diff --git a/apps/e2e/demo-e2e-bundle-size/e2e/browser-bundle-size.e2e.spec.ts b/apps/e2e/demo-e2e-bundle-size/e2e/browser-bundle-size.e2e.spec.ts
index a0604c357..e4bc6c730 100644
--- a/apps/e2e/demo-e2e-bundle-size/e2e/browser-bundle-size.e2e.spec.ts
+++ b/apps/e2e/demo-e2e-bundle-size/e2e/browser-bundle-size.e2e.spec.ts
@@ -25,10 +25,10 @@ import {
 
 const BUDGETS: Array<{ entry: string; label: string; maxBytes: number }> = [
   { entry: 'create-tool.ts', label: 'create() + one tool', maxBytes: 1_340_056 },
-  { entry: 'create-webmcp-tool.ts', label: 'create() + WebMcpPlugin + one tool', maxBytes: 1_346_921 },
+  { entry: 'create-webmcp-tool.ts', label: 'create() + WebMcpPlugin + one tool', maxBytes: 1_347_378 },
 ];
 
-const VITE_INITIAL_CHUNK_MAX_BYTES = 964_390;
+const VITE_INITIAL_CHUNK_MAX_BYTES = 964_827;
 
 // Packages create() never needs: the JavaScript parser behind @enclave-vm/ast (#885) and the MCP client (#886)
 const NEVER_BUNDLED: Record<string, RegExp> = {
diff --git a/libs/lazy-zod/src/__tests__/internal-z.spec.ts b/libs/lazy-zod/src/__tests__/internal-z.spec.ts
index e4d061e01..76930a1b7 100644
--- a/libs/lazy-zod/src/__tests__/internal-z.spec.ts
+++ b/libs/lazy-zod/src/__tests__/internal-z.spec.ts
@@ -472,4 +472,57 @@ describe('internalZ', () => {
     const typed: internalZ.ZodType<Parsed> = schema;
     expect(typed.parse(value)).toEqual({ a: 'x' });
   });
+
+  it('makes no function per stand-in: two alike stand-ins hold the same functions', () => {
+    const functionsOf = (schema: unknown) =>
+      Object.values(targetOf(schema)).filter((value) => typeof value === 'function');
+    const builds: Array<() => unknown> = [
+      () => internalZ.string(),
+      () => internalZ.object({ a: internalZ.number() }),
+      () => internalZ.string().min(1).optional(),
+      () => internalZ.union([internalZ.string(), internalZ.number()]).optional(),
+    ];
+
+    for (const build of builds) {
+      const first = functionsOf(build());
+      expect(first.length).toBeGreaterThan(0);
+      expect(functionsOf(build())).toEqual(first);
+    }
+  });
+
+  it('lets a built stand-in go of the arguments it was made with', () => {
+    const shape = { name: internalZ.string() };
+    const options = [internalZ.literal('a'), internalZ.literal('b')];
+    const object = internalZ.object(shape);
+    const union = internalZ.union(options).optional();
+
+    object.parse({ name: 'x' });
+    union.parse('a');
+
+    for (const target of [object, union].map(targetOf)) {
+      const held = Object.values(target).flatMap((value) => (Array.isArray(value) ? value : [value]));
+      expect(held).not.toContain(shape);
+      expect(held).not.toContain(options);
+    }
+  });
+
+  it('lets a built chained stand-in go of the stand-in it was chained on', () => {
+    const parent = internalZ.object({ a: internalZ.string() });
+    const extended = parent.extend({ b: internalZ.number() });
+
+    extended.parse({ a: 'x', b: 1 });
+
+    expect(Object.values(targetOf(extended))).not.toContain(targetOf(parent));
+  });
+
+  it('builds from the same arguments when a build that threw is tried again', () => {
+    const shape = { a: internalZ.string() };
+    const conflicting = { message: 'm', error: 'e' };
+    const object = internalZ.object(shape, conflicting);
+    const optional = internalZ.object(shape, conflicting).optional();
+
+    for (const schema of [object, optional, object, optional]) {
+      expect(() => schema.parse({ a: 1 })).toThrow('Cannot specify both `message` and `error` params');
+    }
+  });
 });
diff --git a/libs/lazy-zod/src/internal-z.ts b/libs/lazy-zod/src/internal-z.ts
index fad0779ba..0ad04ff86 100644
--- a/libs/lazy-zod/src/internal-z.ts
+++ b/libs/lazy-zod/src/internal-z.ts
@@ -2,37 +2,76 @@ import { z as realZ, type ZodTypeAny } from 'zod';
 
 import { HEAVY_FACTORIES, LEAF_FACTORIES } from './factories';
 import { LAZY_TARGET, LazyZodSchema, wrapLazy } from './lazy-schema';
-import { createLazyZ, type StandInFactory, type z } from './lazy-z';
+import { createLazyZ, type SchemaFactory, type StandInFactory, type z } from './lazy-z';
 import { withoutRecursionWalk } from './memoizer';
 import { isLazy } from './utils';
 
 const BUILT_WITH_PARENT: unique symbol = Symbol('builtWithParent');
 
-class InternalLazySchema extends LazyZodSchema {
+const NO_ARGS: readonly unknown[] = Object.freeze([]);
+
+/** A stand-in keeps what it is built from in fields: a closure per stand-in would cost more than the stand-in. */
+abstract class InternalLazySchema extends LazyZodSchema {
   readonly [BUILT_WITH_PARENT]: boolean;
+  private args: readonly unknown[];
 
