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diff --git a/‎packages/lint/src/validate-component-props.test.ts‎
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diff --git a/‎packages/spec/src/data/inline-grid-columns.ts‎
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@@ -0,0 +1,36 @@
+---
+'@objectstack/spec': minor
+---
+
+feat(spec)!: an `object-master-detail-form` detail entry's `sortField` is retired — the console derives the line-position field from the child object and reads no authored value (#21589)
+
+Clause-②: no (narrowing)
+
+<!-- adr-0087: registered object-master-detail-form-detail-sort-field-removed, object-master-detail-form-detail-sort-field-retired -->
+
+**BREAKING** — an accept-set narrowing on a published authoring surface, shipped as `minor` under the launch-window convention for accept-set narrowings.
+
+`ComponentPropsMap['object-master-detail-form'].details[].sortField` named the child field the line grid stamps with each line's position on drag-reorder. The console stopped reading it: the field it stamps is derived from the child object, and the pinned console crossed that change while the spec still declared the key. So an authored `sortField` went through `os validate` clean and was dropped, and a drag-reorder stamped the derived field, or none (ADR-0049 enforce-or-remove).
+
+### FROM → TO
+
+| before | what to write instead |
+| --- | --- |
+| `details: [{ childObject: 'crm_invoice_line', sortField: 'line_no' }]` | delete `sortField`. The grid stamps the child object's first field named `position`, `sort_order`, `sequence`, `line_no`, `line_number` or `sort`. |
+| `sortField` naming a field outside that list | give the child object one of those fields; the line order is kept there. |
+| an entry that names `relationshipField` and at least one column and gives every column a `type` | unchanged: the renderer keeps that entry exactly as authored, loads no child schema for it, and stamps no line position, before and after the upgrade alike. |
+
+**The one-line fix: delete `sortField` from every `object-master-detail-form` detail entry.** `os migrate meta --from 17` lists the mechanical edits for existing sources; apply them by hand.
+
+**What an author now sees.** Writing the key fails `tsc` (its input type is the retired-key mark), and `os validate`, `os build` and `os lint` report it as a `component-props-invalid` warning carrying the prescription at `properties.details.N.sortField`. A page that carries it still saves and loads: a page component's `properties` is not parsed on the metadata save or load path.
+
+### The retirement kit
+
+- **Tombstone.** `sortField` is a `retiredKey()` on the strict detail entry. Its prescription prints the derived field names from their one declaration, a module reached by relative import only (`data/inline-grid-sort-fields.ts`), which the derived inline-grid columns read too.
+- **D2 conversion `object-master-detail-form-detail-sort-field-removed`** (step 18, retired from the load path): a lossless delete of `sortField` from every `properties.details[]` entry of an `object-master-detail-form`, scoped by component type and by position. Stored `sys_metadata` pages and built artifacts replay it, one notice per entry.
+- **D3 entry `object-master-detail-form-detail-sort-field-retired`** carries the judgment the delete cannot make: whether the child object declares the field the line order is kept in.
+- **`RETIRED_KEYS_BY_MAJOR[18]`** registers the nested key `ui/ObjectMasterDetailFormProps:details.sortField`.
+- **`record:line_items`' answer to `sortField`** no longer sends the author to the detail entry: no block takes an authored `sortField` any more.
+- **No deprecation window**: the writer census is zero.
+
+**Measured producers: none.** On origin/main 9a4182a752, no `object-master-detail-form` detail entry in `examples/`, `apps/`, `packages/`, `skills/` or `content/docs/` writes `sortField`, against the sibling detail-entry key `addLabel` on the showcase project workspace's entry as the control, through the same instrument. At the objectui pin `89cad75d5570` the only detail entries that write it are probes asserting that nothing reads it. Deployed metadata NOT MEASURED.
@@ -1084,7 +1084,7 @@ Sort field and direction pair
 | **formFields** | `string[]` | optional | Child field names for the per-row expand form. When omitted they are derived from the child object's fields — except on an entry that names both `relationshipField` and at least one column, which is kept as authored: nothing is derived, and the per-row form is offered only when `inlineMode` is 'form', where it draws the child object's full field list |
 | **inlineMode** | `Enum<'grid' \| 'form'>` | optional | Inline-edit form factor: 'grid' = editable cells; 'form' = read-only list + per-row full form. When omitted it is resolved from the relationship field's `inlineEdit`, else from the child object's shape — except on an entry that names both `relationshipField` and at least one column, which is kept as authored: nothing is resolved, the collection renders as a grid, and the per-row form is offered only when `formFields` lists more fields than `columns` |
 | **amountField** | `string` | optional | Numeric child column summed for the running total and the `totalField` rollup. When omitted it is picked from the grid's number and currency columns: a computed one, else one named `amount`, `total`, `subtotal`, `line_total`, `line_amount` or `net_amount`, else the last currency column, else the last numeric one — except on an entry that names `relationshipField` and at least one column and gives every column a `type`. The renderer keeps that entry exactly as authored, so nothing is picked. With no `amountField` authored or picked, the sums read a child column named `amount`, and the grid shows a running total only when `totalField` is set |
-| **sortField** | `string` | optional | Child field holding the line sort position, stamped on drag-reorder. When omitted it is the child object's first field named `position`, `sort_order`, `sequence`, `line_no`, `line_number` or `sort`, if it has one — except on an entry that names `relationshipField` and at least one column and gives every column a `type`. The renderer keeps that entry exactly as authored: nothing is derived, the grid stamps no line position, and a drag-reorder is not saved |
+| **sortField** | `never` | optional | [REMOVED] `object-master-detail-form` property `details[].sortField` was removed in @objectstack/spec 17 (ADR-0087 D2) — the console reads no authored value: the field the line grid stamps with each line's position on drag-reorder is derived from the child object, so an authored `sortField` was accepted and dropped. Delete the key. For a drag-reorder to be saved, give the child object a field named `position` / `sort_order` / `sequence` / `line_no` / `line_number` / `sort`: the renderer stamps the child's first field with one of those names — except on an entry that names `relationshipField` and at least one column and gives every column a `type`, which the renderer keeps exactly as authored and stamps no line position on. Run `os migrate meta --from 17` to list the mechanical edits for existing sources; apply them by hand. |
 | **totalField** | `string` | optional | Parent field to receive the rolled-up sum |
 | **title** | `string` | optional | Section title |
 | **minRows** | `number` | optional | Minimum number of rows |
 
