diff --git a/frontend/src/components/settings/SettingField.vue b/frontend/src/components/settings/SettingField.vue
index dba8c89b5..e0afe02f4 100644
--- a/frontend/src/components/settings/SettingField.vue
+++ b/frontend/src/components/settings/SettingField.vue
@@ -137,6 +137,14 @@
 
       <!-- textarea (multi-line text, e.g. MCP server instructions) -->
       <div v-else-if="field.control === 'textarea'" class="flex flex-col items-stretch w-full sm:w-96">
+        <div v-if="field.resetDefault != null" class="flex justify-end mb-1">
+          <button
+            type="button"
+            class="btn btn-ghost btn-xs text-base-content/50 hover:text-base-content"
+            :data-test="`setting-reset-${field.key}`"
+            @click="emitText(field.resetDefault ?? '')"
+          >↩ Reset to default</button>
+        </div>
         <textarea
           class="textarea textarea-bordered textarea-sm w-full font-mono leading-snug"
           :class="{ 'textarea-error': validationError }"
diff --git a/frontend/src/components/settings/SettingsSection.vue b/frontend/src/components/settings/SettingsSection.vue
index 724c780f2..a9425bcea 100644
--- a/frontend/src/components/settings/SettingsSection.vue
+++ b/frontend/src/components/settings/SettingsSection.vue
@@ -5,7 +5,7 @@
       :key="f.key"
       :field="f"
       :model-value="getPath(working, f.key)"
-      :dirty="!!dirty[f.key]"
+      :dirty="isFieldDirty(f.key)"
       @update:model-value="onChange(f, $event)"
     />
     <p v-if="!fields.length" class="text-sm text-base-content/50 py-4">No settings match your search.</p>
@@ -98,7 +98,25 @@ const confirmEl = ref<HTMLDialogElement | null>(null)
 const pendingMessages = ref<string[]>([])
 const pendingInfoOnly = ref(false)
 
-const dirtyKeys = computed(() => Object.keys(dirty.value))
+// A field is dirty if the user changed it through a control (tracked in the
+// `dirty` ref by onChange) OR its working value diverges from the last-saved
+// original. The latter catches values written onto `state.working` from OUTSIDE
+// a control — notably the instructions prefill in Settings.vue (MCP-2484).
+// Without it, "Save without editing" after a prefill would PATCH nothing because
+// the field was never marked dirty.
+const dirtyKeys = computed(() => {
+  const keys = new Set(Object.keys(dirty.value))
+  for (const f of props.fields) {
+    if (!eq(getPath(props.working, f.key), getPath(props.original, f.key))) keys.add(f.key)
+  }
+  return [...keys]
+})
+
+function isFieldDirty(key: string): boolean {
+  if (key in dirty.value) return true
+  const f = props.fields.find((x) => x.key === key)
+  return f != null && !eq(getPath(props.working, key), getPath(props.original, key))
+}
 
 // Block Save only when a CHANGED field is invalid — a pre-existing value the
 // user hasn't touched must never block saving unrelated edits.
diff --git a/frontend/src/views/Settings.vue b/frontend/src/views/Settings.vue
index c841626c9..c2d246619 100644
--- a/frontend/src/views/Settings.vue
+++ b/frontend/src/views/Settings.vue
@@ -174,7 +174,7 @@
 </template>
 
 <script setup lang="ts">
-import { ref, reactive, computed, onMounted, onUnmounted } from 'vue'
+import { ref, reactive, computed, watch, onMounted, onUnmounted } from 'vue'
 import { RouterLink } from 'vue-router'
 import { VueMonacoEditor } from '@guolao/vue-monaco-editor'
 import { useServersStore } from '@/stores/servers'
@@ -192,6 +192,7 @@ import {
   SERVER_EDITION_SECTION_TITLE,
   DOCS_BASE,
   docsUrl,
+  isBlankInstructions,
   type SettingField,
   type SettingsAccordion,
 } from '@/views/settings/fields'
@@ -209,21 +210,35 @@ const serverEditionTitle = SERVER_EDITION_SECTION_TITLE
 // backend so the `instructions` textarea placeholder never drifts from Go.
 const defaultInstructions = ref<string>('')
 
