@@ -0,0 +1,24 @@
+---
+'@object-ui/types': patch
+'@object-ui/core': patch
+'@object-ui/components': patch
+'@object-ui/plugin-form': patch
+---
+
+fix(types,core,components,plugin-form)!: `FormField` drops its open index signature, and a form-view field's `immutable` locks the control once the record exists
+
+Clause-②: yes (narrowing)
+Metadata-protocol: none
+
+<!-- adr-0087: not-required (runtime-interface-only ui/packages/types/src/form.ts#FormField, ui/packages/core/src/validation/server-owned-value.ts#FieldWithDeclaredDefault) Two published TypeScript interfaces each lose their open index signature and nothing else: no member is removed or renamed, no metadata key, accepted value, export or error code moves, the zod mirror `FormFieldSchema` and the strict authoring face are unchanged, and `objectstack migrate meta` has nothing to rewrite. The affected party is a TypeScript consumer, reached by its own compiler (excess-property error naming the undeclared key). -->
+
+**BREAKING** type-surface narrowing, compile-time only.
+
+- **`@object-ui/types`.** `FormField` no longer ends in `[key: string]: any`. A key no member declares — a misspelling included — is now a compile error on a fresh `FormField` literal, as it already was on every node type since `BaseSchema` closed. The declared members are unchanged, and so are the zod mirror `FormFieldSchema` and the strict face `StrictAnyComponentSchema`, which already refused undeclared keys.
+- **`@object-ui/core`.** `FieldWithDeclaredDefault`, the parameter type of `isServerOwnedValue`, drops its `[key: string]: unknown` signature, so a `FormField` value is assignable to it. It still reads only `field.defaultValue`.
+- **`@object-ui/components`.** The form renderer types the field's raw metadata carrier as `Record<string, unknown> | FormField` (the stashed object field, else the field config itself), instead of reading the config through the signature.
+- **`@object-ui/plugin-form`.** The host-written `group` key (copied from the object field for the field-group fallback) and the section-divider disclosure keys (`collapsible`, `collapsed`, `onToggle`) are typed on package-local hand-off types instead of riding the signature.
+
+**Fix (`@object-ui/plugin-form`).** A form-view field declaring `immutable: true` (spec `FormField.immutable`, "editable on create, locked once the record exists") is now drawn disabled when the form edits an existing record, and stays editable on a create form. It used to be copied onto the runtime field as `immutable`, which no renderer reads, so an edit form left the control editable.
+
+What to do: nothing if your `FormField` literals use only declared keys. Otherwise the compiler names each undeclared key on the literal that carries it (an excess-property error): correct a misspelling to the declared member, or, for a key only your host writes and reads, type it on a local intersection such as `FormField & { group?: string }` where its producer and reader live.
@@ -821,7 +821,7 @@ function dependentLookupParents(f: FormFieldConfig | undefined): string[] {
   if (typeof widget !== 'string' || !EXPANDABLE_FIELD_TYPES.has(normalizeFieldType(widget))) {
     return [];
   }
-  const carrier: Record<string, unknown> = f.field || (f as Record<string, unknown>);
+  const carrier: Record<string, unknown> | FormFieldConfig = f.field || f;
   const inner = carrier.field;
   const meta =
     inner !== null && typeof inner === 'object' && ('reference' in inner || 'type' in inner)
@@ -4369,7 +4369,7 @@ interface RenderFieldProps {
   /** react-hook-form's ref binding, handed to the built-in control by name. */
   ref?: RefCallBack;
   /** The field's raw metadata object (`field.field`, else the field config). */
-  field?: Record<string, unknown>;
+  field?: Record<string, unknown> | FormFieldConfig;
   /** A dependent select's gate hint while its controlling field is unset (#2284). */
   emptyHint?: string;
   [key: string]: unknown;
@@ -4729,7 +4729,7 @@ const BuiltinSelectControl = React.forwardRef<HTMLButtonElement, BuiltinSelectCo
  * wrote it until objectui#12130; `objectFormNumericStep-9574.test.tsx` pins
  * both of its routes.
  */
-function nativeNumberStep(carrier: Record<string, unknown> | undefined): number | 'any' {
+function nativeNumberStep(carrier: RenderFieldProps['field']): number | 'any' {
   const step = carrier?.step;
   if (typeof step === 'number') return step;
   const scale = carrier?.scale;
 
@@ -20,6 +20,7 @@
 
 import { describe, it, expect } from 'vitest';
 import { DEFAULT_VALUE_TOKENS } from '@objectstack/spec/data';
+import type { FormField } from '@object-ui/types';
 import { isRuntimeDefault, isServerOwnedValue } from '../server-owned-value.js';
 
