diff --git a/periscope/src/frontend/src/hooks/use-reviewed-count.ts b/periscope/src/frontend/src/hooks/use-reviewed-count.ts
new file mode 100644
index 0000000..289dfba
--- /dev/null
+++ b/periscope/src/frontend/src/hooks/use-reviewed-count.ts
@@ -0,0 +1,33 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import {
+  legacyReviewedFindingsKey,
+  migrateLocalKey,
+  reviewedFindingsKey,
+} from "@/lib/storage-keys";
+
+/** Dashboard-only: length of the reviewed-finding id list in localStorage. */
+export function useReviewedCount(projectId: string): number {
+  const [count, setCount] = useState(0);
+
+  useEffect(() => {
+    try {
+      const raw = migrateLocalKey(
+        reviewedFindingsKey(projectId),
+        legacyReviewedFindingsKey(projectId),
+      );
+      if (!raw) {
+        setCount(0);
+        return;
+      }
+      const parsed: unknown = JSON.parse(raw);
+      setCount(Array.isArray(parsed) ? parsed.length : 0);
+    } catch (err) {
+      console.warn("useReviewedCount: could not read reviewed ids", err);
+      setCount(0);
+    }
+  }, [projectId]);
+
+  return count;
+}
diff --git a/periscope/src/frontend/src/hooks/use-reviewed-findings.ts b/periscope/src/frontend/src/hooks/use-reviewed-findings.ts
new file mode 100644
index 0000000..32963f9
--- /dev/null
+++ b/periscope/src/frontend/src/hooks/use-reviewed-findings.ts
@@ -0,0 +1,79 @@
+"use client";
+
+import { useCallback, useEffect, useMemo, useRef, useState } from "react";
+import type { Finding } from "@/lib/types";
+import { getFindingKey } from "@/lib/utils";
+import {
+  legacyReviewedFindingsKey,
+  migrateLocalKey,
+  reviewedFindingsKey,
+} from "@/lib/storage-keys";
+
+function readStoredIds(projectId: string): Set<string> {
+  try {
+    const raw = migrateLocalKey(
+      reviewedFindingsKey(projectId),
+      legacyReviewedFindingsKey(projectId),
+    );
+    if (!raw) return new Set();
+    const parsed: unknown = JSON.parse(raw);
+    if (!Array.isArray(parsed)) return new Set();
+    return new Set(parsed.filter((id): id is string => typeof id === "string"));
+  } catch (err) {
+    console.warn("useReviewedFindings: could not read reviewed ids", err);
+    return new Set();
+  }
+}
+
+export function useReviewedFindings(projectId: string, findings: Finding[]) {
+  const liveKeys = useMemo(() => {
+    const keys = new Set<string>();
+    findings.forEach((finding, index) => keys.add(getFindingKey(finding, index)));
+    return keys;
+  }, [findings]);
+
+  const [reviewedIds, setReviewedIds] = useState<Set<string>>(() => {
+    if (typeof window === "undefined") return new Set();
+    return readStoredIds(projectId);
+  });
+
+  const skipFirstWrite = useRef(true);
+  useEffect(() => {
+    if (skipFirstWrite.current) {
+      skipFirstWrite.current = false;
+      return;
+    }
+    localStorage.setItem(reviewedFindingsKey(projectId), JSON.stringify([...reviewedIds]));
+  }, [projectId, reviewedIds]);
+
+  useEffect(() => {
+    if (findings.length === 0) return;
+    setReviewedIds((prev) => {
+      let dropped = false;
+      const next = new Set<string>();
+      for (const key of prev) {
+        if (liveKeys.has(key)) next.add(key);
+        else dropped = true;
+      }
+      return dropped ? next : prev;
+    });
+  }, [findings.length, liveKeys]);
+
+  const toggleReviewed = useCallback((key: string) => {
+    setReviewedIds((prev) => {
+      const next = new Set(prev);
+      if (next.has(key)) next.delete(key);
+      else next.add(key);
+      return next;
+    });
+  }, []);
+
+  const isReviewed = useCallback((key: string) => reviewedIds.has(key), [reviewedIds]);
+
+  return {
+    reviewedIds,
+    toggleReviewed,
+    isReviewed,
+    reviewedCount: reviewedIds.size,
+  };
+}
diff --git a/tests/test_periscope_frontend_reviewed_hooks_rewrite.py b/tests/test_periscope_frontend_reviewed_hooks_rewrite.py
new file mode 100644
index 0000000..d28da00
--- /dev/null
+++ b/tests/test_periscope_frontend_reviewed_hooks_rewrite.py
@@ -0,0 +1,33 @@
+"""Reviewed-finding localStorage hooks live under periscope/src."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+SRC = ROOT / "periscope" / "src" / "frontend" / "src"
+
+
+def test_reviewed_hooks_are_src():
+    for rel in ("hooks/use-reviewed-count.ts", "hooks/use-reviewed-findings.ts"):
+        text = (SRC / rel).read_text(encoding="utf-8")
+        assert "Native Periscope overlay" not in text[:400]
+
+
+def test_reviewed_count_reads_migrated_key():
+    text = (SRC / "hooks/use-reviewed-count.ts").read_text(encoding="utf-8")
+    assert "export function useReviewedCount" in text
+    assert "reviewedFindingsKey" in text
+    assert "legacyReviewedFindingsKey" in text
+    assert "migrateLocalKey" in text
+
+
+def test_reviewed_findings_return_shape():
+    text = (SRC / "hooks/use-reviewed-findings.ts").read_text(encoding="utf-8")
+    assert "export function useReviewedFindings" in text
+    assert "toggleReviewed" in text
+    assert "isReviewed" in text
+    assert "reviewedCount" in text
+    assert "reviewedIds" in text
+    assert "getFindingKey" in text
+    assert "skipFirstWrite" in text or "hydrated" in text