diff --git a/packages/core/test/lib/base.test.ts b/packages/core/test/lib/base.test.ts
index 9ccd6ee2b53b..e41dce9e2702 100644
--- a/packages/core/test/lib/base.test.ts
+++ b/packages/core/test/lib/base.test.ts
@@ -888,7 +888,7 @@ describe('BaseClient', () => {
     });
 
     test('calls `beforeSend` and uses original event without any changes', () => {
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSend = jest.fn(event => event);
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSend });
@@ -896,11 +896,12 @@ describe('BaseClient', () => {
 
       client.captureEvent({ message: 'hello' });
 
+      expect(beforeSend).toHaveBeenCalled();
       expect(TestClient.instance!.event!.message).toEqual('hello');
     });
 
     test('calls `beforeSendTransaction` and uses original event without any changes', () => {
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSendTransaction = jest.fn(event => event);
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSendTransaction });
@@ -908,11 +909,12 @@ describe('BaseClient', () => {
 
       client.captureEvent({ transaction: '/dogs/are/great', type: 'transaction' });
 
+      expect(beforeSendTransaction).toHaveBeenCalled();
       expect(TestClient.instance!.event!.transaction).toBe('/dogs/are/great');
     });
 
     test('calls `beforeSend` and uses the modified event', () => {
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSend = jest.fn(event => {
         event.message = 'changed1';
@@ -923,11 +925,12 @@ describe('BaseClient', () => {
 
       client.captureEvent({ message: 'hello' });
 
+      expect(beforeSend).toHaveBeenCalled();
       expect(TestClient.instance!.event!.message).toEqual('changed1');
     });
 
     test('calls `beforeSendTransaction` and uses the modified event', () => {
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSendTransaction = jest.fn(event => {
         event.transaction = '/adopt/dont/shop';
@@ -938,11 +941,12 @@ describe('BaseClient', () => {
 
       client.captureEvent({ transaction: '/dogs/are/great', type: 'transaction' });
 
+      expect(beforeSendTransaction).toHaveBeenCalled();
       expect(TestClient.instance!.event!.transaction).toBe('/adopt/dont/shop');
     });
 
     test('calls `beforeSend` and discards the event', () => {
-      expect.assertions(3);
+      expect.assertions(4);
 
       const beforeSend = jest.fn(() => null);
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSend });
@@ -952,6 +956,7 @@ describe('BaseClient', () => {
 
       client.captureEvent({ message: 'hello' });
 
+      expect(beforeSend).toHaveBeenCalled();
       expect(TestClient.instance!.event).toBeUndefined();
       // This proves that the reason the event didn't send/didn't get set on the test client is not because there was an
       // error, but because `beforeSend` returned `null`
@@ -960,7 +965,7 @@ describe('BaseClient', () => {
     });
 
     test('calls `beforeSendTransaction` and discards the event', () => {
-      expect.assertions(3);
+      expect.assertions(4);
 
       const beforeSendTransaction = jest.fn(() => null);
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSendTransaction });
@@ -970,6 +975,7 @@ describe('BaseClient', () => {
 
       client.captureEvent({ transaction: '/dogs/are/great', type: 'transaction' });
 
+      expect(beforeSendTransaction).toHaveBeenCalled();
       expect(TestClient.instance!.event).toBeUndefined();
       // This proves that the reason the event didn't send/didn't get set on the test client is not because there was an
       // error, but because `beforeSendTransaction` returned `null`
@@ -979,7 +985,7 @@ describe('BaseClient', () => {
 
     test('calls `beforeSend` and logs info about invalid return value', () => {
       const invalidValues = [undefined, false, true, [], 1];
-      expect.assertions(invalidValues.length * 2);
+      expect.assertions(invalidValues.length * 3);
 
       for (const val of invalidValues) {
         const beforeSend = jest.fn(() => val);
@@ -990,6 +996,7 @@ describe('BaseClient', () => {
 
         client.captureEvent({ message: 'hello' });
 
