diff --git a/web/src/components/AssistantChat/ComposerButtons.test.tsx b/web/src/components/AssistantChat/ComposerButtons.test.tsx
index 183b970f..b2663df4 100644
--- a/web/src/components/AssistantChat/ComposerButtons.test.tsx
+++ b/web/src/components/AssistantChat/ComposerButtons.test.tsx
@@ -1,8 +1,32 @@
-import type { ReactElement } from 'react'
-import { cleanup, fireEvent, render, screen } from '@testing-library/react'
+import {
+    AssistantRuntimeProvider,
+    type ChatModelAdapter,
+    useLocalRuntime,
+} from '@assistant-ui/react'
+import type { ReactElement, ReactNode } from 'react'
+import { cleanup, fireEvent, render, screen, within } from '@testing-library/react'
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { I18nProvider } from '@/lib/i18n-context'
-import { DictationButton, UnifiedButton } from './ComposerButtons'
+import {
+    ComposerButtons,
+    ComposerExpandButton,
+    DictationButton,
+    getComposerToolbarJustifyContent,
+    UnifiedButton,
+} from './ComposerButtons'
+
+const adapter: ChatModelAdapter = {
+    async *run() {},
+}
+
+function RuntimeProviders(props: { children: ReactNode }) {
+    const runtime = useLocalRuntime(adapter)
+    return (
+        <AssistantRuntimeProvider runtime={runtime}>
+            <I18nProvider>{props.children}</I18nProvider>
+        </AssistantRuntimeProvider>
+    )
+}
 
 function renderInProviders(ui: ReactElement) {
     return render(<I18nProvider>{ui}</I18nProvider>)
@@ -103,3 +127,83 @@ describe('DictationButton', () => {
         expect(onVoiceToggle).toHaveBeenCalledOnce()
     })
 })
+
+describe('ComposerExpandButton', () => {
+    afterEach(() => {
+        cleanup()
+    })
+
+    it('announces and triggers expansion', () => {
+        const onToggle = vi.fn()
+        renderInProviders(<ComposerExpandButton expanded={false} onToggle={onToggle} />)
+
+        const button = getButton('Expand message editor')
+        expect(button.getAttribute('aria-pressed')).toBe('false')
+        fireEvent.click(button)
+        expect(onToggle).toHaveBeenCalledOnce()
+    })
+
+    it('announces the collapse action while expanded', () => {
+        renderInProviders(<ComposerExpandButton expanded onToggle={() => {}} />)
+
+        const button = getButton('Collapse message editor')
+        expect(button.getAttribute('aria-pressed')).toBe('true')
+        expect(button.className).toContain('text-[var(--app-link)]')
+    })
+})
+
+describe('ComposerButtons responsive toolbar', () => {
+    afterEach(cleanup)
+
+    it('keeps toolbar actions non-shrinking inside a horizontal scroll area', () => {
+        const noop = () => {}
+        render(
+            <RuntimeProviders>
+                <div style={{ width: 320 }}>
+                    <ComposerButtons
+                        canSend
+                        controlsDisabled={false}
+                        showSettingsButton
+                        onSettingsToggle={noop}
+                        expanded={false}
+                        onExpandedToggle={noop}
+                        showTerminalButton
+                        terminalDisabled={false}
+                        terminalLabel="Terminal"
+                        onTerminal={noop}
+                        showAbortButton
+                        abortDisabled={false}
+                        isAborting={false}
+                        onAbort={noop}
+                        showSwitchButton
+                        switchDisabled={false}
+                        isSwitching={false}
+                        onSwitch={noop}
+                        voiceEnabled
+                        dictationEnabled
+                        voiceStatus="disconnected"
+                        onVoiceToggle={noop}
+                        onSend={noop}
+                        onSchedule={noop}
+                        onScratchlistToggle={noop}
+                    />
+                </div>
+            </RuntimeProviders>,
+        )
+
+        const toolbar = screen.getByTestId('composer-toolbar-items')
+        expect(toolbar.className).toContain('overflow-x-auto')
+        const toolbarButtons = within(toolbar).getAllByRole('button')
+        expect(toolbarButtons.length).toBeGreaterThanOrEqual(8)
+        for (const button of toolbarButtons) {
+            expect(button.closest('.shrink-0')).not.toBeNull()
+        }
+    })
+
+    it('uses overflow-safe alignment for centered and right-aligned layouts', () => {
+        expect(getComposerToolbarJustifyContent('center')).toBe('safe center')
+        expect(getComposerToolbarJustifyContent('right')).toBe('safe end')
+        expect(getComposerToolbarJustifyContent('left')).toBe('flex-start')
+        expect(getComposerToolbarJustifyContent('split')).toBe('flex-start')
+    })
+})
diff --git a/web/src/components/AssistantChat/ComposerButtons.tsx b/web/src/components/AssistantChat/ComposerButtons.tsx
index 2701502f..61fa9213 100644
--- a/web/src/components/AssistantChat/ComposerButtons.tsx
+++ b/web/src/components/AssistantChat/ComposerButtons.tsx
@@ -18,7 +18,11 @@ function OrderedToolbarItems(props: { layout: ComposerToolbarLayout; children: R
         (child): child is ReactElement<{ item: ComposerToolbarItemId; children: ReactNode }> => isValidElement(child),
     )
     const slotsByItem = new Map(slots.map((slot) => [slot.props.item, slot]))
-    const renderItems = (items: ComposerToolbarItemId[]) => items.map((item) => slotsByItem.get(item) ?? null)
+    const renderItems = (items: ComposerToolbarItemId[]) => items.map((item) => {
+        const slot = slotsByItem.get(item)
+        if (!slot || slot.props.children == null) return null
+        return <div key={item} className="shrink-0">{slot}</div>
+    })
 
