diff --git a/frontend/src/components/MessageInput.test.tsx b/frontend/src/components/MessageInput.test.tsx
index ca88fdf0..ff658ed7 100644
--- a/frontend/src/components/MessageInput.test.tsx
+++ b/frontend/src/components/MessageInput.test.tsx
@@ -18,6 +18,7 @@ import uploadReducer, {
   type Attachment,
 } from '../upload/uploadSlice';
 import MessageInput from './MessageInput';
+import { UPLOAD_STALL_TIMEOUT_MS } from './message-input/uploadStallGuard';
 
 Object.assign(globalThis, { IS_REACT_ACT_ENVIRONMENT: true });
 
@@ -41,14 +42,53 @@ const att = (over: Partial<Attachment> = {}): Attachment => ({
   ...over,
 });
 
+/** Just enough of XMLHttpRequest for the composer's upload paths. */
+class FakeXHR extends EventTarget {
+  static instances: FakeXHR[] = [];
+
+  upload = new EventTarget();
+  status = 0;
+  responseText = '';
+  timeout = 0;
+  onload: (() => void) | null = null;
+  onerror: (() => void) | null = null;
+  onabort: (() => void) | null = null;
+  ontimeout: (() => void) | null = null;
+
+  open() {}
+  setRequestHeader() {}
+  send() {
+    FakeXHR.instances.push(this);
+  }
+
+  private finish(handler: (() => void) | null, type: string) {
+    handler?.();
+    this.dispatchEvent(new Event(type));
+    this.dispatchEvent(new Event('loadend'));
+  }
+
+  /** What Android Chrome reported: no response, status 0. */
+  failNetwork() {
+    this.status = 0;
+    this.finish(this.onerror, 'error');
+  }
+
+  abort() {
+    this.finish(this.onabort, 'abort');
+  }
+}
+
 describe('MessageInput send with a failed attachment', () => {
   let container: HTMLDivElement;
   let root: Root;
   let store: TestStore;
   let onSubmit: ReturnType<typeof vi.fn<(text: string) => void>>;
+  const realXHR = globalThis.XMLHttpRequest;
 
   beforeEach(() => {
     localStorage.clear();
+    FakeXHR.instances = [];
+    globalThis.XMLHttpRequest = FakeXHR as unknown as typeof XMLHttpRequest;
     container = document.createElement('div');
     document.body.appendChild(container);
     root = createRoot(container);
@@ -59,6 +99,8 @@ describe('MessageInput send with a failed attachment', () => {
   afterEach(async () => {
     await act(async () => root.unmount());
     container.remove();
+    globalThis.XMLHttpRequest = realXHR;
+    vi.useRealTimers();
   });
 
   const render = async (loading = false) => {
@@ -100,6 +142,27 @@ describe('MessageInput send with a failed attachment', () => {
     });
   };
 
+  const attachFile = async (name: string) => {
+    const input = container.querySelector<HTMLInputElement>(
+      'label input[type="file"]',
+    )!;
+    const file = new File(['%PDF-1.4'], name, { type: 'application/pdf' });
+    Object.defineProperty(input, 'files', {
+      value: [file],
+      configurable: true,
+    });
+    const sent = FakeXHR.instances.length;
+    await act(async () => {
+      input.dispatchEvent(new Event('change', { bubbles: true }));
+    });
+    // partitionAttachmentFiles sniffs the file asynchronously.
+    await act(async () => {
+      await vi.waitFor(() =>
+        expect(FakeXHR.instances.length).toBeGreaterThan(sent),
+      );
+    });
+  };
+
   it('sends the question and drops the failed file', async () => {
     store.dispatch(addAttachment(att()));
     store.dispatch(
@@ -127,6 +190,24 @@ describe('MessageInput send with a failed attachment', () => {
     expect(textarea().value).toBe('');
   });
 
