diff --git a/.changeset/lazy-schema-tojsonschema-identity.md b/.changeset/lazy-schema-tojsonschema-identity.md
new file mode 100644
index 0000000000..fe906d21bd
--- /dev/null
+++ b/.changeset/lazy-schema-tojsonschema-identity.md
@@ -0,0 +1,11 @@
+---
+'@objectstack/spec': patch
+---
+
+fix(spec): keep `lazySchema` proxies identity-compatible with `z.toJSONSchema` (objectui#2561)
+
+zod's `toJSONSchema` keys its `seen` map on the node object it traverses — the `lazySchema` Proxy wherever a schema is referenced lazily (`z.lazy(() => X)` recursion getters, direct conversion roots) — while its wrapper-type processors (pipe/lazy/optional/default/…) look themselves up via the REAL instance captured at construction (`inst._zod.processJSONSchema = (ctx, …) => pipeProcessor(inst, …)`). The identity mismatch crashed conversion with `Cannot set properties of undefined (setting 'ref')`.
+
+This stayed latent while lazy-referenced schemas were plain objects (the object processor never looks itself up); ADR-0089 D3a turned `PageComponentSchema` / `FormFieldSchema` into `.strict().transform(…)` **pipes**, which broke ObjectUI Studio's spec-derived Page/View inspector JSONSchema derivation under spec 15.
+
+Fix: the proxy now serves a memoised `_zod` facade that prototype-delegates to the real internals and wraps only `processJSONSchema` to alias the proxy's `seen` entry onto the real instance before delegating. Parse behavior is unchanged; `OS_EAGER_SCHEMAS=1` remains the bypass. Regression tests cover the D3a pipe shape, recursion through `z.lazy(() => proxy)`, mixed proxy+real traversal, and the full `PageSchema` / `ViewSchema` Studio derivation paths.
diff --git a/packages/spec/src/shared/lazy-schema.test.ts b/packages/spec/src/shared/lazy-schema.test.ts
index 516ea6b18b..5c0de97dd9 100644
--- a/packages/spec/src/shared/lazy-schema.test.ts
+++ b/packages/spec/src/shared/lazy-schema.test.ts
@@ -56,3 +56,64 @@ describe('lazySchema', () => {
     }
   });
 });
+
+/**
+ * zod's `toJSONSchema` keys its `seen` map on the traversed node (the Proxy),
+ * while its wrapper-type processors (pipe/lazy/optional/…) look themselves up
+ * via the construction-time REAL instance. Without the `_zod` facade aliasing
+ * the two identities, any lazySchema wrapping a non-object root — e.g. the
+ * ADR-0089 D3a `.strict().transform(…)` pipes — crashed with
+ * `Cannot set properties of undefined (setting 'ref')` (objectui#2561).
+ */
+describe('lazySchema × z.toJSONSchema identity', () => {
+  const TO_JSON = { io: 'input', unrepresentable: 'any' } as const;
+
+  it('converts a lazy `.strict().transform(…)` pipe (ADR-0089 D3a shape)', () => {
+    const schema = lazySchema(() =>
+      z.object({ name: z.string() }).strict().transform((v) => v),
+    );
+    const json = z.toJSONSchema(schema, TO_JSON) as Record<string, any>;
+    expect(json.properties?.name).toBeDefined();
+  });
+
+  it('converts recursion reaching the pipe through `z.lazy(() => proxy)` (FormFieldSchema shape)', () => {
+    const NodeSchema: z.ZodType<any> = lazySchema(() =>
+      z
+        .object({
+          field: z.string(),
+          fields: z.array(z.lazy(() => NodeSchema)).optional(),
+        })
+        .strict()
+        .transform((v) => v),
+    );
+    const json = z.toJSONSchema(NodeSchema, TO_JSON);
+    expect(JSON.stringify(json)).toContain('field');
+  });
+
+  it('does not crash when one conversion sees both the proxy and the real instance', () => {
+    const Leaf: z.ZodType<any> = lazySchema(() =>
+      z.object({ id: z.string() }).strict().transform((v) => v),
+    );
+    // `.optional()` resolves through the proxy and captures the REAL pipe as
+    // its innerType; the `z.lazy` getter hands zod the PROXY — one traversal
+    // meets both identities in either order.