     if (props.layout.mode === 'split') {
         return <>{renderItems(props.layout.left)}<span className="flex-1" aria-hidden="true" />{renderItems(props.layout.right)}</>
@@ -26,6 +30,16 @@ function OrderedToolbarItems(props: { layout: ComposerToolbarLayout; children: R
     return <>{renderItems([...props.layout.left, ...props.layout.right])}</>
 }
 
+export function getComposerToolbarJustifyContent(
+    mode: ComposerToolbarLayout['mode'],
+): 'safe center' | 'safe end' | 'flex-start' {
+    return mode === 'center'
+        ? 'safe center'
+        : mode === 'right'
+            ? 'safe end'
+            : 'flex-start'
+}
+
 function ChevronIcon() {
     return <svg width="10" height="10" viewBox="0 0 10 10" fill="none" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round"><path d="M2.5 3.75L5 6.25L7.5 3.75" /></svg>
 }
@@ -171,6 +185,77 @@ function AttachmentIcon() {
     )
 }
 
+function ComposerExpandIcon(props: { expanded: boolean }) {
+    return props.expanded ? (
+        <svg
+            xmlns="http://www.w3.org/2000/svg"
+            width="18"
+            height="18"
+            viewBox="0 0 24 24"
+            fill="none"
+            stroke="currentColor"
+            strokeWidth="2"
+            strokeLinecap="round"
+            strokeLinejoin="round"
+        >
+            <path d="M8 3v5H3" />
+            <path d="m3 3 5 5" />
+            <path d="M16 3v5h5" />
+            <path d="m21 3-5 5" />
+            <path d="M8 21v-5H3" />
+            <path d="m3 21 5-5" />
+            <path d="M16 21v-5h5" />
+            <path d="m21 21-5-5" />
+        </svg>
+    ) : (
+        <svg
+            xmlns="http://www.w3.org/2000/svg"
+            width="18"
+            height="18"
+            viewBox="0 0 24 24"
+            fill="none"
+            stroke="currentColor"
+            strokeWidth="2"
+            strokeLinecap="round"
+            strokeLinejoin="round"
+        >
+            <path d="M8 3H3v5" />
+            <path d="m3 3 5 5" />
+            <path d="M16 3h5v5" />
+            <path d="m21 3-5 5" />
+            <path d="M8 21H3v-5" />
+            <path d="m3 21 5-5" />
+            <path d="M16 21h5v-5" />
+            <path d="m21 21-5-5" />
+        </svg>
+    )
+}
+
+export function ComposerExpandButton(props: {
+    expanded: boolean
+    onToggle: () => void
+}) {
+    const { t } = useTranslation()
+    const label = props.expanded ? t('composer.collapse') : t('composer.expand')
+
+    return (
+        <button
+            type="button"
+            aria-label={label}
+            title={label}
+            aria-pressed={props.expanded}
+            className={`flex h-8 w-8 items-center justify-center rounded-full transition-colors ${
+                props.expanded
+                    ? 'bg-[var(--app-bg)] text-[var(--app-link)]'
+                    : 'text-[var(--app-fg)]/60 hover:bg-[var(--app-bg)] hover:text-[var(--app-fg)]'
+            }`}
+            onClick={props.onToggle}
+        >
+            <ComposerExpandIcon expanded={props.expanded} />
+        </button>
+    )
+}
+
 function AbortIcon(props: { spinning: boolean }) {
     if (props.spinning) {
         return (
@@ -248,6 +333,7 @@ export function ComposerToolbarItemPreview(props: { item: ComposerToolbarItemId;
         switch (props.item) {
             case 'attachment': return <AttachmentIcon />
             case 'settings': return <SettingsIcon />
+            case 'expand': return <ComposerExpandIcon expanded={false} />
             case 'terminal': return <TerminalIcon />
             case 'abort': return <AbortIcon spinning={false} />
             case 'switch': return <SwitchToRemoteIcon />
@@ -515,6 +601,8 @@ export function ComposerButtons(props: {
     controlsDisabled: boolean
     showSettingsButton: boolean
     onSettingsToggle: () => void
+    expanded: boolean
+    onExpandedToggle: () => void
     showTerminalButton: boolean
     terminalDisabled: boolean
     terminalLabel: string
@@ -567,15 +655,15 @@ export function ComposerButtons(props: {
 
