diff --git a/lib/__tests__/jmap-calendar-tasks.test.ts b/lib/__tests__/jmap-calendar-tasks.test.ts
new file mode 100644
index 000000000..9657aef0d
--- /dev/null
+++ b/lib/__tests__/jmap-calendar-tasks.test.ts
@@ -0,0 +1,124 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { JMAPClient } from '../jmap/client';
+import type { CalendarTask } from '../jmap/types';
+
+function makeSession() {
+  return {
+    capabilities: { 'urn:ietf:params:jmap:core': {}, 'urn:ietf:params:jmap:calendars': {} },
+    accounts: {
+      'acct-1': { name: 'test', isPersonal: true, accountCapabilities: { 'urn:ietf:params:jmap:calendars': {} } },
+    },
+    primaryAccounts: { 'urn:ietf:params:jmap:mail': 'acct-1', 'urn:ietf:params:jmap:calendars': 'acct-1' },
+    apiUrl: 'https://mail.example.com/jmap/api',
+    downloadUrl: 'https://mail.example.com/jmap/download/{accountId}/{blobId}/{name}',
+    uploadUrl: 'https://mail.example.com/jmap/upload/{accountId}/',
+    eventSourceUrl: '',
+  };
+}
+
+function jsonResponse(body: unknown): Response {
+  return new Response(JSON.stringify(body), { status: 200, headers: { 'Content-Type': 'application/json' } });
+}
+
+type Call = { method: string; args: Record<string, unknown> };
+
+describe('JMAPClient task operations (#958)', () => {
+  let client: JMAPClient;
+  let sentCalls: Call[];
+
+  beforeEach(() => {
+    sentCalls = [];
+    client = new JMAPClient('https://mail.example.com', 'test', 'pass');
+    // Simulate active session
+    (client as unknown as { session: unknown; accountId: string; apiUrl: string }).session = makeSession();
+    (client as unknown as { accountId: string }).accountId = 'acct-1';
+    (client as unknown as { apiUrl: string }).apiUrl = 'https://mail.example.com/jmap/api';
+    (client as unknown as { accounts: Record<string, unknown> }).accounts = makeSession().accounts;
+
+    vi.stubGlobal('fetch', vi.fn(async (_url: string, init?: RequestInit) => {
+      const body = JSON.parse((init?.body as string) || '{}');
+      const calls = (body.methodCalls || []) as [string, Record<string, unknown>, string][];
+      const methodResponses: [string, Record<string, unknown>, string][] = [];
+
+      for (const [method, args, tag] of calls) {
+        sentCalls.push({ method, args });
+
+        if (method === 'CalendarEvent/set') {
+          const update = args.update as Record<string, unknown> | undefined;
+          const create = args.create as Record<string, unknown> | undefined;
+          const updated: Record<string, unknown> = {};
+          const created: Record<string, unknown> = {};
+
+          if (update) {
+            for (const id of Object.keys(update)) {
+              updated[id] = { id };
+            }
+          }
+          if (create) {
+            for (const id of Object.keys(create)) {
+              created[id] = { id: 'created-id-123' };
+            }
+          }
+
+          methodResponses.push(['CalendarEvent/set', { updated, created, notUpdated: {}, notCreated: {} }, tag]);
+        } else if (method === 'CalendarEvent/get') {
+          methodResponses.push([
+            'CalendarEvent/get',
+            {
+              list: [
+                {
+                  id: 'created-id-123',
+                  '@type': 'Task',
+                  uid: 'uid-123',
+                  title: 'Task Title',
+                  progress: 'needs-action',
+                  calendarIds: { 'cal-1': true },
+                },
+              ],
+              notFound: [],
+            },
+            tag,
+          ]);
+        }
+      }
+
+      return jsonResponse({ methodResponses });
+    }));
+  });
+
+  it('updateCalendarTask strips progressUpdated before sending CalendarEvent/set (#958)', async () => {
+    const updates: Partial<CalendarTask> & { progressUpdated?: string | null } = {
+      progress: 'completed',
+      progressUpdated: '2026-09-06T12:00:00Z',
+    };
+
+    await client.updateCalendarTask('task-123', updates as Partial<CalendarTask>);
+
+    const setCall = sentCalls.find((c) => c.method === 'CalendarEvent/set');
+    expect(setCall).toBeDefined();
+
+    const updatePayload = (setCall?.args.update as Record<string, Record<string, unknown>>)?.['task-123'];
+    expect(updatePayload).toBeDefined();
+    expect(updatePayload.progress).toBe('completed');
+    expect(updatePayload).not.toHaveProperty('progressUpdated');
+  });
+
+  it('createCalendarTask strips progressUpdated before sending CalendarEvent/set', async () => {
