diff --git a/admin/tests/form/FormView.test.ts b/admin/tests/form/FormView.test.ts
index 37213a73..875c3f58 100644
--- a/admin/tests/form/FormView.test.ts
+++ b/admin/tests/form/FormView.test.ts
@@ -960,3 +960,35 @@ describe('picked relation labels across tab remount', () => {
     expect((maker().element as HTMLInputElement).value).not.toBe('1')
   })
 })
+
+describe('form extensions in compiled order (UI-SPEC C1, D-91)', () => {
+  /** The untabbed schema with an extension field cloned from name and placed by the server. */
+  function extendedSchema(place: (fields: ReturnType<typeof flatSchema>['body']['data']['fields'], field: (typeof formSchemaFixture)['data']['fields'][number]) => void) {
+    const schema = flatSchema()
+    const source = schema.body.data.fields.find((field) => field.name === 'name')!
+    const extension = { ...clone(source), name: 'subtitle', label: 'Subtitle' }
+    place(schema.body.data.fields, extension)
+    return schema
+  }
+
+  it('renders an extension field between two owner fields in compiled order', async () => {
+    // cabana compiled "subtitle after name": the SPA keeps the order it receives.
+    const schema = extendedSchema((fields, extension) => fields.splice(1, 0, extension))
+    const { wrapper } = await mountApp('/acme/demo/widgets/1', routes({ [`GET ${BASE}/schema/form`]: schema }))
+    expect(wrapper.findAll('[data-field]').map((field) => field.attributes('data-field'))).toEqual(['name', 'subtitle', 'code', 'weight'])
+    expect(wrapper.find('[role="tablist"]').exists()).toBe(false)
+  })
+
+  it('a tab-less form with an extension tab renders Basic, then the new tab', async () => {
+    const schema = extendedSchema((fields, extension) => fields.push({ ...extension, tab: 'Publishing' }))
+    const { wrapper } = await mountApp('/acme/demo/widgets/1', routes({ [`GET ${BASE}/schema/form`]: schema }))
+    const tabs = wrapper.findAll('[role="tab"]')
+    expect(tabs.map((tab) => tab.attributes('data-tab'))).toEqual(['default', 'tab:Publishing'])
+    expect(tabs[0]!.text()).toContain('Podstawowe')
+    expect(tabs[1]!.text()).toContain('Publishing')
+    expect(wrapper.findAll('[data-field]').map((field) => field.attributes('data-field'))).toEqual(['name', 'code', 'weight'])
+    await tabs[1]!.trigger('click')
+    await flushPromises()
+    expect(wrapper.findAll('[data-field]').map((field) => field.attributes('data-field'))).toEqual(['subtitle'])
+  })
+})