+        expect(beforeSend).toHaveBeenCalled();
         expect(TestClient.instance!.event).toBeUndefined();
         expect(loggerWarnSpy).toBeCalledWith(new SentryError('`beforeSend` must return `null` or a valid event.'));
       }
@@ -997,7 +1004,7 @@ describe('BaseClient', () => {
 
     test('calls `beforeSendTransaction` and logs info about invalid return value', () => {
       const invalidValues = [undefined, false, true, [], 1];
-      expect.assertions(invalidValues.length * 2);
+      expect.assertions(invalidValues.length * 3);
 
       for (const val of invalidValues) {
         const beforeSendTransaction = jest.fn(() => val);
@@ -1008,6 +1015,7 @@ describe('BaseClient', () => {
 
         client.captureEvent({ transaction: '/dogs/are/great', type: 'transaction' });
 
+        expect(beforeSendTransaction).toHaveBeenCalled();
         expect(TestClient.instance!.event).toBeUndefined();
         expect(loggerWarnSpy).toBeCalledWith(
           new SentryError('`beforeSendTransaction` must return `null` or a valid event.'),
@@ -1017,7 +1025,7 @@ describe('BaseClient', () => {
 
     test('calls async `beforeSend` and uses original event without any changes', done => {
       jest.useFakeTimers();
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSend = jest.fn(
         async event =>
@@ -1034,6 +1042,7 @@ describe('BaseClient', () => {
       jest.runOnlyPendingTimers();
 
       TestClient.sendEventCalled = (event: Event) => {
+        expect(beforeSend).toHaveBeenCalled();
         expect(event.message).toEqual('hello');
       };
 
@@ -1046,7 +1055,7 @@ describe('BaseClient', () => {
 
     test('calls async `beforeSendTransaction` and uses original event without any changes', done => {
       jest.useFakeTimers();
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSendTransaction = jest.fn(
         async event =>
@@ -1063,6 +1072,7 @@ describe('BaseClient', () => {
       jest.runOnlyPendingTimers();
 
       TestClient.sendEventCalled = (event: Event) => {
+        expect(beforeSendTransaction).toHaveBeenCalled();
         expect(event.transaction).toBe('/dogs/are/great');
       };
 
@@ -1075,7 +1085,7 @@ describe('BaseClient', () => {
 
     test('calls async `beforeSend` and uses the modified event', done => {
       jest.useFakeTimers();
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSend = jest.fn(async event => {
         event.message = 'changed2';
@@ -1092,6 +1102,7 @@ describe('BaseClient', () => {
       jest.runOnlyPendingTimers();
 
       TestClient.sendEventCalled = (event: Event) => {
+        expect(beforeSend).toHaveBeenCalled();
         expect(event.message).toEqual('changed2');
       };
 
@@ -1104,7 +1115,7 @@ describe('BaseClient', () => {
 
     test('calls async `beforeSendTransaction` and uses the modified event', done => {
       jest.useFakeTimers();
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSendTransaction = jest.fn(async event => {
         event.transaction = '/adopt/dont/shop';
@@ -1121,6 +1132,7 @@ describe('BaseClient', () => {
       jest.runOnlyPendingTimers();
 
       TestClient.sendEventCalled = (event: Event) => {
+        expect(beforeSendTransaction).toHaveBeenCalled();
         expect(event.transaction).toBe('/adopt/dont/shop');
       };
 
@@ -1133,7 +1145,7 @@ describe('BaseClient', () => {
 
     test('calls async `beforeSend` and discards the event', () => {
       jest.useFakeTimers();
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSend = jest.fn(
         async () =>
@@ -1149,12 +1161,13 @@ describe('BaseClient', () => {
       client.captureEvent({ message: 'hello' });
       jest.runAllTimers();
 
