diff --git a/components/email/__tests__/composer-upload-timeout.test.tsx b/components/email/__tests__/composer-upload-timeout.test.tsx
new file mode 100644
index 000000000..0afc1df55
--- /dev/null
+++ b/components/email/__tests__/composer-upload-timeout.test.tsx
@@ -0,0 +1,326 @@
+import { render, screen, within, fireEvent, act, cleanup } from '@testing-library/react';
+import { describe, it, expect, vi, afterEach, beforeEach } from 'vitest';
+import React from 'react';
+import { EmailComposer } from '../email-composer';
+import { useAuthStore } from '@/stores/auth-store';
+
+// Keep the real composer queue and JMAP client; stub unrelated UI and stores.
+vi.mock('@/components/email/rich-text-editor', () => ({
+  RichTextEditor: () => React.createElement('div', { 'data-testid': 'rich-text-editor' }),
+}));
+
+vi.mock('@/components/plugins/plugin-slot', () => ({ PluginSlot: () => null }));
+vi.mock('@/components/identity/sub-address-helper', () => ({ SubAddressHelper: () => null }));
+vi.mock('@/components/templates/template-picker', () => ({ TemplatePicker: () => null }));
+vi.mock('@/components/templates/template-form', () => ({ TemplateForm: () => null }));
+vi.mock('@/components/files/file-preview-modal', () => ({ FilePreviewModal: () => null }));
+vi.mock('@/hooks/use-focus-trap', () => ({
+  useFocusTrap: () => ({ current: null }),
+}));
+// The selected identity is namespaced with the local account that owns it.
+vi.mock('@/hooks/use-pro-multi-account-identities', () => ({
+  useProMultiAccountIdentities: () => ({ enabled: false, groups: [], allIdentities: [] }),
+  stripCrossAccountIdentityPrefix: (id: string) => {
+    const idx = id.indexOf('::');
+    return idx === -1
+      ? { localAccountId: null, rawId: id }
+      : { localAccountId: id.slice(0, idx), rawId: id.slice(idx + 2) };
+  },
+}));
+
+// ─── Store mocks ──────────────────────────────────────────────────────────────
+
+vi.mock('@/stores/auth-store', () => {
+  const state = {
+    client: null,
+    identities: [],
+    primaryIdentity: null,
+    isAuthenticated: false,
+    isDemoMode: false,
+    activeAccountId: null,
+    connectionLost: false,
+    getClientForAccount: () => undefined,
+    getAllConnectedClients: () => new Map(),
+    syncIdentities: () => {},
+    refreshIdentities: async () => {},
+  };
+  const hook = (sel?: (s: typeof state) => unknown) =>
+    typeof sel === 'function' ? sel(state) : state;
+  hook.getState = () => state;
+  hook.setState = (p: Partial<typeof state>) => Object.assign(state, p);
+  return { useAuthStore: hook };
+});
+
+vi.mock('@/stores/identity-store', () => {
+  const state = {
+    identities: [{ id: 'acct-2::id-other', email: 'other@example.com', name: 'Other' }],
+    defaultIdentityId: 'acct-2::id-other',
+  };
+  const hook = (sel?: (s: typeof state) => unknown) =>
+    typeof sel === 'function' ? sel(state) : state;
+  hook.getState = () => state;
+  hook.setState = (p: Partial<typeof state>) => Object.assign(state, p);
+  return { useIdentityStore: hook };
+});
+
+vi.mock('@/stores/account-store', () => {
+  const state = { accounts: [], getAccountById: () => undefined };
+  const hook = (sel?: (s: typeof state) => unknown) =>
+    typeof sel === 'function' ? sel(state) : state;
+  hook.getState = () => state;
+  hook.setState = (p: Partial<typeof state>) => Object.assign(state, p);
+  return { useAccountStore: hook };
+});
+
+vi.mock('@/stores/email-store', () => {
+  const state = {
+    draftSaveEnabled: false,
+    sendRawEmail: async () => ({ sent: true }),
+  };
+  const hook = (sel?: (s: typeof state) => unknown) =>
+    typeof sel === 'function' ? sel(state) : state;
+  hook.getState = () => state;
+  hook.setState = (p: Partial<typeof state>) => Object.assign(state, p);
+  return { useEmailStore: hook };
+});
+
