diff --git a/lib/__tests__/patch-object.test.ts b/lib/__tests__/patch-object.test.ts
new file mode 100644
index 000000000..ece2627d4
--- /dev/null
+++ b/lib/__tests__/patch-object.test.ts
@@ -0,0 +1,30 @@
+import { describe, expect, it } from 'vitest';
+import { applyPatchObject, patchPath, patchedProperties } from '../jmap/patch-object';
+
+describe('patch-object', () => {
+  it('escapes path segments as JSON Pointer does', () => {
+    expect(patchPath('relatedTo', '<a/b~c@x>')).toBe('relatedTo/<a~1b~0c@x>');
+  });
+
+  it('applies plain keys and member paths without touching the target', () => {
+    const target = { title: 'T', alerts: { a: { acknowledged: null, action: 'display' }, b: { acknowledged: null } } };
+    const after = applyPatchObject(target, { title: 'U', 'alerts/a/acknowledged': '2026-05-04T07:00:00Z', 'alerts/b': null, 'links/x~1y': { href: 'h' } });
+    expect(after).toEqual({
+      title: 'U',
+      alerts: { a: { acknowledged: '2026-05-04T07:00:00Z', action: 'display' } },
+      links: { 'x/y': { href: 'h' } },
+    });
+    expect(target.alerts.b).toEqual({ acknowledged: null });
+  });
+
+  it('keeps __proto__ as a plain key', () => {
+    const after = applyPatchObject({ alerts: {} }, { 'alerts/__proto__': { polluted: true } }) as { alerts: object };
+    expect(Object.getPrototypeOf(after.alerts)).toBe(Object.prototype);
+    expect(Object.keys(after.alerts)).toEqual(['__proto__']);
+    expect(({} as Record<string, unknown>).polluted).toBeUndefined();
+  });
+
+  it('shows a path patch as whole properties', () => {
+    expect(patchedProperties({ alerts: { a: { x: 1 }, b: { x: 2 } } }, { 'alerts/a/x': 3 })).toEqual({ alerts: { a: { x: 3 }, b: { x: 2 } } });
+  });
+});
diff --git a/lib/__tests__/plugin-host-calendar-tasks.test.ts b/lib/__tests__/plugin-host-calendar-tasks.test.ts
index 4d561a38d..7023a6d15 100644
--- a/lib/__tests__/plugin-host-calendar-tasks.test.ts
+++ b/lib/__tests__/plugin-host-calendar-tasks.test.ts
@@ -1,7 +1,7 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import type { InstalledPlugin } from '../plugin-types';
 import type { IJMAPClient } from '../jmap/client-interface';
-import type { Calendar, CalendarEvent, CalendarTask } from '../jmap/types';
+import type { Calendar, CalendarEvent, CalendarEventAlert, CalendarTask } from '../jmap/types';
 import { useAuthStore } from '@/stores/auth-store';
 import { useCalendarStore } from '@/stores/calendar-store';
 import { useTaskStore } from '@/stores/task-store';
@@ -243,3 +243,242 @@ describe('api.tasks', () => {
     await expect(call(WRITE, 'tasks.delete', 'nope')).rejects.toThrow(/no task with id "nope"/);
   });
 });
+
+describe('api.tasks alerts', () => {
+  const at = (when: string, over: Partial<CalendarEventAlert> = {}): CalendarEventAlert => ({
+    '@type': 'Alert', trigger: { '@type': 'AbsoluteTrigger', when }, action: 'display', acknowledged: null, relatedTo: null, ...over,
+  });
+  const updates = () => client.updateCalendarTask.mock.calls.map((c) => c[1] as Record<string, unknown>);
+
+  it('creates a task with alerts, turning the calendar\'s defaults off', async () => {
+    await call(WRITE, 'tasks.create', {
+      title: 'Follow up: Offer',
+      due: '2026-05-04',
+      alerts: {
+        'follow-up': { trigger: { at: '2026-05-04T07:00:00Z' } },
+        'follow-up-mail': { trigger: { offset: '-PT1H' }, action: 'email' },
+      },
+    });
+    expect(client.createCalendarTask).toHaveBeenCalledWith(expect.objectContaining({
+      due: '2026-05-04', showWithoutTime: true, useDefaultAlerts: false,
+      alerts: {
+        'follow-up': at('2026-05-04T07:00:00Z'),
+        'follow-up-mail': {
+          '@type': 'Alert', trigger: { '@type': 'OffsetTrigger', offset: '-PT1H', relativeTo: 'end' },
+          action: 'email', acknowledged: null, relatedTo: null,
+        },
+      },
+    }));
+  });
+
+  it('returns them, each alert with whether it was acknowledged, and whether the defaults are in use', async () => {
+    client.getCalendarTasks.mockResolvedValue([task({
+      alerts: {
+        a: at('2026-05-04T07:00:00Z', { acknowledged: '2026-05-04T07:01:00Z' }),
+        b: { '@type': 'Alert', trigger: { '@type': 'OffsetTrigger', offset: '-PT15M', relativeTo: 'start' }, action: 'email', acknowledged: null, relatedTo: null },
+      },
+    })]);
+    expect(await call(READ, 'tasks.get', 't1')).toEqual(expect.objectContaining({
+      alerts: {
+        a: { trigger: { at: '2026-05-04T07:00:00Z' }, action: 'display', acknowledged: '2026-05-04T07:01:00Z' },
+        b: { trigger: { offset: '-PT15M', relativeTo: 'start' }, action: 'email', acknowledged: null },
+      },
+      useDefaultAlerts: false,
+    }));
+    const [listed] = await call(READ, 'tasks.list') as { alerts: object }[];
+    expect(Object.keys(listed.alerts)).toEqual(['a', 'b']);
+  });
+
+  it('refuses bad triggers, ids and too many alerts', async () => {
+    const create = (over: Record<string, unknown>) => call(WRITE, 'tasks.create', { title: 'x', ...over });
+    await expect(create({ alerts: [{ id: 'a', trigger: { offset: 'PT0S' } }] })).rejects.toThrow(/object keyed by id/);
+    const six = Object.fromEntries([0, 1, 2, 3, 4, 5].map((i) => [`a${i}`, { trigger: { offset: 'PT0S' } }]));
+    await expect(create({ alerts: six })).rejects.toThrow(/at most 5 entries/);
+    await expect(create({ alerts: { a: { trigger: { offset: 'P400D' } } } })).rejects.toThrow(/at most 366 days/);
+    await expect(create({ alerts: { a: { trigger: { offset: 'P' } } } })).rejects.toThrow(/ISO 8601 duration/);
+    await expect(create({ alerts: { a: { trigger: { at: '2026-05-04T09:00' } } } })).rejects.toThrow(/in UTC/);
+    await expect(create({ alerts: { a: { trigger: { offset: 'PT0S' }, action: 'audio' } } })).rejects.toThrow(/display or email/);
+    await expect(create({ alerts: { 'a b': { trigger: { offset: 'PT0S' } } } })).rejects.toThrow(/letters, digits/);
+    expect(client.createCalendarTask).not.toHaveBeenCalled();
+  });
+
+  it('merges by id with patch paths, leaving alerts it did not name and other fields alone', async () => {
+    useTaskStore.setState({ tasks: [task({
+      title: 'Kept',
+      alerts: { foreign: at('2026-05-04T06:00:00Z', { action: 'email' }), old: at('2026-05-04T05:00:00Z') },
+    })] });
+    await call(WRITE, 'tasks.update', 't1', {
+      alerts: { 'follow-up': { trigger: { at: '2026-05-04T07:00:00Z' } }, old: null },
+    });
+    expect(updates()[0]).toEqual({
+      'alerts/follow-up': at('2026-05-04T07:00:00Z'),
+      'alerts/old': null,
+    });
+    const held = useTaskStore.getState().tasks[0];
+    expect(held.title).toBe('Kept');
+    expect(Object.keys(held.alerts ?? {})).toEqual(['foreign', 'follow-up']);
+
+    await call(WRITE, 'tasks.update', 't1', { alerts: null });
+    expect(updates()[1]).toEqual({ alerts: null });
+  });
+
+  it('writes the alerts whole on a task that has none yet', async () => {
+    useTaskStore.setState({ tasks: [task({ alerts: null })] });
+    await call(WRITE, 'tasks.update', 't1', { alerts: { a: { trigger: { at: '2026-05-04T07:00:00Z' } } } });
+    expect(updates()[0]).toEqual({ alerts: { a: at('2026-05-04T07:00:00Z') } });
+  });
+
+  it('keeps an unchanged alert acknowledged and lets a snoozed one fire again', async () => {
+    useTaskStore.setState({ tasks: [task({
+      alerts: {
+        a: at('2026-05-04T07:00:00Z', { acknowledged: '2026-05-04T07:01:00Z' }),
+        b: at('2026-05-04T07:00:00Z', { acknowledged: '2026-05-04T07:01:00Z' }),
+      },
+    })] });
+    await call(WRITE, 'tasks.update', 't1', { alerts: {
+      a: { trigger: { at: '2026-05-04T07:00:00Z' } },
+      b: { trigger: { at: '2026-05-04T08:00:00Z' } },
+    } });
+    expect(updates()[0]).toEqual({ 'alerts/b': at('2026-05-04T08:00:00Z') });
+    const held = useTaskStore.getState().tasks[0];
+    expect(held.alerts?.a.acknowledged).toBe('2026-05-04T07:01:00Z');
+    expect(held.alerts?.b.acknowledged).toBeNull();
+  });
+
+  it('refuses a sixth alert counting those another app wrote', async () => {
+    useTaskStore.setState({ tasks: [task({
+      alerts: Object.fromEntries([1, 2, 3, 4, 5].map((i) => [`x${i}`, at('2026-05-04T07:00:00Z')])),
+    })] });
+    await expect(call(WRITE, 'tasks.update', 't1', { alerts: { new: { trigger: { offset: 'PT0S' } } } })).rejects.toThrow(/more than 5 alerts/);
+    await call(WRITE, 'tasks.update', 't1', { alerts: { x1: null, new: { trigger: { offset: 'PT0S' } } } });
+    expect(updates()[0]).toEqual({ 'alerts/x1': null, 'alerts/new': expect.objectContaining({ action: 'display' }) });
+  });
+
+  it('makes the calendar\'s default alerts the task\'s own under new ids, refusing a plugin alert under a default\'s id', async () => {
+    useTaskStore.setState({ tasks: [task({
+      useDefaultAlerts: true,
+      alerts: { d1: at('2026-05-04T06:00:00Z'), d2: at('2026-05-04T06:30:00Z') },
+    })] });
+    expect(await call(READ, 'tasks.get', 't1')).toEqual(expect.objectContaining({ useDefaultAlerts: true }));
