diff --git a/.changeset/field-inline-edit.md b/.changeset/field-inline-edit.md
new file mode 100644
index 000000000..f29521a21
--- /dev/null
+++ b/.changeset/field-inline-edit.md
@@ -0,0 +1,13 @@
+---
+'@objectstack/spec': minor
+---
+
+feat(spec): `inlineEdit` on relationship fields for declarative master-detail
+
+A `master_detail`/`lookup` field can now declare `inlineEdit: true` (plus
+optional `inlineTitle` / `inlineColumns` / `inlineAmountField`) to mean "these
+child records are entered/edited inline within the parent's form". The intent
+lives in the data model: the parent's standard create/edit form then renders an
+atomic master-detail form (object fields + an editable child grid) with no form
+view config and no bespoke page. Use for line-item/composition children; leave
+off for associations (comments, attachments). Renderer support is in objectui.
diff --git a/examples/app-showcase/src/objects/task.object.ts b/examples/app-showcase/src/objects/task.object.ts
index 927bc5b43..8e0a2d356 100644
--- a/examples/app-showcase/src/objects/task.object.ts
+++ b/examples/app-showcase/src/objects/task.object.ts
@@ -24,7 +24,11 @@ export const Task = ObjectSchema.create({
 
   fields: {
     title: Field.text({ label: 'Title', required: true, searchable: true, maxLength: 200 }),
-    project: Field.masterDetail('showcase_project', { label: 'Project', required: true }),
+    // `inlineEdit` declares (in the data model) that tasks are entered inline
+    // within their project's form — so the standard New/Edit Project form
+    // auto-renders an atomic Tasks subtable, with no form view config and no
+    // bespoke page.
+    project: Field.masterDetail('showcase_project', { label: 'Project', required: true, inlineEdit: true, inlineTitle: 'Tasks' }),
     assignee: Field.text({ label: 'Assignee', maxLength: 200 }),
     status: Field.select({
       label: 'Status',
diff --git a/examples/app-showcase/src/views/project.view.ts b/examples/app-showcase/src/views/project.view.ts
index 4cb1d4963..e79b735e3 100644
--- a/examples/app-showcase/src/views/project.view.ts
+++ b/examples/app-showcase/src/views/project.view.ts
@@ -44,10 +44,10 @@ export const ProjectViews = defineView({
         { label: 'Project', columns: 2, fields: ['name', 'account', 'status', 'health', 'owner'] },
         { label: 'Budget & Schedule', columns: 2, fields: ['budget', 'spent', 'start_date', 'end_date'] },
       ],
-      // Config-driven master-detail (Tier 0): the standard New/Edit Project form
-      // renders its Tasks inline (FK + columns derived from showcase_task),
-      // saved as one atomic transaction — no bespoke page.
-      subforms: [{ childObject: 'showcase_task', title: 'Tasks', addLabel: 'Add task' }],
+      // No subforms here: the Tasks subtable is derived from the data model —
+      // showcase_task.project declares `inlineEdit: true`, so every standard
+      // Project form auto-renders it. (A view could still add `subforms` to
+      // override the derived columns/order.)
     },
   },
 });
diff --git a/packages/spec/src/data/field.zod.ts b/packages/spec/src/data/field.zod.ts
index 151785554..a52307636 100644
--- a/packages/spec/src/data/field.zod.ts
+++ b/packages/spec/src/data/field.zod.ts
@@ -403,6 +403,23 @@ export const FieldSchema = lazySchema(() => z.object({
   referenceFilters: z.array(z.string()).optional().describe('Filters applied to lookup dialogs (e.g. "active = true")'),
   writeRequiresMasterRead: z.boolean().optional().describe('If true, user needs read access to master record to edit this field'),
   deleteBehavior: z.enum(['set_null', 'cascade', 'restrict']).optional().default('set_null').describe('What happens if referenced record is deleted'),
+  /**
+   * Master-detail INLINE EDITING. On a child's `master_detail`/`lookup` field
+   * (whose `reference` is the parent object), set `inlineEdit: true` to declare
+   * "this child is entered/edited inline within the parent's form". The
+   * parent's standard create/edit form then renders an editable grid for these
+   * children and saves parent + children in ONE atomic transaction — no form
+   * view config and no bespoke page. The intent lives here in the data model;
+   * forms derive the UI. Use for true line-item/composition children (invoice
+   * lines, order items); leave off for associations (comments, attachments).
+   */
+  inlineEdit: z.boolean().optional().describe('Edit these child records inline within the parent object\'s form (atomic master-detail).'),
+  /** Optional section title for the inline grid (defaults to the child object label). */
+  inlineTitle: z.string().optional().describe('Title for the inline master-detail grid'),
+  /** Optional explicit grid columns for the inline editor (derived from the child object when omitted). */
+  inlineColumns: z.array(z.any()).optional().describe('Explicit columns for the inline grid (derived from the child object when omitted)'),
+  /** Optional numeric child field summed for the inline grid running total. */
+  inlineAmountField: z.string().optional().describe('Numeric child field summed for the inline grid total'),
 
   /** Calculation — CEL formula. Plain string accepted for back-compat; build emits canonical envelope. */
   expression: ExpressionInputSchema.optional().describe('Formula expression (CEL). e.g. F`record.amount * 0.1`'),