+vi.mock('@/stores/settings-store', () => {
+  const state = {
+    timeFormat: '24h',
+    plainTextMode: false,
+    subAddressDelimiter: '+',
+    autoSelectReplyIdentity: true,
+    attachmentReminderEnabled: false,
+    attachmentReminderKeywords: [],
+    emptySubjectWarningEnabled: true,
+    sendDelaySeconds: 0,
+    signaturePosition: 'above_quote',
+    signatureSeparatorEnabled: false,
+    requestReadReceiptDefault: false,
+    addTrustedSender: () => {},
+    trustedSendersAddressBook: null,
+    updateSetting: () => {},
+  };
+  const hook = (sel?: (s: typeof state) => unknown) =>
+    typeof sel === 'function' ? sel(state) : state;
+  hook.getState = () => state;
+  hook.setState = (p: Partial<typeof state>) => Object.assign(state, p);
+  return { useSettingsStore: hook };
+});
+
+vi.mock('@/stores/contact-store', () => {
+  const state = {
+    contacts: [],
+    getAutocomplete: async () => [],
+    addToTrustedSendersBook: async () => {},
+  };
+  const hook = (sel?: (s: typeof state) => unknown) =>
+    typeof sel === 'function' ? sel(state) : state;
+  hook.getState = () => state;
+  hook.setState = (p: Partial<typeof state>) => Object.assign(state, p);
+  return { useContactStore: hook };
+});
+
+vi.mock('@/stores/template-store', () => {
+  const state = { templates: [], addTemplate: async () => {} };
+  const hook = (sel?: (s: typeof state) => unknown) =>
+    typeof sel === 'function' ? sel(state) : state;
+  hook.getState = () => state;
+  hook.setState = (p: Partial<typeof state>) => Object.assign(state, p);
+  return { useTemplateStore: hook };
+});
+
+// ─── Misc dependency mocks ────────────────────────────────────────────────────
+
+vi.mock('@/stores/toast-store', () => ({
+  toast: { info: vi.fn(), error: vi.fn(), success: vi.fn() },
+}));
+
+vi.mock('@/lib/plugin-hooks', () => ({
+  identityHooks: {
+    onSignatureRender: { size: 0, transform: async (value: unknown) => value },
+    onIdentitySelect: { size: 0, emit: async () => {} },
+  },
+  templateHooks: {
+    onTemplateRender: { size: 0, transform: async (value: unknown) => value },
+    onTemplateApply: { size: 0, emit: async () => {} },
+  },
+  emailHooks: {
+    onComposerOpen: { call: async () => [] },
+    onRecipientChange: { call: async () => [] },
+    getRecipientSuggestions: { call: async () => [] },
+    onRecipientChipsChange: { transform: async (chips: unknown) => chips },
+    onDraftChange: { emit: () => {} },
+    onBeforeDraftAutoSave: { transform: async (draft: unknown) => draft },
+    onBeforeEmailSend: { intercept: async () => true, interceptOutcome: async () => 'allowed' },
+    onComposeSend: { takeOver: async () => 'default' },
+    onTransformOutgoingEmail: { transform: async (email: unknown) => email },
+    onBeforeAttachmentUpload: { intercept: async () => true },
+    onBeforeBlobUpload: { transform: async (fileId: unknown) => fileId },
+    onAfterAttachmentUpload: { emit: vi.fn() },
+  },
+  contactHooks: {
+    search: { call: async () => [] },
+    onProvideRecipientSuggestions: { transform: async (initial: unknown) => initial },
+  },
+  isExternalAttachmentResult: () => false,
+}));
+
+const { deleteStagedFile } = vi.hoisted(() => ({
+  deleteStagedFile: vi.fn(async (_id: string) => {}),
+}));
+
+vi.mock('@/lib/plugin-storage', () => ({
+  fileStorage: {
+    saveFile: async () => {},
+    getFile: async () => null,
+    deleteFile: deleteStagedFile,
+  },
+}));
+
+const progressCleanup = vi.hoisted(() => vi.fn());
+vi.mock('@/lib/upload-progress', async (importOriginal) => {
+  const original = await importOriginal<typeof import('@/lib/upload-progress')>();
+  return { ...original, onUploadProgress: (id: string, listener: (loaded: number, total: number) => void) => {
+    const unsubscribe = original.onUploadProgress(id, listener);
+    return () => { unsubscribe(); progressCleanup(); };
