diff --git a/.changeset/adr-0089-d3a-strict-visibility-schemas.md b/.changeset/adr-0089-d3a-strict-visibility-schemas.md
new file mode 100644
index 0000000000..03470bb8f6
--- /dev/null
+++ b/.changeset/adr-0089-d3a-strict-visibility-schemas.md
@@ -0,0 +1,11 @@
+---
+"@objectstack/spec": major
+---
+
+ADR-0089 D3a: flip `.strict()` on the view form + page component schemas so a mis-layered or stale conditional-visibility key is a **loud parse error** instead of a silent strip.
+
+`FormFieldSchema`, `FormSectionSchema` (`view.zod.ts`) and `PageComponentSchema` (`page.zod.ts`) now reject unknown keys. Previously zod's default strip mode discarded any key these schemas did not declare — including a `visibleWhen` typo, a page-only `visibility` pasted onto a view field (or vice-versa), or a key surviving past its deprecation window — with no diagnostic, shipping inert metadata (ADR-0049 enforce-or-remove, ADR-0078 no-silently-inert).
+
+- **Breaking:** metadata carrying a key not declared by these three schemas now fails validation at parse. A monorepo + examples sweep found a single offender (a test fixture using `id`/`title` on a form section instead of the canonical `name`/`label`); all first-party apps and platform metadata parse clean.
+- The deprecated `visibleOn` (view form) / `visibility` (page component) aliases are **declared** keys, so they keep parsing and normalizing to `visibleWhen` — unchanged.
+- Rejection messages name the offending key(s) and, when a key looks like the visibility predicate, point the author at the canonical `visibleWhen` (new `strictVisibilityError` zod error map, exported from `shared/visibility`).
diff --git a/content/docs/protocol/objectui/layout-dsl.mdx b/content/docs/protocol/objectui/layout-dsl.mdx
index 69a9066090..ceef8f1171 100644
--- a/content/docs/protocol/objectui/layout-dsl.mdx
+++ b/content/docs/protocol/objectui/layout-dsl.mdx
@@ -818,6 +818,13 @@ The legacy spellings `visibleOn` (view) and `visibility` (page) are `@deprecated
 aliases: still accepted and folded into `visibleWhen` at parse time, so existing
 metadata keeps working. Author new metadata with `visibleWhen`.
 
+Since `@objectstack/spec` 15 (ADR-0089 D3a) the form field/section and page
+component schemas are **strict**: a key they do not declare — a `visibleWhen`
+typo, a page-only `visibility` pasted onto a view field (or vice-versa), or a
+stale key — is now a **loud parse error** instead of being silently dropped. The
+error names the offending key and, when it looks like a visibility predicate,
+points you back at `visibleWhen`.
+
 Breakpoint-based show/hide is handled separately via the component's
 `responsive.hiddenOn` array (e.g. `hiddenOn: ['xs', 'sm']`), see
 `packages/spec/src/ui/responsive.zod.ts`.
diff --git a/docs/adr/0089-unify-visibility-predicate-naming.md b/docs/adr/0089-unify-visibility-predicate-naming.md
index dee2e5c2d3..a3698696ea 100644
--- a/docs/adr/0089-unify-visibility-predicate-naming.md
+++ b/docs/adr/0089-unify-visibility-predicate-naming.md
@@ -1,6 +1,6 @@
 # ADR-0089: Unify the conditional-visibility predicate under one name (`visibleWhen`), alias the rest
 