+    const task: Partial<CalendarTask> & { progressUpdated?: string | null } = {
+      title: 'New Task',
+      progress: 'needs-action',
+      progressUpdated: '2026-09-06T12:00:00Z',
+    };
+
+    await client.createCalendarTask(task as Partial<CalendarTask>);
+
+    const setCall = sentCalls.find((c) => c.method === 'CalendarEvent/set');
+    expect(setCall).toBeDefined();
+
+    const createPayload = (setCall?.args.create as Record<string, Record<string, unknown>>)?.['new-task'];
+    expect(createPayload).toBeDefined();
+    expect(createPayload['@type']).toBe('Task');
+    expect(createPayload.title).toBe('New Task');
+    expect(createPayload).not.toHaveProperty('progressUpdated');
+  });
+});
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index d01e35e8b..bf0923d50 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -344,7 +344,6 @@ const CALENDAR_TASK_PROPERTIES = [
   'utcStart',
   'utcEnd',
   'progress',
-  'progressUpdated',
   'priority',
   'privacy',
   'color',
@@ -6516,7 +6515,7 @@ export class JMAPClient implements IJMAPClient {
     const accountId = targetAccountId || this.getCalendarsAccountId();
 
     // Strip client-only and server-immutable fields before sending to JMAP
-    const { id: _id, uid: _uid, '@type': _typ, created: _cr, updated: _up, sequence: _sq, isOrigin: _io, isDraft: _idr, originalId: _oi, baseEventId: _be, originalCalendarIds: _oc, accountId: _ai, accountName: _an, isShared: _is, ...cleanUpdates } = updates as CalendarEvent;
+    const { id: _id, uid: _uid, '@type': _typ, created: _cr, updated: _up, sequence: _sq, isOrigin: _io, isDraft: _idr, originalId: _oi, baseEventId: _be, originalCalendarIds: _oc, accountId: _ai, accountName: _an, isShared: _is, progressUpdated: _pu, ...cleanUpdates } = updates as CalendarEvent & { progressUpdated?: string | null };
     cleanRecurrenceRules(cleanUpdates as unknown as Record<string, unknown>);
 
     const setArgs: Record<string, unknown> = {
@@ -6770,7 +6769,7 @@ export class JMAPClient implements IJMAPClient {
 
   async createCalendarTask(task: Partial<CalendarTask>, targetAccountId?: string): Promise<CalendarTask> {
     const accountId = targetAccountId || this.getCalendarsAccountId();
-    const { '@type': _type, ...taskData } = task;
+    const { '@type': _type, progressUpdated: _pu, ...taskData } = task as Partial<CalendarTask> & { progressUpdated?: string | null };
     const cleanTask = { ...taskData, '@type': 'Task' };
 
     debug.group('CalendarTask/create', 'tasks');
diff --git a/lib/jmap/types.ts b/lib/jmap/types.ts
index 5e93f475b..824e2c425 100644
--- a/lib/jmap/types.ts
+++ b/lib/jmap/types.ts
@@ -792,7 +792,7 @@ export interface CalendarTask {
   timeZone: string | null;
   showWithoutTime: boolean;
   progress: 'needs-action' | 'in-process' | 'completed' | 'cancelled';
-  progressUpdated: string | null;
+  progressUpdated?: string | null;
   priority: number;
   privacy: 'public' | 'private' | 'secret';
   keywords: Record<string, boolean> | null;
diff --git a/stores/__tests__/task-store.test.ts b/stores/__tests__/task-store.test.ts
new file mode 100644
index 000000000..e2074c57b
--- /dev/null
+++ b/stores/__tests__/task-store.test.ts
@@ -0,0 +1,135 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { useTaskStore } from '../task-store';
+import type { CalendarTask } from '@/lib/jmap/types';
+import type { IJMAPClient } from '@/lib/jmap/client-interface';
+
+function makeTask(overrides: Partial<CalendarTask> = {}): CalendarTask {
+  return {
+    id: 'task-1',
+    calendarIds: { 'cal-1': true },
+    '@type': 'Task',
+    uid: 'task-uid-1',
+    title: 'Test task',
+    description: '',
+    due: '2026-09-08T10:00:00',
+    start: null,
+    duration: null,
+    timeZone: null,
+    showWithoutTime: false,
+    progress: 'needs-action',
+    priority: 0,
+    privacy: 'public',
+    keywords: null,
+    categories: null,
+    color: null,
+    created: '2026-09-01T10:00:00Z',
+    updated: '2026-09-01T10:00:00Z',
+    recurrenceRules: null,
+    alerts: null,
+    relatedTo: null,
+    ...overrides,
+  };
+}
+
+describe('task-store', () => {
+  let mockClient: Partial<IJMAPClient>;
+
+  beforeEach(() => {
+    useTaskStore.setState({
+      tasks: [],
+      selectedTaskId: null,
+      filter: 'all',
+      showCompleted: false,
+      isLoading: false,
+      error: null,
+    });
+
+    mockClient = {
+      getCalendarTasks: vi.fn().mockResolvedValue([makeTask()]),
