diff --git a/‎.changeset/21091-inline-row-form-join-key.md‎
Lines changed: 20 additions & 0 deletions b/‎.changeset/21091-inline-row-form-join-key.md‎
Lines changed: 20 additions & 0 deletions
diff --git a/‎packages/lint/src/validate-field-consumers.test.ts‎
Lines changed: 346 additions & 7 deletions b/‎packages/lint/src/validate-field-consumers.test.ts‎
Lines changed: 346 additions & 7 deletions
diff --git a/‎packages/lint/src/validate-field-consumers.ts‎
Lines changed: 236 additions & 13 deletions b/‎packages/lint/src/validate-field-consumers.ts‎
Lines changed: 236 additions & 13 deletions
diff --git a/‎packages/spec/api-surface/data.json‎
Lines changed: 2 additions & 0 deletions b/‎packages/spec/api-surface/data.json‎
Lines changed: 2 additions & 0 deletions
diff --git a/‎packages/spec/export-origins/data.json‎
Lines changed: 2 additions & 0 deletions b/‎packages/spec/export-origins/data.json‎
Lines changed: 2 additions & 0 deletions
diff --git a/‎packages/spec/src/data/index.ts‎
Lines changed: 4 additions & 2 deletions b/‎packages/spec/src/data/index.ts‎
Lines changed: 4 additions & 2 deletions
diff --git a/‎packages/spec/src/data/inline-grid-columns.test.ts‎
Lines changed: 126 additions & 1 deletion b/‎packages/spec/src/data/inline-grid-columns.test.ts‎
Lines changed: 126 additions & 1 deletion
diff --git a/‎packages/spec/src/data/inline-grid-columns.ts‎
Lines changed: 88 additions & 1 deletion b/‎packages/spec/src/data/inline-grid-columns.ts‎
Lines changed: 88 additions & 1 deletion
@@ -0,0 +1,20 @@
+---
+'@objectstack/spec': minor
+'@objectstack/lint': patch
+---
+
+`deriveInlineRowFormFields` and `isInlineRowFormOffered` (`@objectstack/spec/data`) state which fields an inline master-detail grid's per-row expand form draws and when that form is offered, and `field-no-consumers` stops calling four more kinds of in-use child field "inert" (#21091).
+
+Clause-②: yes (widening)
+
+- **`@objectstack/spec`.** Two new exports from `@objectstack/spec/data`, beside `deriveInlineGridColumns`:
+  - `deriveInlineRowFormFields(def, { relationshipField?, exclude? })` returns the child field names of the per-row expand form, in the child's field order. It skips the same system, audit, tenancy, ownership and sort-position names as the grid, the relationship field, `exclude`, `system` and `hidden` fields, and the computed types (`formula`, `summary`, `rollup`, `autonumber`, `auto_number`). Unlike the grid it keeps `readonly` fields and the rich types a cell cannot edit (`richtext`, `json`, `markdown`, …), so the derived grid's columns are always a subset of its fields.
+  - `isInlineRowFormOffered({ inlineMode?, formFields?, columns? })` is `true` when the form factor is `form`, or when the form has more fields than the grid has columns.
+  - Both are the renderer's current rule, reproduced exactly. No schema accepts anything new or refuses anything new.
+- **`@objectstack/lint`.** `os validate` no longer warns that these fields are inert:
+  - a `lookup` field that sets `inlineEdit`: it is the inline grid's join key, read whatever columns the grid draws, as a `master_detail` field already was;
+  - a field a derived inline grid's per-row expand form draws, through `deriveInlineRowFormFields`, such as a `readonly`, `richtext` or `json` child field;
+  - a field named in an `object-master-detail-form` detail entry's `formFields`, now read against the entry's `childObject` instead of the block's object. When the form is never offered for the list, the list is reported as a carrier. That is judged on an entry that names both its `relationshipField` and its `columns` under its declared `inlineMode` or none. On any other entry it is judged under a declared `inlineMode` where the grid can be counted: authored `columns`, or the derived grid of a named `relationshipField`. Otherwise the list is credited as drawn;
+  - a field named in a `record:line_items` block's `columns`, `relationshipField`, `amountField`, `sort` or `filter`, now read against the block's `childObject`.
+
+  A parent field that shares a name with one of those child fields was credited in the child's place, and is now reported if nothing else reads it. A child field nothing draws or names, such as a `hidden` one, is still reported.
@@ -803,6 +803,7 @@
     "defineSeed (function)",
     "deriveFieldGroupLayout (function)",
     "deriveInlineGridColumns (function)",
+    "deriveInlineRowFormFields (function)",
     "deriveRecordFlowSurface (function)",
     "deriveRecordSurface (function)",
     "describeManagedApiMethodConflicts (function)",
@@ -852,6 +853,7 @@
     "isGlobalUnique (function)",
     "isIncoherentAggregate (function)",
     "isInjectedColumnDefinition (function)",
+    "isInlineRowFormOffered (function)",
     "isKnownFilterToken (function)",
     "isLegacyApiMethod (function)",
     "isMaskedOnReadFieldType (function)",
 
