diff --git a/.changeset/9491-walkabledef-rest-null.md b/.changeset/9491-walkabledef-rest-null.md
new file mode 100644
index 0000000000..a3455ed458
--- /dev/null
+++ b/.changeset/9491-walkabledef-rest-null.md
@@ -0,0 +1,31 @@
+---
+'@object-ui/types': patch
+---
+
+Declare `WalkableDef.rest` as `z.ZodType | null` (objectui#9491).
+
+`packages/types/src/zod/node-derivation.ts` declares the def member set both zod walkers
+in this package read. It declared `rest?: z.ZodType` — i.e. `z.ZodType | undefined` — while
+zod 4 spells "this tuple has no rest element" as an OWN `rest` key holding `null`, minted by
+`const rest = hasRest ? _paramsOrRest : null` in its `tuple` factory.
+
+**This is the declaration, not a behaviour change.** Nothing here changes what either walker
+does with the value; the accept set of every exported schema is untouched. objectui#9088
+already repaired the one arm the inaccurate type misled — the `tuple` arm in
+`zod/imported-defaults.ts`, which normalised the absent case to `undefined` because the
+declared type said that was the absent case, and so rebuilt every rest-less tuple through a
+`===` comparison that could never match. This change corrects the type that licensed it, so
+the next arm written against it is told the truth and `tsc` agrees with the truth instead of
+with the mistake.
+
+One read needed adjusting, contrary to the expectation the card recorded: the local
+`unchanged` helper in `zod/imported-defaults.ts` declares its comparison pairs
+`z.ZodType | undefined`, and the objectui#9088 repair hands it `def.rest` RAW — comparing
+like with like is the whole of that repair. Its parameter now admits `null` too. The
+comparison itself is still `===`: `null` matches only `null`, `undefined` only `undefined`.
+
+`WalkableDef` is emitted into the published `dist/` but is reachable through no entry in the
+package `exports` map, so no consumer outside this package can name the widened member; the
+new pin `types/src/__tests__/walkable-def-null-mint-9491.test.ts` re-derives against the
+INSTALLED zod that `rest` is minted `null` and that no other member the walkers read ever is,
+so a zod bump that moves either half goes red here.
diff --git a/packages/types/src/__tests__/walkable-def-null-mint-9491.test.ts b/packages/types/src/__tests__/walkable-def-null-mint-9491.test.ts
new file mode 100644
index 0000000000..85ec211a35
--- /dev/null
+++ b/packages/types/src/__tests__/walkable-def-null-mint-9491.test.ts
@@ -0,0 +1,192 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * THE DECLARATION ADMITS WHAT ZOD ACTUALLY MINTS — AND NOTHING MORE (objectui#9491).
+ *
+ * `../zod/node-derivation.ts` declares `WalkableDef`, the def member set both
+ * walkers in this package read. Every member but one is declared
+ * `z.ZodType | undefined`, and for every member but one that is the truth. The
+ * exception is `rest`: zod 4 spells "this tuple has no rest element" as an OWN
+ * `rest` key holding `null`.
+ *
+ * ## Why a test rather than a comment
+ *
+ * The declaration used to say `rest?: z.ZodType`, and that is not a cosmetic
+ * drift — it is what licensed objectui#9088. The `tuple` arm in
+ * `../zod/imported-defaults.ts` normalised the absent case to `undefined`
+ * because the declared type said `undefined` was the absent case; `unchanged`
+ * compares by `===`; `null === undefined` is false; and so EVERY rest-less
+ * tuple was rebuilt. The author read the type, the type was wrong, and `tsc`
+ * agreed with them. A wrong type is more expensive than a wrong implementation
+ * because it makes the next author do every step right and still be wrong.
+ *
+ * ⇒ what has to stay true is an AGREEMENT between two things that move
+ * independently: a hand-written declaration in this repository, and a value
+ * minted inside an installed dependency. Prose cannot hold an agreement like
+ * that; only something that re-derives both halves can. So this file probes the
+ * INSTALLED zod rather than restating a version-stamped reading of it — a zod
+ * bump that moves either half reddens here instead of being rediscovered from
+ * a rebuilt schema months later.