 /** A CEL Expression envelope — the non-token half of "the server resolves it". */
@@ -71,7 +72,8 @@ describe('isServerOwnedValue (#4085)', () => {
 
   it('a field declaring no default is never server-owned', () => {
     expect(isServerOwnedValue(withDefault(undefined), true)).toBe(false);
-    expect(isServerOwnedValue({ name: 'title' }, true)).toBe(false);
+    const declaresNoStash: FormField = { name: 'title' };
+    expect(isServerOwnedValue(declaresNoStash, true)).toBe(false);
   });
 
   it('a hand-authored FormField carries no object metadata, so it is untouched', () => {
 
@@ -85,16 +85,16 @@ export function isRuntimeDefault(v: unknown): boolean {
 
 /**
  * A runtime `FormField` as far as this module is concerned: the resolved
- * object-field metadata stash, and whatever else the field carries.
+ * object-field metadata stash.
  *
  * Stated structurally rather than as `@object-ui/types`' `FormField` so the
- * one key this module reads is visible in the signature — and with an index
- * signature so a caller (or a test) may hand over an object literal carrying
- * the rest of the field without tripping TypeScript's excess-property check.
+ * one key this module reads is visible in the signature. No index signature:
+ * `FormField` declares its keys and carries none (objectstack#25923), and an
+ * interface without one is not assignable to a type that has one — a
+ * `FormField` value is accepted here because it declares this `field` slot.
  */
 export interface FieldWithDeclaredDefault {
   field?: { defaultValue?: unknown } | null;
-  [key: string]: unknown;
 }
 
 /**
 
@@ -45,7 +45,7 @@ import {
   filterSystemFields,
   sectionedGridLayout,
 } from './autoLayout';
-import { deriveFieldGroupSections, projectSectionDivider, resolveSectionCollapse } from './fieldGroups';
+import { deriveFieldGroupSections, projectSectionDivider, resolveSectionCollapse, type FormFieldWithGroup } from './fieldGroups';
 import { mergeCustomFields } from './customFieldsMerge';
 import { hasSectionGroupReference, resolveSectionGroupReferences } from './sectionGroups';
 import {
@@ -958,7 +958,7 @@ const SimpleObjectForm: React.FC<{ schema: LocalizedObjectFormSchema; dataSource
       const field = objectSchema.fields?.[name];
       if (field) {
         // Auto-generate field from schema
-        const formField: FormField = {
+        const formField: FormFieldWithGroup = {
           name: name,
           label: fieldLabel(schema.objectName, name, field.label || fieldName),
           // (type, multiple) decides the widget, not the type alone: a `select`
 
@@ -21,6 +21,30 @@
 import { deriveFieldGroupLayout } from '@objectstack/spec/data';
 import type { FormField, ObjectFormSection } from '@object-ui/types';
 
+/**
+ * A runtime `FormField` carrying its object field's group membership.
+ *
+ * `group` is written by the HOST, never authored on a form field: the
+ * object-bound form copies it from the object field (`Field.group`, the key of
+ * one of the object's `fieldGroups`) and {@link deriveFieldGroupSections} reads
+ * it back. So it is typed here, where its producer and consumer live, and not
+ * on the `FormField` authoring face (ui AGENTS.md #0.1, objectstack#25923).
+ */
+export type FormFieldWithGroup = FormField & { group?: string };
+
+/**
+ * A `section-divider` row as {@link projectSectionDivider} writes it: a
+ * runtime `FormField` plus the disclosure control the HOST resolves for it
+ * (`resolveSectionCollapse`). The form renderer's divider branch reads the
+ * three keys off the row; none of them is authored on a form field, so they
+ * stay off the `FormField` authoring face (ui AGENTS.md #0.1, objectstack#25923).
+ */
+type SectionDividerRow = FormField & {
+  collapsible?: boolean;
+  collapsed?: boolean;
+  onToggle?: () => void;
+};
+
 /**
  * Derive form sections from an object's declared `fieldGroups` and each
  * rendered field's `group`.
@@ -49,7 +73,7 @@ export function deriveFieldGroupSections(
     // Pseudo-def over the rendered fields: membership is the only input the
     // derivation needs per field.
     fields: Object.fromEntries(
-      fields.map((f) => [f.name, { group: (f as { group?: unknown }).group }]),
+      fields.map((f) => [f.name, { group: (f as FormFieldWithGroup).group }]),
     ),
   });
   if (!derived) return null;
@@ -135,7 +159,7 @@ export interface SectionDividerSource {
   /** The authored blurb (spec `FormSection.description`). */
   description?: string;
   /** ADR-0089 `FormSection.visibleWhen` (#6111), unevaluated. */
-  visibleWhen?: unknown;
+  visibleWhen?: FormField['visibleWhen'];
   /**
    * The fault direction of {@link visibleWhen}, copied onto the row unchanged
    * (`ObjectFormSection.visibleWhenFault`, objectui#13055).
@@ -335,11 +359,11 @@ export function projectSectionDivider(source: SectionDividerSource): FormField[]
         ...(visibleWhenFault !== undefined ? { visibleWhenFault } : {}),
         fields: members,
         colSpan: DIVIDER_COL_SPAN,
-      } as unknown as FormField,
+      } satisfies SectionDividerRow,
     ];
   }
 