+    await expect(call(WRITE, 'tasks.update', 't1', { alerts: { d1: { trigger: { offset: 'PT0S' } } } })).rejects.toThrow(/default alerts.*another id/);
+    await expect(call(WRITE, 'tasks.acknowledgeAlert', 't1', 'd1')).rejects.toThrow(/useDefaultAlerts/);
+    expect(client.updateCalendarTask).not.toHaveBeenCalled();
+
+    await call(WRITE, 'tasks.update', 't1', { alerts: { d2: null, mine: { trigger: { at: '2026-05-04T07:00:00Z' } } } });
+    expect(updates()[0]).toEqual({
+      useDefaultAlerts: false,
+      alerts: { alert1: at('2026-05-04T06:00:00Z'), mine: at('2026-05-04T07:00:00Z') },
+    });
+    useTaskStore.setState({ tasks: [task({ useDefaultAlerts: true, alerts: { d1: at('2026-05-04T06:00:00Z') } })] });
+    await call(WRITE, 'tasks.update', 't1', { alerts: {} });
+    expect(updates()[1]).toEqual({ useDefaultAlerts: false, alerts: { alert1: at('2026-05-04T06:00:00Z') } });
+  });
+
+  it('takes ids such as __proto__ and constructor as plain keys', async () => {
+    await call(WRITE, 'tasks.create', {
+      title: 'x',
+      alerts: JSON.parse('{"__proto__": {"trigger": {"offset": "PT0S"}}, "constructor": {"trigger": {"offset": "PT0S"}}}'),
+    });
+    const created = client.createCalendarTask.mock.calls[0][0] as CalendarTask;
+    expect(Object.getPrototypeOf(created.alerts)).toBe(Object.prototype);
+    expect(Object.keys(created.alerts ?? {})).toEqual(['__proto__', 'constructor']);
+
+    useTaskStore.setState({ tasks: [task({ alerts: { a: at('2026-05-04T07:00:00Z') } })] });
+    await expect(call(WRITE, 'tasks.acknowledgeAlert', 't1', 'constructor')).rejects.toThrow(/no alert "constructor"/);
+    await expect(call(WRITE, 'tasks.acknowledgeAlert', 't1', '__proto__')).rejects.toThrow(/no alert "__proto__"/);
+    await call(WRITE, 'tasks.update', 't1', { alerts: JSON.parse('{"__proto__": {"trigger": {"offset": "PT0S"}}}') });
+    expect(updates()[0]).toEqual(JSON.parse('{"alerts/__proto__": {"@type": "Alert", "trigger": {"@type": "OffsetTrigger", "offset": "PT0S", "relativeTo": "end"}, "action": "display", "acknowledged": null, "relatedTo": null}}'));
+    const held = useTaskStore.getState().tasks[0];
+    expect(Object.getPrototypeOf(held.alerts)).toBe(Object.prototype);
+    expect(Object.keys(held.alerts ?? {})).toEqual(['a', '__proto__']);
+  });
+
+  it('acknowledges an alert by its path, now or at a given time, and undoes it', async () => {
+    vi.useFakeTimers({ now: new Date('2026-05-04T07:02:03.456Z'), toFake: ['Date'] });
+    try {
+      useTaskStore.setState({ tasks: [task({ alerts: { a: at('2026-05-04T07:00:00Z'), b: at('2026-05-04T07:00:00Z', { action: 'email' }) } })] });
+      await call(WRITE, 'tasks.acknowledgeAlert', 't1', 'a');
+      expect(client.updateCalendarTask).toHaveBeenLastCalledWith('t1', { 'alerts/a/acknowledged': '2026-05-04T07:02:03Z' });
+      expect(useTaskStore.getState().tasks[0].alerts?.a.acknowledged).toBe('2026-05-04T07:02:03Z');
+      await call(WRITE, 'tasks.acknowledgeAlert', 't1', 'b', '2026-05-04T06:00:00Z');
+      expect(useTaskStore.getState().tasks[0].alerts?.b.acknowledged).toBe('2026-05-04T06:00:00Z');
+      await call(WRITE, 'tasks.acknowledgeAlert', 't1', 'a', null);
+      expect(client.updateCalendarTask).toHaveBeenLastCalledWith('t1', { 'alerts/a/acknowledged': null });
+      expect(await call(READ, 'tasks.get', 't1')).toEqual(expect.objectContaining({
+        alerts: expect.objectContaining({ a: expect.objectContaining({ acknowledged: null }) }),
+      }));
+
+      client.updateCalendarTask.mockClear();
+      await call(WRITE, 'tasks.acknowledgeAlert', 't1', 'a', null);
+      expect(client.updateCalendarTask).not.toHaveBeenCalled();
+      await expect(call(WRITE, 'tasks.acknowledgeAlert', 't1', 'zz')).rejects.toThrow(/no alert "zz"/);
+      await expect(call(WRITE, 'tasks.acknowledgeAlert', 't1', 'a', '2026-05-04T06:00')).rejects.toThrow(/in UTC/);
+      await expect(call(READ, 'tasks.acknowledgeAlert', 't1', 'a')).rejects.toThrow(/lacks permission "tasks:write"/);
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it('acknowledges two alerts at once one after the other, neither undoing the other', async () => {
+    useTaskStore.setState({ tasks: [task({ alerts: { a: at('2026-05-04T07:00:00Z'), b: at('2026-05-04T07:00:00Z') } })] });
+    let inFlight = 0;
+    let most = 0;
+    client.updateCalendarTask.mockImplementation(async (id: string) => {
+      most = Math.max(most, ++inFlight);
+      await new Promise((resolve) => setTimeout(resolve, 5));
+      inFlight -= 1;
+      return id;
+    });
+    await Promise.all([
+      call(WRITE, 'tasks.acknowledgeAlert', 't1', 'a', '2026-05-04T07:01:00Z'),
+      call(WRITE, 'tasks.acknowledgeAlert', 't1', 'b', '2026-05-04T07:02:00Z'),
+    ]);
+    // Stalwart refuses a second set on an object while one is under way.
+    expect(most).toBe(1);
+    expect(updates()).toEqual([
+      { 'alerts/a/acknowledged': '2026-05-04T07:01:00Z' },
+      { 'alerts/b/acknowledged': '2026-05-04T07:02:00Z' },
+    ]);
+    const alerts = useTaskStore.getState().tasks[0].alerts;
+    expect([alerts?.a.acknowledged, alerts?.b.acknowledged]).toEqual(['2026-05-04T07:01:00Z', '2026-05-04T07:02:00Z']);
+  });
+
+  it('acknowledges an alert of a task the list does not hold, emitting the whole task', async () => {
+    const full = task({ title: 'From the server', alerts: { a: at('2026-05-04T07:00:00Z'), b: at('2026-05-04T08:00:00Z') } });
+    client.getCalendarTasks.mockResolvedValue([full]);
+    const seen: { patch: Record<string, unknown> }[] = [];
+    const emitted: { title: string; alerts: Record<string, { acknowledged: string | null }> }[] = [];
+    taskHooks.onBeforeTaskUpdate.register('watch', (e: unknown) => { seen.push(e as { patch: Record<string, unknown> }); return true; });
+    taskHooks.onTaskUpdate.register('watch', (v: unknown) => { emitted.push(v as never); });
+    try {
+      await call(WRITE, 'tasks.acknowledgeAlert', 't1', 'a', '2026-05-04T07:01:00Z');
+      expect(updates()).toEqual([{ 'alerts/a/acknowledged': '2026-05-04T07:01:00Z' }]);
+      expect(Object.keys(seen[0].patch.alerts as object)).toEqual(['a', 'b']);
+      await vi.waitFor(() => expect(emitted).toHaveLength(1));
+      expect(emitted[0].title).toBe('From the server');
+      expect(emitted[0].alerts.a.acknowledged).toBe('2026-05-04T07:01:00Z');
+      expect(emitted[0].alerts.b.acknowledged).toBeNull();
+    } finally {
+      taskHooks.onBeforeTaskUpdate.removePlugin('watch');
+      taskHooks.onTaskUpdate.removePlugin('watch');
+    }
+  });
+
+  it('decides on the alerts as the server holds them, not an older copy in the list', async () => {
+    useTaskStore.setState({ tasks: [task({ alerts: null })] });
+    client.getCalendarEvent.mockImplementation(async (id: string) => (id === 't1'
+      ? { id: 't1', alerts: { other: at('2026-05-04T06:00:00Z', { action: 'email' }) } }
+      : null));
+    await call(WRITE, 'tasks.update', 't1', { alerts: { mine: { trigger: { at: '2026-05-04T07:00:00Z' } } } });
+    expect(updates()[0]).toEqual({ 'alerts/mine': at('2026-05-04T07:00:00Z') });
+    const held = useTaskStore.getState().tasks[0];
+    expect(Object.keys(held.alerts ?? {})).toEqual(['other', 'mine']);
+    expect(client.getCalendarEvent).toHaveBeenCalledWith('t1', undefined);
+  });
+
+  it('turns the calendar\'s default alerts off when every alert is removed', async () => {
+    useTaskStore.setState({ tasks: [task({ useDefaultAlerts: true, alerts: { d1: at('2026-05-04T06:00:00Z') } })] });
+    await call(WRITE, 'tasks.update', 't1', { alerts: null });
+    expect(updates()[0]).toEqual({ alerts: null, useDefaultAlerts: false });
+  });
+});
diff --git a/lib/__tests__/plugin-permission-coverage.test.ts b/lib/__tests__/plugin-permission-coverage.test.ts
index c34925f2f..1ac16d526 100644
--- a/lib/__tests__/plugin-permission-coverage.test.ts
+++ b/lib/__tests__/plugin-permission-coverage.test.ts
@@ -67,6 +67,7 @@ describe('plugin permissions', () => {
       'calendar.createEvent': 'calendar:write',
       'tasks.list': 'tasks:read',
       'tasks.setComplete': 'tasks:write',
+      'tasks.acknowledgeAlert': 'tasks:write',
       'templates.list': 'templates:read',
       'templates.create': 'templates:write',
       'identities.update': 'identity:write',
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index 2d7adf72b..d5314bbb8 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -30,6 +30,7 @@ import { buildShareSetCalls, failAll, parseShareTargets, readShareSetResults, SE
 import type { ShareKind } from "./share-rights";
 import { ARCHIVED_ITEM_USING, STALWART_CAPABILITY, buildRestoreCalls, hasArchivedEmail, listArchivedEmailsPaged, readRestoreResults, type ArchiveIo, type ArchiveListResult, type ArchiveQueryPage, type RestoreResult } from "@/lib/recovery/archived-items";
 import { RescheduleRestoredError } from "./reschedule-error";
+import { applyPatchObject } from "./patch-object";
 
