diff --git a/package.json b/package.json
index 2eb5eb56..b9aaa8e8 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "hawk.api",
-  "version": "1.5.3",
+  "version": "1.5.0",
   "main": "index.ts",
   "license": "BUSL-1.1",
   "scripts": {
diff --git a/src/models/eventsFactory.js b/src/models/eventsFactory.js
index fccae66d..e7234566 100644
--- a/src/models/eventsFactory.js
+++ b/src/models/eventsFactory.js
@@ -889,7 +889,7 @@ class EventsFactory extends Factory {
    * @param {string|ObjectId} userId - id of the user who is visiting events
    * @returns {Promise<{ updatedCount: number, updatedEventIds: string[], failedEventIds: string[] }>}
    */
-  async bulkVisitEvent(eventIds, userId) {
+  async bulkVisitEvents(eventIds, userId) {
     const {
       collection,
       found,
diff --git a/src/resolvers/event.js b/src/resolvers/event.js
index af1d6f70..dd6f12b6 100644
--- a/src/resolvers/event.js
+++ b/src/resolvers/event.js
@@ -6,6 +6,7 @@ const {
 } = require('./helpers/bulkEvents');
 const { aiService } = require('../services/ai');
 const { UserInputError } = require('apollo-server-express');
+const { ObjectId } = require('mongodb');
 
 /**
  * See all types and fields here {@see ../typeDefs/event.graphql}
@@ -161,7 +162,7 @@ module.exports = {
       }
 
       const factory = getEventsFactory(context, projectId);
-      const result = await factory.bulkVisitEvent(validEventIds, user.id);
+      const result = await factory.bulkVisitEvents(validEventIds, user.id);
 
       return {
         ...result,
@@ -317,6 +318,10 @@ module.exports = {
       const factory = getEventsFactory(context, projectId);
 
       if (assignee) {
+        if (!ObjectId.isValid(String(assignee))) {
+          throw new UserInputError('assignee must be a valid id or null');
+        }
+
         const userExists = await factories.usersFactory.findById(assignee);
 
         if (!userExists) {
diff --git a/src/resolvers/helpers/bulkEvents.js b/src/resolvers/helpers/bulkEvents.js
index 4d0238a5..50dedb6f 100644
--- a/src/resolvers/helpers/bulkEvents.js
+++ b/src/resolvers/helpers/bulkEvents.js
@@ -2,6 +2,8 @@ const sendPersonalNotification = require('../../utils/personalNotifications').de
 const { UserInputError } = require('apollo-server-express');
 const { ObjectId } = require('mongodb');
 
+const ASSIGNEE_NOTIFICATIONS_CHUNK_SIZE = 25;
+
 /**
  * Enqueue assignee notifications in background (do not block resolver response)
  *
@@ -26,20 +28,40 @@ function fireAndForgetAssigneeNotifications({
     return;
   }
 
-  Promise.allSettled(eventIds.map(eventId => sendPersonalNotification(assigneeData, {
-    type: 'assignee',
-    payload: {
-      assigneeId,
-      projectId,
-      whoAssignedId,
-      eventId,
-    },
-  })))
-    .then((results) => {
-      const failedResults = results.filter(result => result.status === 'rejected');
+  Promise.resolve()
+    .then(async () => {
+      const failedResults = [];
+
+      for (let i = 0; i < eventIds.length; i += ASSIGNEE_NOTIFICATIONS_CHUNK_SIZE) {
+        const chunk = eventIds.slice(i, i + ASSIGNEE_NOTIFICATIONS_CHUNK_SIZE);
+        const results = await Promise.allSettled(chunk.map(eventId => sendPersonalNotification(assigneeData, {
+          type: 'assignee',
+          payload: {
+            assigneeId,
+            projectId,
+            whoAssignedId,
+            eventId,
+          },
+        })));
+
+        failedResults.push(...results.filter(result => result.status === 'rejected'));
+      }
 
       if (failedResults.length > 0) {
-        console.error('Failed to enqueue assignee notifications', failedResults);
+        const failedMessages = failedResults.map((result) => {
+          const reason = result && result.reason;
+
+          if (reason && typeof reason.message === 'string') {
+            return reason.message;
+          }
+
+          return String(reason || 'Unknown error');
+        });
+
+        console.error('Failed to enqueue assignee notifications', {
+          failedCount: failedResults.length,
+          errors: failedMessages,
+        });
       }
     })
     .catch((error) => {
diff --git a/src/typeDefs/event.ts b/src/typeDefs/event.ts
index 849c708a..1d792aff 100644
--- a/src/typeDefs/event.ts
+++ b/src/typeDefs/event.ts
@@ -487,7 +487,7 @@ type BulkUpdateAssigneeResponse {
 }
 
