diff --git a/web/src/lib/arraypolicy.ts b/web/src/lib/arraypolicy.ts
new file mode 100644
index 0000000..23b614a
--- /dev/null
+++ b/web/src/lib/arraypolicy.ts
@@ -0,0 +1,54 @@
+// Pure helpers for array-valued local state: parse the declared initial value
+// and enforce the declared sizing policy (dynamic / capped / fixed). Shared by
+// localstate.ts (write path) and logic.ts (node handlers + migration).
+
+import type { StateVar } from './types';
+
+export type ArrVal = number | ArrVal[];
+export const ARRAY_MAX = 1_000_000;
+
+// zeroFill builds a length-n array of zeros (flat; fixed nested init must come
+// from an explicit `initial` literal).
+function zeroFill(n: number): ArrVal[] {
+  return new Array(Math.max(0, n)).fill(0);
+}
+
+// parseInitialArray returns the starting contents of an array local.
+export function parseInitialArray(sv: StateVar): ArrVal[] {
+  const cap = sv.capacity ?? 0;
+  const raw = (sv.initial ?? '').trim();
+  let parsed: ArrVal[] | null = null;
+  if (raw) {
+    try {
+      const j = JSON.parse(raw);
+      if (Array.isArray(j)) parsed = j as ArrVal[];
+    } catch { parsed = null; }
+  }
+  if (sv.sizing === 'fixed') {
+    if (!parsed) return zeroFill(cap);
+    // truncate / zero-pad to capacity
+    const out = parsed.slice(0, cap);
+    while (out.length < cap) out.push(0);
+    return out;
+  }
+  return parsed ?? [];
+}
+
+// applySizing returns arr clamped to the declared policy.
+//   dynamic → unchanged (but globally capped at ARRAY_MAX, dropping oldest)
+//   capped  → keep at most capacity elements, dropping oldest (ring/FIFO)
+//   fixed   → exactly capacity elements (truncate / zero-pad); never grow/shrink
+export function applySizing(arr: ArrVal[], sv: StateVar): ArrVal[] {
+  const cap = sv.capacity ?? 0;
+  switch (sv.sizing) {
+    case 'fixed': {
+      const out = arr.slice(0, cap);
+      while (out.length < cap) out.push(0);
+      return out;
+    }
+    case 'capped':
+      return arr.length > cap ? arr.slice(arr.length - cap) : arr;
+    default:
+      return arr.length > ARRAY_MAX ? arr.slice(arr.length - ARRAY_MAX) : arr;
+  }
+}