diff --git a/src/components/agent/AgentAssistantPanel.vue b/src/components/agent/AgentAssistantPanel.vue
index 9ee51b42..088cb9b7 100644
--- a/src/components/agent/AgentAssistantPanel.vue
+++ b/src/components/agent/AgentAssistantPanel.vue
@@ -228,6 +228,7 @@ const historyLoading = ref(false)
 const historyLoadingMore = ref(false)
 const historyPage = ref(1)
 const historyHasMore = ref(true)
+const fullscreen = ref(false)
 const slashCommands = ref<AgentSlashCommand[]>([])
 const slashCommandsLoading = ref(false)
 const slashCommandsLoaded = ref(false)
@@ -293,8 +294,10 @@ const recordingTimeText = computed(() => {
 
   return `${minutes}:${String(remainSeconds).padStart(2, '0')}`
 })
-// 窄屏下直接全屏，避免聊天内容被压成半屏窄栏。
-const drawerWidth = computed(() => (display.mdAndDown.value ? '100vw' : '30rem'))
+// 窄屏下直接全屏，桌面端则允许用户按需扩展对话区域。
+const drawerWidth = computed(() => (display.mdAndDown.value || fullscreen.value ? '100vw' : '30rem'))
+// 仅桌面宽屏展示全屏开关，窄屏已默认占满视口。
+const canToggleFullscreen = computed(() => !display.mdAndDown.value)
 const hasMessages = computed(() => messages.value.length > 0)
 const hasHistorySessions = computed(() => historySessions.value.length > 0)
 const currentUserName = computed(() => userStore.getUserName || t('common.user'))
@@ -2180,6 +2183,11 @@ function closeDrawer() {
   isOpen.value = false
 }
 
+// 在桌面侧栏与全屏对话模式之间切换。
+function toggleFullscreen() {
+  fullscreen.value = !fullscreen.value
+}
+
 // 同步面板打开状态到全局 DOM，供悬浮入口避让面板宽度。
 function syncAgentAssistantOpenState(isOpen: boolean) {
   if (typeof document === 'undefined') return
@@ -2238,10 +2246,12 @@ function handleInputKeydown(event: KeyboardEvent) {
   sendMessage()
 }
 
+// 开始输入法组合输入，避免回车确认候选词时误发送消息。
 function handleCompositionStart() {
   isComposing.value = true
 }
 
+// 结束输入法组合输入，恢复回车发送能力。
 function handleCompositionEnd() {
   isComposing.value = false
 }
@@ -2288,7 +2298,11 @@ onScopeDispose(() => {
   <aside
     v-show="isOpen"
     class="agent-assistant-panel"
-    :class="{ 'is-motion-paused': !props.motionActive, 'is-open': isOpen }"
+    :class="{
+      'is-fullscreen': fullscreen,
+      'is-motion-paused': !props.motionActive,
+      'is-open': isOpen,
+    }"
     :style="drawerStyle"
     role="dialog"
     :aria-label="t('agentAssistant.title')"
@@ -2404,6 +2418,16 @@ onScopeDispose(() => {
           >
             <VIcon icon="mdi-message-plus-outline" />
           </IconBtn>
+          <IconBtn
+            v-if="canToggleFullscreen"
+            class="agent-assistant-fullscreen-toggle"
+            :title="t(fullscreen ? 'agentAssistant.exitFullscreen' : 'agentAssistant.enterFullscreen')"
+            :aria-label="t(fullscreen ? 'agentAssistant.exitFullscreen' : 'agentAssistant.enterFullscreen')"
+            :aria-pressed="fullscreen"
+            @click="toggleFullscreen"
+          >
+            <VIcon :icon="fullscreen ? 'mdi-fullscreen-exit' : 'mdi-fullscreen'" />
+          </IconBtn>
           <IconBtn :title="t('common.close')" :aria-label="t('common.close')" @click="closeDrawer">
             <VIcon icon="mdi-close" />
           </IconBtn>
@@ -2713,6 +2737,27 @@ onScopeDispose(() => {
   overscroll-behavior: contain;
 }
 
+.agent-assistant-panel.is-fullscreen {
+  border-inline-start: 0;
+  box-shadow: none;
+
+  .agent-assistant-message__bubble {
+    max-inline-size: min(100%, 64rem);
+  }
+
+  .agent-assistant-segments,
+  .agent-assistant-choices,
+  .agent-assistant-attachments {
+    inline-size: min(100%, 64rem);
+  }
+
+  .agent-assistant-composer {
+    inline-size: min(calc(100% - 2rem), 72rem);
+    inset-inline: 0;
+    margin-inline: auto;
+  }
+}
+
 @supports (block-size: 100lvh) {
   .agent-assistant-panel {
     block-size: 100lvh !important;
@@ -3048,6 +3093,11 @@ onScopeDispose(() => {
   min-block-size: 100%;
 }
 
+.agent-assistant-panel.is-fullscreen .agent-assistant-messages__content {
+  inline-size: min(100%, 72rem);
+  margin-inline: auto;
+}
+
 /* 只有消息态预留输入框空间，避免 iOS 空态被 padding 撑出不可滚动的滚动条。 */
 .agent-assistant-messages--has-content {
   padding-block-end: calc(env(safe-area-inset-bottom, 0px) + 8.75rem);
diff --git a/src/components/agent/__tests__/AgentAssistantPanel.spec.ts b/src/components/agent/__tests__/AgentAssistantPanel.spec.ts
index 0ebb1889..2400f7db 100644
--- a/src/components/agent/__tests__/AgentAssistantPanel.spec.ts
+++ b/src/components/agent/__tests__/AgentAssistantPanel.spec.ts
@@ -2,12 +2,14 @@ import { flushPromises, shallowMount } from '@vue/test-utils'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import AgentAssistantPanel from '@/components/agent/AgentAssistantPanel.vue'
 