-// Inject the live default as the `instructions` field placeholder. Until the
-// fetch resolves (or against an older core that doesn't expose it) the static
-// catalogue placeholder ("Loading built-in default…") is used.
+// Inject the live default as the `instructions` field placeholder AND as the
+// resetDefault value (drives the "Reset to default" button). Until the fetch
+// resolves the static catalogue placeholder is used.
 const advancedAccordions = computed<SettingsAccordion[]>(() =>
   ADVANCED_ACCORDIONS.map((acc) => {
     if (!defaultInstructions.value || !acc.fields.some((f) => f.key === 'instructions')) return acc
     return {
       ...acc,
       fields: acc.fields.map((f) =>
-        f.key === 'instructions' ? { ...f, placeholder: defaultInstructions.value } : f
+        f.key === 'instructions'
+          ? { ...f, placeholder: defaultInstructions.value, resetDefault: defaultInstructions.value }
+          : f
       ),
     }
   })
 )
 
+// Prefill the instructions textarea with the built-in default when the config
+// field is empty (fresh install or blank override). Only runs when BOTH the
+// config AND the default are loaded; whichever resolves second triggers it.
+// Never overwrites a user's saved value (non-empty instructions stay as-is).
+function maybePrefillInstructions() {
+  if (!defaultInstructions.value) return
+  if (!loaded.value) return
+  if (isBlankInstructions(state.working.instructions)) {
+    state.working.instructions = defaultInstructions.value
+  }
+}
+
 // ---- form state ----
 const loading = ref(false)
 const loaded = ref(false)
@@ -332,6 +347,7 @@ async function loadConfig() {
       configJson.value = JSON.stringify(cfg, null, 2)
       configStatus.value = { valid: true }
       loaded.value = true
+      maybePrefillInstructions()
     } else {
       loadError.value = response.error || 'Failed to load configuration'
     }
@@ -426,12 +442,20 @@ async function loadDefaultInstructions() {
     const resp = await api.getStatus()
     if (resp.success && typeof resp.data?.default_instructions === 'string') {
       defaultInstructions.value = resp.data.default_instructions
+      // In case loadConfig already ran and set loaded=true, prefill now.
+      maybePrefillInstructions()
     }
   } catch {
     /* leave placeholder as the static fallback */
   }
 }
 
+// Also trigger prefill if defaultInstructions resolves after config is loaded
+// (handles the common case where /api/v1/status is slower than /api/v1/config).
+watch(defaultInstructions, () => {
+  maybePrefillInstructions()
+})
+
 onMounted(() => {
   loadConfig()
   loadDefaultInstructions()
diff --git a/frontend/src/views/settings/fields.ts b/frontend/src/views/settings/fields.ts
index 277ba7ab3..0ec23984f 100644
--- a/frontend/src/views/settings/fields.ts
+++ b/frontend/src/views/settings/fields.ts
@@ -49,6 +49,7 @@ export interface SettingField {
   docs?: string // doc page path on docs.mcpproxy.app, e.g. "/features/docker-isolation"
   valueKind?: ValueKind // extra format validation for text/secret fields
   optional?: boolean // when true, an empty value is valid (skips kind validation)
+  resetDefault?: string // when set, render an inline "Reset to default" button that emits this value
 }
 
 export interface SettingsAccordion {
@@ -277,6 +278,12 @@ export const SERVER_EDITION_FIELDS: SettingField[] = [
   { key: 'server_edition.max_user_servers', label: 'Max servers per user', control: 'number', min: 0 },
 ]
 