 """
-Result of bulk toggling event marks (resolve / ignore)
+Result of bulk toggling event marks (resolve / ignore / starred)
 """
 type BulkToggleEventMarksResult {
   """
@@ -602,7 +602,8 @@ extend type Mutation {
 
   """
   Toggle the same mark on many original events at once (resolved, ignored or starred).
-  Same toggle semantics as toggleEventMark per event.
+  Uses bulk semantics: if every selected event already has the mark, clear it for all;
+  otherwise set it on each selected event that does not have it yet.
   """
   bulkToggleEventMarks(
     """
diff --git a/test/models/eventsFactory-bulk-visit.test.ts b/test/models/eventsFactory-bulk-visit.test.ts
index d5b19feb..7a1b1627 100644
--- a/test/models/eventsFactory-bulk-visit.test.ts
+++ b/test/models/eventsFactory-bulk-visit.test.ts
@@ -35,7 +35,7 @@ jest.mock('../../src/mongo', () => ({
 // eslint-disable-next-line @typescript-eslint/no-var-requires, @typescript-eslint/no-explicit-any -- CJS class
 const EventsFactory = require('../../src/models/eventsFactory') as any;
 
-describe('EventsFactory.bulkVisitEvent', () => {
+describe('EventsFactory.bulkVisitEvents', () => {
   const projectId = '507f1f77bcf86cd799439011';
 
   beforeEach(() => {
@@ -57,7 +57,7 @@ describe('EventsFactory.bulkVisitEvent', () => {
     });
     collectionMock.updateMany.mockResolvedValue({ modifiedCount: 1 });
 
-    const result = await factory.bulkVisitEvent([ a.toString(), b.toString() ], userId.toString());
+    const result = await factory.bulkVisitEvents([ a.toString(), b.toString() ], userId.toString());
 
     expect(result.updatedCount).toBe(1);
     expect(result.updatedEventIds).toEqual([ b.toString() ]);
@@ -72,7 +72,7 @@ describe('EventsFactory.bulkVisitEvent', () => {
       toArray: () => Promise.resolve([]),
     });
 
-    const result = await factory.bulkVisitEvent([ missing.toString() ], new ObjectId().toString());
+    const result = await factory.bulkVisitEvents([ missing.toString() ], new ObjectId().toString());
 
     expect(result.updatedCount).toBe(0);
     expect(result.updatedEventIds).toEqual([]);
diff --git a/test/resolvers/event-bulk-update-assignee.test.ts b/test/resolvers/event-bulk-update-assignee.test.ts
index 3ab7ce76..9fcc093d 100644
--- a/test/resolvers/event-bulk-update-assignee.test.ts
+++ b/test/resolvers/event-bulk-update-assignee.test.ts
@@ -26,16 +26,17 @@ const eventResolvers = require('../../src/resolvers/event') as {
 };
 
 const bulkUpdateAssignee = jest.fn();
+const ASSIGNEE_ID = '507f1f77bcf86cd799439012';
 
 describe('EventsMutations.bulkUpdateAssignee', () => {
   const ctx = {
     user: { id: 'u1' },
     factories: {
       usersFactory: {
-        findById: jest.fn().mockResolvedValue({ id: 'assignee-1' }),
+        findById: jest.fn().mockResolvedValue({ id: ASSIGNEE_ID }),
         dataLoaders: {
           userById: {
-            load: jest.fn().mockResolvedValue({ id: 'assignee-1', email: 'a@a.a' }),
+            load: jest.fn().mockResolvedValue({ id: ASSIGNEE_ID, email: 'a@a.a' }),
           },
         },
       },
@@ -44,7 +45,7 @@ describe('EventsMutations.bulkUpdateAssignee', () => {
       },
       workspacesFactory: {
         findById: jest.fn().mockResolvedValue({
-          getMemberInfo: jest.fn().mockResolvedValue({ userId: 'assignee-1' }),
+          getMemberInfo: jest.fn().mockResolvedValue({ userId: ASSIGNEE_ID }),
         }),
       },
     },
@@ -66,13 +67,31 @@ describe('EventsMutations.bulkUpdateAssignee', () => {
     await expect(
       eventResolvers.EventsMutations.bulkUpdateAssignee(
         {},
-        { input: { projectId: 'p1', eventIds: [], assignee: 'assignee-1' } },
+        { input: { projectId: 'p1', eventIds: [], assignee: ASSIGNEE_ID } },
         ctx
       )
     ).rejects.toThrow(UserInputError);
     expect(bulkUpdateAssignee).not.toHaveBeenCalled();
   });
 