     const hasSchedule = props.pendingSchedule != null
     const hasAttachments = props.hasAttachments ?? false
-    const toolbarAlignmentClass = layout.mode === 'center'
-        ? 'justify-center'
-        : layout.mode === 'right'
-            ? 'justify-end'
-            : 'justify-start'
+    const toolbarJustifyContent = getComposerToolbarJustifyContent(layout.mode)
 
     return (
-        <div className="flex items-center gap-1 px-2 pb-2">
-            <div className={`flex min-w-0 flex-1 items-center gap-1 ${toolbarAlignmentClass}`}>
+        <div className="flex shrink-0 items-center gap-1 px-2 pb-2">
+            <div
+                data-testid="composer-toolbar-items"
+                className="flex min-w-0 flex-1 items-center gap-1 overflow-x-auto [scrollbar-width:none] [&::-webkit-scrollbar]:hidden"
+                style={{ justifyContent: toolbarJustifyContent }}
+            >
                 <OrderedToolbarItems layout={layout}>
                 <ToolbarItemSlot item="attachment">
                 <ComposerPrimitive.AddAttachment
@@ -603,6 +691,13 @@ export function ComposerButtons(props: {
                 ) : null}
                 </ToolbarItemSlot>
 
+                <ToolbarItemSlot item="expand">
+                <ComposerExpandButton
+                    expanded={props.expanded}
+                    onToggle={props.onExpandedToggle}
+                />
+                </ToolbarItemSlot>
+
                 <ToolbarItemSlot item="piModel">
                 {props.piModelLabel ? (
                     <button
diff --git a/web/src/components/AssistantChat/HappyComposer.expandSelection.test.tsx b/web/src/components/AssistantChat/HappyComposer.expandSelection.test.tsx
new file mode 100644
index 00000000..c480d73a
--- /dev/null
+++ b/web/src/components/AssistantChat/HappyComposer.expandSelection.test.tsx
@@ -0,0 +1,105 @@
+import {
+    AssistantRuntimeProvider,
+    type ChatModelAdapter,
+    useLocalRuntime,
+} from '@assistant-ui/react'
+import { fireEvent, render, screen, waitFor } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { HappyComposer } from './HappyComposer'
+
+vi.mock('@/components/AssistantChat/ComposerButtons', () => ({
+    ComposerButtons: (props: { expanded: boolean; onExpandedToggle: () => void }) => (
+        <button
+            type="button"
+            aria-label={props.expanded ? 'Collapse message editor' : 'Expand message editor'}
+            onClick={props.onExpandedToggle}
+        />
+    ),
+}))
+
+vi.mock('@/components/AssistantChat/StatusBar', () => ({ StatusBar: () => null }))
+vi.mock('@/hooks/useComposerDraft', () => ({ useComposerDraft: () => {} }))
+vi.mock('@/hooks/usePlatform', () => ({
+    usePlatform: () => ({
+        isTelegram: false,
+        isTouch: false,
+        haptic: {
+            impact: () => {},
+            notification: () => {},
+            selection: () => {},
+        },
+    }),
+}))
+vi.mock('@/hooks/usePWAInstall', () => ({
+    usePWAInstall: () => ({
+        installState: 'idle',
+        canInstall: false,
+        canInstallIOS: false,
+        isStandalone: false,
+        isIOS: false,
+        promptInstall: async () => false,
+        dismissInstall: () => {},
+    }),
+}))
+vi.mock('@/lib/use-translation', () => ({
+    useTranslation: () => ({