 export { SchedulingDeniedError, RescheduleRestoredError };
 import { acceptedFileName, fileNameRulesFrom, type FileNameRules } from "@/lib/file-name-rules";
@@ -9933,7 +9934,8 @@ export class JMAPClient implements IJMAPClient {
 
     // Drop what the server sets itself; keep the uid so it stays the same task.
     const { id: _id, created: _cr, updated: _up, sequence: _sq, isDraft: _idr, isOrigin: _io, baseEventId: _be, blobId: _bl, utcStart: _us, utcEnd: _ue, ...rest } = original as Record<string, unknown>;
-    const created = await this.createCalendarTask({ ...rest, ...patch } as Partial<CalendarTask>, destAccountId);
+    // The patch may name members by path ("alerts/<id>/acknowledged").
+    const created = await this.createCalendarTask(applyPatchObject(rest, patch) as Partial<CalendarTask>, destAccountId);
     await this.deleteCalendarEvent(source.id, false, source.accountId);
     return created.id;
   }
diff --git a/lib/jmap/patch-object.ts b/lib/jmap/patch-object.ts
new file mode 100644
index 000000000..20c9b0dae
--- /dev/null
+++ b/lib/jmap/patch-object.ts
@@ -0,0 +1,70 @@
+/**
+ * JMAP PatchObject paths (RFC 8620 §5.3): a key such as
+ * "alerts/a1/acknowledged" changes that one member and leaves the rest of the
+ * object as the server holds it, so a change made meanwhile elsewhere (another
+ * device, another plugin call) is not overwritten.
+ */
+
+/** The JSON Pointer path of nested members, each segment escaped (RFC 6901). */
+export function patchPath(...segments: string[]): string {
+  return segments.map((s) => s.replace(/~/g, '~0').replace(/\//g, '~1')).join('/');
+}
+
+function unescapeSegment(segment: string): string {
+  return segment.replace(/~1/g, '/').replace(/~0/g, '~');
+}
+
+/** A shallow copy that keeps every own key, `__proto__` included, as data. */
+function copyOf(object: object | null | undefined): Record<string, unknown> {
+  return Object.fromEntries(Object.entries(object ?? {}));
+}
+
+/** Sets `key` as an own data property, never the prototype. */
+function put(object: Record<string, unknown>, key: string, value: unknown): void {
+  Object.defineProperty(object, key, { value, enumerable: true, configurable: true, writable: true });
+}
+
+/**
+ * The object as the server holds it after `patch`: plain keys replace the
+ * property, path keys change the member they name (null removes a map
+ * member). Nothing is changed in place.
+ */
+export function applyPatchObject<T extends object>(target: T, patch: object): T {
+  let result = copyOf(target);
+  for (const [key, value] of Object.entries(patch)) {
+    if (!key.includes('/')) {
+      put(result, key, value);
+      continue;
+    }
+    result = setIn(result, key.split('/').map(unescapeSegment), value);
+  }
+  return result as T;
+}
+
+function setIn(object: Record<string, unknown>, [head, ...rest]: string[], value: unknown): Record<string, unknown> {
+  const copy = copyOf(object);
+  if (rest.length === 0) {
+    if (value === null) delete copy[head];
+    else put(copy, head, value);
+    return copy;
+  }
+  const child = Object.hasOwn(copy, head) ? copy[head] : undefined;
+  put(copy, head, setIn(child && typeof child === 'object' ? (child as Record<string, unknown>) : {}, rest, value));
+  return copy;
+}
+
+/**
+ * The top-level properties `patch` changes, each whole as it is after the
+ * patch: a path key ("alerts/a1/acknowledged") becomes its property
+ * ("alerts") with every member as the patch leaves it. For whoever reads a
+ * patch as plain properties (plugin hooks), while the server gets the paths.
+ */
+export function patchedProperties<T extends object>(target: T, patch: object): Partial<T> {
+  const after = applyPatchObject(target, patch) as Record<string, unknown>;
+  const changed: Record<string, unknown> = {};
+  for (const key of Object.keys(patch)) {
+    const property = unescapeSegment(key.split('/')[0]);
+    put(changed, property, Object.hasOwn(after, property) ? after[property] : null);
+  }
+  return changed as Partial<T>;
+}
diff --git a/lib/plugin-projection.ts b/lib/plugin-projection.ts
index c950b4d02..c8cbf838b 100644
--- a/lib/plugin-projection.ts
+++ b/lib/plugin-projection.ts
@@ -175,6 +175,14 @@ export function taskToView(task: CalendarTask): TaskView {
     priority: task.priority ?? 0,
     calendarId: trueKeys(task.calendarIds)[0] || '',
     keywords: trueKeys(task.keywords),
+    alerts: Object.fromEntries(Object.entries(task.alerts || {}).filter(([, alert]) => alert?.trigger).map(([id, alert]) => [id, {
+      trigger: alert.trigger['@type'] === 'AbsoluteTrigger'
+        ? { at: alert.trigger.when }
+        : { offset: alert.trigger.offset, relativeTo: alert.trigger.relativeTo === 'end' ? 'due' as const : 'start' as const },
+      action: alert.action === 'email' ? 'email' as const : 'display' as const,
+      acknowledged: alert.acknowledged || null,
+    }])),
+    useDefaultAlerts: !!task.useDefaultAlerts,
   };
 }
 