@@ -790,6 +790,7 @@
     "defineSeed": "src/data/seed.zod.ts#defineSeed (function)",
     "deriveFieldGroupLayout": "src/data/field-group-layout.ts#deriveFieldGroupLayout (function)",
     "deriveInlineGridColumns": "src/data/inline-grid-columns.ts#deriveInlineGridColumns (function)",
+    "deriveInlineRowFormFields": "src/data/inline-grid-columns.ts#deriveInlineRowFormFields (function)",
     "deriveRecordFlowSurface": "src/data/record-surface.ts#deriveRecordFlowSurface (function)",
     "deriveRecordSurface": "src/data/record-surface.ts#deriveRecordSurface (function)",
     "describeManagedApiMethodConflicts": "src/data/managed-api-affordance.ts#describeManagedApiMethodConflicts (function)",
@@ -839,6 +840,7 @@
     "isGlobalUnique": "src/data/field.zod.ts#isGlobalUnique (function)",
     "isIncoherentAggregate": "src/data/aggregation-policy.ts#isIncoherentAggregate (function)",
     "isInjectedColumnDefinition": "src/data/injected-system-column-provenance.ts#isInjectedColumnDefinition (function)",
+    "isInlineRowFormOffered": "src/data/inline-grid-columns.ts#isInlineRowFormOffered (function)",
     "isKnownFilterToken": "src/data/context-tokens.zod.ts#isKnownFilterToken (function)",
     "isLegacyApiMethod": "src/data/api-derivation.ts#isLegacyApiMethod (function)",
     "isMaskedOnReadFieldType": "src/data/masked-field-types.ts#isMaskedOnReadFieldType (function)",
 
@@ -306,8 +306,10 @@ export * from './search-fields';
 export * from './field-group-layout';
 
 // Default inline-grid columns — the single source of which child fields an
-// inline master-detail grid draws when its author listed none. Consumed by the
-// renderer and credited by lint's `field-no-consumers`, so the two agree.
+// inline master-detail grid draws when its author listed none, and of which
+// fields its per-row expand form draws (and when that form is offered).
+// Consumed by the renderer and credited by lint's `field-no-consumers`, so the
+// two agree.
 export * from './inline-grid-columns';
 
 // record-surface derivation (ADR-0085 §5) — the single source for how a record's
 
@@ -1,7 +1,12 @@
 // Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
 
 import { describe, it, expect } from 'vitest';
-import { DEFAULT_MAX_INLINE_GRID_COLUMNS, deriveInlineGridColumns } from './inline-grid-columns';
+import {
+  DEFAULT_MAX_INLINE_GRID_COLUMNS,
+  deriveInlineGridColumns,
+  deriveInlineRowFormFields,
+  isInlineRowFormOffered,
+} from './inline-grid-columns';
 import { InlineGridColumnSchema } from './field.zod';
 
