diff --git a/app/(main)/admin/__tests__/extension-multiline-config.test.tsx b/app/(main)/admin/__tests__/extension-multiline-config.test.tsx
new file mode 100644
index 000000000..40501bd9b
--- /dev/null
+++ b/app/(main)/admin/__tests__/extension-multiline-config.test.tsx
@@ -0,0 +1,55 @@
+import { fireEvent, render, screen, waitFor } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { useAdminUnsavedStore } from '@/stores/admin-unsaved-store';
+
+// A plugin setting declared `multiline` (the assistant's instructions) gets a
+// text box that keeps line breaks; other strings stay one-line inputs.
+
+const calls: Array<{ url: string; method: string; body: Record<string, unknown> }> = [];
+const routes: Record<string, unknown> = {};
+vi.mock('@/lib/browser-navigation', () => ({
+  apiFetch: vi.fn(async (url: string, init?: RequestInit) => {
+    const method = init?.method ?? 'GET';
+    calls.push({ url, method, body: JSON.parse(String(init?.body ?? '{}')) });
+    const data = method === 'GET' ? routes[url.split('?')[0]] ?? {} : { ok: true };
+    return new Response(JSON.stringify(data), { status: 200, headers: { 'content-type': 'application/json' } });
+  }),
+  withBasePath: (p: string) => p,
+}));
+vi.mock('next/link', () => ({ default: ({ children }: { children: React.ReactNode }) => children }));
+vi.mock('../_pages/use-extension-icons', () => ({ useExtensionIcons: () => () => null }));
+vi.mock('@/hooks/use-plugin-slot-offers', () => ({ usePluginSlotOffers: () => [] }));
+vi.mock('@/components/plugins/plugin-iframe-slot', () => ({ PluginIframeSlot: () => null }));
+
+import { ExtensionDetailPage } from '../_pages/extension-detail';
+
+beforeEach(() => {
+  calls.length = 0;
+  routes['/api/admin/plugins'] = [{
+    id: 'ai-assistant', name: 'AI Writing Assistant', version: '2.1.1', enabled: true, author: 'x', description: '', permissions: [],
+    configSchema: {
+      model: { type: 'string', label: 'Model' },
+      instructions: { type: 'string', label: 'Instructions for the assistant', multiline: true, description: 'Added to every request.' },
+    },
+  }];
+  routes['/api/admin/plugins/ai-assistant/config'] = { instructions: 'Write in British English.\nSign off with "Kind regards".' };
+});
+afterEach(() => {
+  useAdminUnsavedStore.setState({ dirty: {}, pendingLeave: null, pendingScope: null });
+});
+
+describe('plugin settings with several lines', () => {
+  it('shows a text box for a multiline setting and saves its line breaks', async () => {
+    render(<ExtensionDetailPage kind="plugin" id="ai-assistant" />);
+    const box = await screen.findByRole('textbox', { name: 'Instructions for the assistant' });
+    expect(box.tagName).toBe('TEXTAREA');
+    await waitFor(() => expect((box as HTMLTextAreaElement).value).toBe('Write in British English.\nSign off with "Kind regards".'));
+    expect(screen.getByRole('textbox', { name: 'Model' }).tagName).toBe('INPUT');
+
+    fireEvent.change(box, { target: { value: 'Line one\nLine two' } });
+    fireEvent.click(screen.getByRole('button', { name: 'Save' }));
+    await waitFor(() => expect(calls.filter((c) => c.method !== 'GET')).toEqual([
+      expect.objectContaining({ url: '/api/admin/plugins/ai-assistant/config', body: { key: 'instructions', value: 'Line one\nLine two' } }),
+    ]));
+  });
+});
diff --git a/app/(main)/admin/_pages/extension-detail.tsx b/app/(main)/admin/_pages/extension-detail.tsx
index 0680c1915..68f6e5c82 100644
--- a/app/(main)/admin/_pages/extension-detail.tsx
+++ b/app/(main)/admin/_pages/extension-detail.tsx
@@ -1,6 +1,6 @@
 'use client';
 