diff --git a/lib/plugin-sandbox/host-api-calendar.ts b/lib/plugin-sandbox/host-api-calendar.ts
index 4c6a36be2..c3574975a 100644
--- a/lib/plugin-sandbox/host-api-calendar.ts
+++ b/lib/plugin-sandbox/host-api-calendar.ts
@@ -10,7 +10,9 @@ import { createRecurrenceRule } from '@/lib/recurrence-rule';
 import type {
   CalendarEventInput, CalendarEventView, CalendarView, TaskInput, TaskView,
 } from '../plugin-types';
-import type { Calendar, CalendarEvent, CalendarParticipant, CalendarRecurrenceRule, CalendarTask } from '../jmap/types';
+import type {
+  Calendar, CalendarEvent, CalendarEventAlert, CalendarParticipant, CalendarRecurrenceRule, CalendarTask,
+} from '../jmap/types';
 import type { IJMAPClient } from '../jmap/client-interface';
 import { useCalendarStore } from '@/stores/calendar-store';
 import { useTaskStore } from '@/stores/task-store';
@@ -19,10 +21,12 @@ import { eventToView, taskToView } from '../plugin-projection';
 import { expandRecurringEvents } from '../recurrence-expansion';
 import { buildParticipantMap } from '../calendar-participants';
 import { parseDurationSeconds } from '../calendar-event-normalization';
+import { parseAlertOffset, withOwnAlertIds } from '../calendar-alerts';
+import { patchPath } from '../jmap/patch-object';
 import { getEffectiveTimeZone } from '../timezone';
 import { generateUUID } from '../utils';
 import {
-  readBoolean, readId, readInteger, readObject, readString, sessionClient,
+  isPlainRecord, readBoolean, readId, readInteger, readObject, readString, sessionClient,
 } from './host-api-input';
 