-**Status**: Accepted (2026-07-14) — D1 + D2 + codemod + docs implemented in #2642. D3a (`.strict()` flip) and D3b (lint rule) deferred to follow-up per the ADR's staged rollout (sweep first, then flip).
+**Status**: Accepted (2026-07-14) — D1 + D2 + codemod + docs implemented in #2642/#2900; renderer reads in objectui#2490 (#2904); D3b lint rule in #2900 + bidirectional binding-root follow-up (#2903). **D3a (`.strict()` flip) implemented (#2902)** — the monorepo + examples sweep found a single offender (a test fixture), and the strict flip ships as a major (`@objectstack/spec` 15.0.0).
 **Deciders**: ObjectStack Protocol Architects
 **Builds on**: [ADR-0049](./0049-no-unenforced-security-properties.md) (enforce-or-remove — a declared-but-unchecked visibility key is exactly this class), [ADR-0078](./0078-no-silently-inert-metadata.md) (no silently-inert metadata — a mis-layered visibility key that zod strips is inert by accident), [ADR-0085](./0085-object-semantic-roles-over-surface-hint-blocks.md) (the enforce-or-remove pass that already deleted a consumer-less `visibleOn` from `fieldGroups`), [ADR-0033](./0033-ai-assisted-metadata-authoring.md) (the AI-authoring population this ADR optimizes for), [ADR-0058](./0058-expression-and-predicate-surface.md) (the CEL predicate surface these keys all belong to), [ADR-0087](./0087-metadata-protocol-upgrade-contract.md) (conversion-over-notification; this rename ships as an L1 invisible break via the alias mechanism)
 **Consumers**: `@objectstack/spec` (the field / view / page zod schemas + the alias normalization), `@objectstack/objectql` (`rule-validator` — the server-side enforcer), `@objectstack/lint` (the new mis-layer / wrong-root rule), ObjectUI form + page renderers, and every AI author of `*.object.ts` / `*.view.ts` / `*.page.ts`
diff --git a/packages/objectql/src/protocol-meta.test.ts b/packages/objectql/src/protocol-meta.test.ts
index 8047052805..c2ba396fbe 100644
--- a/packages/objectql/src/protocol-meta.test.ts
+++ b/packages/objectql/src/protocol-meta.test.ts
@@ -408,7 +408,7 @@ describe('ObjectStackProtocolImplementation - Metadata Persistence', () => {
                     form: {
                         type: 'tabbed',
                         data: { provider: 'object', object: 'lead' },
-                        sections: [{ id: 'main', title: 'Main', fields: [{ field: 'first_name' }] }],
+                        sections: [{ name: 'main', label: 'Main', fields: [{ field: 'first_name' }] }],
                     },
                 };
                 await expect(
diff --git a/packages/spec/api-surface.json b/packages/spec/api-surface.json
index af11f4f6bf..ba7c261406 100644
--- a/packages/spec/api-surface.json
+++ b/packages/spec/api-surface.json
@@ -4457,6 +4457,7 @@
     "resilientFetch (function)",
     "safeParsePretty (function)",
     "singularToPlural (function)",
+    "strictVisibilityError (const)",
     "suggestFieldType (function)",
     "tmpl (function)"
   ]
diff --git a/packages/spec/src/shared/visibility.ts b/packages/spec/src/shared/visibility.ts
index 1ee90f4c8a..f2050e4b9b 100644
--- a/packages/spec/src/shared/visibility.ts
+++ b/packages/spec/src/shared/visibility.ts
@@ -1,5 +1,7 @@
 // Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
 