+const displayState = vi.hoisted(() => ({ mdAndDown: { value: true } }))
+
 vi.mock('vue-i18n', () => ({
   useI18n: () => ({ t: (key: string) => key }),
 }))
 
 vi.mock('vuetify', () => ({
-  useDisplay: () => ({ mdAndDown: { value: true } }),
+  useDisplay: () => displayState,
 }))
 
 vi.mock('@/stores', () => ({
@@ -47,9 +49,77 @@ describe('AgentAssistantPanel stream recovery', () => {
   })
 
   afterEach(() => {
+    displayState.mdAndDown.value = true
     vi.useRealTimers()
   })
 
+  it('toggles the desktop assistant between the side panel and fullscreen modes', async () => {
+    displayState.mdAndDown.value = false
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async () => createAgentResponse([])),
+    )
+
+    const wrapper = shallowMount(AgentAssistantPanel, {
+      props: { modelValue: true },
+      global: {
+        stubs: {
+          AgentMarkdownContent: agentMarkdownContentStub,
+          IconBtn: { template: '<button><slot /></button>' },
+          PerfectScrollbar: { template: '<div><slot /></div>' },
+          VIcon: true,
+        },
+      },
+    })
+    await flushPromises()
+
+    const panel = wrapper.get('.agent-assistant-panel')
+    const fullscreenToggle = wrapper.get('.agent-assistant-fullscreen-toggle')
+    expect(panel.classes()).not.toContain('is-fullscreen')
+    expect(panel.attributes('style')).toContain('--agent-assistant-panel-width: 30rem')
+    expect(fullscreenToggle.attributes('title')).toBe('agentAssistant.enterFullscreen')
+    expect(fullscreenToggle.attributes('aria-pressed')).toBe('false')
+
+    await fullscreenToggle.trigger('click')
+
+    expect(panel.classes()).toContain('is-fullscreen')
+    expect(panel.attributes('style')).toContain('--agent-assistant-panel-width: 100vw')
+    expect(fullscreenToggle.attributes('title')).toBe('agentAssistant.exitFullscreen')
+    expect(fullscreenToggle.attributes('aria-pressed')).toBe('true')
+
+    await fullscreenToggle.trigger('click')
+
+    expect(panel.classes()).not.toContain('is-fullscreen')
+    expect(panel.attributes('style')).toContain('--agent-assistant-panel-width: 30rem')
+
+    wrapper.unmount()
+  })
+
+  it('keeps the narrow assistant fullscreen without a redundant toggle', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async () => createAgentResponse([])),
+    )
+
+    const wrapper = shallowMount(AgentAssistantPanel, {
+      props: { modelValue: true },
+      global: {
+        stubs: {
+          AgentMarkdownContent: agentMarkdownContentStub,
+          IconBtn: { template: '<button><slot /></button>' },
+          PerfectScrollbar: { template: '<div><slot /></div>' },
+          VIcon: true,
+        },
+      },
+    })
+    await flushPromises()
+
+    expect(wrapper.get('.agent-assistant-panel').attributes('style')).toContain('--agent-assistant-panel-width: 100vw')
+    expect(wrapper.find('.agent-assistant-fullscreen-toggle').exists()).toBe(false)
+
+    wrapper.unmount()
+  })
+
   it('restores a legacy empty message to loading and waits for the final reply after a PWA reload', async () => {
     const startedAt = Date.now() - 5000
     const localSessionId = 'web-local-session'
diff --git a/src/locales/en-US.ts b/src/locales/en-US.ts
index 84f361c3..c4aa4b32 100644
--- a/src/locales/en-US.ts
+++ b/src/locales/en-US.ts
@@ -847,6 +847,8 @@ export default {
     ready: 'Ready',
     thinking: 'Thinking',
     newChat: 'New Chat',
+    enterFullscreen: 'Enter fullscreen',
+    exitFullscreen: 'Exit fullscreen',
     history: 'Chat History',
     historyLoading: 'Loading chat history...',
     historyLoadFailed: 'Failed to load chat history',
diff --git a/src/locales/zh-CN.ts b/src/locales/zh-CN.ts
index f1049482..49dd451b 100644
--- a/src/locales/zh-CN.ts
+++ b/src/locales/zh-CN.ts
@@ -836,6 +836,8 @@ export default {
     ready: '随时待命',
     thinking: '思考中',
     newChat: '新会话',
+    enterFullscreen: '全屏显示',
+    exitFullscreen: '退出全屏',
     history: '历史会话',
     historyLoading: '正在加载历史会话...',
     historyLoadFailed: '历史会话加载失败',
diff --git a/src/locales/zh-TW.ts b/src/locales/zh-TW.ts
index 19e00212..f2b12c3f 100644
--- a/src/locales/zh-TW.ts
+++ b/src/locales/zh-TW.ts
@@ -836,6 +836,8 @@ export default {
     ready: '隨時待命',
     thinking: '思考中',
     newChat: '新會話',
+    enterFullscreen: '全屏顯示',
+    exitFullscreen: '退出全屏',
     history: '歷史會話',
     historyLoading: '正在載入歷史會話...',
     historyLoadFailed: '歷史會話載入失敗',