-import { useCallback, useEffect, useMemo, useState } from 'react';
+import { useCallback, useEffect, useId, useMemo, useState } from 'react';
 import { Puzzle, SwatchBook, Loader2, Eye, EyeOff, Lock, RotateCcw, Trash2, Shield, ShieldAlert, Store, Upload } from '@/components/icons';
 import { apiFetch } from '@/lib/browser-navigation';
 import { adminPath } from '@/lib/admin/admin-nav';
@@ -31,6 +31,8 @@ export interface ConfigField {
   default?: unknown;
   placeholder?: string;
   options?: { label: string; value: string }[];
+  /** A string that takes several lines (instructions, templates): a full-width text box. */
+  multiline?: boolean;
 }
 
 /** A plugin or theme as /api/admin/plugins and /api/admin/themes list it. */
@@ -421,7 +423,14 @@ export function ExtensionDetailPage({ kind, id }: Props) {
         {/* The plugin's own settings come last: they are the long part. */}
         {kind === 'plugin' && hasSchema && schema && (
           <SettingsSection id="settings" title="Settings" description="What this plugin reads from the server. Saved for every user.">
-            {Object.entries(schema).map(([key, field]) => (
+            {Object.entries(schema).map(([key, field]) => field.type === 'string' && field.multiline ? (
+              <MultilineConfigItem
+                key={key}
+                field={field}
+                value={formValues[key] ?? ''}
+                onChange={(v) => setFormValues((prev) => ({ ...prev, [key]: v }))}
+              />
+            ) : (
               <SettingItem
                 key={key}
                 label={field.required ? `${field.label} *` : field.label}
@@ -510,6 +519,35 @@ function ConfigFieldControl({ field, value, stored, revealed, onReveal, onChange
   );
 }
 
+/**
+ * A setting that takes several lines: label and description on top, a
+ * full-width text box under them, as wide as the section.
+ */
+function MultilineConfigItem({ field, value, onChange }: {
+  field: ConfigField;
+  value: string;
+  onChange: (value: string) => void;
+}) {
+  const id = useId();
+  return (
+    <div data-search-label={field.label} className="py-3 border-b border-border last:border-0">
+      <label htmlFor={id} className="text-sm font-medium text-foreground">
+        {field.required ? `${field.label} *` : field.label}
+      </label>
+      {field.description && <p id={`${id}-desc`} className="text-xs text-muted-foreground mt-1">{field.description}</p>}
+      <textarea
+        id={id}
+        value={value}
+        onChange={(e) => onChange(e.target.value)}
+        placeholder={field.placeholder || ''}
+        aria-describedby={field.description ? `${id}-desc` : undefined}
+        rows={5}
+        className={cn(inputClass, 'mt-2 block h-auto min-h-24 w-full resize-y py-2 leading-relaxed')}
+      />
+    </div>
+  );
+}
+
 /**
  * The plugin's own `admin-plugin-page` slot, if it offers one. Sandboxed
  * plugins ship a React component under `slots['admin-plugin-page']` and the
diff --git a/lib/admin/plugin-registry.ts b/lib/admin/plugin-registry.ts
index 17a873113..09b932e6b 100644
--- a/lib/admin/plugin-registry.ts
+++ b/lib/admin/plugin-registry.ts
@@ -25,6 +25,8 @@ export interface PluginConfigField {
   default?: unknown;
   placeholder?: string;
   options?: { label: string; value: string }[];
+  /** `type: 'string'` only: the admin form shows a full-width text box. */
+  multiline?: boolean;
 }
 
 /**
diff --git a/lib/extension-package.ts b/lib/extension-package.ts
index 9df41e2ba..4ed0183dd 100644
--- a/lib/extension-package.ts
+++ b/lib/extension-package.ts
@@ -307,6 +307,8 @@ export interface ConfigFieldSchema {
   default?: unknown;
   placeholder?: string;
   options?: { label: string; value: string }[];
+  /** `type: 'string'` only: the admin form shows a full-width text box. */
+  multiline?: boolean;
 }
 
 function validateConfigSchema(raw: unknown, errors: string[]): Record<string, ConfigFieldSchema> | undefined {