diff --git a/src/lib/util/persist.svelte.test.ts b/src/lib/util/persist.svelte.test.ts
new file mode 100644
index 00000000..3776bb54
--- /dev/null
+++ b/src/lib/util/persist.svelte.test.ts
@@ -0,0 +1,54 @@
+import { beforeEach, describe, expect, it } from 'vitest';
+import { persisted, readJSON, writeJSON } from './persist.svelte';
+
+beforeEach(() => {
+  window.localStorage.clear();
+});
+
+describe('readJSON', () => {
+  it('returns the fallback when the key is missing', () => {
+    expect(readJSON('missing', 'fallback')).toBe('fallback');
+  });
+
+  it('returns the parsed value when present', () => {
+    window.localStorage.setItem('key', '{"a":1}');
+    expect(readJSON<{ a: number }>('key', { a: 0 })).toEqual({ a: 1 });
+  });
+
+  it('returns the fallback when the stored value is corrupt', () => {
+    window.localStorage.setItem('corrupt', '{oops');
+    expect(readJSON('corrupt', 'fallback')).toBe('fallback');
+    // The previous persistence layer could write the literal string "undefined".
+    window.localStorage.setItem('legacy', 'undefined');
+    expect(readJSON('legacy', 'fallback')).toBe('fallback');
+  });
+});
+
+describe('writeJSON', () => {
+  it('round-trips values through localStorage as JSON', () => {
+    writeJSON('key', { nested: { value: 2 } });
+    expect(window.localStorage.getItem('key')).toBe('{"nested":{"value":2}}');
+    expect(readJSON('key', {})).toEqual({ nested: { value: 2 } });
+  });
+});
+
+describe('persisted', () => {
+  it('initialises from storage when a value exists', () => {
+    window.localStorage.setItem('counter', '5');
+    const counter = persisted('counter', 0);
+    expect(counter.value).toBe(5);
+  });
+
+  it('uses the initial value when storage is empty, without writing it', () => {
+    const counter = persisted('counter', 7);
+    expect(counter.value).toBe(7);
+    expect(window.localStorage.getItem('counter')).toBeNull();
+  });
+
+  it('persists on assignment and exposes the new value', () => {
+    const counter = persisted('counter', 0);
+    counter.value = 42;
+    expect(counter.value).toBe(42);
+    expect(window.localStorage.getItem('counter')).toBe('42');
+  });
+});
diff --git a/src/lib/util/persist.svelte.ts b/src/lib/util/persist.svelte.ts
new file mode 100644
index 00000000..5204af25
--- /dev/null
+++ b/src/lib/util/persist.svelte.ts
@@ -0,0 +1,46 @@
+/**
+ * Runes-based localStorage persistence, shared by the persisted state in
+ * `state.svelte.ts`, `migrations.svelte.ts`, `promo.svelte.ts` and History.
+ *
+ * Values are stored as plain JSON. Reads of missing or corrupt values fall
+ * back to the provided default, so values written by older versions of the
+ * editor (which serialized plain objects to the same JSON shape) stay loadable.
+ */
+
+const hasStorage = (): boolean => typeof window !== 'undefined' && !!window.localStorage;
+
+export const readJSON = <T>(key: string, fallback: T): T => {
+  if (!hasStorage()) {
+    return fallback;
+  }
+  try {
+    const raw = window.localStorage.getItem(key);
+    return raw === null ? fallback : (JSON.parse(raw) as T);
+  } catch {
+    return fallback;
+  }
+};
+
+export const writeJSON = (key: string, value: unknown): void => {
+  if (hasStorage()) {
+    window.localStorage.setItem(key, JSON.stringify(value));
+  }
+};
+
+export interface Persisted<T> {
+  value: T;
+}
+
+// A localStorage-backed reactive value. Reads on init, writes on every set.
+export const persisted = <T>(key: string, initial: T): Persisted<T> => {
+  let value = $state<T>(readJSON(key, initial));
+  return {
+    get value() {
+      return value;
+    },
+    set value(next: T) {
+      value = next;
+      writeJSON(key, next);
+    }
+  };
+};