+import type { z } from 'zod';
+
 /**
  * # Conditional-visibility predicate normalization (ADR-0089)
  *
@@ -64,3 +66,46 @@ export function normalizeVisibleWhen<T extends WithVisibilityAliases>(
   }
   return { ...rest, visibleWhen: canonical } as Omit<T, 'visibleOn' | 'visibility'>;
 }
+
+/** A key that is (or is a likely mis-spelling of) the visibility predicate. */
+function looksLikeVisibilityKey(key: string): boolean {
+  return /vis|conceal|hidden|show.?when/i.test(key);
+}
+
+/**
+ * Custom zod `error` for the `.strict()` view/page schemas (ADR-0089 D3a).
+ *
+ * With `.strict()`, a key these schemas do not declare — a stale `visibleOn` past
+ * removal, a `visibleWhen` typo, or a wrong-layer paste — is now a **loud parse
+ * error** instead of a silent strip (ADR-0049 enforce-or-remove, ADR-0078
+ * no-silently-inert). This error map turns that rejection into a *fixable* one: it
+ * always names the offending key(s), and when a key looks like the
+ * conditional-visibility predicate it points the author at the canonical
+ * `visibleWhen`. Every other issue code defers to zod's default (`undefined`).
+ *
+ * Wire it as the object's `error` alongside `.strict()`:
+ *
+ * ```ts
+ * z.object({ ..., visibleWhen: Expr.optional() }, { error: strictVisibilityError })
+ *   .strict()
+ *   .transform(normalizeVisibleWhen)
+ * ```
+ */
+export const strictVisibilityError: z.core.$ZodErrorMap = (issue) => {
+  if (issue.code !== 'unrecognized_keys') return undefined;
+  const keys = (issue as { keys?: readonly string[] }).keys ?? [];
+  const list = keys.map((k) => `\`${k}\``).join(', ');
+  const base =
+    `Unrecognized key(s) on this view/page schema: ${list}. ` +
+    `Before ADR-0089 D3a these were dropped silently, shipping inert metadata; ` +
+    `a mis-layered or stale key is now a loud parse error.`;
+  if (keys.some(looksLikeVisibilityKey)) {
+    return (
+      base +
+      ' If this is the conditional-visibility predicate, the canonical key is ' +
+      '`visibleWhen` (ADR-0089) — `visibleOn` (view form) and `visibility` (page ' +
+      'component) are still accepted as deprecated aliases.'
+    );
+  }
+  return base;
+};
diff --git a/packages/spec/src/ui/page.test.ts b/packages/spec/src/ui/page.test.ts
index acd18dbdae..0cebfbac77 100644
--- a/packages/spec/src/ui/page.test.ts
+++ b/packages/spec/src/ui/page.test.ts
@@ -1016,3 +1016,30 @@ describe('ADR-0089 — visibleWhen unification (page component)', () => {
     expect((parsed as Record<string, unknown>).visibility).toBeUndefined();
   });
 });
+
+describe('ADR-0089 D3a — strict page component schema (loud mis-layered keys)', () => {
+  it('rejects an unknown key on a page component instead of silently stripping it', () => {
+    const res = PageComponentSchema.safeParse({ type: 'element:text', notARealKey: 1 });
+    expect(res.success).toBe(false);
+    if (!res.success) {
+      expect(res.error.issues[0].code).toBe('unrecognized_keys');
+    }
+  });
+
+  it('a visibility-ish typo is rejected AND the message points at `visibleWhen`', () => {
+    const res = PageComponentSchema.safeParse({ type: 'element:text', visibilty: "page.a == 1" });
+    expect(res.success).toBe(false);
+    if (!res.success) {
+      expect(res.error.issues[0].message).toContain('visibleWhen');
+    }
+  });
+
+  it('the deprecated `visibility` alias is still accepted under strict (declared key)', () => {
+    expect(() => PageComponentSchema.parse({ type: 'element:text', visibility: "page.a == 1" })).not.toThrow();
+  });
+
+  it('rejects a stale `visibleOn` key on a page component (that is the view-form alias, not a page one)', () => {
+    const res = PageComponentSchema.safeParse({ type: 'element:text', visibleOn: "page.a == 1" });
+    expect(res.success).toBe(false);
+  });
+});
diff --git a/packages/spec/src/ui/page.zod.ts b/packages/spec/src/ui/page.zod.ts
index 379c201d8e..d4e9689339 100644
--- a/packages/spec/src/ui/page.zod.ts
+++ b/packages/spec/src/ui/page.zod.ts
@@ -3,7 +3,7 @@
 import { z } from 'zod';
 import { SnakeCaseIdentifierSchema } from '../shared/identifiers.zod';
 import { ExpressionInputSchema } from '../shared/expression.zod';
-import { normalizeVisibleWhen } from '../shared/visibility';
+import { normalizeVisibleWhen, strictVisibilityError } from '../shared/visibility';
 import { SortItemSchema } from '../shared/enums.zod';
 import { FilterConditionSchema } from '../data/filter.zod';
 import { I18nLabelSchema, AriaPropsSchema } from './i18n.zod';
@@ -123,7 +123,7 @@ export const PageComponentSchema = lazySchema(() => z.object({
 
   /** ARIA accessibility attributes */
   aria: AriaPropsSchema.optional().describe('ARIA accessibility attributes'),
-}).transform(normalizeVisibleWhen));
+}, { error: strictVisibilityError }).strict().transform(normalizeVisibleWhen));
 