+      createCalendarTask: vi.fn().mockImplementation(async (t) => makeTask({ id: 'task-created', ...t })),
+      updateCalendarTask: vi.fn().mockResolvedValue(undefined),
+      deleteCalendarTask: vi.fn().mockResolvedValue(undefined),
+    };
+  });
+
+  describe('toggleTaskComplete', () => {
+    it('toggles needs-action to completed without sending progressUpdated in payload (#958)', async () => {
+      const task = makeTask({ id: 't1', progress: 'needs-action' });
+      useTaskStore.setState({ tasks: [task] });
+
+      await useTaskStore.getState().toggleTaskComplete(mockClient as IJMAPClient, task);
+
+      expect(mockClient.updateCalendarTask).toHaveBeenCalledTimes(1);
+      const [calledId, calledUpdates] = (mockClient.updateCalendarTask as ReturnType<typeof vi.fn>).mock.calls[0];
+      expect(calledId).toBe('t1');
+      expect(calledUpdates).toEqual({
+        progress: 'completed',
+      });
+      // Stalwart rejects progressUpdated with invalidProperties error
+      expect(calledUpdates).not.toHaveProperty('progressUpdated');
+
+      const updatedTask = useTaskStore.getState().tasks.find((t) => t.id === 't1');
+      expect(updatedTask?.progress).toBe('completed');
+    });
+
+    it('toggles completed back to needs-action without sending progressUpdated in payload', async () => {
+      const task = makeTask({ id: 't2', progress: 'completed' });
+      useTaskStore.setState({ tasks: [task] });
+
+      await useTaskStore.getState().toggleTaskComplete(mockClient as IJMAPClient, task);
+
+      expect(mockClient.updateCalendarTask).toHaveBeenCalledTimes(1);
+      const [calledId, calledUpdates] = (mockClient.updateCalendarTask as ReturnType<typeof vi.fn>).mock.calls[0];
+      expect(calledId).toBe('t2');
+      expect(calledUpdates).toEqual({
+        progress: 'needs-action',
+      });
+      expect(calledUpdates).not.toHaveProperty('progressUpdated');
+
+      const updatedTask = useTaskStore.getState().tasks.find((t) => t.id === 't2');
+      expect(updatedTask?.progress).toBe('needs-action');
+    });
+  });
+
+  describe('CRUD operations', () => {
+    it('fetches tasks and updates store state', async () => {
+      await useTaskStore.getState().fetchTasks(mockClient as IJMAPClient, ['cal-1']);
+
+      expect(mockClient.getCalendarTasks).toHaveBeenCalledWith(['cal-1']);
+      expect(useTaskStore.getState().tasks).toHaveLength(1);
+      expect(useTaskStore.getState().isLoading).toBe(false);
+      expect(useTaskStore.getState().error).toBeNull();
+    });
+
+    it('creates a task and appends to store', async () => {
+      const created = await useTaskStore.getState().createTask(mockClient as IJMAPClient, { title: 'New Task' });
+
+      expect(mockClient.createCalendarTask).toHaveBeenCalledWith({ title: 'New Task' });
+      expect(created.id).toBe('task-created');
+      expect(useTaskStore.getState().tasks).toContainEqual(created);
+    });
+
+    it('updates a task and preserves unchanged fields', async () => {
+      const task = makeTask({ id: 't3', title: 'Original Title' });
+      useTaskStore.setState({ tasks: [task] });
+
+      await useTaskStore.getState().updateTask(mockClient as IJMAPClient, 't3', { title: 'Updated Title' });
+
+      expect(mockClient.updateCalendarTask).toHaveBeenCalledWith('t3', { title: 'Updated Title' });
+      const updated = useTaskStore.getState().tasks.find((t) => t.id === 't3');
+      expect(updated?.title).toBe('Updated Title');
+      expect(updated?.progress).toBe('needs-action');
+    });
+
+    it('deletes a task and resets selectedTaskId if selected', async () => {
+      const task = makeTask({ id: 't4' });
+      useTaskStore.setState({ tasks: [task], selectedTaskId: 't4' });
+
+      await useTaskStore.getState().deleteTask(mockClient as IJMAPClient, 't4');
+
+      expect(mockClient.deleteCalendarTask).toHaveBeenCalledWith('t4');
+      expect(useTaskStore.getState().tasks).toHaveLength(0);
+      expect(useTaskStore.getState().selectedTaskId).toBeNull();
+    });
+  });
+});
diff --git a/stores/task-store.ts b/stores/task-store.ts
index a2f7d0702..b14f9e191 100644
--- a/stores/task-store.ts
+++ b/stores/task-store.ts
@@ -83,7 +83,6 @@ export const useTaskStore = create<TaskStore>((set, get) => ({
     const newProgress = task.progress === 'completed' ? 'needs-action' : 'completed';
     const updates: Partial<CalendarTask> = {
       progress: newProgress,
-      progressUpdated: new Date().toISOString(),
     };
     await client.updateCalendarTask(task.id, updates);
     set({