 /** Longest range calendar.getEvents covers in one call. */
@@ -486,7 +490,218 @@ export async function doCalendarDeleteEvent(id: unknown): Promise<void> {
 
 // ─── Tasks ────────────────────────────────────────────────────
 
-const TASK_FIELDS = ['title', 'description', 'due', 'priority', 'calendarId'] as const;
+const TASK_FIELDS = ['title', 'description', 'due', 'priority', 'calendarId', 'alerts'] as const;
+/** Most alerts a plugin may give a task. */
+export const MAX_PLUGIN_TASK_ALERTS = 5;
+const MAX_ALERT_OFFSET_MS = MAX_PLUGIN_EVENT_RANGE_DAYS * DAY_MS;
+/** Longest key a plugin may name to remove an entry it did not write. */
+const MAX_REMOVED_KEY = 1024;
+
+const MEMBER_ID_RE = /^[A-Za-z0-9._-]{1,64}$/;
+const OFFSET_RE = /^-?P(?:\d+W|(?:\d+D)?(?:T(?:\d+H)?(?:\d+M)?(?:\d+S)?)?)$/;
+const CONTROL_RE = /\p{Cc}/u;
+
+/** The maps a plugin edits on a task, each keyed as JSCalendar keys it. */
+const TASK_MAPS = ['alerts'] as const;
+type TaskMap = typeof TASK_MAPS[number];
+
+/**
+ * A map change: `null` for the whole map (removes every entry), else the
+ * entries named, each a value to set or null to remove. Entries not named
+ * are left as they are.
+ */
+type MapChange<T> = null | [key: string, value: T | null][];
+
+/** An instant in UTC, as JSCalendar's UTCDateTime (`2026-05-01T07:30:00Z`). */
+function readInstant(value: unknown, method: string, field: string): string {
+  const when = readWhen(value, method, field);
+  if (!when.utc || when.time === null) throw new Error(`${method}: ${field} must be a time in UTC, like 2026-05-01T07:30:00Z`);
+  return `${when.date}T${when.time}Z`;
+}
+
+function utcNow(): string {
+  return new Date().toISOString().replace(/\.\d{3}Z$/, 'Z');
+}
+
+/**
+ * `{ key: value | null }` → its entries, each value read by `read`. A key
+ * set must pass `keyOk`; a key removed (null) may be any key the task could
+ * hold, so a plugin can remove entries another app wrote.
+ */
+function readMapChange<T>(
+  value: unknown,
+  method: string,
+  field: string,
+  max: number,
+  keyOk: (key: string) => boolean,
+  keyRule: string,
+  read: (value: unknown, field: string) => T,
+): MapChange<T> {
+  if (value === null) return null;
+  if (!isPlainRecord(value)) throw new Error(`${method}: ${field} must be an object keyed by id, or null`);
+  const entries = Object.entries(value);
+  if (entries.length > max) throw new Error(`${method}: ${field} may name at most ${max} entries`);
+  return entries.map(([key, entry]) => {
+    const where = `${field}["${key.slice(0, 64)}"]`;
+    if (entry === null) {
+      if (!key || key.length > MAX_REMOVED_KEY || CONTROL_RE.test(key)) throw new Error(`${method}: ${where} is not a key a task holds`);
+      return [key, null];
+    }
+    if (!keyOk(key)) throw new Error(`${method}: ${field} key "${key.slice(0, 64)}" must be ${keyRule}`);
+    return [key, read(entry, where)];
+  });
+}
+
+function readTrigger(value: unknown, method: string, field: string): CalendarEventAlert['trigger'] {
+  if (isPlainRecord(value) && Object.hasOwn(value, 'at')) {
+    const t = readObject(value, method, field, ['at']);
+    return { '@type': 'AbsoluteTrigger', when: readInstant(t.at, method, `${field}.at`) };
+  }
+  const t = readObject(value, method, field, ['offset', 'relativeTo']);
+  if (typeof t.offset !== 'string' || !OFFSET_RE.test(t.offset) || !/\d/.test(t.offset) || t.offset.endsWith('T')) {
+    throw new Error(`${method}: ${field}.offset must be an ISO 8601 duration such as -PT15M, or give ${field}.at`);
+  }
+  const ms = parseAlertOffset(t.offset);
+  if (ms === null || Math.abs(ms) > MAX_ALERT_OFFSET_MS) {
+    throw new Error(`${method}: ${field}.offset may be at most ${MAX_PLUGIN_EVENT_RANGE_DAYS} days`);
+  }
+  if (t.relativeTo !== undefined && t.relativeTo !== 'due' && t.relativeTo !== 'start') {
+    throw new Error(`${method}: ${field}.relativeTo must be due or start`);
+  }
+  // JSCalendar names a task's due its "end".
+  return { '@type': 'OffsetTrigger', offset: t.offset, relativeTo: t.relativeTo === 'start' ? 'start' : 'end' };
+}
+
+function alertsOf(value: unknown, method: string): MapChange<CalendarEventAlert> {
+  return readMapChange(value, method, 'alerts', MAX_PLUGIN_TASK_ALERTS, (k) => MEMBER_ID_RE.test(k),
+    '1-64 letters, digits or . _ -', (entry, field) => {
+      const a = readObject(entry, method, field, ['trigger', 'action']);
+      if (a.action !== undefined && a.action !== 'display' && a.action !== 'email') {
+        throw new Error(`${method}: ${field}.action must be display or email`);
+      }
+      return {
+        '@type': 'Alert',
+        trigger: readTrigger(a.trigger, method, `${field}.trigger`),
+        action: a.action === 'email' ? 'email' : 'display',
+        acknowledged: null,
+        relatedTo: null,
+      };
+    });
+}
+
+interface TaskMapChanges {
+  alerts?: MapChange<CalendarEventAlert>;
+}
+
+/** The plugin's map changes, read and checked before anything is asked of the server. */
+function mapChangesOf(data: Record<string, unknown>, method: string): TaskMapChanges {
+  const changes: TaskMapChanges = {};
+  if (data.alerts !== undefined) changes.alerts = alertsOf(data.alerts, method);
+  return changes;
+}
+
+/** The entries set, as a map (null when none), for a task that has none yet. */
+function wholeMap<T>(change: MapChange<T>): Record<string, T> | null {
+  const set = (change ?? []).filter((entry): entry is [string, T] => entry[1] !== null);
+  return set.length ? Object.fromEntries(set) : null;
+}
+
+/** Map changes on a new task: the entries set. Alerts turn the calendar's defaults off. */
+function createMaps(changes: TaskMapChanges): Partial<CalendarTask> {
+  const task: Partial<CalendarTask> = {};
+  for (const name of TASK_MAPS) {
+    const change = changes[name];
+    if (change !== undefined) Object.assign(task, { [name]: wholeMap(change as MapChange<unknown>) });
+  }
+  if (task.alerts) task.useDefaultAlerts = false;
+  return task;
+}
+
+function sameAlert(a: CalendarEventAlert, b: CalendarEventAlert): boolean {
+  const key = (t: CalendarEventAlert['trigger']) => t['@type'] === 'AbsoluteTrigger'
+    ? `at ${Date.parse(t.when)}`
+    : `offset ${parseAlertOffset(t.offset)} ${t.relativeTo === 'end' ? 'end' : 'start'}`;
+  return a.action === b.action && key(a.trigger) === key(b.trigger);
+}
+
+function countAfter(held: object | null | undefined, change: [string, unknown][]): number {
+  const keys = new Set(Object.keys(held ?? {}));
+  for (const [key, value] of change) {
+    if (value === null) keys.delete(key);
+    else keys.add(key);
+  }
+  return keys.size;
+}
+
+/**
+ * The patch for a task's maps: each entry named by its JMAP path, so entries
+ * the plugin did not name, and members changed meanwhile elsewhere, stay as
+ * the server holds them. A map the task does not have yet is written whole.
+ * An alert set with the trigger and action it has keeps its `acknowledged`;
+ * a new trigger (a snooze) makes it fire again.
+ */
+function updateMaps(method: string, task: CalendarTask, changes: TaskMapChanges): Record<string, unknown> {
+  const patch: Record<string, unknown> = {};
+  const limits: Record<TaskMap, number> = {
+    alerts: MAX_PLUGIN_TASK_ALERTS,
+  };
+  for (const name of TASK_MAPS) {
+    const change = changes[name] as MapChange<unknown> | undefined;
+    if (change === undefined) continue;
+    if (change === null) {
+      patch[name] = null;
+      // Else the calendar's defaults would still show and fire.
+      if (name === 'alerts' && task.useDefaultAlerts) patch.useDefaultAlerts = false;
+      continue;
+    }
+    const held = task[name] as Record<string, unknown> | null | undefined;
+    const adds = change.some(([key, value]) => value !== null && !(held && Object.hasOwn(held, key)));
+    if (adds && countAfter(held, change) > limits[name]) {
+      throw new Error(`${method}: the task would hold more than ${limits[name]} ${name}`);
+    }
+    if (name === 'alerts' && task.useDefaultAlerts) {
+      Object.assign(patch, ownAlerts(method, task, change as [string, CalendarEventAlert | null][]));
+      continue;
+    }
+    if (!held || Object.keys(held).length === 0) {
+      patch[name] = wholeMap(change);
+      continue;
+    }
+    for (const [key, value] of change) {
+      if (name === 'alerts' && value !== null && Object.hasOwn(held, key) && sameAlert(held[key] as CalendarEventAlert, value as CalendarEventAlert)) {
+        continue;
+      }
+      patch[patchPath(name, key)] = value;
+    }
+  }
+  return patch;
+}
+
+/**
+ * Alerts of a task that uses its calendar's default alerts. Stalwart shows
+ * the defaults in `alerts` under the defaults' ids and drops on save every
+ * alert under one of those ids, so the defaults the plugin keeps become the
+ * task's own under new ids (as the task form does), and an alert of the
+ * plugin's under a default's id is refused rather than lost.
+ */
+function ownAlerts(
+  method: string,
+  task: CalendarTask,
+  change: [string, CalendarEventAlert | null][],
+): Partial<CalendarTask> {
+  const defaults = task.alerts ?? {};
+  const removed = new Set<string>();
+  const set: [string, CalendarEventAlert][] = [];
+  for (const [key, value] of change) {
+    if (value === null) removed.add(key);
+    else if (Object.hasOwn(defaults, key)) {
+      throw new Error(`${method}: alert "${key}" is one of the calendar's default alerts the task uses (useDefaultAlerts); give yours another id`);
+    } else set.push([key, value]);
+  }
+  const kept = Object.entries(defaults).filter(([key]) => !removed.has(key)).map(([, alert]) => alert);
+  const alerts = { ...withOwnAlertIds(kept, [...Object.keys(defaults), ...set.map(([key]) => key)]), ...Object.fromEntries(set) };
+  return { useDefaultAlerts: false, alerts: Object.keys(alerts).length ? alerts : null };
+}
 
