diff --git a/lib/storage/investigation-storage.js b/lib/storage/investigation-storage.js
new file mode 100644
index 0000000..67b6995
--- /dev/null
+++ b/lib/storage/investigation-storage.js
@@ -0,0 +1,5 @@
+// investigation-storage — generic persistence boundary
+// Exposes loadInvestigation / saveInvestigation / clearInvestigation
+// and delegates internally to the concrete localStorage provider.
+
+export { loadInvestigation, saveInvestigation, clearInvestigation } from "./providers/local-storage.js";
diff --git a/lib/storage/providers/local-storage.js b/lib/storage/providers/local-storage.js
new file mode 100644
index 0000000..c0a8163
--- /dev/null
+++ b/lib/storage/providers/local-storage.js
@@ -0,0 +1,111 @@
+// ── canonical identifiers ────────────────────────────────────────────
+
+const CANONICAL_KEY  = "confidence-engine-investigation";
+const LEGACY_KEY     = "confidence-engine-session";
+const SCHEMA_VERSION = 1;
+
+// ── helpers ──────────────────────────────────────────────────────────
+
+function safeGet(storage, key) {
+  if (!storage || typeof storage.getItem !== "function") return null;
+  try { return storage.getItem(key); } catch (_) { return null; }
+}
+
+function isPlainObject(value) {
+  return (
+    value !== null &&
+    typeof value === "object" &&
+    !Array.isArray(value) &&
+    Object.prototype.toString.call(value) === "[object Object]"
+  );
+}
+
+// ── loadInvestigation ────────────────────────────────────────────────
+
+/**
+ * Returns the persisted investigation snapshot (normalised to current schema)
+ * or null when no usable state exists.
+ */
+export function loadInvestigation() {
+  const storage = _getTargetStorage();
+  if (!storage) return null;
+
+  // 1 – canonical key first
+  let raw = safeGet(storage, CANONICAL_KEY);
+  if (raw !== null) {
+    try {
+      const parsed = JSON.parse(raw);
+      if (isPlainObject(parsed)) {
+        if (!("schemaVersion" in parsed)) parsed.schemaVersion = SCHEMA_VERSION;
+        return parsed;
+      }
+    } catch (_) { /* malformed — fall through to legacy */ }
+  }
+
+  // 2 – legacy fallback
+  const legacyStorage = _getLegacyStorage();
+  if (!legacyStorage) return null;
+
+  raw = safeGet(legacyStorage, LEGACY_KEY);
+  if (raw === null) return null;
+  let parsed;
+  try {
+    parsed = JSON.parse(raw);
+  } catch (_) { return null; }
+
+  if (!isPlainObject(parsed)) return null;
+
+  // Normalise schemaVersion for legacy data
+  if (!("schemaVersion" in parsed)) parsed.schemaVersion = SCHEMA_VERSION;
+
+  // Migrate into canonical key
+  _persist(storage, CANONICAL_KEY, JSON.stringify(parsed));
+
+  return parsed;
+}
+
+// ── saveInvestigation ────────────────────────────────────────────────
+
+/**
+ * Persists the supplied snapshot to the canonical localStorage key.
+ * The caller's object is never mutated.
+ */
+export function saveInvestigation(snapshot) {
+  const storage = _getTargetStorage();
+  if (!storage) return; // silently no-op in non-browser
+
+  try {
+    const record = JSON.parse(JSON.stringify(snapshot));
+    record.schemaVersion = SCHEMA_VERSION;
+    _persist(storage, CANONICAL_KEY, JSON.stringify(record));
+  } catch (_) { /* storage errors must not crash caller */ }
+}
+
+// ── clearInvestigation ───────────────────────────────────────────────
+
+/** Removes only the canonical investigation key. */
+export function clearInvestigation() {
+  const storage = _getTargetStorage();
+  if (!storage) return;
+  try { storage.removeItem(CANONICAL_KEY); } catch (_) {}
+}
+
+// ── internals ────────────────────────────────────────────────────────
+
+function _persist(storage, key, value) {
+  try { storage.setItem(key, value); } catch (_) {}
+}
+
+function _getTargetStorage() {
+  if (typeof globalThis.window === "undefined") return null;
+  const s = globalThis.window.localStorage;
+  if (!s || typeof s.getItem !== "function") return null;
+  return s;
+}
+
+function _getLegacyStorage() {
+  if (typeof globalThis.window === "undefined") return null;
+  const s = globalThis.window.sessionStorage;
+  if (!s || typeof s.getItem !== "function") return null;
+  return s;
+}
diff --git a/tests/storage/investigation-storage.test.js b/tests/storage/investigation-storage.test.js
new file mode 100644
index 0000000..e35a6a7
--- /dev/null
+++ b/tests/storage/investigation-storage.test.js
@@ -0,0 +1,228 @@
+/**
+ * Focused provider-level contract tests for investigation-storage.