+  it('should throw when assignee id is invalid', async () => {
+    await expect(
+      eventResolvers.EventsMutations.bulkUpdateAssignee(
+        {},
+        {
+          input: {
+            projectId: 'p1',
+            eventIds: [ '507f1f77bcf86cd799439011' ],
+            assignee: 'not-an-object-id',
+          },
+        },
+        ctx
+      )
+    ).rejects.toThrow(UserInputError);
+
+    expect(bulkUpdateAssignee).not.toHaveBeenCalled();
+  });
+
   it('should call factory for bulk assign', async () => {
     const result = await eventResolvers.EventsMutations.bulkUpdateAssignee(
       {},
@@ -80,7 +99,7 @@ describe('EventsMutations.bulkUpdateAssignee', () => {
         input: {
           projectId: 'p1',
           eventIds: [ '507f1f77bcf86cd799439011' ],
-          assignee: 'assignee-1',
+          assignee: ASSIGNEE_ID,
         },
       },
       ctx
@@ -89,15 +108,15 @@ describe('EventsMutations.bulkUpdateAssignee', () => {
     expect(result.updatedCount).toBe(1);
     expect(bulkUpdateAssignee).toHaveBeenCalledWith(
       [ '507f1f77bcf86cd799439011' ],
-      'assignee-1'
+      ASSIGNEE_ID
     );
     expect(sendPersonalNotification).toHaveBeenCalledTimes(1);
     expect(sendPersonalNotification).toHaveBeenCalledWith(
-      expect.objectContaining({ id: 'assignee-1' }),
+      expect.objectContaining({ id: ASSIGNEE_ID }),
       expect.objectContaining({
         type: 'assignee',
         payload: expect.objectContaining({
-          assigneeId: 'assignee-1',
+          assigneeId: ASSIGNEE_ID,
           projectId: 'p1',
           whoAssignedId: 'u1',
           eventId: '507f1f77bcf86cd799439011',
@@ -119,7 +138,7 @@ describe('EventsMutations.bulkUpdateAssignee', () => {
         input: {
           projectId: 'p1',
           eventIds: [ '507f1f77bcf86cd799439011', 'invalid-id' ],
-          assignee: 'assignee-1',
+          assignee: ASSIGNEE_ID,
         },
       },
       ctx
@@ -127,7 +146,7 @@ describe('EventsMutations.bulkUpdateAssignee', () => {
 
     expect(bulkUpdateAssignee).toHaveBeenCalledWith(
       [ '507f1f77bcf86cd799439011' ],
-      'assignee-1'
+      ASSIGNEE_ID
     );
     expect(result).toEqual({
       updatedCount: 1,
@@ -143,7 +162,7 @@ describe('EventsMutations.bulkUpdateAssignee', () => {
         input: {
           projectId: 'p1',
           eventIds: [ 'bad-1', 'bad-2' ],
-          assignee: 'assignee-1',
+          assignee: ASSIGNEE_ID,
         },
       },
       ctx
diff --git a/test/resolvers/event-bulk-visit.test.ts b/test/resolvers/event-bulk-visit.test.ts
index 632d4a71..e99ce988 100644
--- a/test/resolvers/event-bulk-visit.test.ts
+++ b/test/resolvers/event-bulk-visit.test.ts
@@ -17,7 +17,7 @@ const eventResolvers = require('../../src/resolvers/event') as {
   };
 };
 
-const bulkVisitEvent = jest.fn();
+const bulkVisitEvents = jest.fn();
 
 describe('Mutation.bulkVisitEvents', () => {
   const ctx = {
@@ -26,11 +26,11 @@ describe('Mutation.bulkVisitEvents', () => {
 
   beforeEach(() => {
     jest.clearAllMocks();
-    (getEventsFactory as unknown as jest.Mock).mockReturnValue({ bulkVisitEvent });
+    (getEventsFactory as unknown as jest.Mock).mockReturnValue({ bulkVisitEvents });
   });
 
   it('should call factory with valid ids only and merge invalid ids', async () => {
-    bulkVisitEvent.mockResolvedValue({
+    bulkVisitEvents.mockResolvedValue({
       updatedCount: 1,
       updatedEventIds: [ '507f1f77bcf86cd799439012' ],
       failedEventIds: [ '507f1f77bcf86cd799439099' ],
@@ -42,7 +42,7 @@ describe('Mutation.bulkVisitEvents', () => {
       ctx
     );
 
-    expect(bulkVisitEvent).toHaveBeenCalledWith(
+    expect(bulkVisitEvents).toHaveBeenCalledWith(
       [ '507f1f77bcf86cd799439012' ],
       '507f1f77bcf86cd799439011'
     );
@@ -60,7 +60,7 @@ describe('Mutation.bulkVisitEvents', () => {
       ctx
     );
 
-    expect(bulkVisitEvent).not.toHaveBeenCalled();
+    expect(bulkVisitEvents).not.toHaveBeenCalled();
     expect(result).toEqual({
       updatedCount: 0,
       updatedEventIds: [],