+  it('sends when the only attachment failed on the network', async () => {
+    await render();
+    await attachFile('scan.pdf');
+    expect(FakeXHR.instances).toHaveLength(1);
+
+    await act(async () => FakeXHR.instances[0].failNetwork());
+    const [chip] = store.getState().upload.attachments;
+    expect(chip.status).toBe('failed');
+    expect(chip.errorMessage).toBe('conversation.attachments.uploadFailed');
+
+    await type('Summarise this');
+    await pressEnter();
+
+    expect(onSubmit).toHaveBeenCalledTimes(1);
+    expect(onSubmit).toHaveBeenCalledWith('Summarise this');
+    expect(store.getState().upload.attachments).toEqual([]);
+  });
+
   it('shows no send-blocked alert after a failed send attempt', async () => {
     store.dispatch(addAttachment(att({ id: 'bad-1', status: 'failed' })));
     await render();
@@ -137,6 +218,26 @@ describe('MessageInput send with a failed attachment', () => {
     expect(container.querySelector('[role="alert"]')).toBeNull();
   });
 
+  it('fails a stalled upload and then flushes the queued send', async () => {
+    vi.useFakeTimers();
+    await render();
+    await attachFile('huge.pdf');
+    expect(store.getState().upload.attachments[0].status).toBe('uploading');
+
+    await type('Is it done?');
+    await pressEnter();
+    // Pending: the send is queued, not dropped.
+    expect(onSubmit).not.toHaveBeenCalled();
+
+    await act(async () => {
+      vi.advanceTimersByTime(UPLOAD_STALL_TIMEOUT_MS);
+    });
+
+    expect(onSubmit).toHaveBeenCalledTimes(1);
+    expect(onSubmit).toHaveBeenCalledWith('Is it done?');
+    expect(store.getState().upload.attachments).toEqual([]);
+  });
+
   it('keeps a queued question while another answer is streaming', async () => {
     store.dispatch(addAttachment(att({ status: 'processing', progress: 30 })));
     await render();
diff --git a/frontend/src/components/MessageInput.tsx b/frontend/src/components/MessageInput.tsx
index 05920510..1fd733df 100644
--- a/frontend/src/components/MessageInput.tsx
+++ b/frontend/src/components/MessageInput.tsx
@@ -51,6 +51,7 @@ import {
   ToolsTrigger,
 } from './message-input';
 import { useArmedSend } from './message-input/armedSend';
+import { guardUploadStall } from './message-input/uploadStallGuard';
 import { cannotReadAttachment } from './message-input/attachmentReadability';
 import { handleAbort } from '../conversation/conversationSlice';
 import {
@@ -555,6 +556,7 @@ export default function MessageInput({
       const files = supported;
 
       const apiHost = envVar('VITE_API_HOST');
+      const uploadFailedMessage = t('conversation.attachments.uploadFailed');
 
       if (files.length > 1) {
         const formData = new FormData();
@@ -830,20 +832,29 @@ export default function MessageInput({
           }
         };
 
-        xhr.onerror = () => {
-          console.error('Upload network error');
-          Object.values(indexToUiId).forEach((id) =>
-            dispatch(
-              updateAttachment({
-                id,
-                updates: { status: 'failed' },
-              }),
-            ),
-          );
-        };
+        // No response at all (status 0): a file the browser couldn't read,
+        // a dropped connection, or a stall the guard aborted.
+        xhr.onerror =
+          xhr.onabort =
+          xhr.ontimeout =
+            () => {
+              console.error('Upload network error');
+              Object.values(indexToUiId).forEach((id) =>
+                dispatch(
+                  updateAttachment({
+                    id,
+                    updates: {
+                      status: 'failed',
+                      errorMessage: uploadFailedMessage,
+                    },
+                  }),
+                ),
+              );
+            };
 
         xhr.open('POST', `${apiHost}${endpoints.USER.STORE_ATTACHMENT}`);
         if (token) xhr.setRequestHeader('Authorization', `Bearer ${token}`);
+        guardUploadStall(xhr);
         xhr.send(formData);
         return;
       }
@@ -951,17 +962,24 @@ export default function MessageInput({
           }
         };
 