+ *
+ * Deterministic in-memory mocks — no React, no Playwright, no LLM calls.
+ */
+
+import { describe, it, expect, vi } from "vitest";
+
+// ── test-time storage mock (injected into globalThis.window) ───────────
+
+class MockStorageMap {
+  constructor() { this._data = new Map(); }
+  getItem(k)   { return this._data.has(k) ? this._data.get(k) : null; }
+  setItem(k, v){ this._data.set(k, String(v)); }
+  removeItem(k){ this._data.delete(k); }
+  clear()      { this._data.clear(); }
+}
+
+function installMockStorage() {
+  // Define window with mock localStorage and sessionStorage as properties
+  Object.defineProperty(globalThis, "window", {
+    value: globalThis.window || {},
+    writable: true,
+    configurable: true,
+  });
+  if (!globalThis.window.localStorage) {
+    Object.defineProperty(globalThis.window, "localStorage", {
+      value: new MockStorageMap(),
+      writable: true,
+      configurable: true,
+    });
+  }
+  if (!globalThis.window.sessionStorage) {
+    Object.defineProperty(globalThis.window, "sessionStorage", {
+      value: new MockStorageMap(),
+      writable: true,
+      configurable: true,
+    });
+  }
+}
+
+function uninstallMockStorage() {
+  const desc = Object.getOwnPropertyDescriptor(globalThis, "window");
+  if (desc && !("localStorage" in globalThis.window)) return;
+  delete globalThis.window.localStorage;
+  delete globalThis.window.sessionStorage;
+  if (Object.getOwnPropertyNames(globalThis.window).length === 0) {
+    delete globalThis.window;
+  }
+}
+
+// ── lifecycle ──────────────────────────────────────────────────────────
+
+beforeEach(() => { installMockStorage(); });
+afterEach  (() => { uninstallMockStorage(); });
+
+// ── import under test (fresh module each time) ────────────────────────
+
+async function getModule() {
+  // Clear all cached modules to get fresh bindings per test
+  vi.resetModules();
+  const m = await import("../../lib/storage/providers/local-storage.js");
+  return { load: m.loadInvestigation, save: m.saveInvestigation, clear: m.clearInvestigation };
+}
+
+// ── shared snapshot factories ──────────────────────────────────────────
+
+function makeSnapshot(overrides = {}) {
+  return {
+    schemaVersion: overrides.schemaVersion ?? 1,
+    situationGraph: { nodes: [{ id: "n1", label: "A" }], edges: [] },
+    openQuestions: ["q1"],
+    assumptions: [],
+    observations: [],
+    uncertainties: [],
+    relationships: [],
+    messages: [],
+    ...overrides,
+  };
+}
+
+function legacySnapshot() {
+  return { situationGraph: { nodes: [{ id: "old" }], edges: [] }, legacy: true };
+}
+
+// ════════════════════════════════════════════════════════════════
+// Tests
+// ════════════════════════════════════════════════════════════════
+
+describe("investigation-storage provider", () => {
+
+  // ── 1. save → load round trip ───────────────────────────────────
+
+  it("save then load returns the same snapshot (round-trip)", async () => {
+    const { load, save } = await getModule();
+    const snap = makeSnapshot({ situationGraph: { nodes: [{ id: "x1" }], edges: [] }, openQuestions: ["why?"] });
+
+    save(snap);
+    const loaded = load();
+
+    expect(loaded).not.toBeNull();
+    expect(loaded.situationGraph.nodes[0].id).toBe("x1");
+    expect(loaded.openQuestions).toEqual(["why?"]);
+  });
+
+  // ── 2. schemaVersion = 1 persisted ──────────────────────────────
+
+  it("persisted record includes schemaVersion = 1", async () => {
+    const { load, save } = await getModule();
+    const snap = makeSnapshot({ schemaVersion: undefined });
+    save(snap);
+    const loaded = load();
+    expect(loaded.schemaVersion).toBe(1);
+  });
+
+  // ── 3. caller snapshot object not mutated ───────────────────────
+
+  it("save does not mutate the caller's snapshot object", async () => {
+    const { save } = await getModule();
+    const snap = { nodes: [{ id: "a" }] };