-  constructor(factory: () => ZodTypeAny, builtWithParent: boolean) {
-    super(factory);
+  protected constructor(args: readonly unknown[], builtWithParent: boolean) {
+    super(InternalLazySchema.buildOnce);
+    this.args = args;
     this[BUILT_WITH_PARENT] = builtWithParent;
   }
 
+  private static buildOnce(this: InternalLazySchema): ZodTypeAny {
+    const built = this.buildWith(this.args);
+    this.args = NO_ARGS;
+    return built;
+  }
+
+  protected abstract buildWith(args: readonly unknown[]): ZodTypeAny;
+
   protected override build(factory: () => ZodTypeAny): ZodTypeAny {
-    return withoutRecursionWalk(factory);
+    return withoutRecursionWalk(() => factory.call(this));
   }
 
   override chain(method: string, args: unknown[]): unknown {
-    return internalStandIn(
-      () => {
-        const real = this.materialize() as unknown as Record<string, (...a: unknown[]) => ZodTypeAny>;
-        return real[method](...args.map(withBuiltChildren));
-      },
-      this[BUILT_WITH_PARENT] && args.every(holdsOnlyLeaves),
-    );
+    return wrapLazy(new ChainedStandIn(this, method, args, this[BUILT_WITH_PARENT] && args.every(holdsOnlyLeaves)));
+  }
+}
+
+class FactoryStandIn extends InternalLazySchema {
+  constructor(
+    private readonly factory: SchemaFactory,
+    private readonly namespace: object,
+    args: readonly unknown[],
+    builtWithParent: boolean,
+  ) {
+    super(args, builtWithParent);
+  }
+
+  protected buildWith(args: readonly unknown[]): ZodTypeAny {
+    return this.factory.apply(this.namespace, args.map(withBuiltChildren));
   }
 }
 
-function internalStandIn(build: () => ZodTypeAny, builtWithParent: boolean): unknown {
-  return wrapLazy(new InternalLazySchema(build, builtWithParent));
+class ChainedStandIn extends InternalLazySchema {
+  private parent: InternalLazySchema | undefined;
+
+  constructor(
+    parent: InternalLazySchema,
+    private readonly method: string,
+    args: readonly unknown[],
+    builtWithParent: boolean,
+  ) {
+    super(args, builtWithParent);
+    this.parent = parent;
+  }
+
+  protected buildWith(args: readonly unknown[]): ZodTypeAny {
+    const real = (this.parent as InternalLazySchema).materialize() as unknown as Record<string, SchemaFactory>;
+    const built = real[this.method](...args.map(withBuiltChildren));
+    this.parent = undefined;
+    return built;
+  }
 }
 
 function targetOf(value: unknown): LazyZodSchema | undefined {
@@ -85,8 +124,8 @@ function withBuiltChildren(arg: unknown): unknown {
   return changed ? Object.create(Object.getPrototypeOf(arg), descriptors) : arg;
 }
 
-const deferEveryFactory: StandInFactory = (name, args, build) =>
-  internalStandIn(() => build(args.map(withBuiltChildren)), LEAF_FACTORIES.has(name) && args.every(holdsOnlyLeaves));
+const deferEveryFactory: StandInFactory = (name, args, factory, namespace) =>
+  wrapLazy(new FactoryStandIn(factory, namespace, args, LEAF_FACTORIES.has(name) && args.every(holdsOnlyLeaves)));
 
 /** FrontMCP's own `z`: every factory returns a stand-in that builds its leaf children with it. Not public API. */
 export const internalZ: typeof realZ = createLazyZ(
diff --git a/libs/lazy-zod/src/lazy-z.ts b/libs/lazy-zod/src/lazy-z.ts
index 9268737bd..7abc7c317 100644
--- a/libs/lazy-zod/src/lazy-z.ts
+++ b/libs/lazy-zod/src/lazy-z.ts
@@ -19,9 +19,12 @@ import { LazyZodSchema, wrapLazy } from './lazy-schema';
  * target owns nothing, so no invariant applies; every trap forwards to
  * `namespace` instead. Exported for tests — the public entry point is `z`.
  */
-export type StandInFactory = (name: string, args: unknown[], build: (args: unknown[]) => ZodTypeAny) => unknown;
+export type SchemaFactory = (...args: unknown[]) => ZodTypeAny;
 
-const compoundStandIn: StandInFactory = (_name, args, build) => wrapLazy(new LazyZodSchema(() => build(args)));
+export type StandInFactory = (name: string, args: unknown[], factory: SchemaFactory, namespace: object) => unknown;
+
+const compoundStandIn: StandInFactory = (_name, args, factory, namespace) =>
+  wrapLazy(new LazyZodSchema(() => factory.apply(namespace, args)));
 
 export function createLazyZ(
   namespace: typeof realZ,
@@ -31,8 +34,8 @@ export function createLazyZ(
   return new Proxy({} as typeof realZ, {
     get(_target, key, receiver) {
       if (typeof key === 'string' && factories.has(key)) {
-        const realFactory = (namespace as unknown as Record<string, (...a: unknown[]) => ZodTypeAny>)[key];
-        return (...args: unknown[]) => standIn(key, args, (built) => realFactory.apply(namespace, built));
+        const realFactory = (namespace as unknown as Record<string, SchemaFactory>)[key];
+        return (...args: unknown[]) => standIn(key, args, realFactory, namespace);
       }
       return Reflect.get(namespace, key, receiver);
     },