-        xhr.onerror = () => {
-          dispatch(
-            updateAttachment({
-              id: uniqueId,
-              updates: { status: 'failed' },
-            }),
-          );
-        };
+        xhr.onerror =
+          xhr.onabort =
+          xhr.ontimeout =
+            () => {
+              dispatch(
+                updateAttachment({
+                  id: uniqueId,
+                  updates: {
+                    status: 'failed',
+                    errorMessage: uploadFailedMessage,
+                  },
+                }),
+              );
+            };
 
         xhr.open('POST', `${apiHost}${endpoints.USER.STORE_ATTACHMENT}`);
         if (token) xhr.setRequestHeader('Authorization', `Bearer ${token}`);
+        guardUploadStall(xhr);
         xhr.send(formData);
       });
     },
diff --git a/frontend/src/components/message-input/uploadStallGuard.test.ts b/frontend/src/components/message-input/uploadStallGuard.test.ts
new file mode 100644
index 00000000..02262b5e
--- /dev/null
+++ b/frontend/src/components/message-input/uploadStallGuard.test.ts
@@ -0,0 +1,54 @@
+import { guardUploadStall, UPLOAD_STALL_TIMEOUT_MS } from './uploadStallGuard';
+
+class FakeXHR extends EventTarget {
+  upload = new EventTarget();
+  abort = vi.fn(() => {
+    this.dispatchEvent(new Event('abort'));
+    this.dispatchEvent(new Event('loadend'));
+  });
+}
+
+const make = () => {
+  const xhr = new FakeXHR();
+  guardUploadStall(xhr as unknown as XMLHttpRequest);
+  return xhr;
+};
+
+describe('guardUploadStall', () => {
+  beforeEach(() => vi.useFakeTimers());
+  afterEach(() => vi.useRealTimers());
+
+  it('aborts an upload that never reports progress', () => {
+    const xhr = make();
+    vi.advanceTimersByTime(UPLOAD_STALL_TIMEOUT_MS - 1);
+    expect(xhr.abort).not.toHaveBeenCalled();
+    vi.advanceTimersByTime(1);
+    expect(xhr.abort).toHaveBeenCalledTimes(1);
+  });
+
+  it('lets a slow upload run while it keeps making progress', () => {
+    const xhr = make();
+    for (let i = 0; i < 5; i += 1) {
+      vi.advanceTimersByTime(UPLOAD_STALL_TIMEOUT_MS - 1);
+      xhr.upload.dispatchEvent(new Event('progress'));
+    }
+    expect(xhr.abort).not.toHaveBeenCalled();
+  });
+
+  it('gives the server a fresh window once the body is sent', () => {
+    const xhr = make();
+    vi.advanceTimersByTime(UPLOAD_STALL_TIMEOUT_MS - 1);
+    xhr.upload.dispatchEvent(new Event('load'));
+    vi.advanceTimersByTime(UPLOAD_STALL_TIMEOUT_MS - 1);
+    expect(xhr.abort).not.toHaveBeenCalled();
+    vi.advanceTimersByTime(1);
+    expect(xhr.abort).toHaveBeenCalledTimes(1);
+  });
+
+  it('stops watching once the request settles', () => {
+    const xhr = make();
+    xhr.dispatchEvent(new Event('loadend'));
+    vi.advanceTimersByTime(UPLOAD_STALL_TIMEOUT_MS * 2);
+    expect(xhr.abort).not.toHaveBeenCalled();
+  });
+});
diff --git a/frontend/src/components/message-input/uploadStallGuard.ts b/frontend/src/components/message-input/uploadStallGuard.ts
new file mode 100644
index 00000000..bf79cad5
--- /dev/null
+++ b/frontend/src/components/message-input/uploadStallGuard.ts
@@ -0,0 +1,33 @@
+// How long an attachment upload may go without any sign of life. It is an
+// idle window, not a total cap: a large file on a slow link keeps resetting
+// it with progress events, so only a silent stall is cut off.
+export const UPLOAD_STALL_TIMEOUT_MS = 120_000;
+
+/**
+ * Abort ``xhr`` once it makes no progress for ``idleMs``.
+ *
+ * ``xhr.timeout`` would cap the whole request, which a large file on a slow
+ * connection can legitimately exceed. This resets on every upload progress
+ * event, and again when the body finishes sending so the server gets its own
+ * window to answer. The abort fires the request's ``onabort`` handler, which
+ * is where the caller marks the attachment failed.