+  } };
+});
+
+vi.mock('@/lib/email-sanitization', () => ({
+  sanitizeSignatureHtml: (v: string) => v,
+  sanitizeSignatureHtmlForDisplay: (v: string) => v,
+  sanitizeEmailHtml: (v: string) => v,
+  sanitizePluginBodyHtml: (v: string) => v,
+  escapeHtml: (v: string) => v,
+  parseHtmlSafely: (html: string) => new DOMParser().parseFromString(html, 'text/html'),
+}));
+
+vi.mock('@/lib/email-threading', () => ({
+  computeReplyThreadingHeaders: () => ({ inReplyTo: [], references: [] }),
+}));
+vi.mock('@/lib/signature-utils', () => ({
+  appendPlainTextSignature: (body: string) => body,
+  getPlainTextSignature: () => '',
+  plainTextBodyHasSignature: () => false,
+  plainTextBodyWithoutSignature: (body: string) => body,
+}));
+vi.mock('@/lib/sub-addressing', () => ({ generateSubAddress: () => '' }));
+vi.mock('@/lib/debug', () => ({ debug: { log: () => {}, warn: () => {}, error: () => {} } }));
+vi.mock('@/components/email/quoted-html', () => ({
+  buildQuotedHtmlBlock: () => '',
+  serializeEditorContent: () => '',
+}));
+vi.mock('@/lib/template-utils', () => ({ substitutePlaceholders: (s: string) => s }));
+
+
+import { RequestTimeoutError } from '@/lib/jmap/client';
+import { UploadXHR, uploadClient } from '@/lib/__tests__/helpers/upload-xhr';
+import { toast } from '@/stores/toast-store';
+import { emailHooks } from '@/lib/plugin-hooks';
+
+const INACTIVITY_MS = 300_000;
+const file = (name: string) => new File(['0123456789'], name, { type: 'text/plain' });
+
+async function advance(ms = 0) {
+  await act(async () => { await vi.advanceTimersByTimeAsync(ms); });
+}
+
+async function select(input: HTMLInputElement, files: File[]) {
+  await act(async () => {
+    fireEvent.change(input, { target: { files } });
+    await vi.advanceTimersByTimeAsync(0);
+  });
+}
+
+function chip(name: string) {
+  const progress = screen.queryByRole('progressbar', { name });
+  // Each attachment chip contains its label and cancel/remove button.
+  return progress?.parentElement ?? screen.getByText(name).parentElement!.parentElement!.parentElement!;
+}
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  vi.clearAllMocks();
+  UploadXHR.instances = [];
+  vi.stubGlobal('XMLHttpRequest', UploadXHR);
+  vi.stubGlobal('fetch', vi.fn(() => { throw new Error('Unexpected networking'); }));
+});
+
+afterEach(async () => {
+  cleanup();
+  for (const xhr of UploadXHR.instances) {
+    xhr.abort();
+    // Drain any queued requests before restoring the synthetic transport.
+    await vi.advanceTimersByTimeAsync(0);
+  }
+  vi.clearAllTimers();
+  vi.useRealTimers();
+  vi.unstubAllGlobals();
+  vi.restoreAllMocks();
+  useAuthStore.setState({ client: null });
+});
+
+describe('composer XHR inactivity recovery', () => {
+  it.each([0, 10])('releases stalled A after %i uploaded bytes, starts B, and permits removing/re-adding A', async loaded => {
+    const client = uploadClient();
+    const upload = vi.spyOn(client, 'uploadBlob');
+    useAuthStore.setState({ client: client as never });
+    render(<EmailComposer
+      initialData={{ to: '', cc: '', bcc: '', subject: '', body: '', showCc: false, showBcc: false,
+        selectedIdentityId: 'acct-2::id-other', subAddressTag: '', mode: 'compose', draftId: null }}
+      onClose={vi.fn()}
+    />);
+    const input = document.querySelector('input[type="file"]') as HTMLInputElement;
+    const a = file('A.txt');
+    await select(input, [a, file('B.txt')]);
+    expect(upload).toHaveBeenCalledTimes(1);
+    const aXHR = UploadXHR.instances[0];
+    if (loaded) {
+      await advance(120_000);
+      act(() => aXHR.progress(loaded));
+    }
+    const aOutcome = upload.mock.results[0].value as ReturnType<typeof client.uploadBlob>;
+    // Attach the rejection assertion before the fake clock expires the request.