-  const row: Record<string, unknown> = {
+  const row: SectionDividerRow = {
     name: `__section_${key}`,
     label: title,
     type: 'section-divider',
@@ -355,5 +379,5 @@ export function projectSectionDivider(source: SectionDividerSource): FormField[]
     row.collapsed = collapse.collapsed;
     row.onToggle = collapse.onToggle;
   }
-  return [row as FormField];
+  return [row];
 }
@@ -135,8 +135,10 @@ describe('buildFlatFields WARNS on an unresolvable top-level member (route 1, ob
 describe('buildFlatFields ↔ normalizeSectionField parity (#4755)', () => {
   it('agrees key-by-key with the sectioned producer, adding only `group`', () => {
     for (const name of Object.keys(OBJECT_SCHEMA.fields)) {
-      const flat = buildFlatFields({ ...ctx, fields: [name] })[0] as Record<string, unknown>;
-      const sectioned = normalizeSectionField(name, ctx) as Record<string, unknown>;
+      // Spread into plain records: the pin enumerates whatever keys each
+      // producer wrote, declared on `FormField` or not.
+      const flat: Record<string, unknown> = { ...buildFlatFields({ ...ctx, fields: [name] })[0] };
+      const sectioned: Record<string, unknown> = { ...normalizeSectionField(name, ctx) };
 
       expect(new Set(Object.keys(flat))).toEqual(new Set([...Object.keys(sectioned), 'group']));
       for (const key of Object.keys(sectioned)) {
 
@@ -36,6 +36,7 @@
 import type { FormField } from '@object-ui/types';
 import { fromObjectSchema, warnUnresolvedTopLevelField, type SectionFieldsContext } from './sectionFields';
 import { mergeCustomFields } from './customFieldsMerge';
+import type { FormFieldWithGroup } from './fieldGroups';
 
 export interface FlatFieldsContext extends SectionFieldsContext {
   /**
@@ -92,6 +93,6 @@ export function buildFlatFields(ctx: FlatFieldsContext): FormField[] {
       // it the moment explicit sections exist, so stamping `group` onto
       // section-built fields would add a key with no reader.
       group: (field as any).group,
-    };
+    } satisfies FormFieldWithGroup;
   });
 }
@@ -106,7 +106,14 @@ const TABLE: Record<string, (f: (def: Record<string, unknown>) => any) => void>
   placeholder: (f) => expect(f({ placeholder: 'Type…' }).placeholder).toBe('Type…'),
   helpText: (f) => expect(f({ helpText: 'Hint' }).description).toBe('Hint'),
   readonly: (f) => expect(f({ readonly: true }).disabled).toBe(true),
-  immutable: (f) => expect(f({ immutable: true }).immutable).toBe(true),
+  immutable: (f) => {
+    // Locked once the record exists: an edit form draws it disabled, a create
+    // form leaves it editable, and no `immutable` key is carried (objectstack#25923).
+    const edit = normalizeSectionField({ field: 'ghost', immutable: true }, { ...ctx, mode: 'edit', recordId: 'r1' }) as any;
+    expect(edit.disabled).toBe(true);
+    expect(edit.immutable).toBeUndefined();
+    expect(f({ immutable: true }).disabled).toBeFalsy();
+  },
   required: (f) => expect(f({ required: true }).required).toBe(true),
   hidden: (f) => expect(f({ hidden: true }).hidden).toBe(true),
   colSpan: (f) => expect(f({ colSpan: 2 }).colSpan).toBe(2),
 
@@ -414,7 +414,14 @@ function resolveSectionEntry(
     );
   }
   if (fd.readonly != null) base.disabled = fd.readonly || base.disabled;
-  if (fd.immutable != null) base.immutable = fd.immutable;
+  // `immutable`: editable on create, locked once the record exists (spec
+  // FormField.immutable). It lands on `disabled`, the slot `readonly` above
+  // locks a control through, and asks the create-mode question `required`
+  // asks (`isCreateFormMode`), so a form seeded as create never locks it and an
+  // edit form always does. It used to be copied through as `immutable`, a key
+  // no renderer reads, so an edit form left the control editable
+  // (objectstack#25923).
+  if (fd.immutable === true && !isCreateFormMode(ctx)) base.disabled = true;
   if (fd.hidden != null) base.hidden = fd.hidden;
   if (fd.colSpan != null) base.colSpan = fd.colSpan;
   if (fd.span != null) base.span = fd.span;