 function isDone(task: CalendarTask): boolean {
   return task.progress === 'completed' || task.progress === 'cancelled';
@@ -505,6 +720,22 @@ export async function doTasksList(options: unknown): Promise<TaskView[]> {
     .map(taskToView);
 }
 
+// A plugin's writes to one task, one after another: Stalwart refuses a
+// CalendarEvent/set while another is under way on the same object, and each
+// write is worked out against the task as the one before left it.
+const taskWrites = new Map<string, Promise<unknown>>();
+
+function oneAtATime<T>(taskId: string, write: () => Promise<T>): Promise<T> {
+  const before = taskWrites.get(taskId) ?? Promise.resolve();
+  const next = before.catch(() => undefined).then(write);
+  const settled = next.catch(() => undefined);
+  taskWrites.set(taskId, settled);
+  void settled.then(() => {
+    if (taskWrites.get(taskId) === settled) taskWrites.delete(taskId);
+  });
+  return next;
+}
+
 async function loadTask(client: IJMAPClient, id: string): Promise<CalendarTask | null> {
   const inStore = useTaskStore.getState().tasks.find((t) => t.id === id);
   if (inStore) return inStore;
@@ -517,6 +748,20 @@ async function requireTask(client: IJMAPClient, method: string, id: string): Pro
   return task;
 }
 