+// isBlankInstructions returns true when a saved instructions value is empty /
+// whitespace-only / null / undefined — i.e. eligible for prefill from the built-in default.
+export function isBlankInstructions(v: string | null | undefined): boolean {
+  return !v || v.trim() === ''
+}
+
 // ---- Section 3: Advanced (subsystem accordions) ----
 export const ADVANCED_ACCORDIONS: SettingsAccordion[] = [
   {
diff --git a/frontend/tests/unit/instructions-prefill.spec.ts b/frontend/tests/unit/instructions-prefill.spec.ts
new file mode 100644
index 000000000..33b7eca4e
--- /dev/null
+++ b/frontend/tests/unit/instructions-prefill.spec.ts
@@ -0,0 +1,95 @@
+import { describe, it, expect } from 'vitest'
+import { isBlankInstructions, type SettingField } from '../../src/views/settings/fields'
+
+// MCP-2484: instructions textarea prefill + reset-to-default
+
+// Unit: SettingField interface accepts resetDefault (compile-time test via import)
+describe('SettingField.resetDefault (MCP-2484)', () => {
+  it('allows a resetDefault property on SettingField without TS error', () => {
+    const f: SettingField = {
+      key: 'instructions',
+      label: 'Server instructions',
+      control: 'textarea',
+      optional: true,
+      resetDefault: 'built-in default text',
+    }
+    expect(f.resetDefault).toBe('built-in default text')
+  })
+
+  it('resetDefault is optional — omitting it does not cause a TS error', () => {
+    const f: SettingField = {
+      key: 'instructions',
+      label: 'Server instructions',
+      control: 'textarea',
+      optional: true,
+    }
+    expect(f.resetDefault).toBeUndefined()
+  })
+})
+
+// Unit: isBlankInstructions helper
+describe('isBlankInstructions (MCP-2484)', () => {
+  it('treats unset/empty/whitespace as blank (eligible for prefill)', () => {
+    expect(isBlankInstructions(undefined)).toBe(true)
+    expect(isBlankInstructions(null)).toBe(true)
+    expect(isBlankInstructions('')).toBe(true)
+    expect(isBlankInstructions('  \n ')).toBe(true)
+  })
+
+  it('treats a real saved value as non-blank (never overwrite)', () => {
+    expect(isBlankInstructions('my custom instructions')).toBe(false)
+  })
+})
+
+// Unit: maybePrefillInstructions logic (isolated, not mounting the full component)
+describe('maybePrefillInstructions logic (MCP-2484)', () => {
+  function prefill(working: Record<string, any>, defaultVal: string, isLoaded: boolean) {
+    if (!defaultVal) return
+    if (!isLoaded) return
+    if (isBlankInstructions(working.instructions)) {
+      working.instructions = defaultVal
+    }
+  }
+
+  it('prefills instructions when config is empty and default is available', () => {
+    const w: Record<string, any> = { instructions: '' }
+    prefill(w, 'built-in default', true)
+    expect(w.instructions).toBe('built-in default')
+  })
+
+  it('prefills when instructions is null', () => {
+    const w: Record<string, any> = { instructions: null }
+    prefill(w, 'built-in default', true)
+    expect(w.instructions).toBe('built-in default')
+  })
+
+  it('prefills when instructions is undefined', () => {
+    const w: Record<string, any> = {}
+    prefill(w, 'built-in default', true)
+    expect(w.instructions).toBe('built-in default')
+  })
+
+  it('prefills when instructions is whitespace-only', () => {
+    const w: Record<string, any> = { instructions: '  \n  ' }
+    prefill(w, 'built-in default', true)
+    expect(w.instructions).toBe('built-in default')
+  })
+
+  it('does NOT overwrite a saved custom instruction', () => {
+    const w: Record<string, any> = { instructions: 'my custom instructions' }
+    prefill(w, 'built-in default', true)
+    expect(w.instructions).toBe('my custom instructions')
+  })
+
+  it('does NOT prefill when config is not yet loaded', () => {