+      expect(beforeSend).toHaveBeenCalled();
       expect(TestClient.instance!.event).toBeUndefined();
     });
 
     test('calls async `beforeSendTransaction` and discards the event', () => {
       jest.useFakeTimers();
-      expect.assertions(1);
+      expect.assertions(2);
 
       const beforeSendTransaction = jest.fn(
         async () =>
@@ -1170,11 +1183,12 @@ describe('BaseClient', () => {
       client.captureEvent({ transaction: '/dogs/are/great', type: 'transaction' });
       jest.runAllTimers();
 
+      expect(beforeSendTransaction).toHaveBeenCalled();
       expect(TestClient.instance!.event).toBeUndefined();
     });
 
     test('`beforeSend` gets access to a hint as a second argument', () => {
-      expect.assertions(2);
+      expect.assertions(3);
 
       const beforeSend = jest.fn((event, hint) => ({ ...event, data: hint.data }));
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSend });
@@ -1182,12 +1196,13 @@ describe('BaseClient', () => {
 
       client.captureEvent({ message: 'hello' }, { data: 'someRandomThing' });
 
+      expect(beforeSend).toHaveBeenCalledWith(expect.any(Object), expect.objectContaining({ data: 'someRandomThing' }));
       expect(TestClient.instance!.event!.message).toEqual('hello');
       expect((TestClient.instance!.event! as any).data).toEqual('someRandomThing');
     });
 
     test('`beforeSendTransaction` gets access to a hint as a second argument', () => {
-      expect.assertions(2);
+      expect.assertions(3);
 
       const beforeSendTransaction = jest.fn((event, hint) => ({ ...event, data: hint.data }));
       const options = getDefaultTestClientOptions({ dsn: PUBLIC_DSN, beforeSendTransaction });
@@ -1198,19 +1213,22 @@ describe('BaseClient', () => {
         { data: { dogs: 'yes', cats: 'maybe' } },
       );
 
+      expect(beforeSendTransaction).toHaveBeenCalledWith(
+        expect.any(Object),
+        expect.objectContaining({ data: { dogs: 'yes', cats: 'maybe' } }),
+      );
       expect(TestClient.instance!.event!.transaction).toBe('/dogs/are/great');
       expect((TestClient.instance!.event! as any).data).toEqual({ dogs: 'yes', cats: 'maybe' });
     });
 
     test('`beforeSend` records dropped events', () => {
-      expect.assertions(1);
+      expect.assertions(2);
 
+      const beforeSend = jest.fn(() => null);
       const client = new TestClient(
         getDefaultTestClientOptions({
           dsn: PUBLIC_DSN,
-          beforeSend() {
-            return null;
-          },
+          beforeSend,
         }),
       );
 
@@ -1218,18 +1236,19 @@ describe('BaseClient', () => {
 
       client.captureEvent({ message: 'hello' }, {});
 
+      expect(beforeSend).toHaveBeenCalled();
       expect(recordLostEventSpy).toHaveBeenCalledWith('before_send', 'error');
     });
 
     test('`beforeSendTransaction` records dropped events', () => {
-      expect.assertions(1);
+      expect.assertions(2);
+
+      const beforeSendTransaction = jest.fn(() => null);
 
       const client = new TestClient(
         getDefaultTestClientOptions({
           dsn: PUBLIC_DSN,
-          beforeSendTransaction() {
-            return null;
-          },
+          beforeSendTransaction,
         }),
       );
 
@@ -1237,6 +1256,7 @@ describe('BaseClient', () => {
 
       client.captureEvent({ transaction: '/dogs/are/great', type: 'transaction' });
 
+      expect(beforeSendTransaction).toHaveBeenCalled();
       expect(recordLostEventSpy).toHaveBeenCalledWith('before_send', 'transaction');
     });
 
@@ -1361,6 +1381,7 @@ describe('BaseClient', () => {
         },
       });
 
+      expect(beforeSendTransaction).toHaveBeenCalled();
       expect(TestClient.instance!.event!.transaction).toBe('/adopt/dont/shop');
       expect(TestClient.instance!.event!.transaction_info).toEqual({
         source: 'custom',