+        t: (key: string) => key === 'misc.typeAMessage' ? 'Type a message' : key,
+    }),
+}))
+
+const adapter: ChatModelAdapter = {
+    async *run() {},
+}
+
+function TestRuntime() {
+    const runtime = useLocalRuntime(adapter)
+    return (
+        <AssistantRuntimeProvider runtime={runtime}>
+            <HappyComposer />
+        </AssistantRuntimeProvider>
+    )
+}
+
+describe('HappyComposer plain-text expansion', () => {
+    beforeEach(() => {
+        localStorage.clear()
+        localStorage.setItem('hapi.composer.richMentions', '0')
+    })
+
+    it('preserves draft text and selection across expand and collapse', async () => {
+        render(<TestRuntime />)
+
+        const draft = 'A long draft with a selection in the middle that must survive both editor layout changes.'
+        const collapsedInput = screen.getByRole('textbox') as HTMLTextAreaElement
+        fireEvent.change(collapsedInput, { target: { value: draft } })
+        collapsedInput.setSelectionRange(12, 36, 'forward')
+
+        fireEvent.click(screen.getByRole('button', { name: 'Expand message editor' }))
+
+        await waitFor(() => {
+            expect(screen.getByRole('button', { name: 'Collapse message editor' })).toBeInTheDocument()
+            const expandedInput = screen.getByRole('textbox') as HTMLTextAreaElement
+            expect(expandedInput).not.toBe(collapsedInput)
+            expect(expandedInput.value).toBe(draft)
+            expect(expandedInput.selectionStart).toBe(12)
+            expect(expandedInput.selectionEnd).toBe(36)
+            expect(expandedInput.selectionDirection).toBe('forward')
+            expect(document.activeElement).toBe(expandedInput)
+        })
+
+        const expandedInput = screen.getByRole('textbox') as HTMLTextAreaElement
+        expandedInput.setSelectionRange(42, 67, 'backward')
+        fireEvent.click(screen.getByRole('button', { name: 'Collapse message editor' }))
+
+        await waitFor(() => {
+            expect(screen.getByRole('button', { name: 'Expand message editor' })).toBeInTheDocument()
+            const nextCollapsedInput = screen.getByRole('textbox') as HTMLTextAreaElement
+            expect(nextCollapsedInput).not.toBe(expandedInput)
+            expect(nextCollapsedInput.value).toBe(draft)
+            expect(nextCollapsedInput.selectionStart).toBe(42)
+            expect(nextCollapsedInput.selectionEnd).toBe(67)
+            expect(nextCollapsedInput.selectionDirection).toBe('backward')
+            expect(document.activeElement).toBe(nextCollapsedInput)
+        })
+    })
+})
diff --git a/web/src/components/AssistantChat/HappyComposer.richBridge.test.tsx b/web/src/components/AssistantChat/HappyComposer.richBridge.test.tsx
index 34bcab4d..7a01c88c 100644
--- a/web/src/components/AssistantChat/HappyComposer.richBridge.test.tsx
+++ b/web/src/components/AssistantChat/HappyComposer.richBridge.test.tsx
@@ -1,7 +1,7 @@
 import { act, fireEvent, render, screen } from '@testing-library/react'
 import { useEffect, useState } from 'react'
 import { describe, expect, it, vi } from 'vitest'
-import { type TextInputState, useRichComposerBridge } from './HappyComposer'
+import { getComposerEscapeAction, type TextInputState, useRichComposerBridge } from './HappyComposer'
 
 const events = vi.hoisted(() => [] as string[])
 