+ *
+ * ## Both halves, because "only `rest`" is load-bearing
+ *
+ *  1. **`rest` IS minted `null`** — subject, plus the rest-BEARING control that
+ *     fires the other way, so a green cannot come from the probe reading
+ *     nothing.
+ *  2. **⭐ No other member is** — swept across the node kinds the walkers meet.
+ *     Without this half, the honest repair for `rest` reads like a licence to
+ *     spell `| null` on any member that looks similar, which would declare
+ *     something zod does not do and reopen the same class from the other side.
+ *  3. **The declaration agrees with 1 and 2** — pinned through `tsc` (this
+ *     package's `type-check` compiles `src/**\/*.test.ts` via
+ *     `tsconfig.test.json`), so narrowing `rest` back, or widening a sibling to
+ *     match it, fails to COMPILE rather than failing quietly.
+ *
+ * ⛔ Nothing here asserts walker BEHAVIOUR: the rest-less-tuple identity
+ * property and the rebuild half are `imported-defaults-rest-less-tuple-9088.test.ts`'s
+ * subject and stay there. This file is about the type telling the truth.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { z } from 'zod';
+import { internals, type WalkableDef } from '../zod/node-derivation.js';
+
+/**
+ * Does `T` admit `null`? Resolved by the compiler; the runtime assertions below
+ * exist so the answer is also READ — an unused type alias proves nothing and a
+ * lint rule would be right to delete it.
+ */
+type AdmitsNull<T> = null extends T ? true : false;
+
+/** Every member of `WalkableDef` — i.e. the whole surface the walkers read. */
+const WALKABLE_MEMBERS = [
+  'type', 'shape', 'options', 'items', 'element', 'rest', 'valueType', 'keyType',
+  'left', 'right', 'in', 'out', 'innerType', 'catchall', 'getter',
+] as const;
+
+const defOf = (node: z.ZodType): WalkableDef => internals(node)._zod.def;
+
+/**
+ * The node kinds these walkers can meet, each built the way a schema author
+ * would build it. ⚠️ The sweep below is only as wide as this matrix — so a node
+ * kind added to either walker's `switch` belongs here on the same change.
+ */
+const leaf = z.number();
+const MATRIX: Record<string, z.ZodType> = {
+  'tuple (rest-less)': z.tuple([z.number(), z.number()]),
+  'tuple (empty)': z.tuple([]),
+  'tuple (with rest)': z.tuple([z.number()], z.string()),
+  object: z.object({ a: leaf }),
+  'object (strict)': z.object({ a: leaf }).strict(),
+  'object (loose)': z.object({ a: leaf }).loose(),
+  union: z.union([z.number(), z.string()]),
+  discriminatedUnion: z.discriminatedUnion('type', [
+    z.object({ type: z.literal('a') }),
+    z.object({ type: z.literal('b') }),
+  ]),
+  array: z.array(leaf),
+  record: z.record(z.string(), leaf),
+  map: z.map(z.string(), leaf),
+  set: z.set(leaf),
+  intersection: z.intersection(z.object({ a: leaf }), z.object({ b: leaf })),
+  'pipe (transform)': z.string().transform((s) => s.length),
+  'pipe (preprocess)': z.preprocess((v) => v, z.string()),
+  lazy: z.lazy(() => leaf),
+  optional: z.optional(leaf),
+  nullable: z.nullable(leaf),
+  default: leaf.default(1),
+  prefault: leaf.prefault(1),
+  catch: leaf.catch(0),
+  readonly: z.object({ a: leaf }).readonly(),
+  nonoptional: z.optional(leaf).nonoptional(),
+  string: z.string(),
+  literal: z.literal('a'),
+  enum: z.enum(['a', 'b']),
+  never: z.never(),
+};
+
+/** `[member, node label]` for every walkable member observed holding `null`. */
+const nullMints = (): [string, string][] => {
+  const hits: [string, string][] = [];
+  for (const [label, node] of Object.entries(MATRIX)) {
+    const def = defOf(node) as unknown as Record<string, unknown>;
+    for (const member of WALKABLE_MEMBERS) {
+      if (Object.hasOwn(def, member) && def[member] === null) hits.push([member, label]);
+    }
+  }
+  return hits;
+};
+
+describe('zod mints `null` into exactly one walkable def member (objectui#9491)', () => {
+  it('⭐ a rest-less tuple carries an OWN `rest` key holding `null`', () => {
+    const def = defOf(z.tuple([z.number(), z.number()])) as unknown as Record<string, unknown>;
+
+    // Three separate claims, and the fix rests on all three. An absent key, or
+    // an `undefined` value, would each make `: undefined` the correct spelling
+    // in the `tuple` arm again — and each would arrive without a symptom.