@@ -91,6 +91,32 @@ describe('validateComponentProps — undeclared keys', () => {
     }
   });
 
+  // #21589 — an `object-master-detail-form` detail entry's `sortField` is a
+  // retiredKey tombstone, one array level down. The same door, the same
+  // warning: the finding names the entry's key, and the page is never refused.
+  it('reports a retired detail-entry `sortField` as a warning carrying the prescription, at the entry\'s key', () => {
+    const findings = validateComponentProps(
+      stackWith([
+        {
+          type: 'object-master-detail-form',
+          properties: {
+            objectName: 'invoice',
+            details: [
+              { title: 'Payments', childObject: 'invoice_payment' },
+              { title: 'Lines', childObject: 'invoice_line', sortField: 'line_no' },
+            ],
+          },
+        },
+      ]),
+    );
+    expect(findings).toHaveLength(1);
+    const [f] = findings;
+    expect(f.severity).toBe('warning');
+    expect(f.rule).toBe(COMPONENT_PROPS_INVALID);
+    expect(f.path).toBe('pages[0].regions[0].components[0].properties.details.1.sortField');
+    expect(f.message).toContain('`object-master-detail-form` property `details[].sortField` was removed in @objectstack/spec 17');
+  });
+
   it('walks components nested inside `properties` (tabs items → children)', () => {
     const findings = validateComponentProps(
       stackWith([
 
@@ -12473,6 +12473,208 @@ const dashboardWidgetChartConfigStructureRemoved: MetadataConversion = {
   },
 };
 