@@ -78,3 +78,23 @@ describe('useRichComposerBridge', () => {
         ])
     })
 })
+
+describe('getComposerEscapeAction', () => {
+    it('clears suggestions before aborting, and aborts before collapsing', () => {
+        expect(getComposerEscapeAction({
+            hasSuggestions: true,
+            threadIsRunning: true,
+            isExpanded: true,
+        })).toBe('clearSuggestions')
+        expect(getComposerEscapeAction({
+            hasSuggestions: false,
+            threadIsRunning: true,
+            isExpanded: true,
+        })).toBe('abort')
+        expect(getComposerEscapeAction({
+            hasSuggestions: false,
+            threadIsRunning: false,
+            isExpanded: true,
+        })).toBe('collapse')
+    })
+})
diff --git a/web/src/components/AssistantChat/HappyComposer.tsx b/web/src/components/AssistantChat/HappyComposer.tsx
index 38766c91..186c281f 100644
--- a/web/src/components/AssistantChat/HappyComposer.tsx
+++ b/web/src/components/AssistantChat/HappyComposer.tsx
@@ -56,6 +56,17 @@ export interface TextInputState {
     selection: { start: number; end: number }
 }
 
+export function getComposerEscapeAction(input: {
+    hasSuggestions: boolean
+    threadIsRunning: boolean
+    isExpanded: boolean
+}): 'clearSuggestions' | 'abort' | 'collapse' | null {
+    if (input.hasSuggestions) return 'clearSuggestions'
+    if (input.threadIsRunning) return 'abort'
+    if (input.isExpanded) return 'collapse'
+    return null
+}
+
 /**
  * One rejected send.  `id` is bumped per failure so two failures with the
  * same `text` still trigger a fresh restore (the dedupe key is the id, not
@@ -382,6 +393,7 @@ export function HappyComposer(props: {
         text: '',
         selection: { start: 0, end: 0 }
     })
+    const [isExpanded, setIsExpanded] = useState(false)
     const [showSettings, setShowSettings] = useState(false)
     const [showPiModelPanel, setShowPiModelPanel] = useState(false)
     const [showPiThinkingPanel, setShowPiThinkingPanel] = useState(false)
@@ -608,6 +620,39 @@ export function HappyComposer(props: {
         }
     }, [platformHaptic])
 
+    const handleExpandedToggle = useCallback(() => {
+        const currentInput = textareaRef.current
+        const selection = currentInput ? {
+            start: currentInput.selectionStart,
+            end: currentInput.selectionEnd,
+            direction: currentInput.selectionDirection,
+        } : null
+
+        setIsExpanded((expanded) => !expanded)
+        haptic('light')
+        setTimeout(() => {
+            if (richMentionsEnabled) {
+                richInputRef.current?.focus()
+                return
+            }
+            const input = textareaRef.current
+            if (!input) return
+            try {
+                input.focus({ preventScroll: true })
+            } catch {
+                input.focus()
+            }
+            if (selection) {
+                const maxOffset = input.value.length
+                input.setSelectionRange(
+                    Math.min(selection.start, maxOffset),
+                    Math.min(selection.end, maxOffset),
+                    selection.direction,
+                )
+            }
+        }, 0)
+    }, [haptic, richMentionsEnabled])
+
     const handleSuggestionSelect = useCallback((index: number) => {
         const suggestion = suggestions[index]
         if (!suggestion) return
@@ -873,17 +918,21 @@ export function HappyComposer(props: {
                 handleSuggestionSelect(indexToSelect)
                 return
             }
-            if (key === 'Escape') {
-                e.preventDefault()
-                clearSuggestions()
-                return
-            }
         }
 
-        if (key === 'Escape' && threadIsRunning) {
-            e.preventDefault()
-            handleAbort()
-            return
+        if (key === 'Escape') {
+            const action = getComposerEscapeAction({
+                hasSuggestions: suggestions.length > 0,
+                threadIsRunning,
+                isExpanded,
+            })
+            if (action) {
+                e.preventDefault()
+                if (action === 'clearSuggestions') clearSuggestions()
+                else if (action === 'abort') handleAbort()
+                else handleExpandedToggle()
+                return
+            }
         }
 
         if (key === 'Tab' && e.shiftKey && onPermissionModeChange && permissionModes.length > 0) {
@@ -912,6 +961,8 @@ export function HappyComposer(props: {
         composerEnterBehavior,
         richMentionsEnabled,
         flushAndSend,
+        isExpanded,
+        handleExpandedToggle,
     ])
 
     useEffect(() => {
@@ -1124,6 +1175,10 @@ export function HappyComposer(props: {
         haptic('light')
     }, [controlsDisabled, haptic])
 