 /**
@@ -166,3 +171,123 @@ describe('deriveInlineGridColumns', () => {
     for (const col of cols) expect(InlineGridColumnSchema.parse(col)).toEqual(col);
   });
 });
+
+/**
+ * [#21091] The fields of an inline grid's per-row expand form. The first two
+ * fixtures are the renderer's own `deriveFormFields` cases (objectui
+ * `packages/plugin-form/src/deriveMasterDetail.test.ts` at the `.objectui-sha`
+ * pin `31971ff1e28f`), asserted here as whole lists rather than as the
+ * `toContain` probes they are there.
+ */
+describe('deriveInlineRowFormFields', () => {
+  const taskSchema = {
+    name: 'showcase_task',
+    fields: {
+      id: { type: 'text', system: true },
+      title: { type: 'text', label: 'Title', required: true },
+      status: { type: 'select', label: 'Status', options: [{ label: 'To Do', value: 'todo' }] },
+      estimate_hours: { type: 'number', label: 'Estimate (h)' },
+      budget: { type: 'currency', label: 'Budget' },
+      due_date: { type: 'date', label: 'Due Date' },
+      assignee: { type: 'lookup', label: 'Assignee', reference: 'user' },
+      project: { type: 'master_detail', label: 'Project', reference: 'showcase_project', required: true },
+      health: { type: 'formula', label: 'Health', expression: 'x' },
+      created_at: { type: 'datetime' },
+    },
+  };
+
+  it('returns the business fields in field order, skipping system, audit, the relationship and computed types', () => {
+    expect(deriveInlineRowFormFields(taskSchema, { relationshipField: 'project' })).toEqual([
+      'title', 'status', 'estimate_hours', 'budget', 'due_date', 'assignee',
+    ]);
+  });
+
+  it('keeps the rich input types the grid omits, and drops the computed ones', () => {
+    const rich = {
+      fields: {
+        title: { type: 'text', required: true },
+        parent: { type: 'master_detail', reference: 'p', required: true },
+        notes: { type: 'textarea' },
+        cover: { type: 'image' },
+        attachment: { type: 'file' },
+        total: { type: 'summary' },
+      },
+    };
+    expect(deriveInlineRowFormFields(rich, { relationshipField: 'parent' })).toEqual(['title', 'notes', 'cover', 'attachment']);
+  });
+
+  it('keeps `readonly` fields and every type a cell cannot edit; drops `system`, `hidden`, sort positions and `exclude`', () => {
+    const def = {
+      fields: {
+        line_no: { type: 'number' },
+        sort_order: { type: 'number' },
+        frozen: { type: 'text', readonly: true },
+        secret: { type: 'text', hidden: true },
+        internal: { type: 'text', system: true },
+        owner: { type: 'lookup', reference: 'sys_user' },
+        body: { type: 'richtext' },
+        meta: { type: 'json' },
+        place: { type: 'location' },
+        page: { type: 'html' },
+        doc: { type: 'markdown' },
+        seq: { type: 'autonumber' },
+        roll: { type: 'rollup' },
+        note: { type: 'text' },
+      },
+    };
+    expect(deriveInlineRowFormFields(def, { exclude: ['note'] })).toEqual(['frozen', 'body', 'meta', 'place', 'page', 'doc']);
+  });
+
+  it('without a relationship field, the relationship is an ordinary field', () => {
+    expect(deriveInlineRowFormFields(taskSchema)).toContain('project');
+  });
+
+  it('returns no fields for a definition with no field map', () => {
+    expect(deriveInlineRowFormFields(undefined)).toEqual([]);
+    expect(deriveInlineRowFormFields(null)).toEqual([]);
+    expect(deriveInlineRowFormFields('line')).toEqual([]);
+    expect(deriveInlineRowFormFields({ name: 'line' })).toEqual([]);
+  });
+
+  it('the derived grid draws a subset of the derived form: the form has every column, in the same order', () => {
+    const wide = {
+      fields: {
+        ...taskSchema.fields,
+        body: { type: 'richtext' },
+        frozen: { type: 'number', readonly: true },
+        ...Object.fromEntries(Array.from({ length: 6 }, (_, i) => [`f${i}`, { type: 'text' }])),
+      },
+    };
+    const opts = { relationshipField: 'project' };
+    const form = deriveInlineRowFormFields(wide, opts);
+    const columns = deriveInlineGridColumns(wide, opts).map((c) => c.name);
+    expect(columns.length).toBeGreaterThan(0);
+    expect(form.filter((name) => columns.includes(name))).toEqual(columns);
+    expect(form.filter((name) => !columns.includes(name))).toEqual(['body', 'frozen']);
+  });
+});
+
+/**
+ * [#21091] When an inline grid offers its per-row expand form — the condition
+ * objectui's `MasterDetailForm` applies at the `.objectui-sha` pin
+ * `31971ff1e28f` before it hands a row an expand control.
+ */
+describe('isInlineRowFormOffered', () => {
+  it('always in the `form` factor: the row form IS the editor', () => {
+    expect(isInlineRowFormOffered({ inlineMode: 'form', formFields: ['a'], columns: [{ name: 'a' }, { name: 'b' }] })).toBe(true);
+    expect(isInlineRowFormOffered({ inlineMode: 'form' })).toBe(true);
+  });
+
+  it('in the `grid` factor, only when the form has more fields than the grid has columns', () => {
+    expect(isInlineRowFormOffered({ inlineMode: 'grid', formFields: ['a', 'b', 'c'], columns: [{ name: 'a' }, { name: 'b' }] })).toBe(true);
+    expect(isInlineRowFormOffered({ inlineMode: 'grid', formFields: ['a', 'b'], columns: [{ name: 'a' }, { name: 'b' }] })).toBe(false);
+    expect(isInlineRowFormOffered({ inlineMode: 'grid', formFields: ['a'], columns: [{ name: 'a' }, { name: 'b' }] })).toBe(false);
+  });
+
+  it('with no form factor, the same count decides; an absent list counts as empty', () => {
+    expect(isInlineRowFormOffered({ formFields: ['a'], columns: [] })).toBe(true);
+    expect(isInlineRowFormOffered({ formFields: ['a'] })).toBe(true);
+    expect(isInlineRowFormOffered({ columns: [{ name: 'a' }] })).toBe(false);
+    expect(isInlineRowFormOffered({})).toBe(false);
+  });
+});
@@ -2,7 +2,8 @@
 