+    const result = aOutcome.catch((error: unknown) => error);
+    await advance(INACTIVITY_MS - 1);
+    expect(screen.getByRole('progressbar', { name: 'A.txt' })).toHaveAttribute('aria-valuenow', String(loaded * 10));
+    expect(screen.getByRole('progressbar', { name: 'B.txt' })).not.toHaveAttribute('aria-valuenow');
+    expect(toast.error).not.toHaveBeenCalled();
+    expect(deleteStagedFile).not.toHaveBeenCalled();
+    await advance(1);
+
+    // Timeout must let the existing serial loop reach B.
+    expect(upload).toHaveBeenCalledTimes(2);
+    expect(await result).toBeInstanceOf(RequestTimeoutError);
+    expect(aXHR.abort).toHaveBeenCalledOnce();
+    expect(within(chip('A.txt')).getByText('attachment_upload_failed')).toBeTruthy();
+    expect(within(chip('A.txt')).queryByRole('progressbar')).toBeNull();
+    expect(screen.getByRole('progressbar', { name: 'B.txt' })).toHaveAttribute('aria-valuenow', '0');
+    expect(toast.error).toHaveBeenCalledExactlyOnceWith('upload_failed');
+    expect(deleteStagedFile).toHaveBeenCalledTimes(1);
+    expect(progressCleanup).toHaveBeenCalledTimes(1);
+
+    act(() => UploadXHR.instances[1].load());
+    await advance();
+    expect(screen.queryByRole('progressbar')).toBeNull();
+    expect(input.value).toBe('');
+    expect(progressCleanup).toHaveBeenCalledTimes(2);
+    fireEvent.click(within(chip('A.txt')).getByRole('button', { name: 'recipient_remove A.txt' }));
+    expect(screen.queryByText('A.txt')).toBeNull();
+
+    await select(input, [a]);
+    expect(upload).toHaveBeenCalledTimes(3);
+    expect(upload.mock.calls[2][0]).toBe(a);
+    act(() => UploadXHR.instances[2].load());
+    await advance();
+    expect(screen.queryByRole('progressbar')).toBeNull();
+    expect(screen.queryByText('attachment_upload_failed')).toBeNull();
+    expect(toast.error).toHaveBeenCalledTimes(1);
+    expect(deleteStagedFile).toHaveBeenCalledTimes(3);
+    expect(progressCleanup).toHaveBeenCalledTimes(3);
+    expect(emailHooks.onAfterAttachmentUpload.emit).toHaveBeenCalledTimes(2);
+    expect(UploadXHR.instances).toHaveLength(3);
+    expect(fetch).not.toHaveBeenCalled();
+  });
+});
diff --git a/lib/__tests__/helpers/upload-xhr.ts b/lib/__tests__/helpers/upload-xhr.ts
new file mode 100644
index 000000000..5ded4faac
--- /dev/null
+++ b/lib/__tests__/helpers/upload-xhr.ts
@@ -0,0 +1,55 @@
+import { vi } from 'vitest';
+import { JMAPClient } from '@/lib/jmap/client';
+
+// No native networking: tests explicitly deliver progress and terminal events.
+export class UploadXHR {
+  static instances: UploadXHR[] = [];
+  method = '';
+  url = '';
+  headers: Record<string, string> = {};
+  responseType = '';
+  responseText = '';
+  status = 0;
+  body: File | null = null;
+  upload = { onprogress: null as ((event: ProgressEvent) => void) | null };
+  onload: (() => void) | null = null;
+  onerror: (() => void) | null = null;
+  onabort: (() => void) | null = null;
+  ontimeout: (() => void) | null = null;
+  private done = false;
+
+  constructor() { UploadXHR.instances.push(this); }
+  open(method: string, url: string) { this.method = method; this.url = url; }
+  setRequestHeader(name: string, value: string) { this.headers[name] = value; }
+  send(body: File) { this.body = body; }
+  abort = vi.fn(() => {
+    // Native abort emits no abort event before send or after DONE.
