diff --git a/web/src/components/AssistantChat/DragDropZone.test.tsx b/web/src/components/AssistantChat/DragDropZone.test.tsx
new file mode 100644
index 00000000..6f3737d7
--- /dev/null
+++ b/web/src/components/AssistantChat/DragDropZone.test.tsx
@@ -0,0 +1,79 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { render, fireEvent } from '@testing-library/react'
+
+const addAttachment = vi.fn()
+
+vi.mock('@assistant-ui/react', () => ({
+    useAssistantApi: () => ({
+        composer: () => ({ addAttachment }),
+    }),
+}))
+
+vi.mock('@/lib/use-translation', () => ({
+    useTranslation: () => ({ t: (key: string) => key }),
+}))
+
+import { DragDropZone } from './DragDropZone'
+
+function createDropEvent(types: string[], files: File[]): Event {
+    const event = new Event('drop', { bubbles: true, cancelable: true })
+    Object.defineProperty(event, 'dataTransfer', {
+        value: { types, files },
+        configurable: true,
+    })
+    return event
+}
+
+describe('DragDropZone drop handling', () => {
+    beforeEach(() => {
+        vi.clearAllMocks()
+    })
+
+    it('adds dropped files as attachments and cancels the browser default', () => {
+        const { container } = render(
+            <DragDropZone>
+                <div />
+            </DragDropZone>
+        )
+        const zone = container.firstChild as HTMLElement
+        const file = new File(['x'], 'a.txt', { type: 'text/plain' })
+        const event = createDropEvent(['Files'], [file])
+
+        fireEvent(zone, event)
+
+        expect(event.defaultPrevented).toBe(true)
+        expect(addAttachment).toHaveBeenCalledTimes(1)
+        expect(addAttachment).toHaveBeenCalledWith(file)
+    })
+
+    it('ignores non-file drops so the browser keeps its default (e.g. text into composer)', () => {
+        const { container } = render(
+            <DragDropZone>
+                <div />
+            </DragDropZone>
+        )
+        const zone = container.firstChild as HTMLElement
+        const event = createDropEvent(['text/plain'], [])
+
+        fireEvent(zone, event)
+
+        expect(event.defaultPrevented).toBe(false)
+        expect(addAttachment).not.toHaveBeenCalled()
+    })
+
+    it('does not attach when disabled but still cancels the file default', () => {
+        const { container } = render(
+            <DragDropZone disabled>
+                <div />
+            </DragDropZone>
+        )
+        const zone = container.firstChild as HTMLElement
+        const file = new File(['x'], 'a.txt', { type: 'text/plain' })
+        const event = createDropEvent(['Files'], [file])
+
+        fireEvent(zone, event)
+
+        expect(event.defaultPrevented).toBe(true)
+        expect(addAttachment).not.toHaveBeenCalled()
+    })
+})
diff --git a/web/src/components/AssistantChat/DragDropZone.tsx b/web/src/components/AssistantChat/DragDropZone.tsx
new file mode 100644
index 00000000..102ac5d1
--- /dev/null
+++ b/web/src/components/AssistantChat/DragDropZone.tsx
@@ -0,0 +1,60 @@
+import { useCallback } from 'react'
+import { useAssistantApi } from '@assistant-ui/react'
+import { useDragOver } from '@/hooks/useDragOver'
+import { useTranslation } from '@/lib/use-translation'
+
+export function DragDropZone({
+    children,
+    disabled,
+}: {
+    children: React.ReactNode
+    disabled?: boolean
+}) {
+    const api = useAssistantApi()
+    const isDragging = useDragOver()
+    const { t } = useTranslation()
+
+    const onDragOver = useCallback((e: React.DragEvent) => {
+        if (e.dataTransfer.types.includes('Files')) {
+            e.preventDefault()
+            e.dataTransfer.dropEffect = disabled ? 'none' : 'copy'
+        }
+    }, [disabled])
+
+    const onDrop = useCallback(
+        async (e: React.DragEvent) => {
+            // Let non-file drops (e.g. selected text into the composer) keep
+            // their default browser behaviour instead of being cancelled.