+    const overlayPositionClass = isExpanded
+        ? 'absolute z-10 bottom-12 mb-2'
+        : 'absolute z-10 bottom-[100%] mb-2'
+
     const overlays = useMemo(() => {
         // Pi flavor: separate floating panels for model and thinking level.
         // (Pi RPC mode has no runtime permission switching → no permission panel.)
@@ -1134,7 +1189,7 @@ export function HappyComposer(props: {
             if (showPiModelPanel && piModels && piModels.length > 0) {
                 const currentPiModel = selectedPiModel ?? null
                 panels.push(
-                    <div key="model" className="absolute bottom-[100%] mb-2 left-2 w-64">
+                    <div key="model" className={`${overlayPositionClass} left-2 w-64`}>
                         <PiModelPanel
                             models={piModels}
                             currentModel={currentPiModel ? { provider: currentPiModel.provider, modelId: currentPiModel.modelId } : null}
@@ -1151,7 +1206,7 @@ export function HappyComposer(props: {
             // Thinking level panel
             if (showPiThinkingPanel && selectedPiModel?.reasoning !== false) {
                 panels.push(
-                    <div key="thinking" className="absolute bottom-[100%] mb-2 left-2 w-48">
+                    <div key="thinking" className={`${overlayPositionClass} left-2 w-48`}>
                         <PiThinkingLevelPanel
                             currentLevel={effort}
                             reasoning={selectedPiModel?.reasoning}
@@ -1170,7 +1225,7 @@ export function HappyComposer(props: {
         // Non-Pi flavors: original unified gear menu
         if (showSettings && (showCollaborationSettings || showPermissionSettings || showModelSettings || showModelEffortSettings || showModelReasoningEffortSettings || showEffortSettings || showFastModeSettings)) {
             return (
-                <div className="absolute bottom-[100%] mb-2 w-full">
+                <div className={`${overlayPositionClass} w-full`}>
                     <FloatingOverlay maxHeight={320}>
                         {showCollaborationSettings ? (
                             <div className="py-2">
@@ -1482,7 +1537,7 @@ export function HappyComposer(props: {
 
         if (suggestions.length > 0) {
             return (
-                <div className="absolute bottom-[100%] mb-2 w-full">
+                <div className={`${overlayPositionClass} w-full`}>
                     <FloatingOverlay>
                         <Autocomplete
                             suggestions={suggestions}
@@ -1535,13 +1590,27 @@ export function HappyComposer(props: {
         handleEffortChange,
         handleServiceTierChange,
         handleSuggestionSelect,
+        overlayPositionClass,
         t
     ])
 
+    const shellClassName = isExpanded
+        ? `z-[60] flex min-h-0 flex-col bg-[var(--app-bg)] px-3 ${bottomPaddingClass} max-sm:fixed max-sm:inset-x-0 max-sm:top-0 max-sm:h-[var(--tg-viewport-stable-height,var(--app-viewport-height,100dvh))] max-sm:pt-[calc(0.5rem+env(safe-area-inset-top))] sm:absolute sm:inset-0 sm:pt-2`
+        : `bg-[var(--app-bg)] px-3 ${bottomPaddingClass} pt-2`
+    const innerClassName = isExpanded
+        ? 'mx-auto flex min-h-0 w-full max-w-content flex-1 flex-col'
+        : 'mx-auto w-full max-w-content'
+    const rootClassName = isExpanded
+        ? 'relative flex min-h-0 flex-1 flex-col'
+        : 'relative'
+    const editorClassName = isExpanded
+        ? 'h-full min-h-[1.5rem] flex-1 overflow-y-auto whitespace-pre-wrap break-words bg-transparent text-base leading-snug text-[var(--app-fg)] focus:outline-none'
+        : 'max-h-[7.5rem] min-h-[1.5rem] flex-1 overflow-y-auto whitespace-pre-wrap break-words bg-transparent text-base leading-snug text-[var(--app-fg)] focus:outline-none'
+
     return (
-        <div className={`px-3 ${bottomPaddingClass} pt-2 bg-[var(--app-bg)]`}>
-            <div className="mx-auto w-full max-w-content">
-                <ComposerPrimitive.Root className="relative" onSubmit={handleSubmit}>
+        <div className={shellClassName} data-testid="composer-shell" data-expanded={isExpanded || undefined}>
+            <div className={innerClassName}>
+                <ComposerPrimitive.Root className={rootClassName} onSubmit={handleSubmit}>
                     {overlays}
 