+    expect(Object.hasOwn(def, 'rest')).toBe(true);
+    expect(def.rest).toBeNull();
+    expect(def.rest).not.toBeUndefined();
+
+    // PROVING REMOVAL: declare `rest` absent-as-undefined here (`def.rest`
+    // asserted `toBeUndefined`) and this reddens on the installed zod.
+  });
+
+  it('the control fires the other way: a tuple WITH a rest element holds a node there', () => {
+    const def = defOf(z.tuple([z.number()], z.string()));
+
+    expect(def.rest).not.toBeNull();
+    expect(def.rest && '_zod' in def.rest).toBe(true);
+  });
+
+  it('⭐ no OTHER member the walkers read is ever minted `null`', () => {
+    const members = [...new Set(nullMints().map(([member]) => member))].sort();
+
+    // The uniqueness claim, as a set rather than a count — a count says nothing
+    // about WHICH member moved, and which member moved is the entire question.
+    expect(members).toEqual(['rest']);
+
+    // ⇒ if a later zod mints `null` into another member, this is the assertion
+    // that reddens, and the repair is to widen THAT member in `WalkableDef` and
+    // pin it below — ⛔ not to relax this expectation.
+    //
+    // PROVING REMOVAL: add `element` to the expected set and this reddens,
+    // which is what says the sweep is reading the matrix rather than a
+    // hard-coded answer.
+  });
+
+  it('⭐ every member observed holding `null` is minted by a tuple, not by a wrapper', () => {
+    // Guards the reading above against a matrix that happens to contain only
+    // tuples: the labels are carried so the source of each hit is named.
+    const labels = [...new Set(nullMints().map(([, label]) => label))].sort();
+
+    expect(labels).toEqual(['tuple (empty)', 'tuple (rest-less)']);
+  });
+});
+
+describe('`WalkableDef` agrees with that mint, and the agreement is compiled (objectui#9491)', () => {
+  it('⭐ `rest` admits `null`', () => {
+    // ⛔ Not a restatement of the line in `node-derivation.ts`: `tsc` resolves
+    // `AdmitsNull` from the declaration itself, so narrowing `rest` back to
+    // `z.ZodType | undefined` makes this a type ERROR — the package's
+    // `type-check` compiles this file, so the error is a gate and not a
+    // suggestion. That compile failure is the real pin; the `expect` is here so
+    // the value is read.
+    const restAdmitsNull: AdmitsNull<WalkableDef['rest']> = true;
+
+    expect(restAdmitsNull).toBe(true);
+  });
+
+  it('⭐ the siblings do NOT admit `null`, which is the half that keeps this honest', () => {
+    // `out` is named first because it is the near miss: a pipe's `out` holds an
+    // opaque transform, it sits one arm away from `rest` in both walkers, and
+    // the sweep above measures it clean. Copying `| null` onto it would declare
+    // an absent case zod never produces, and every read guarding against it
+    // would be dead code no test could ever reach.
+    const outAdmitsNull: AdmitsNull<WalkableDef['out']> = false;
+    const elementAdmitsNull: AdmitsNull<WalkableDef['element']> = false;
+    const itemsAdmitsNull: AdmitsNull<WalkableDef['items']> = false;
+
+    expect([outAdmitsNull, elementAdmitsNull, itemsAdmitsNull]).toEqual([false, false, false]);
+  });
+});
diff --git a/packages/types/src/zod/imported-defaults.ts b/packages/types/src/zod/imported-defaults.ts
index 42765e66a0..2aab2660ab 100644
--- a/packages/types/src/zod/imported-defaults.ts
+++ b/packages/types/src/zod/imported-defaults.ts
@@ -158,9 +158,19 @@ const walk = (schema: z.ZodType): z.ZodType => {
    * the day `@objectstack/spec` adopts the same principle, with nothing to roll
    * back, and today it leaves every already-clean imported schema binding by
    * reference exactly as it was before batch #90.