 /**
  * Page Variable Schema
diff --git a/packages/spec/src/ui/view.test.ts b/packages/spec/src/ui/view.test.ts
index 65288e1e32..9bed1ecbfc 100644
--- a/packages/spec/src/ui/view.test.ts
+++ b/packages/spec/src/ui/view.test.ts
@@ -2536,3 +2536,44 @@ describe('ADR-0089 — visibleWhen unification (view form)', () => {
     expect(parsed.visibleOn).toBeUndefined();
   });
 });
+
+describe('ADR-0089 D3a — strict view form schemas (loud mis-layered keys)', () => {
+  it('rejects an unknown key on a form field instead of silently stripping it', () => {
+    const res = FormFieldSchema.safeParse({ field: 'state', notARealKey: 1 });
+    expect(res.success).toBe(false);
+    if (!res.success) {
+      expect(res.error.issues[0].code).toBe('unrecognized_keys');
+    }
+  });
+
+  it('a visibility-ish typo is rejected AND the message points at `visibleWhen`', () => {
+    const res = FormFieldSchema.safeParse({ field: 'state', visibleWhenn: "record.a == 1" });
+    expect(res.success).toBe(false);
+    if (!res.success) {
+      expect(res.error.issues[0].message).toContain('visibleWhen');
+    }
+  });
+
+  it('the deprecated `visibleOn` alias is still accepted under strict (declared key)', () => {
+    expect(() => FormFieldSchema.parse({ field: 'state', visibleOn: "record.a == 1" })).not.toThrow();
+  });
+
+  it('rejects a stale `visibility` key on a form field (that is the page-component alias, not a view one)', () => {
+    const res = FormFieldSchema.safeParse({ field: 'state', visibility: "record.a == 1" });
+    expect(res.success).toBe(false);
+  });
+
+  it('a non-visibility unknown key is rejected without the visibleWhen hint', () => {
+    const res = FormSectionSchema.safeParse({ label: 'S', fields: [], bogusKey: true });
+    expect(res.success).toBe(false);
+    if (!res.success) {
+      expect(res.error.issues[0].message).not.toContain('visibleWhen');
+      expect(res.error.issues[0].message).toContain('bogusKey');
+    }
+  });
+
+  it('a strict form section still accepts canonical + alias keys', () => {
+    expect(() => FormSectionSchema.parse({ label: 'S', visibleWhen: 'record.a == 1', fields: [] })).not.toThrow();
+    expect(() => FormSectionSchema.parse({ label: 'S', visibleOn: 'record.a == 1', fields: [] })).not.toThrow();
+  });
+});
diff --git a/packages/spec/src/ui/view.zod.ts b/packages/spec/src/ui/view.zod.ts
index eedfbdf327..676a3b81e0 100644
--- a/packages/spec/src/ui/view.zod.ts
+++ b/packages/spec/src/ui/view.zod.ts
@@ -5,7 +5,7 @@ import { ProtectionSchema } from '../shared/protection.zod';
 import { MetadataProtectionFields } from '../kernel/metadata-protection.zod';
 import { SnakeCaseIdentifierSchema } from '../shared/identifiers.zod';
 import { ExpressionInputSchema } from '../shared/expression.zod';
-import { normalizeVisibleWhen } from '../shared/visibility';
+import { normalizeVisibleWhen, strictVisibilityError } from '../shared/visibility';
 import { I18nLabelSchema, AriaPropsSchema } from './i18n.zod';
 import { SharingConfigSchema } from './sharing.zod';
 import { ResponsiveConfigSchema, PerformanceConfigSchema } from './responsive.zod';
@@ -770,7 +770,7 @@ export const FormFieldSchema: z.ZodType<any> = lazySchema(() => z.object({
   /** @deprecated ADR-0089 — use `visibleWhen`. Accepted and normalized to `visibleWhen` at parse. */
   visibleOn: ExpressionInputSchema.optional().describe('[DEPRECATED → `visibleWhen`] Visibility predicate (CEL). Normalized to `visibleWhen` at parse.'),
   disclosure: z.enum(['inline', 'popover']).optional().describe('Composite rendering: inline bordered box (default) or a summary line + gear popover (progressive disclosure).'),
-}).transform(normalizeVisibleWhen));
+}, { error: strictVisibilityError }).strict().transform(normalizeVisibleWhen));
 
 /**
  * Form Layout Section
@@ -807,7 +807,7 @@ export const FormSectionSchema = lazySchema(() => z.object({
     z.string(), // Legacy: simple field name
     FormFieldSchema, // Enhanced: detailed field config
   ])),
-}).transform(normalizeVisibleWhen));
+}, { error: strictVisibilityError }).strict().transform(normalizeVisibleWhen));
 
 /**
  * Form View Schema