+/**
+ * `object-master-detail-form`'s detail entry `sortField` leaves the contract
+ * (protocol 18, #21589 — ADR-0049 enforce-or-remove; the spec half of
+ * objectui#11070 round 9, the direction recorded on #21220's landing and
+ * mirrored on objectui#11396 ③: tombstone plus ADR-0087).
+ *
+ * **A pure lossless delete.** The console stopped reading the authored
+ * override at objectui `0a3e5409f`, and the `.objectui-sha` pin
+ * (`89cad75d5570`) is past it: `MasterDetailDetailConfig` has no `sortField`
+ * member (`plugin-form/src/MasterDetailForm.tsx:83`), and the field the line
+ * grid stamps with each line's position is the one `deriveDetail` derives from
+ * the child object (`deriveMasterDetail.ts:540`), handed to the grid as
+ * `sort_field` (`:874`). An authored value changed nothing on that console, so
+ * deleting it preserves observed behaviour exactly; the line order is kept by
+ * the child object's own field, which the entry's tombstone names. The
+ * tombstone refuses the key for a live author (advisory, via the props lint:
+ * `PageComponentSchema.properties` is an open bag).
+ *
+ * ⚠️ Scoped by component `type` and by POSITION — `properties.details[]` of an
+ * `object-master-detail-form` — never by key name: `sortField` is an ordinary
+ * name for an open-namespace component's own prop, and the fixture's
+ * non-carrier control is such a component authoring the same shape. A
+ * `details` entry that is not an object rides through untouched (the props
+ * gate reports it; it is not this entry's to fix).
+ *
+ * Zero authored occurrences in this repo's corpora at the retirement — no
+ * detail entry under `examples/`, `apps/`, `packages/`, `skills/` or
+ * `content/docs/` writes the key (control: the sibling detail-entry key
+ * `addLabel`, authored on the showcase project workspace's entry, same
+ * instrument, origin/main 9a4182a752) — so this entry exists for stored
+ * `sys_metadata` rows and for authors outside the repo, which the census could
+ * not measure.
+ */
+const objectMasterDetailFormDetailSortFieldRemoved: MetadataConversion = {
+  id: 'object-master-detail-form-detail-sort-field-removed',
+  toMajor: 18,
+  retiredFromLoadPath: true,
+  retiredAfter: '17.6.0',
+  surface: 'page.component.object-master-detail-form.details[].sortField',
+  summary:
+    "object-master-detail-form detail entry prop 'sortField' removed (the console reads no authored "
+    + 'value: the line grid stamps the field it derives from the child object, so the key was '
+    + "accepted and dropped; delete the key — the child object's own position field keeps the line order)",
+  apply(stack, emit) {
+    return mapPageComponents(stack, (component, path) => {
+      if (component.type !== 'object-master-detail-form') return component;
+      const properties = component.properties;
+      if (!isDict(properties) || !Array.isArray(properties.details)) return component;
+      const details = properties.details as unknown[];
+      let changed = false;
+      const nextDetails = details.map((entry, i) => {
+        if (!isDict(entry) || !('sortField' in entry)) return entry;
+        changed = true;
+        return stripKeys(entry, ['sortField'], emit, `${path}.properties.details[${i}]`);
+      });
+      if (!changed) return component;
+      return { ...component, properties: { ...properties, details: nextDetails } };
+    });
+  },
+  fixture: {
+    before: {
+      pages: [
+        {
+          name: 'invoice_entry',
+          regions: [
+            {
+              name: 'main',
+              components: [
+                // The carrier: a detail entry authoring the retired override,
+                // beside an entry WITHOUT it, which rides through untouched.
+                {
+                  type: 'object-master-detail-form',
+                  id: 'm1',
+                  properties: {
+                    objectName: 'crm_invoice',
+                    details: [
+                      { childObject: 'crm_invoice_line', title: 'Lines', sortField: 'line_no' },
+                      { childObject: 'crm_invoice_payment', title: 'Payments' },
+                    ],
+                  },
+                },
+                // ⚠️ The same shape on a component that is NOT an
+                // `object-master-detail-form` — its own prop, not this entry's
+                // key. Untouched: the strip is scoped by component type.
+                {
+                  type: 'acme:line_editor',
+                  id: 'x1',
+                  properties: { details: [{ childObject: 'crm_invoice_line', sortField: 'position' }] },
+                },
+                // A block whose entries carry no `sortField`, and one with a
+                // non-object entry: both ride through untouched, by reference.
+                {
+                  type: 'object-master-detail-form',
+                  id: 'm2',
+                  properties: { objectName: 'crm_order', details: [{ childObject: 'crm_order_line' }, 'crm_order_note'] },
+                },
+                // The nested position (#6775's lesson): a block inside a
+                // card's `children` is still a component.
+                {
+                  type: 'page:card',
+                  id: 'c1',
+                  properties: {
+                    children: [
+                      {
+                        type: 'object-master-detail-form',
+                        id: 'm3',
+                        properties: {
+                          objectName: 'crm_quote',
+                          details: [{ childObject: 'crm_quote_line', sortField: 'position' }],
+                        },
+                      },
+                    ],
+                  },
+                },
+              ],
+            },
+          ],
+        },
+        // The named-slot shape (#6776): the block authored into a slotted page.
+        {
+          name: 'invoice_entry_detail',
+          kind: 'slotted',
+          regions: [],
+          slots: {
+            details: {
+              type: 'object-master-detail-form',
+              id: 'm4',
+              properties: {
+                objectName: 'crm_invoice',
+                details: [{ childObject: 'crm_invoice_line', sortField: 'sequence' }],
+              },
+            },
+          },
+        },
+      ],
+    },
+    after: {
+      pages: [
+        {
+          name: 'invoice_entry',
+          regions: [
+            {
+              name: 'main',
+              components: [
+                {
+                  type: 'object-master-detail-form',
+                  id: 'm1',
+                  properties: {
+                    objectName: 'crm_invoice',
+                    details: [
+                      { childObject: 'crm_invoice_line', title: 'Lines' },
+                      { childObject: 'crm_invoice_payment', title: 'Payments' },
+                    ],
+                  },
+                },
+                {
+                  type: 'acme:line_editor',
+                  id: 'x1',
+                  properties: { details: [{ childObject: 'crm_invoice_line', sortField: 'position' }] },
+                },
+                {
+                  type: 'object-master-detail-form',
+                  id: 'm2',
+                  properties: { objectName: 'crm_order', details: [{ childObject: 'crm_order_line' }, 'crm_order_note'] },
+                },
+                {
+                  type: 'page:card',
+                  id: 'c1',
+                  properties: {
+                    children: [
+                      {
+                        type: 'object-master-detail-form',
+                        id: 'm3',
+                        properties: { objectName: 'crm_quote', details: [{ childObject: 'crm_quote_line' }] },
+                      },
+                    ],
+                  },
+                },
+              ],
+            },
+          ],
+        },
+        {
+          name: 'invoice_entry_detail',
+          kind: 'slotted',
+          regions: [],
+          slots: {
+            details: {
+              type: 'object-master-detail-form',
+              id: 'm4',
+              properties: { objectName: 'crm_invoice', details: [{ childObject: 'crm_invoice_line' }] },
+            },
+          },
+        },
+      ],
+    },
+    // Three notices: the region-level entry, the nested one and the slotted
+    // one. The open-namespace sibling and the entries without the key emit none.
+    expectedNotices: 3,
+  },
+};
+
 /**
  * `object.tenancy.organizationField` leaves the authorable surface (protocol
  * 18, #19054 — ADR-0049 enforce-or-remove; maintainer ruling 2026-09-18,
@@ -14598,6 +14800,7 @@ const MAJOR_18_CONVERSIONS: readonly OrderedConversion[] = [
   { conversion: objectGridDefaultSortRemoved, order: 14 },
   { conversion: objectGridResizableColumnsRemoved, order: 57 },
   { conversion: objectKanbanQuickAddRemoved, order: 15 },
+  { conversion: objectMasterDetailFormDetailSortFieldRemoved, order: 60 },
   { conversion: objectTenancyOrganizationFieldRemoved, order: 35 },
   { conversion: pageAssignedProfilesRemoved, order: 31 },
   { conversion: pageComponentFilterRecordToRuleArray, order: 36 },
 