+/**
+ * The task with its alerts as the server holds them now: the store's copy may be older, and a map the task seems
+ * not to have is written whole, which would drop what the server gained.
+ */
+async function withServerMaps(client: IJMAPClient, task: CalendarTask): Promise<CalendarTask> {
+  const server = await client.getCalendarEvent(task.originalId ?? task.id, task.accountId);
+  if (!server) return task;
+  return {
+    ...task,
+    alerts: server.alerts ?? null,
+    useDefaultAlerts: server.useDefaultAlerts,
+  };
+}
+
 export async function doTasksGet(id: unknown): Promise<TaskView | null> {
   const method = 'tasks.get';
   const task = await loadTask(sessionClient(method), readId(id, method));
@@ -547,6 +792,7 @@ export async function doTasksCreate(input: unknown): Promise<TaskView | null> {
     ...(data.due === undefined ? { due: null, showWithoutTime: true } : dueOf(data.due, method)),
     priority: data.priority === undefined ? 0 : readInteger(data.priority, method, 'priority', 0, 9),
     progress: 'needs-action',
+    ...createMaps(mapChangesOf(data, method)),
     calendarIds: { [await pickCalendar(client, method, data.calendarId, true)]: true },
   };
   const created = await useTaskStore.getState().createTask(client, task);
@@ -569,8 +815,37 @@ export async function doTasksUpdate(id: unknown, patch: unknown): Promise<void>
   if (data.calendarId !== undefined) {
     updates.calendarIds = { [await pickCalendar(client, method, data.calendarId, true)]: true };
   }
-  await requireTask(client, method, taskId);
-  await useTaskStore.getState().updateTask(client, taskId, updates);
+  const maps = mapChangesOf(data, method);
+  await oneAtATime(taskId, async () => {
+    let task = await requireTask(client, method, taskId);
+    if (Object.keys(maps).length) task = await withServerMaps(client, task);
+    Object.assign(updates, updateMaps(method, task, maps));
+    if (Object.keys(updates).length === 0) return;
+    await useTaskStore.getState().updateTask(client, taskId, updates, task);
+  });
+}
+
+/**
+ * Mark one of a task's alerts dismissed at `when` (UTC; now when left out),
+ * or not dismissed with null. Only that alert's `acknowledged` is written.
+ */
+export async function doTasksAcknowledgeAlert(id: unknown, alertId: unknown, when: unknown): Promise<void> {
+  const method = 'tasks.acknowledgeAlert';
+  const taskId = readId(id, method);
+  const key = readId(alertId, method, 'alertId');
+  const acknowledged = when === undefined ? utcNow() : when === null ? null : readInstant(when, method, 'when');
+  const client = sessionClient(method);
+  await oneAtATime(taskId, async () => {
+    const task = await withServerMaps(client, await requireTask(client, method, taskId));
+    const alert = task.alerts && Object.hasOwn(task.alerts, key) ? task.alerts[key] : null;
+    if (!alert) throw new Error(`${method}: the task has no alert "${key}"`);
+    if (task.useDefaultAlerts) {
+      throw new Error(`${method}: the task uses its calendar's default alerts (useDefaultAlerts); make them its own with tasks.update(id, { alerts: {} }) first`);
+    }
+    if ((alert.acknowledged ?? null) === acknowledged) return;
+    const patch = { [patchPath('alerts', key, 'acknowledged')]: acknowledged } as Partial<CalendarTask>;
+    await useTaskStore.getState().updateTask(client, taskId, patch, task);
+  });
 }
 
 export async function doTasksSetComplete(id: unknown, complete: unknown): Promise<void> {
@@ -578,9 +853,11 @@ export async function doTasksSetComplete(id: unknown, complete: unknown): Promis
   const taskId = readId(id, method);
   const done = readBoolean(complete, method, 'complete');
   const client = sessionClient(method);
-  const task = await requireTask(client, method, taskId);
-  if ((task.progress === 'completed') === done) return;
-  await useTaskStore.getState().toggleTaskComplete(client, task);
+  await oneAtATime(taskId, async () => {
+    const task = await requireTask(client, method, taskId);
+    if ((task.progress === 'completed') === done) return;
+    await useTaskStore.getState().toggleTaskComplete(client, task);
+  });
 }
 
 export async function doTasksDelete(id: unknown): Promise<void> {
diff --git a/lib/plugin-sandbox/host-api.ts b/lib/plugin-sandbox/host-api.ts
index 7c611e4b7..cefeb157c 100644
--- a/lib/plugin-sandbox/host-api.ts
+++ b/lib/plugin-sandbox/host-api.ts
@@ -51,7 +51,7 @@ import { EncryptionAtRestConfig, PublicKeyInfo, PublicKeyInput, useAccountSecuri
 import {
   doCalendarCreateEvent, doCalendarDeleteEvent, doCalendarGetEvent, doCalendarGetEvents,
   doCalendarListCalendars, doCalendarUpdateEvent, doTasksCreate, doTasksDelete, doTasksGet,
-  doTasksList, doTasksSetComplete, doTasksUpdate,
+  doTasksAcknowledgeAlert, doTasksList, doTasksSetComplete, doTasksUpdate,
 } from './host-api-calendar';
 import {
   doIdentitiesUpdate, doTemplatesCreate, doTemplatesDelete, doTemplatesGet, doTemplatesList,
@@ -277,6 +277,7 @@ export const PERM_PER_METHOD: Readonly<Record<string, Permission | null>> = {
   'tasks.create': 'tasks:write',
   'tasks.update': 'tasks:write',
   'tasks.setComplete': 'tasks:write',
+  'tasks.acknowledgeAlert': 'tasks:write',
   'tasks.delete': 'tasks:write',
   // message templates
   'templates.list': 'templates:read',
@@ -2336,6 +2337,7 @@ export async function dispatchApiCall(
     case 'tasks.create': return doTasksCreate(args[0]);
     case 'tasks.update': return doTasksUpdate(args[0], args[1]);
     case 'tasks.setComplete': return doTasksSetComplete(args[0], args[1]);
+    case 'tasks.acknowledgeAlert': return doTasksAcknowledgeAlert(args[0], args[1], args[2]);
     case 'tasks.delete': return doTasksDelete(args[0]);
 