+    if (this.body && !this.done) { this.done = true; this.onabort?.(); }
+  });
+  progress(loaded: number, total = this.body?.size ?? 0, lengthComputable = true) {
+    this.upload.onprogress?.(new ProgressEvent('progress', { loaded, total, lengthComputable }));
+  }
+  load(status = 201, text = JSON.stringify({ blobId: `blob-${this.body?.name}`, size: this.body?.size, type: this.body?.type })) {
+    this.done = true;
+    this.status = status;
+    this.responseText = text;
+    this.onload?.();
+  }
+  error() { this.done = true; this.onerror?.(); }
+  timeout() { this.done = true; this.ontimeout?.(); }
+}
+
+export function uploadClient() {
+  const client = new JMAPClient('https://upload.invalid', 'synthetic', 'synthetic');
+  // Seed only a synthetic connected session; avoid keep-alive/reconnect traffic.
+  Object.assign(client, {
+    accountId: 'synthetic',
+    session: {
+      capabilities: { 'urn:ietf:params:jmap:core': {} },
+      accounts: { synthetic: { accountCapabilities: {} } },
+      primaryAccounts: {},
+      uploadUrl: 'https://upload.invalid/upload/{accountId}',
+    },
+  });
+  return client;
+}
diff --git a/lib/__tests__/jmap-upload-inactivity.test.ts b/lib/__tests__/jmap-upload-inactivity.test.ts
new file mode 100644
index 000000000..38457ddeb
--- /dev/null
+++ b/lib/__tests__/jmap-upload-inactivity.test.ts
@@ -0,0 +1,213 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { RequestTimeoutError } from '../jmap/client';
+import { UploadXHR, uploadClient } from './helpers/upload-xhr';
+
+const INACTIVITY_MS = 300_000;
+const file = () => new File(['0123456789'], 'A.txt', { type: 'text/plain' });
+
+async function start(options: { onProgress?: (loaded: number, total: number) => void; signal?: AbortSignal; accountId?: string } = { onProgress: vi.fn() }) {
+  const client = uploadClient();
+  const body = file();
+  const completed = vi.fn();
+  const promise = client.uploadBlob(body, options);
+  void promise.then(completed, completed);
+  await vi.advanceTimersByTimeAsync(0);
+  return { promise, completed, body, xhr: UploadXHR.instances[0] };
+}
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  UploadXHR.instances = [];
+  vi.stubGlobal('XMLHttpRequest', UploadXHR);
+  vi.stubGlobal('fetch', vi.fn(() => { throw new Error('Unexpected networking'); }));
+});
+
+afterEach(() => {
+  for (const xhr of UploadXHR.instances) xhr.abort();
+  vi.clearAllTimers();
+  vi.useRealTimers();
+  vi.unstubAllGlobals();
+  vi.restoreAllMocks();
+});
+
+describe('JMAP XHR upload inactivity recovery', () => {
+  it('rejects a silent upload at five minutes, aborts it and does not retry', async () => {
+    const { promise, completed, xhr } = await start();
+    await vi.advanceTimersByTimeAsync(INACTIVITY_MS - 1);
+    expect(completed).not.toHaveBeenCalled();
+    await vi.advanceTimersByTimeAsync(1);
+    expect(completed).toHaveBeenCalledExactlyOnceWith(expect.any(RequestTimeoutError));
+    await expect(promise).rejects.toThrow('Request timed out after 300s');
+    expect(xhr.abort).toHaveBeenCalledOnce();
+    await vi.advanceTimersByTimeAsync(INACTIVITY_MS);
+    expect(UploadXHR.instances).toHaveLength(1);
+    expect(fetch).not.toHaveBeenCalled();
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it('resets the threshold when additional bytes have transferred', async () => {
+    const { completed, xhr } = await start();
+    await vi.advanceTimersByTimeAsync(240_000);
+    xhr.progress(1);
+    await vi.advanceTimersByTimeAsync(240_000);
+    expect(completed).not.toHaveBeenCalled();
+    xhr.progress(2);
+    await vi.advanceTimersByTimeAsync(INACTIVITY_MS - 1);
+    expect(completed).not.toHaveBeenCalled();
+    await vi.advanceTimersByTimeAsync(1);
+    expect(completed).toHaveBeenCalledExactlyOnceWith(expect.any(RequestTimeoutError));
+  });
+
+  it('does not renew for zero, duplicate or decreasing byte counts, or a changed total', async () => {
+    const { completed, xhr } = await start();
+    await vi.advanceTimersByTimeAsync(100_000);
+    xhr.progress(0);
+    await vi.advanceTimersByTimeAsync(100_000);
+    xhr.progress(2);
+    await vi.advanceTimersByTimeAsync(100_000);
+    xhr.progress(2, 20);
+    await vi.advanceTimersByTimeAsync(100_000);
+    xhr.progress(1);
+    await vi.advanceTimersByTimeAsync(99_999);