@@ -86,6 +86,11 @@
  * no expand control.
  */
 
+// The sort-position names live in their own module, reached by relative
+// import only: the retired `object-master-detail-form` detail entry
+// `sortField`'s prescription prints the same list (`ui/component.zod.ts`).
+import { INLINE_GRID_SORT_FIELDS } from './inline-grid-sort-fields';
+
 /** Default-visible column budget of a derived inline grid; the rest are `defaultHidden`. */
 export const DEFAULT_MAX_INLINE_GRID_COLUMNS = 6;
 
@@ -107,11 +112,6 @@ const INLINE_GRID_SYSTEM_FIELDS: ReadonlySet<string> = new Set([
   'organization_id', 'tenant_id', 'space', 'owner',
 ]);
 
-/** Field names that hold a line's sort position: the grid stamps them on drag-reorder. */
-const INLINE_GRID_SORT_FIELDS: ReadonlySet<string> = new Set([
-  'position', 'sort_order', 'sequence', 'line_no', 'line_number', 'sort',
-]);
-
 /** Field types a line-item cell cannot edit. File-family types are absent: they render an upload cell. */
 const INLINE_GRID_NON_EDITABLE_TYPES: ReadonlySet<unknown> = new Set([
   'formula', 'summary', 'rollup', 'autonumber', 'auto_number',