+    const w: Record<string, any> = { instructions: '' }
+    prefill(w, 'built-in default', false)
+    expect(w.instructions).toBe('')
+  })
+
+  it('does NOT prefill when defaultInstructions is empty (API not yet resolved)', () => {
+    const w: Record<string, any> = { instructions: '' }
+    prefill(w, '', true)
+    expect(w.instructions).toBe('')
+  })
+})
diff --git a/frontend/tests/unit/settings-section-prefill-dirty.spec.ts b/frontend/tests/unit/settings-section-prefill-dirty.spec.ts
new file mode 100644
index 000000000..10eedd7e3
--- /dev/null
+++ b/frontend/tests/unit/settings-section-prefill-dirty.spec.ts
@@ -0,0 +1,75 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { mount, flushPromises } from '@vue/test-utils'
+import { reactive } from 'vue'
+
+// MCP-2484 (Codex REQUEST_CHANGES on #685): the instructions prefill writes the
+// built-in default directly onto `state.working` in Settings.vue. SettingsSection
+// PATCHes only its dirty keys, so a value set OUTSIDE a control must still be
+// treated as dirty — otherwise "Save without editing" after a prefill persists
+// nothing. SettingsSection now derives dirtiness from working≠original.
+
+const patchConfig = vi.fn(() =>
+  Promise.resolve({ success: true, data: { changed_fields: ['instructions'], requires_restart: false } })
+)
+
+vi.mock('@/services/api', () => ({
+  default: { patchConfig: (...args: unknown[]) => patchConfig(...args) },
+}))
+vi.mock('@/stores/system', () => ({ useSystemStore: () => ({ addToast: vi.fn() }) }))
+
+import SettingsSection from '@/components/settings/SettingsSection.vue'
+import type { SettingField } from '@/views/settings/fields'
+
+const instructionsField: SettingField = {
+  key: 'instructions',
+  label: 'Server instructions',
+  control: 'textarea',
+  optional: true,
+  resetDefault: 'BUILT-IN DEFAULT',
+}
+
+function mountSection(working: Record<string, unknown>, original: Record<string, unknown>) {
+  return mount(SettingsSection, {
+    props: { sectionId: 'mcp', fields: [instructionsField], working: reactive(working), original: reactive(original) },
+  })
+}
+
+describe('SettingsSection prefill-is-dirty (MCP-2484)', () => {
+  beforeEach(() => patchConfig.mockClear())
+
+  it('treats an externally-prefilled value (working≠original) as dirty and saves it', async () => {
+    // working got the prefilled default written directly; original is still the
+    // saved (empty) value — exactly the post-prefill state from Settings.vue.
+    const w = mountSection({ instructions: 'BUILT-IN DEFAULT' }, { instructions: '' })
+
+    const save = w.find('[data-test="settings-apply-mcp"]')
+    expect(save.exists()).toBe(true)
+    expect((save.element as HTMLButtonElement).disabled).toBe(false)
+
+    // the field itself is flagged dirty in the UI
+    expect(w.find('[data-test="setting-row-instructions"]').classes()).toContain('border-l-warning')
+
+    await save.trigger('click')
+    await flushPromises()
+
+    expect(patchConfig).toHaveBeenCalledTimes(1)
+    expect(patchConfig.mock.calls[0][0]).toMatchObject({ instructions: 'BUILT-IN DEFAULT' })
+  })
+
+  it('is NOT dirty when working equals original (no spurious save)', () => {
+    const w = mountSection({ instructions: 'same' }, { instructions: 'same' })
+    expect((w.find('[data-test="settings-apply-mcp"]').element as HTMLButtonElement).disabled).toBe(true)
+  })
+
+  it('clears dirty after a save commits original = working', async () => {
+    const working = reactive({ instructions: 'BUILT-IN DEFAULT' })
+    const original = reactive({ instructions: '' })
+    const w = mount(SettingsSection, {
+      props: { sectionId: 'mcp', fields: [instructionsField], working, original },
+    })
+    await w.find('[data-test="settings-apply-mcp"]').trigger('click')