+   *
+   * ⚠️ `| null` on the sides, because the `tuple` arm below hands this helper
+   * `def.rest` RAW — comparing like with like is the whole of objectui#9088's
+   * repair, and zod spells a rest-less tuple's `rest` as `null`. ⛔ NOT a
+   * relaxation of the comparison: it stays `===`, so `null` still matches only
+   * `null` and `undefined` still matches only `undefined`. Widening the
+   * parameter is what lets the arm keep passing the value zod actually minted
+   * instead of normalising it back into the objectui#9088 defect to satisfy a
+   * signature (objectui#9491).
    */
-  const unchanged = (children: readonly (readonly [z.ZodType | undefined, z.ZodType | undefined])[]): boolean =>
-    children.every(([before, after]) => before === after);
+  const unchanged = (
+    children: readonly (readonly [z.ZodType | null | undefined, z.ZodType | null | undefined])[],
+  ): boolean => children.every(([before, after]) => before === after);
 
   let out: z.ZodType;
   switch (def.type) {
@@ -237,10 +247,10 @@ const walk = (schema: z.ZodType): z.ZodType => {
     // `null == undefined` true for every arm at once and erase a real zod-4
     // spelling distinction another arm may come to depend on.
     //
-    // ⚠️ `WalkableDef.rest` is declared `z.ZodType | undefined`, which does not
-    // admit the `null` zod actually mints — that inaccurate declaration is what
-    // made `: undefined` look correct. The value flows through untyped here;
-    // widening the shared type is objectui#9491.
+    // ⚠️ `WalkableDef.rest` is declared `z.ZodType | null` (objectui#9491) —
+    // it admits the `null` zod actually mints, so the value below is typed as
+    // what it is. Until that landed the declaration said `z.ZodType |
+    // undefined`, and that is what made `: undefined` look correct here.
     //
     // ⭐ PROVING REMOVAL: put `: undefined` back and
     // `__tests__/imported-defaults-rest-less-tuple-9088.test.ts` reddens on
diff --git a/packages/types/src/zod/node-derivation.ts b/packages/types/src/zod/node-derivation.ts
index 610c23e05c..7ddf40c50b 100644
--- a/packages/types/src/zod/node-derivation.ts
+++ b/packages/types/src/zod/node-derivation.ts
@@ -71,7 +71,30 @@ export interface WalkableDef {
   options?: z.ZodType[];
   items?: z.ZodType[];
   element?: z.ZodType;
-  rest?: z.ZodType;
+  /**
+   * ⭐ `| null`, and the `| null` is not decoration (objectui#9491).
+   *
+   * Zod 4 spells "this tuple has no rest element" as an OWN `rest` key holding
+   * `null`, minted by `const rest = hasRest ? _paramsOrRest : null` in its
+   * `tuple` factory — ⛔ not as an absent key, which is what `rest?: z.ZodType`
+   * alone claims. The inaccurate declaration is what licensed objectui#9088:
+   * the `tuple` arm in `./imported-defaults.ts` normalised the absent case to
+   * `undefined` because the type said that was the absent case, `unchanged`
+   * compares by `===`, and so every rest-less tuple was rebuilt. The author
+   * read the type, the type was wrong, and `tsc` agreed with them.
+   *
+   * ⛔ This is NOT a wider accept set: nothing here changes what the walkers
+   * do with the value. It admits the value zod was already putting there, so
+   * the next arm written against this type is told the truth.
+   *
+   * ⚠️ `rest` is the ONLY member of this interface with that property, and
+   * "only" is the load-bearing half — a sibling arm that copies this `| null`
+   * onto `out` or `element` would be declaring something zod does not do.
+   * `../__tests__/walkable-def-null-mint-9491.test.ts` re-derives BOTH halves
+   * against the installed zod — the `null` here, and its absence everywhere
+   * else — so a zod bump that moves either one goes red rather than quiet.
+   */
+  rest?: z.ZodType | null;
   valueType?: z.ZodType;
   keyType?: z.ZodType;
   left?: z.ZodType;