+            if (!e.dataTransfer.types.includes('Files')) return
+            e.preventDefault()
+            if (disabled) return
+            const files = Array.from(e.dataTransfer.files)
+            if (files.length === 0) return
+            try {
+                for (const file of files) {
+                    await api.composer().addAttachment(file)
+                }
+            } catch (error) {
+                console.error('Error adding dragged file:', error)
+            }
+        },
+        [api, disabled]
+    )
+
+    return (
+        <div
+            className="relative flex min-h-0 flex-1 flex-col"
+            onDragOver={onDragOver}
+            onDrop={onDrop}
+        >
+            {children}
+            {isDragging && !disabled && (
+                <div className="pointer-events-none absolute inset-0 z-50 flex items-center justify-center rounded-lg border-2 border-dashed border-[var(--app-link)] bg-[var(--app-link)]/10">
+                    <div className="rounded-lg bg-[var(--app-bg)] px-4 py-2 text-sm font-medium text-[var(--app-link)] shadow-lg">
+                        {t('composer.dropToAttach')}
+                    </div>
+                </div>
+            )}
+        </div>
+    )
+}
diff --git a/web/src/components/SessionChat.tsx b/web/src/components/SessionChat.tsx
index 8edc884d..57eedc3b 100644
--- a/web/src/components/SessionChat.tsx
+++ b/web/src/components/SessionChat.tsx
@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import { useNavigate } from '@tanstack/react-router'
 import { AssistantRuntimeProvider, useAssistantApi, useAssistantState } from '@assistant-ui/react'
+import { DragDropZone } from '@/components/AssistantChat/DragDropZone'
 import type { ApiClient } from '@/api/client'
 import type {
     AttachmentMetadata,
@@ -1137,7 +1138,8 @@ function SessionChatInner(props: SessionChatProps) {
 
             <AssistantRuntimeProvider runtime={runtime}>
                 <ShareSeedConsumer sessionId={props.session.id} sessionActive={props.session.active} />
-                <div className="relative flex min-h-0 flex-1 flex-col">
+                <DragDropZone disabled={sessionInactive || props.isSending || pendingSchedule != null}>
+
                     <HappyThread
                         // Key with prefix: different components under the same session
                         // (thread, scratchlist, composer) must have distinct keys to avoid
@@ -1337,7 +1339,7 @@ function SessionChatInner(props: SessionChatProps) {
                         sendError={props.sendError ?? null}
                         onClearSendError={props.onClearSendError}
                     />
-                </div>
+                </DragDropZone>
             </AssistantRuntimeProvider>
 
             {/* Voice session component - renders nothing but initializes voice backend */}
diff --git a/web/src/hooks/useDragOver.test.ts b/web/src/hooks/useDragOver.test.ts
new file mode 100644
index 00000000..6a98a47b
--- /dev/null
+++ b/web/src/hooks/useDragOver.test.ts
@@ -0,0 +1,62 @@
+import { describe, it, expect } from 'vitest'
+import { renderHook, act } from '@testing-library/react'
+import { useDragOver } from './useDragOver'
+
+function makeDragEvent(type: string, types: string[]): Event {
+    const event = new Event(type, { bubbles: true, cancelable: true })
+    Object.defineProperty(event, 'dataTransfer', {
+        value: { types },
+        configurable: true,
+    })
+    return event
+}
+
+describe('useDragOver', () => {
+    it('prevents the browser default when a file is dropped outside a zone', () => {
+        // Regression: a file dropped on the document (e.g. the sidebar) must not
+        // trigger the browser's file-open/navigation behaviour.