+    const DocA = z.object({
+      a: (Leaf as any).optional(),
+      b: z.lazy(() => Leaf),
+    });
+    const DocB = z.object({
+      b: z.lazy(() => Leaf),
+      a: (Leaf as any).optional(),
+    });
+    for (const Doc of [DocA, DocB]) {
+      const json = z.toJSONSchema(Doc, TO_JSON) as Record<string, any>;
+      expect(json.properties?.a).toBeDefined();
+      expect(json.properties?.b).toBeDefined();
+    }
+  });
+
+  it('memoises the `_zod` facade (identity-stable across accesses)', () => {
+    const schema = lazySchema(() => z.object({ x: z.number() }).strict().transform((v) => v));
+    expect((schema as any)._zod).toBe((schema as any)._zod);
+  });
+});
diff --git a/packages/spec/src/shared/lazy-schema.ts b/packages/spec/src/shared/lazy-schema.ts
index 9bed23508c..c1f5c14ea3 100644
--- a/packages/spec/src/shared/lazy-schema.ts
+++ b/packages/spec/src/shared/lazy-schema.ts
@@ -31,9 +31,57 @@ export function lazySchema<T extends z.ZodTypeAny>(factory: () => T): T {
 
   const target = function lazyZod() {} as unknown as T;
 
+  /**
+   * Memoised `_zod` facade (one per proxy — `_zod.run` is the parse hot path).
+   *
+   * Why it exists: zod's `toJSONSchema` traversal keys its `seen` map on the
+   * node object it was handed — this Proxy, wherever the schema is referenced
+   * lazily (a `z.lazy(() => X)` recursion getter, or a direct conversion
+   * root). But zod's per-type JSON-Schema hooks close over the REAL instance
+   * at construction time (`inst._zod.processJSONSchema = (ctx, …) =>
+   * pipeProcessor(inst, ctx, …)`), and the wrapper-type processors
+   * (pipe/lazy/optional/default/…) then resolve `ctx.seen.get(inst)` — which
+   * misses when the entry was keyed on the Proxy, crashing with
+   * `Cannot set properties of undefined (setting 'ref')`. Plain-object
+   * schemas never look themselves up, which kept this latent until ADR-0089
+   * D3a turned FormFieldSchema / PageComponentSchema into
+   * `.strict().transform(…)` pipes.
+   *
+   * The facade prototype-delegates every `_zod` read to the real internals,
+   * wrapping only `processJSONSchema` to alias the Proxy's `seen` entry onto
+   * the real instance before delegating, so both identities resolve to the
+   * same entry. If the real instance was already traversed under its own
+   * identity it keeps its entry (alias, never clobber).