                     <StatusBar
@@ -1592,16 +1661,22 @@ export function HappyComposer(props: {
 
                     <div
                         className={`overflow-hidden rounded-[20px] bg-[var(--app-secondary-bg)] ${
+                            isExpanded ? 'flex min-h-0 flex-1 flex-col' : ''
+                        } ${
                             sendError ? 'ring-1 ring-red-500' : ''
                         }`}
                     >
                         {attachments.length > 0 ? (
-                            <div className="flex flex-wrap gap-2 px-4 pt-3">
+                            <div className={`flex flex-wrap gap-2 px-4 pt-3 ${
+                                isExpanded ? 'max-h-[35%] shrink-0 overflow-y-auto' : ''
+                            }`}>
                                 <ComposerPrimitive.Attachments components={{ Attachment: AttachmentItem }} />
                             </div>
                         ) : null}
 
-                        <div className="flex items-center px-4 py-3">
+                        <div className={`flex px-4 py-3 ${
+                            isExpanded ? 'min-h-0 flex-1 items-stretch' : 'items-center'
+                        }`}>
                             {richMentionsEnabled ? (
                                 <RichComposerInput
                                     ref={richInputRef}
@@ -1615,8 +1690,26 @@ export function HappyComposer(props: {
                                     onPaste={handlePaste}
                                     resolveSessionMentionTooltip={resolveSessionMentionTooltip}
                                     onEdit={handleRichEdit}
-                                    className="max-h-[7.5rem] min-h-[1.5rem] flex-1 overflow-y-auto whitespace-pre-wrap break-words bg-transparent text-base leading-snug text-[var(--app-fg)] focus:outline-none"
+                                    className={editorClassName}
                                 />
+                            ) : isExpanded ? (
+                                <ComposerPrimitive.Input
+                                    asChild
+                                    ref={textareaRef}
+                                    autoFocus={!controlsDisabled && !isTouch}
+                                    submitOnEnter={false}
+                                    cancelOnEscape={false}
+                                    onChange={handleChange}
+                                    onSelect={handleSelect}
+                                    onKeyDown={handleKeyDown}
+                                    onPaste={handlePaste}
+                                >
+                                    <textarea
+                                        placeholder={showContinueHint ? t('misc.typeMessage') : t('misc.typeAMessage')}
+                                        disabled={controlsDisabled}
+                                        className="h-full min-h-0 flex-1 resize-none overflow-y-auto bg-transparent text-base leading-snug text-[var(--app-fg)] placeholder-[var(--app-hint)] focus:outline-none disabled:cursor-not-allowed disabled:opacity-50"
+                                    />
+                                </ComposerPrimitive.Input>
                             ) : (
                                 <ComposerPrimitive.Input
                                     ref={textareaRef}
@@ -1640,6 +1733,8 @@ export function HappyComposer(props: {
                             controlsDisabled={controlsDisabled}
                             showSettingsButton={showSettingsButton}
                             onSettingsToggle={handleSettingsToggle}
+                            expanded={isExpanded}
+                            onExpandedToggle={handleExpandedToggle}
                             showTerminalButton={showTerminalButton}
                             terminalDisabled={terminalDisabled}
                             terminalLabel={terminalLabel}