+        const { unmount } = renderHook(() => useDragOver())
+        const event = makeDragEvent('drop', ['Files'])
+        act(() => {
+            document.dispatchEvent(event)
+        })
+        expect(event.defaultPrevented).toBe(true)
+        unmount()
+    })
+
+    it('does not prevent default for a non-file drop', () => {
+        const { unmount } = renderHook(() => useDragOver())
+        const event = makeDragEvent('drop', ['text/plain'])
+        act(() => {
+            document.dispatchEvent(event)
+        })
+        expect(event.defaultPrevented).toBe(false)
+        unmount()
+    })
+
+    it('also prevents default on dragover for files so the drop can be cancelled', () => {
+        const { unmount } = renderHook(() => useDragOver())
+        const event = makeDragEvent('dragover', ['Files'])
+        act(() => {
+            document.dispatchEvent(event)
+        })
+        expect(event.defaultPrevented).toBe(true)
+        unmount()
+    })
+
+    it('tracks file-drag state and clears it on drop', () => {
+        const { result, unmount } = renderHook(() => useDragOver())
+        expect(result.current).toBe(false)
+
+        act(() => {
+            document.dispatchEvent(makeDragEvent('dragenter', ['Files']))
+        })
+        expect(result.current).toBe(true)
+
+        act(() => {
+            document.dispatchEvent(makeDragEvent('drop', ['Files']))
+        })
+        expect(result.current).toBe(false)
+        unmount()
+    })
+})
diff --git a/web/src/hooks/useDragOver.ts b/web/src/hooks/useDragOver.ts
new file mode 100644
index 00000000..6a53e90e
--- /dev/null
+++ b/web/src/hooks/useDragOver.ts
@@ -0,0 +1,59 @@
+import { useEffect, useState } from 'react'
+
+/**
+ * Returns true while the user is dragging files over the browser window.
+ * Also suppresses the browser's default file-open behaviour for drags that
+ * land outside an explicit drop zone.
+ */
+export function useDragOver(): boolean {
+    const [isDraggingFiles, setIsDraggingFiles] = useState(false)
+
+    useEffect(() => {
+        const onDragEnter = (e: DragEvent) => {
+            if (e.dataTransfer?.types.includes('Files')) {
+                setIsDraggingFiles(true)
+            }
+        }
+
+        // Only clear when the drag leaves the browser window entirely
+        // (relatedTarget === null means the pointer moved outside the document)
+        const onDragLeave = (e: DragEvent) => {
+            if (e.relatedTarget === null) {
+                setIsDraggingFiles(false)
+            }
+        }
+
+        const clearDrag = () => setIsDraggingFiles(false)
+
+        // Prevent the browser from opening/navigating to a file dropped outside
+        // an explicit drop zone (e.g. the sidebar). This must run on BOTH
+        // `dragover` and `drop`: preventing only `dragover` still lets the
+        // browser perform its default file-open action on the `drop` event.
+        const preventFileDefault = (e: DragEvent) => {
+            if (e.dataTransfer?.types.includes('Files')) {
+                e.preventDefault()
+            }
+        }
+
+        const onDrop = (e: DragEvent) => {
+            preventFileDefault(e)
+            clearDrag()
+        }
+
+        document.addEventListener('dragenter', onDragEnter)
+        document.addEventListener('dragleave', onDragLeave)
+        document.addEventListener('dragend', clearDrag)
+        document.addEventListener('drop', onDrop)
+        document.addEventListener('dragover', preventFileDefault)
+
+        return () => {
+            document.removeEventListener('dragenter', onDragEnter)
+            document.removeEventListener('dragleave', onDragLeave)
+            document.removeEventListener('dragend', clearDrag)
+            document.removeEventListener('drop', onDrop)
+            document.removeEventListener('dragover', preventFileDefault)
+        }
+    }, [])
+
+    return isDraggingFiles
+}
diff --git a/web/src/lib/locales/en.ts b/web/src/lib/locales/en.ts
index 60b119bc..dcaeeb20 100644
--- a/web/src/lib/locales/en.ts
+++ b/web/src/lib/locales/en.ts
@@ -412,6 +412,7 @@ export default {
   'composer.abort': 'Abort',
   'composer.switchRemote': 'Switch to remote mode',
   'composer.attach': 'Attach file',
+  'composer.dropToAttach': 'Drop to attach',
   'composer.send': 'Send',
   'composer.stop': 'Stop',
   'composer.voice': 'Voice assistant',
diff --git a/web/src/lib/locales/zh-CN.ts b/web/src/lib/locales/zh-CN.ts
index a44efebd..7508af10 100644
--- a/web/src/lib/locales/zh-CN.ts
+++ b/web/src/lib/locales/zh-CN.ts
@@ -416,6 +416,7 @@ export default {
   'composer.abort': '中止',
   'composer.switchRemote': '切换到远程模式',
   'composer.attach': '添加文件',
+  'composer.dropToAttach': '松开以添加文件',
   'composer.send': '发送',
   'composer.stop': '停止',
   'composer.voice': '语音助手',