+ *
+ * Args:
+ *   xhr: The request to watch. Call before ``send()``.
+ *   idleMs: The longest allowed gap between signs of progress.
+ */
+export function guardUploadStall(
+  xhr: XMLHttpRequest,
+  idleMs: number = UPLOAD_STALL_TIMEOUT_MS,
+): void {
+  let timer: ReturnType<typeof setTimeout> | undefined;
+  const restart = () => {
+    clearTimeout(timer);
+    timer = setTimeout(() => xhr.abort(), idleMs);
+  };
+  xhr.upload.addEventListener('progress', restart);
+  xhr.upload.addEventListener('load', restart);
+  xhr.addEventListener('progress', restart);
+  xhr.addEventListener('loadend', () => clearTimeout(timer));
+  restart();
+}
diff --git a/frontend/src/locale/de.json b/frontend/src/locale/de.json
index e788de44..7c35fe9b 100644
--- a/frontend/src/locale/de.json
+++ b/frontend/src/locale/de.json
@@ -1373,6 +1373,7 @@
     "attachments": {
       "attach": "Anhängen",
       "remove": "Anhang entfernen",
+      "uploadFailed": "Upload fehlgeschlagen. Die Datei konnte nicht gelesen werden oder die Verbindung wurde unterbrochen.",
       "attached": "Anhang",
       "failed": "Fehlgeschlagen",
       "attachment": "Anhang"
diff --git a/frontend/src/locale/en.json b/frontend/src/locale/en.json
index fa295e00..986577a1 100644
--- a/frontend/src/locale/en.json
+++ b/frontend/src/locale/en.json
@@ -1379,6 +1379,7 @@
     "attachments": {
       "attach": "Attach",
       "remove": "Remove attachment",
+      "uploadFailed": "Upload failed. The file couldn't be read or the connection dropped.",
       "waitingToSend_one": "Will send when {{count}} file finishes processing…",
       "waitingToSend_other": "Will send when {{count}} files finish processing…",
       "cancelQueuedSend": "Cancel",
diff --git a/frontend/src/locale/es.json b/frontend/src/locale/es.json
index 90b38b89..681e2e0d 100644
--- a/frontend/src/locale/es.json
+++ b/frontend/src/locale/es.json
@@ -1373,6 +1373,7 @@
     "attachments": {
       "attach": "Adjuntar",
       "remove": "Eliminar adjunto",
+      "uploadFailed": "Error al subir. No se pudo leer el archivo o se perdió la conexión.",
       "attached": "Adjunto",
       "failed": "Error",
       "attachment": "Adjunto"
diff --git a/frontend/src/locale/jp.json b/frontend/src/locale/jp.json
index 41cf29f1..9e0fa1dd 100644
--- a/frontend/src/locale/jp.json
+++ b/frontend/src/locale/jp.json
@@ -1370,6 +1370,7 @@
     "attachments": {
       "attach": "添付",
       "remove": "添付ファイルを削除",
+      "uploadFailed": "アップロードに失敗しました。ファイルを読み取れなかったか、接続が切断されました。",
       "attached": "添付ファイル",
       "failed": "失敗",
       "attachment": "添付ファイル"
diff --git a/frontend/src/locale/ru.json b/frontend/src/locale/ru.json
index 7a9d0352..c00face0 100644
--- a/frontend/src/locale/ru.json
+++ b/frontend/src/locale/ru.json
@@ -1417,6 +1417,7 @@
     "attachments": {
       "attach": "Прикрепить",
       "remove": "Удалить вложение",
+      "uploadFailed": "Не удалось загрузить. Файл не удалось прочитать или соединение прервалось.",
       "attached": "Вложение",
       "failed": "Ошибка",
       "attachment": "Вложение"
diff --git a/frontend/src/locale/zh-TW.json b/frontend/src/locale/zh-TW.json
index ff1d2efc..3ca0570b 100644
--- a/frontend/src/locale/zh-TW.json
+++ b/frontend/src/locale/zh-TW.json
@@ -1370,6 +1370,7 @@
     "attachments": {
       "attach": "附件",
       "remove": "刪除附件",
+      "uploadFailed": "上傳失敗。無法讀取檔案或連線已中斷。",
       "attached": "附件",
       "failed": "失敗",
       "attachment": "附件"
diff --git a/frontend/src/locale/zh.json b/frontend/src/locale/zh.json
index 36dc4e58..d41815c1 100644
--- a/frontend/src/locale/zh.json
+++ b/frontend/src/locale/zh.json
@@ -1370,6 +1370,7 @@
     "attachments": {
       "attach": "附件",
       "remove": "删除附件",
+      "uploadFailed": "上传失败。无法读取文件或连接已断开。",
       "attached": "附件",
       "failed": "失败",
       "attachment": "附件"