feat(ui): prefill MCP instructions default + Reset-to-default button (MCP-2484)

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 }"

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'
@@ -209,21 +209,39 @@ 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
+// Inject the live default as the `instructions` field placeholder AND as the
+// `resetDefault` value (drives the compact "Reset to default" button). Until the
 // fetch resolves (or against an older core that doesn't expose it) the static
-// catalogue placeholder ("Loading built-in default…") is used.
+// catalogue placeholder ("Loading built-in default…") is used and no reset
+// button is shown (MCP-2484).
 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 — a non-empty `instructions` stays as-is
+// (MCP-2484). Saving the prefilled text freezes the current default into the
+// config; the "Reset to default" button is the re-sync path.
+function maybePrefillInstructions() {
+  if (!defaultInstructions.value) return
+  if (!loaded.value) return
+  if (!state.working.instructions) {
+    state.working.instructions = defaultInstructions.value
+  }
+}
+
 // ---- form state ----
 const loading = ref(false)
 const loaded = ref(false)
@@ -332,6 +350,7 @@ 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 +445,21 @@ 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 resolved (loaded=true) before this fetch,
+      // prefill now — whichever of the two requests finishes second wins.
+      maybePrefillInstructions()
     }
   } catch {
     /* leave placeholder as the static fallback */
   }
 }
 
+// Also prefill if `defaultInstructions` resolves after the config has loaded
+// (the common case where /api/v1/status is slower than /api/v1/config).
+watch(defaultInstructions, () => {
+  maybePrefillInstructions()
+})
+
 onMounted(() => {
   loadConfig()
   loadDefaultInstructions()

frontend/src/views/settings/fields.ts

@@ -49,6 +49,11 @@ 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)
+  // When set, the control renders a compact "Reset to default" button that
+  // repopulates the editable value with this text. Injected at runtime once the
+  // live default (e.g. the backend-resolved `instructions` default) is fetched
+  // (MCP-2484).
+  resetDefault?: string
 }
 
 export interface SettingsAccordion {

frontend/tests/unit/settings-instructions.spec.ts

@@ -59,3 +59,45 @@ describe('instructions field catalogue (MCP-2175)', () => {
     expect(validateField(f, 'custom instructions text')).toBeNull()
   })
 })
+
+// MCP-2484 — a compact "Reset to default" button next to the instructions
+// textarea. The live default (Go `defaultInstructions`, fetched from
+// /api/v1/status) is injected onto the field as `resetDefault`; clicking the
+// button repopulates the editable value (it flows through the normal change
+// path so the section marks it dirty and Save persists it as `instructions`).
+
+const resettableField: Field = {
+  key: 'instructions',
+  label: 'Server instructions',
+  control: 'textarea',
+  optional: true,
+  placeholder: 'BUILT-IN DEFAULT',
+  resetDefault: 'THE BUILT-IN DEFAULT TEXT',
+}
+
+describe('SettingField "Reset to default" (MCP-2484)', () => {
+  it('renders a compact reset button when the field carries a resetDefault', () => {
+    const w = mount(SettingField, { props: { field: resettableField, modelValue: 'custom' } })
+    expect(w.find('[data-test="setting-reset-instructions"]').exists()).toBe(true)
+  })
+
+  it('emits the default text when the reset button is clicked', async () => {
+    const w = mount(SettingField, { props: { field: resettableField, modelValue: 'custom edited' } })
+    await w.find('[data-test="setting-reset-instructions"]').trigger('click')
+    const emits = w.emitted('update:modelValue')
+    expect(emits).toBeTruthy()
+    expect(emits![emits!.length - 1][0]).toBe('THE BUILT-IN DEFAULT TEXT')
+  })
+
+  it('hides the reset button until the async default has loaded (no resetDefault)', () => {
+    const w = mount(SettingField, {
+      props: { field: { ...resettableField, resetDefault: undefined }, modelValue: '' },
+    })
+    expect(w.find('[data-test="setting-reset-instructions"]').exists()).toBe(false)
+  })
+
+  it('does not render a reset button on a field without resetDefault', () => {
+    const w = mount(SettingField, { props: { field: instrField, modelValue: '' } })
+    expect(w.find('[data-test="setting-reset-instructions"]').exists()).toBe(false)
+  })
+})