+    await flushPromises()
+    // doSave copies working → original on success; section is no longer dirty
+    expect((w.find('[data-test="settings-apply-mcp"]').element as HTMLButtonElement).disabled).toBe(true)
+  })
+})
diff --git a/specs/060-settings-page/verification/instructions-prefill-persist.spec.ts b/specs/060-settings-page/verification/instructions-prefill-persist.spec.ts
new file mode 100644
index 000000000..ae9078c09
--- /dev/null
+++ b/specs/060-settings-page/verification/instructions-prefill-persist.spec.ts
@@ -0,0 +1,71 @@
+import { test, expect, request } from '@playwright/test'
+
+// MCP-2484 (Codex fix on #685): on a fresh install the MCP "Server instructions"
+// textarea is PREFILLED with the built-in default, and clicking Save WITHOUT
+// editing must persist it as config.instructions (the prefilled value is dirty).
+//
+// Scenario: fresh install → open Settings → Advanced → MCP accordion →
+// default text is IN the textarea → Save without editing → reload →
+// instructions still present AND persisted to config.
+
+const BASE = process.env.MCP_BASE || 'http://127.0.0.1:18482'
+const KEY = process.env.MCP_KEY || 'uitest2484'
+
+async function getConfigInstructions(): Promise<string> {
+  const ctx = await request.newContext()
+  const resp = await ctx.get(`${BASE}/api/v1/config`, { headers: { 'X-API-Key': KEY } })
+  const body = await resp.json()
+  await ctx.dispose()
+  return body?.data?.config?.instructions ?? body?.config?.instructions ?? ''
+}
+
+test('prefilled instructions persist on Save without editing', async ({ page }) => {
+  // fresh install: config.instructions starts empty
+  expect(await getConfigInstructions()).toBe('')
+
+  await page.goto(`${BASE}/ui/settings?apikey=${KEY}`)
+  await page.waitForLoadState('domcontentloaded')
+
+  // Advanced tab → expand the "MCP server instructions" accordion
+  await page.locator('[data-test="settings-tab-advanced"]').click()
+  await page.locator('[data-test="settings-accordion-mcp"]').click()
+
+  const textarea = page.locator('[data-test="setting-textarea-instructions"]')
+  await expect(textarea).toBeVisible()
+
+  // the default text is IN the textarea (not just the placeholder)
+  const prefilled = await textarea.inputValue()
+  expect(prefilled.trim().length).toBeGreaterThan(0)
+
+  // Save WITHOUT editing — the prefilled value must be dirty & savable
+  const save = page.locator('[data-test="settings-apply-mcp"]')
+  await expect(save).toBeEnabled()
+  await save.click()
+
+  // persisted to config
+  await expect.poll(async () => await getConfigInstructions(), { timeout: 5000 }).toBe(prefilled)
+
+  // survives a reload (still IN the textarea, loaded from saved config now)
+  await page.reload()
+  await page.waitForLoadState('domcontentloaded')
+  await page.locator('[data-test="settings-tab-advanced"]').click()
+  await page.locator('[data-test="settings-accordion-mcp"]').click()
+  expect((await textarea.inputValue()).trim()).toBe(prefilled.trim())
+})
+
+test('Reset to default repopulates after a custom edit', async ({ page }) => {
+  await page.goto(`${BASE}/ui/settings?apikey=${KEY}`)
+  await page.waitForLoadState('domcontentloaded')
+  await page.locator('[data-test="settings-tab-advanced"]').click()
+  await page.locator('[data-test="settings-accordion-mcp"]').click()
+
+  const textarea = page.locator('[data-test="setting-textarea-instructions"]')
+  const def = (await textarea.inputValue()).trim()
+  expect(def.length).toBeGreaterThan(0)
+
+  await textarea.fill('my custom instructions')
+  expect(await textarea.inputValue()).toBe('my custom instructions')
+
+  await page.locator('[data-test="setting-reset-instructions"]').click()
+  expect((await textarea.inputValue()).trim()).toBe(def)
+})