     case 'templates.list': return doTemplatesList();
diff --git a/lib/plugin-sandbox/runtime.tsx b/lib/plugin-sandbox/runtime.tsx
index eba4da5a2..a39b8c79e 100644
--- a/lib/plugin-sandbox/runtime.tsx
+++ b/lib/plugin-sandbox/runtime.tsx
@@ -996,10 +996,22 @@ function buildPluginApi(manifest: PluginManifest) {
       get: (id: string) => callApi('tasks.get', [id]) as Promise<TaskView | null>,
       /** Resolves to the new task, or null when another plugin stopped it. */
       create: (task: TaskInput) => callApi('tasks.create', [task]) as Promise<TaskView | null>,
-      /** Change the given fields. */
+      /**
+       * Change the given fields. `alerts` merges by id: an alert given is
+       * set, one given null is removed, the rest stay as they are (each
+       * written by its own JMAP patch path); null for the whole map clears it.
+       */
       update: (id: string, patch: Partial<TaskInput>) => callApi('tasks.update', [id, patch]) as Promise<void>,
       /** Mark a task done or not done. */
       setComplete: (id: string, complete: boolean) => callApi('tasks.setComplete', [id, complete]) as Promise<void>,
+      /**
+       * Dismiss one of a task's alerts at `when` (UTC, `2026-05-01T07:30:00Z`;
+       * now when left out), or undo that with null. Bulwark shows no reminder
+       * for a dismissed alert. Refused while the task uses its calendar's
+       * default alerts (TaskView.useDefaultAlerts).
+       */
+      acknowledgeAlert: (id: string, alertId: string, when?: string | null) =>
+        callApi('tasks.acknowledgeAlert', when === undefined ? [id, alertId] : [id, alertId, when]) as Promise<void>,
       delete: (id: string) => callApi('tasks.delete', [id]) as Promise<void>,
     },
     // Message templates (permissions: templates:read / templates:write).
diff --git a/lib/plugin-types.ts b/lib/plugin-types.ts
index 7ed094b70..506fb6e1d 100644
--- a/lib/plugin-types.ts
+++ b/lib/plugin-types.ts
@@ -586,6 +586,33 @@ export interface TaskInput {
   priority?: number;
   /** Defaults to the tasks calendar, else the default calendar. */
   calendarId?: string;
+  /**
+   * The task's reminders by id (1-64 of `A-Z a-z 0-9 . _ -`); a task holds
+   * at most 5. On update only the ids named change: an alert given is set
+   * (one kept with the same trigger and action stays acknowledged; a new
+   * trigger, a snooze, makes it fire again), an id given null is removed,
+   * and alerts not named are left as they are. null for the whole map
+   * removes every alert. While the task uses its calendar's default alerts
+   * (TaskView.useDefaultAlerts), any change makes the defaults the task's
+   * own under new ids; `{}` does only that.
+   */
+  alerts?: Record<string, TaskAlertInput | null> | null;
+}
+
+/**
+ * When a task's alert fires: an ISO 8601 duration from the task's due
+ * (`-PT15M` before, `PT0S` at it; `relativeTo: 'start'` counts from its
+ * start, else from its due), at most a year either way; or an instant in
+ * UTC (`2026-05-01T07:30:00Z`).
+ */
+export type TaskAlertTrigger =
+  | { offset: string; relativeTo?: 'due' | 'start' }
+  | { at: string };
+
+export interface TaskAlertInput {
+  trigger: TaskAlertTrigger;
+  /** `display` (the default) shows a reminder; `email` has the server send one. */
+  action?: 'display' | 'email';
 }
 
 export interface ContactView {
@@ -651,6 +678,22 @@ export interface TaskView {
   priority: number;
   calendarId: string;
   keywords: string[];
+  /** By id; as written by any app, not only by plugins. */
+  alerts: Record<string, TaskAlertView>;
+  /**
+   * True while the task uses its calendar's default alerts: `alerts` then
+   * shows those defaults, which can't be acknowledged until an update of
+   * `alerts` makes them the task's own (under new ids).
+   */
+  useDefaultAlerts: boolean;
+}
+
+export interface TaskAlertView {
+  /** As in TaskAlertInput; `relativeTo` is always given for an offset. */
+  trigger: { offset: string; relativeTo: 'due' | 'start' } | { at: string };
+  action: 'display' | 'email';
+  /** When the alert was dismissed (UTC), or null while it is not. */
+  acknowledged: string | null;
 }
 
 export interface TemplateView {
diff --git a/stores/task-store.ts b/stores/task-store.ts
index e5caa40c4..478c6e1a1 100644
--- a/stores/task-store.ts
+++ b/stores/task-store.ts
@@ -4,6 +4,7 @@ import type { IJMAPClient } from '@/lib/jmap/client-interface';
 import { debug } from '@/lib/debug';
 import { taskHooks } from '@/lib/plugin-hooks';
 import { taskToView } from '@/lib/plugin-projection';
+import { applyPatchObject, patchedProperties } from '@/lib/jmap/patch-object';
 
 export type TaskViewFilter = 'all' | 'pending' | 'completed' | 'overdue';
 
@@ -28,7 +29,12 @@ interface TaskStore {
   /** Resolves to null when a plugin stopped the create. */
   createTask: (client: IJMAPClient, task: Partial<CalendarTask>) => Promise<CalendarTask | null>;
   /** false when a plugin stopped it (the plugin tells the user why). */
-  updateTask: (client: IJMAPClient, id: string, updates: Partial<CalendarTask>) => Promise<boolean>;
+  /**
+   * `updates` may name members by path (`alerts/<id>/acknowledged`). `base`
+   * is the task as the caller read it, for a task the list does not hold
+   * or holds in an older state.
+   */
+  updateTask: (client: IJMAPClient, id: string, updates: Partial<CalendarTask>, base?: CalendarTask) => Promise<boolean>;
   deleteTask: (client: IJMAPClient, id: string) => Promise<boolean>;
   toggleTaskComplete: (client: IJMAPClient, task: CalendarTask) => Promise<void>;
   clearTasks: () => void;
@@ -93,9 +99,12 @@ export const useTaskStore = create<TaskStore>((set, get) => ({
     return created;
   },
 
-  updateTask: async (client, id, updates) => {
-    if (!(await taskHooks.onBeforeTaskUpdate.intercept({ taskId: id, patch: updates }))) return false;
-    const before = get().tasks.find(t => t.id === id);
+  updateTask: async (client, id, updates, base) => {
+    const before = base ?? get().tasks.find(t => t.id === id);
+    // `updates` may name members by path (`alerts/<id>/acknowledged`);
+    // plugins read the patch as whole properties.
+    const patch = patchedProperties(before ?? { id }, updates);
+    if (!(await taskHooks.onBeforeTaskUpdate.intercept({ taskId: id, patch }))) return false;
     const newId = await client.updateCalendarTask(id, updates);
     if (newId && newId !== id) {
       // Moved into a calendar of another account: the server recreated it
@@ -103,11 +112,15 @@ export const useTaskStore = create<TaskStore>((set, get) => ({
       set({ selectedTaskId: get().selectedTaskId === id ? newId : get().selectedTaskId });
       await get().fetchTasks(client);
       const moved = get().tasks.find(t => t.id === newId)
-        ?? ({ ...before, ...updates, id: newId } as CalendarTask);
+        ?? ({ ...applyPatchObject(before ?? {}, updates), id: newId } as CalendarTask);
       void taskHooks.onTaskUpdate.emit(taskToView(moved));
       return true;
     }
-    const updated = { ...(before ?? { id }), ...updates, updated: new Date().toISOString() } as CalendarTask;
+    // Applied to the task the caller read, else as the store holds it now:
+    // another update may have finished meanwhile (two alerts acknowledged
+    // at once).
+    const current = base ?? get().tasks.find(t => t.id === id) ?? { id };
+    const updated = { ...applyPatchObject(current, updates), updated: new Date().toISOString() } as CalendarTask;
     set({
       tasks: get().tasks.map(t => t.id === id ? updated : t),
     });