+   */
+  let zodFacade: object | undefined;
+  const makeZodFacade = (real: T): object | undefined => {
+    const realZod = (real as unknown as { _zod?: Record<string, unknown> })._zod;
+    if (!realZod || typeof realZod.processJSONSchema !== 'function') {
+      return realZod;
+    }
+    const delegate = realZod.processJSONSchema as (...a: unknown[]) => unknown;
+    return Object.create(realZod as object, {
+      processJSONSchema: {
+        enumerable: true,
+        value: (ctx: { seen?: Map<unknown, unknown> }, json: unknown, params: unknown) => {
+          const seen = ctx?.seen;
+          if (seen && typeof seen.get === 'function') {
+            const entry = seen.get(proxy);
+            if (entry !== undefined && !seen.has(real)) seen.set(real, entry);
+          }
+          return delegate(ctx, json, params);
+        },
+      },
+    }) as object;
+  };
+
   const proxy = new Proxy(target as object, {
     get(_t, prop) {
       const real = resolve() as unknown as Record<PropertyKey, unknown>;
+      if (prop === '_zod') {
+        zodFacade ??= makeZodFacade(real as unknown as T);
+        return zodFacade;
+      }
       const value = real[prop];
       if (typeof value === 'function') {
         return (value as (...a: unknown[]) => unknown).bind(real);
diff --git a/packages/spec/src/ui/page.test.ts b/packages/spec/src/ui/page.test.ts
index 0cebfbac77..a1580478db 100644
--- a/packages/spec/src/ui/page.test.ts
+++ b/packages/spec/src/ui/page.test.ts
@@ -1,4 +1,5 @@
 import { describe, it, expect } from 'vitest';
+import { z } from 'zod';
 import {
   PageSchema,
   PAGE_TYPE_ROADMAP,
@@ -1043,3 +1044,25 @@ describe('ADR-0089 D3a — strict page component schema (loud mis-layered keys)'
     expect(res.success).toBe(false);
   });
 });
+
+/**
+ * ObjectUI Studio derives the Page inspector's authoring JSONSchema straight
+ * from these zod schemas via `z.toJSONSchema` (objectui#2561). The lazySchema
+ * proxy must stay identity-compatible with zod's converter — the D3a
+ * `.strict().transform(…)` pipe on PageComponentSchema is exactly the shape
+ * that crashed it before the `_zod` facade fix.
+ */
+describe('PageSchema → JSON Schema derivation (Studio inspector path)', () => {
+  const TO_JSON = { io: 'input', unrepresentable: 'any' } as const;
+
+  it('derives the input-io JSONSchema for the whole Page document', () => {
+    const json = z.toJSONSchema(PageSchema, TO_JSON);
+    const text = JSON.stringify(json);
+    expect(text).toContain('interfaceConfig');
+    expect(text).toContain('regions');
+  });
+
+  it('derives the input-io JSONSchema for a bare PageComponent', () => {
+    expect(() => z.toJSONSchema(PageComponentSchema, TO_JSON)).not.toThrow();
+  });
+});
diff --git a/packages/spec/src/ui/view.test.ts b/packages/spec/src/ui/view.test.ts
index 9bed1ecbfc..7bf9059622 100644
--- a/packages/spec/src/ui/view.test.ts
+++ b/packages/spec/src/ui/view.test.ts
@@ -1,4 +1,5 @@
 import { describe, it, expect } from 'vitest';
+import { z } from 'zod';
 import {
   ViewSchema,
   ListViewSchema,
@@ -2577,3 +2578,22 @@ describe('ADR-0089 D3a — strict view form schemas (loud mis-layered keys)', ()
     expect(() => FormSectionSchema.parse({ label: 'S', visibleOn: 'record.a == 1', fields: [] })).not.toThrow();
   });
 });
+
+/**
+ * ObjectUI Studio derives the View inspector's authoring JSONSchema from
+ * ViewSchema via `z.toJSONSchema` (objectui#2561). FormFieldSchema is a
+ * self-recursive `.strict().transform(…)` pipe reached through
+ * `z.lazy(() => FormFieldSchema)` — the exact lazySchema-proxy identity shape
+ * that crashed zod's converter before the `_zod` facade fix.
+ */
+describe('ViewSchema → JSON Schema derivation (Studio inspector path)', () => {
+  const TO_JSON = { io: 'input', unrepresentable: 'any' } as const;
+
+  it('derives the input-io JSONSchema for the whole View document', () => {
+    expect(() => z.toJSONSchema(ViewSchema, TO_JSON)).not.toThrow();
+  });
+
+  it('derives the input-io JSONSchema for the recursive FormFieldSchema', () => {
+    expect(() => z.toJSONSchema(FormFieldSchema, TO_JSON)).not.toThrow();
+  });
+});