+    expect(completed).not.toHaveBeenCalled();
+    await vi.advanceTimersByTimeAsync(1);
+    expect(completed).toHaveBeenCalledExactlyOnceWith(expect.any(RequestTimeoutError));
+  });
+
+  it('allows a successful sixteen-minute transfer while bytes keep increasing', async () => {
+    const progress = vi.fn();
+    const { promise, completed, xhr, body } = await start({ onProgress: progress });
+    expect(progress).toHaveBeenCalledExactlyOnceWith(0, body.size);
+    for (let loaded = 1; loaded <= 4; loaded++) {
+      await vi.advanceTimersByTimeAsync(240_000);
+      xhr.progress(loaded);
+      expect(completed).not.toHaveBeenCalled();
+    }
+    xhr.load();
+    await expect(promise).resolves.toEqual({ blobId: 'blob-A.txt', size: 10, type: 'text/plain' });
+    expect(progress).toHaveBeenCalledTimes(5);
+    expect(xhr.abort).not.toHaveBeenCalled();
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it.each([5, 10])('expires after progress to %i bytes followed by five silent minutes', async loaded => {
+    const { completed, xhr } = await start();
+    await vi.advanceTimersByTimeAsync(120_000);
+    xhr.progress(loaded);
+    await vi.advanceTimersByTimeAsync(INACTIVITY_MS - 1);
+    expect(completed).not.toHaveBeenCalled();
+    await vi.advanceTimersByTimeAsync(1);
+    expect(completed).toHaveBeenCalledExactlyOnceWith(expect.any(RequestTimeoutError));
+    expect(xhr.abort).toHaveBeenCalledOnce();
+  });
+
+  it('observes byte activity for signal-only callers and keeps the file-size progress fallback', async () => {
+    const controller = new AbortController();
+    const { promise, completed, xhr } = await start({ signal: controller.signal });
+    await vi.advanceTimersByTimeAsync(240_000);
+    xhr.progress(2, 0, false);
+    await vi.advanceTimersByTimeAsync(240_000);
+    expect(completed).not.toHaveBeenCalled();
+    xhr.load();
+    await promise;
+
+    const progress = vi.fn();
+    const next = uploadClient().uploadBlob(file(), { onProgress: progress });
+    await vi.advanceTimersByTimeAsync(0);
+    UploadXHR.instances[1].progress(3, 0, false);
+    expect(progress).toHaveBeenLastCalledWith(3, 10);
+    UploadXHR.instances[1].load();
+    await next;
+  });
+
+  it.each(['success', 'http', 'network', 'cancel', 'native abort', 'native timeout', 'invalid JSON'])('cleans the watchdog and signal listener after %s', async terminal => {
+    const controller = new AbortController();
+    const add = vi.spyOn(controller.signal, 'addEventListener');
+    const remove = vi.spyOn(controller.signal, 'removeEventListener');
+    const { promise, completed, xhr } = await start({ signal: controller.signal });
+    await vi.advanceTimersByTimeAsync(120_000);
+    if (terminal === 'success') xhr.load();
+    else if (terminal === 'http') xhr.load(503, 'synthetic failure');
+    else if (terminal === 'network') xhr.error();
+    else if (terminal === 'cancel') controller.abort();
+    else if (terminal === 'native abort') xhr.abort();
+    else if (terminal === 'native timeout') xhr.timeout();
+    else xhr.load(201, 'invalid JSON');
+    await vi.advanceTimersByTimeAsync(0);
+    expect(completed).toHaveBeenCalledOnce();
+    if (terminal === 'success') await expect(promise).resolves.toHaveProperty('blobId');
+    else if (terminal === 'cancel' || terminal === 'native abort') await expect(promise).rejects.toHaveProperty('name', 'AbortError');
+    else if (terminal === 'native timeout') await expect(promise).rejects.toBeInstanceOf(RequestTimeoutError);
+    else await expect(promise).rejects.toBeInstanceOf(Error);
+    expect(remove).toHaveBeenCalledExactlyOnceWith('abort', add.mock.calls[0][1]);
+    expect(vi.getTimerCount()).toBe(0);
+    await vi.advanceTimersByTimeAsync(INACTIVITY_MS * 2);
+    expect(completed).toHaveBeenCalledOnce();
+  });
+
+  it.each(['timeout', 'cancel', 'load', 'error'])('%s wins the terminal race; retained late callbacks cannot renew or report', async first => {
+    const controller = new AbortController();
+    const progress = vi.fn();
+    const remove = vi.spyOn(controller.signal, 'removeEventListener');
+    const { completed, xhr } = await start({ signal: controller.signal, onProgress: progress });
+    // Retain callbacks even if cleanup detaches them from the XHR.