+    const before = JSON.stringify(snap);
+    save(snap);
+    expect(JSON.stringify(snap)).toBe(before);
+  });
+
+  // ── 4. clear → load returns null ────────────────────────────────
+
+  it("clear then load returns null", async () => {
+    const { load, save, clear } = await getModule();
+    save(makeSnapshot());
+    expect(load()).not.toBeNull();
+    clear();
+    expect(load()).toBeNull();
+  });
+
+  // ── 5. malformed localStorage → null, no throw ─────────────────
+
+  it("malformed localStorage returns null without throwing", async () => {
+    const { load } = await getModule();
+    globalThis.window.localStorage.setItem(
+      "confidence-engine-investigation",
+      "not valid json {{{"
+    );
+    expect(() => load()).not.toThrow();
+    expect(load()).toBeNull();
+  });
+
+  // ── 6. localStorage takes precedence over sessionStorage ────────
+
+  it("localStorage value used even when legacy sessionStorage exists", async () => {
+    const { load, save } = await getModule();
+    const localSnap   = makeSnapshot({ source: "local" });
+    const sessionSnap = legacySnapshot(); // no schemaVersion
+
+    save(localSnap);
+    globalThis.window.sessionStorage.setItem(
+      "confidence-engine-session",
+      JSON.stringify(sessionSnap)
+    );
+
+    const loaded = load();
+    expect(loaded).not.toBeNull();
+    expect(loaded.source).toBe("local");       // from localStorage, not sessionStorage
+  });
+
+  // ── 7. empty localStorage + valid legacy sessionStorage → legacy returned ──
+
+  it("empty canonical key returns legacy snapshot when no local state", async () => {
+    const { load, save } = await getModule();
+    save(makeSnapshot({ source: "local" }));     // populate a value
+    globalThis.window.localStorage.clear();      // remove it (canonical key gone)
+    globalThis.window.sessionStorage.setItem(
+      "confidence-engine-session",
+      JSON.stringify(legacySnapshot())
+    );
+
+    const loaded = load();
+    expect(loaded).not.toBeNull();
+    expect(loaded.legacy).toBe(true);            // came from sessionStorage
+  });
+
+  // ── 8. legacy snapshot copied into localStorage ─────────────────
+
+  it("legacy migration copies snapshot into canonical key", async () => {
+    const { load } = await getModule();
+    globalThis.window.localStorage.clear();
+    globalThis.window.sessionStorage.setItem(
+      "confidence-engine-session",
+      JSON.stringify(legacySnapshot())
+    );
+
+    load(); // triggers migration
+
+    const migrated = globalThis.window.localStorage.getItem("confidence-engine-investigation");
+    expect(migrated).not.toBeNull();
+    expect(JSON.parse(migrated).legacy).toBe(true);
+    expect(JSON.parse(migrated).schemaVersion).toBe(1);
+  });
+
+  // ── 9. unrelated keys untouched ─────────────────────────────────
+
+  it("unrelated storage keys remain untouched", async () => {
+    const { load, save } = await getModule();
+    globalThis.window.localStorage.setItem("unrelated-key", '{"data":42}');
+    globalThis.window.sessionStorage.setItem("unrelated-session", "hello");
+
+    save(makeSnapshot());
+    load();
+
+    expect(globalThis.window.localStorage.getItem("unrelated-key")).toBe('{"data":42}');
+    expect(globalThis.window.sessionStorage.getItem("unrelated-session")).toBe("hello");
+  });
+
+  // ── 10. non-browser environment safely returns null / no-op ─────
+
+  it("non-browser environment returns null load, safe no-op save/clear", async () => {
+    uninstallMockStorage(); // removes window entirely
+
+    const { load, save, clear } = await getModule();
+
+    expect(load()).toBeNull();       // no browser → null
+    expect(() => save(makeSnapshot())).not.toThrow();  // no-op without crash
+    expect(() => clear()).not.toThrow();                 // no-op without crash
+
+    installMockStorage();             // restore for remaining tests
+  });
+
+});