 /**
  * Default inline-grid columns — the single source of WHICH child fields an
- * inline master-detail grid draws when its author listed no columns.
+ * inline master-detail grid draws when its author listed no columns, and of
+ * which fields its per-row expand form draws when its author listed none.
  *
  * Two carriers draw a grid of a child object's records inside the parent's
  * form, and both say "derived from the child object when omitted":
@@ -54,6 +55,35 @@
  * renderer hydrates them from the child field, exactly as it hydrates an
  * identity-only column an author wrote. So the derived list is precisely the
  * `inlineColumns` an author could have written to draw the same grid.
+ *
+ * ## The per-row expand form ({@link deriveInlineRowFormFields})
+ *
+ * Each row of the grid can open a full form for that row, and that form draws
+ * more than the grid: it has room for the rich inputs a cell cannot hold. Its
+ * fields are derived from the child object too, by a broader rule — measured
+ * against objectui at the `.objectui-sha` pin `31971ff1e28f`
+ * (`packages/plugin-form/src/deriveMasterDetail.ts`, `deriveFormFields`).
+ * Every child field, in the field map's own order, except:
+ *
+ *   - a name in {@link INLINE_GRID_SYSTEM_FIELDS} or
+ *     {@link INLINE_GRID_SORT_FIELDS} — the same two sets the grid skips;
+ *   - the relationship field back to the parent, and any name in `exclude`;
+ *   - a field flagged `system` or `hidden` — NOT `readonly`: the form shows a
+ *     read-only value, where a cell would only waste the width;
+ *   - a field whose `type` is in {@link INLINE_ROW_FORM_NON_INPUT_TYPES}, the
+ *     computed types nobody types into. `richtext`, `json`, `markdown` and the
+ *     other types a cell cannot edit stay in.
+ *
+ * Every type the form skips the grid skips too, so with the same
+ * `relationshipField` and `exclude`, the derived grid's columns are always a
+ * subset of the derived form's fields.
+ *
+ * The form is not always offered ({@link isInlineRowFormOffered}). Measured in
+ * the same pin's `MasterDetailForm.tsx`, it is offered when it adds something:
+ * always when the collection's form factor is `form` (the row form IS the
+ * editor there), and otherwise only when the form has more fields than the
+ * grid has columns. A thin grid whose columns already cover every field shows
+ * no expand control.
  */
 