+    const late = { progress: xhr.upload.onprogress, load: xhr.onload, error: xhr.onerror, abort: xhr.onabort, timeout: xhr.ontimeout };
+    await vi.advanceTimersByTimeAsync(INACTIVITY_MS - 1);
+    if (first === 'cancel') controller.abort();
+    else if (first === 'load') xhr.load();
+    else if (first === 'error') xhr.error();
+    await vi.advanceTimersByTimeAsync(1);
+    controller.abort();
+    if (first === 'timeout') expect(completed).toHaveBeenCalledExactlyOnceWith(expect.any(RequestTimeoutError));
+    else if (first === 'cancel') expect(completed).toHaveBeenCalledExactlyOnceWith(expect.objectContaining({ name: 'AbortError' }));
+    else if (first === 'load') expect(completed).toHaveBeenCalledExactlyOnceWith(expect.objectContaining({ blobId: 'blob-A.txt' }));
+    else expect(completed).toHaveBeenCalledExactlyOnceWith(expect.objectContaining({ message: 'Upload network error' }));
+    const progressCalls = progress.mock.calls.length;
+    late.progress?.(new ProgressEvent('progress', { loaded: 9, total: 10, lengthComputable: true }));
+    late.load?.(); late.error?.(); late.abort?.(); late.timeout?.();
+    await vi.advanceTimersByTimeAsync(INACTIVITY_MS * 2);
+    expect(completed).toHaveBeenCalledOnce();
+    expect(progress).toHaveBeenCalledTimes(progressCalls);
+    expect(remove).toHaveBeenCalledOnce();
+    expect(xhr.abort).toHaveBeenCalledTimes(first === 'timeout' || first === 'cancel' ? 1 : 0);
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it('rejects a pre-aborted signal without starting a request or watchdog', async () => {
+    const controller = new AbortController();
+    controller.abort();
+    await expect(uploadClient().uploadBlob(file(), { signal: controller.signal })).rejects.toHaveProperty('name', 'AbortError');
+    expect(UploadXHR.instances).toHaveLength(0);
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it('handles cancellation from the initial progress callback before send', async () => {
+    const controller = new AbortController();
+    const { promise, completed, xhr } = await start({ signal: controller.signal, onProgress: () => controller.abort() });
+    expect(completed).toHaveBeenCalledExactlyOnceWith(expect.objectContaining({ name: 'AbortError' }));
+    await expect(promise).rejects.toHaveProperty('name', 'AbortError');
+    expect(xhr.body).toBeNull();
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it('cleans up a synchronous send failure', async () => {
+    const controller = new AbortController();
+    const remove = vi.spyOn(controller.signal, 'removeEventListener');
+    vi.spyOn(UploadXHR.prototype, 'send').mockImplementationOnce(() => { throw new Error('synthetic send failure'); });
+    const { promise } = await start({ signal: controller.signal });
+    await expect(promise).rejects.toThrow('synthetic send failure');
+    expect(remove).toHaveBeenCalledOnce();
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it('preserves request construction and nested upload results', async () => {
+    const { promise, xhr, body } = await start({ accountId: 'group/a b', onProgress: vi.fn() });
+    expect(xhr.method).toBe('POST');
+    expect(xhr.url).toBe('https://upload.invalid/upload/group%2Fa%20b');
+    expect(xhr.headers).toEqual({ 'Content-Type': 'text/plain', Authorization: uploadClient().getAuthHeader() });
+    expect(xhr.body).toBe(body);
+    expect(xhr.responseType).toBe('text');
+    xhr.load(200, JSON.stringify({ 'group/a b': { blobId: 'nested', size: 10, type: 'text/plain' } }));
+    await expect(promise).resolves.toEqual({ blobId: 'nested', size: 10, type: 'text/plain' });
+  });
+});
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index e7a42f77c..a3e4e4294 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -5806,34 +5806,68 @@ export class JMAPClient implements IJMAPClient {
       xhr.setRequestHeader('Authorization', this.authHeader);
       xhr.responseType = 'text';
 
-      const onAbort = () => xhr.abort();
-      if (signal) signal.addEventListener('abort', onAbort, { once: true });
-      const cleanup = () => signal?.removeEventListener('abort', onAbort);
-
-      if (onProgress) {
-        // Fire 0% immediately so the UI leaves its initial state even
-        // before the first network packet flushes.
