.removeItem(TIMESTAMP_KEY);
+    setIsNearLimit(false);
+  }, []);
+
+  const hasCachedData = useCallback((): boolean => {
+    if (!canUseStorage()) {
+      return false;
+    }
+
+    return Boolean(
+      localStorage.getItem(HEADER_KEY) &&
+        localStorage.getItem(CONFIG_KEY) &&
+        localStorage.getItem(RESULT_KEY) &&
+        localStorage.getItem(TIMESTAMP_KEY),
+    );
+  }, []);
+
+  return {
+    save,
+    load,
+    clear,
+    hasCachedData,
+    isNearLimit,
+  };
+};
+
+export default useAnalysisCache;
diff --git a/frontend/vitest.config.ts b/frontend/vitest.config.ts
index 1cf8aa5..521402b 100644
--- a/frontend/vitest.config.ts
+++ b/frontend/vitest.config.ts
@@ -4,6 +4,7 @@ export default defineConfig({
   test: {
     environment: "jsdom",
     globals: true,
+    setupFiles: ["./vitest.setup.ts"],
     exclude: ["**/e2e/**", "**/node_modules/**", "**/.next/**", "**/dist/**"],
   },
 });
diff --git a/frontend/vitest.setup.ts b/frontend/vitest.setup.ts
new file mode 100644
index 0000000..0d6b092
--- /dev/null
+++ b/frontend/vitest.setup.ts
@@ -0,0 +1,52 @@
+const createLocalStorageMock = () => {
+  let store = new Map<string, string>();
+
+  return {
+    get length() {
+      return store.size;
+    },
+    clear() {
+      store.clear();
+    },
+    getItem(key: string) {
+      return store.has(key) ? store.get(key) ?? null : null;
+    },
+    key(index: number) {
+      if (index < 0 || index >= store.size) {
+        return null;
+      }
+      return Array.from(store.keys())[index] ?? null;
+    },
+    removeItem(key: string) {
+      store.delete(key);
+    },
+    setItem(key: string, value: string) {
+      store.set(String(key), String(value));
+    },
+  };
+};
+
+const ensureLocalStorage = () => {
+  const target = typeof window !== "undefined" ? window : globalThis;
+  const current = target.localStorage as Storage | undefined;
+
+  if (!current || typeof current.clear !== "function") {
+    const mock = createLocalStorageMock();
+    Object.defineProperty(target, "localStorage", {
+      value: mock,
+      configurable: true,
+      enumerable: true,
+      writable: false,
+    });
+    if (target !== globalThis) {
+      Object.defineProperty(globalThis, "localStorage", {
+        value: mock,
+        configurable: true,
+        enumerable: true,
+        writable: false,
+      });
+    }
+  }
+};
+
+ensureLocalStorage();