diff --git a/web/src/components/settings/ComposerToolbarLayoutControl.tsx b/web/src/components/settings/ComposerToolbarLayoutControl.tsx
index 13247779..665fd488 100644
--- a/web/src/components/settings/ComposerToolbarLayoutControl.tsx
+++ b/web/src/components/settings/ComposerToolbarLayoutControl.tsx
@@ -15,6 +15,7 @@ import { SettingsChoiceGroup } from './SettingsPrimitives'
 const ITEM_LABEL_KEYS: Record<ComposerToolbarItemId, string> = {
     attachment: 'settings.chat.composerToolbar.item.attachment',
     settings: 'settings.chat.composerToolbar.item.settings',
+    expand: 'settings.chat.composerToolbar.item.expand',
     piModel: 'settings.chat.composerToolbar.item.piModel',
     piThinking: 'settings.chat.composerToolbar.item.piThinking',
     terminal: 'settings.chat.composerToolbar.item.terminal',
diff --git a/web/src/hooks/useComposerToolbarLayout.test.ts b/web/src/hooks/useComposerToolbarLayout.test.ts
index a1f2baa7..077373d6 100644
--- a/web/src/hooks/useComposerToolbarLayout.test.ts
+++ b/web/src/hooks/useComposerToolbarLayout.test.ts
@@ -11,6 +11,7 @@ describe('DEFAULT_COMPOSER_TOOLBAR_LAYOUT', () => {
         expect(DEFAULT_COMPOSER_TOOLBAR_LAYOUT.left).toEqual([
             'attachment',
             'settings',
+            'expand',
             'piModel',
             'piThinking',
             'terminal',
@@ -80,9 +81,9 @@ describe('normalizeComposerToolbarLayout', () => {
             'piModel',
             'piThinking',
             'terminal',
+            'expand',
             'abort',
             'switch',
-            'voiceMic',
             'attachment',
         ])
         expect(result.left).toHaveLength(layout.left.length)
diff --git a/web/src/hooks/useComposerToolbarLayout.ts b/web/src/hooks/useComposerToolbarLayout.ts
index d4782c30..6576ecb8 100644
--- a/web/src/hooks/useComposerToolbarLayout.ts
+++ b/web/src/hooks/useComposerToolbarLayout.ts
@@ -3,6 +3,7 @@ import { useCallback, useEffect, useState } from 'react'
 export const COMPOSER_TOOLBAR_ITEM_IDS = [
     'attachment',
     'settings',
+    'expand',
     'piModel',
     'piThinking',
     'terminal',
diff --git a/web/src/lib/locales/en.ts b/web/src/lib/locales/en.ts
index f0970ab1..27bfa097 100644
--- a/web/src/lib/locales/en.ts
+++ b/web/src/lib/locales/en.ts
@@ -517,6 +517,8 @@ export default {
   'composer.abort': 'Abort',
   'composer.switchRemote': 'Switch to remote mode',
   'composer.attach': 'Attach file',
+  'composer.expand': 'Expand message editor',
+  'composer.collapse': 'Collapse message editor',
   'composer.dropToAttach': 'Drop to attach',
   'composer.send': 'Send',
   'composer.stop': 'Stop',
@@ -750,6 +752,7 @@ export default {
   'settings.chat.composerToolbar.moveLater': 'Move later',
   'settings.chat.composerToolbar.item.attachment': 'Add attachment',
   'settings.chat.composerToolbar.item.settings': 'Session settings',
+  'settings.chat.composerToolbar.item.expand': 'Expand message editor',
   'settings.chat.composerToolbar.item.piModel': 'Pi model',
   'settings.chat.composerToolbar.item.piThinking': 'Pi thinking level',
   'settings.chat.composerToolbar.item.terminal': 'Terminal',
diff --git a/web/src/lib/locales/zh-CN.ts b/web/src/lib/locales/zh-CN.ts
index 2bf7eb7c..bbf6dc6a 100644
--- a/web/src/lib/locales/zh-CN.ts
+++ b/web/src/lib/locales/zh-CN.ts
@@ -521,6 +521,8 @@ export default {
   'composer.abort': '中止',
   'composer.switchRemote': '切换到远程模式',
   'composer.attach': '添加文件',
+  'composer.expand': '展开消息编辑器',
+  'composer.collapse': '收起消息编辑器',
   'composer.dropToAttach': '松开以添加文件',
   'composer.send': '发送',
   'composer.stop': '停止',
@@ -754,6 +756,7 @@ export default {
   'settings.chat.composerToolbar.moveLater': '向后移动',
   'settings.chat.composerToolbar.item.attachment': '添加附件',
   'settings.chat.composerToolbar.item.settings': '会话设置',
+  'settings.chat.composerToolbar.item.expand': '展开消息编辑器',
   'settings.chat.composerToolbar.item.piModel': 'Pi 模型',
   'settings.chat.composerToolbar.item.piThinking': 'Pi 思考等级',
   'settings.chat.composerToolbar.item.terminal': '终端',