-        onProgress(0, file.size);
-        xhr.upload.onprogress = (ev) => {
-          // ev.total is only meaningful when lengthComputable; fall back
-          // to file.size so callers always get a usable denominator.
-          const total = ev.lengthComputable ? ev.total : file.size;
-          onProgress(ev.loaded, total);
-        };
-      }
-
-      xhr.onload = () => {
+      let settled = false;
+      let timer: ReturnType<typeof setTimeout> | undefined;
+      let uploaded = 0;
+      const cleanup = () => {
+        clearTimeout(timer);
+        signal?.removeEventListener('abort', onAbort);
+        xhr.onload = xhr.onerror = xhr.onabort = xhr.ontimeout = null;
+        xhr.upload.onprogress = null;
+      };
+      const settle = (complete: () => void): boolean => {
+        if (settled) return false;
+        settled = true;
         cleanup();
+        complete();
+        return true;
+      };
+      const abortWith = (error: Error) => {
+        // Settle first: abort() may synchronously dispatch an abort event.
+        if (settle(() => reject(error))) xhr.abort();
+      };
+      const onAbort = () => abortWith(new DOMException('Upload aborted', 'AbortError'));
+      const onTimeout = () => abortWith(new RequestTimeoutError(JMAPClient.TRANSFER_TIMEOUT_MS));
+      const resetWatchdog = () => {
+        clearTimeout(timer);
+        // XHR uses this budget for inactivity, unlike fetch's deadline until
+        // response headers. Additional upload bytes renew it; after 100% it
+        // stays armed while waiting for the terminal response.
+        timer = setTimeout(onTimeout, JMAPClient.TRANSFER_TIMEOUT_MS);
+      };
+
+      xhr.upload.onprogress = (ev) => {
+        if (settled) return;
+        if (ev.loaded > uploaded) {
+          uploaded = ev.loaded;
+          resetWatchdog();
+        }
+        // ev.total is only meaningful when lengthComputable; fall back
+        // to file.size so callers always get a usable denominator.
+        onProgress?.(ev.loaded, ev.lengthComputable ? ev.total : file.size);
+      };
+      xhr.onload = () => settle(() => {
         if (xhr.status >= 200 && xhr.status < 300) {
           resolve(xhr.responseText);
         } else {
           reject(new Error(`Failed to upload file: ${xhr.status} - ${xhr.responseText}`));
         }
-      };
-      xhr.onerror = () => { cleanup(); reject(new Error('Upload network error')); };
-      xhr.onabort = () => { cleanup(); reject(new DOMException('Upload aborted', 'AbortError')); };
+      });
+      xhr.onerror = () => settle(() => reject(new Error('Upload network error')));
+      xhr.onabort = () => settle(() => reject(new DOMException('Upload aborted', 'AbortError')));
+      xhr.ontimeout = onTimeout;
 
-      xhr.send(file);
+      try {
+        signal?.addEventListener('abort', onAbort, { once: true });
+        // Fire 0% immediately so the UI leaves its initial state even
+        // before the first network packet flushes.
+        onProgress?.(0, file.size);
+        if (settled) return;
+        resetWatchdog();
+        xhr.send(file);
+      } catch (error) {
+        settle(() => reject(error));
+      }
     });
   }