 /** Default-visible column budget of a derived inline grid; the rest are `defaultHidden`. */
@@ -209,3 +239,60 @@ export function deriveInlineGridColumns(
   }
   return candidates.map(({ name }) => (visible.has(name) ? { name } : { name, defaultHidden: true }));
 }
+
+/**
+ * Field types the per-row expand form leaves out: the computed, server-derived
+ * values nobody types. Narrower than {@link INLINE_GRID_NON_EDITABLE_TYPES} —
+ * the form has room for the rich inputs a cell cannot hold. As there, the
+ * names that are not `FieldType` members are the renderer's legacy tolerances.
+ */
+const INLINE_ROW_FORM_NON_INPUT_TYPES: ReadonlySet<unknown> = new Set([
+  'formula', 'summary', 'rollup', 'autonumber', 'auto_number',
+]);
+
+/**
+ * Derive the fields of an inline master-detail grid's per-row expand form
+ * from the child object's definition (or any bare record shaped like one:
+ * `{ fields }` with the field map the spec declares). The rule is in the
+ * module note; the renderer offers the form only when
+ * {@link isInlineRowFormOffered} says so.
+ *
+ * `relationshipField` is the child's field back to the parent — excluded, as
+ * in {@link deriveInlineGridColumns}. `exclude` drops further names.
+ *
+ * Returns `[]` when the definition carries no field map.
+ */
+export function deriveInlineRowFormFields(
+  def: unknown,
+  opts: { relationshipField?: string; exclude?: readonly string[] } = {},
+): string[] {
+  const fields = prop(def, 'fields');
+  if (!fields || typeof fields !== 'object') return [];
+  const exclude = new Set<string>([...(opts.exclude ?? []), ...(opts.relationshipField ? [opts.relationshipField] : [])]);
+
+  const out: string[] = [];
+  for (const [name, field] of Object.entries(fields as AnyRec)) {
+    if (INLINE_GRID_SYSTEM_FIELDS.has(name) || exclude.has(name) || INLINE_GRID_SORT_FIELDS.has(name)) continue;
+    if (prop(field, 'system') || prop(field, 'hidden')) continue;
+    if (INLINE_ROW_FORM_NON_INPUT_TYPES.has(prop(field, 'type'))) continue;
+    out.push(name);
+  }
+  return out;
+}
+
+/**
+ * Whether an inline child collection offers its per-row expand form: always
+ * when its form factor is `form`, else only when the form has more fields
+ * than the grid has columns.
+ *
+ * `inlineMode` is the collection's RESOLVED form factor (`grid` / `form`), as
+ * the renderer resolved it. `formFields` and `columns` are the lists the
+ * collection draws, authored or derived; only their lengths are read.
+ */
+export function isInlineRowFormOffered(opts: {
+  inlineMode?: 'grid' | 'form';
+  formFields?: readonly unknown[];
+  columns?: readonly unknown[];
+}): boolean {
+  return opts.inlineMode === 'form' || (opts.formFields?.length ?? 0) > (opts.columns?.length ?? 0);
+}