diff --git a/packages/browser/src/client/client.ts b/packages/browser/src/client/client.ts
index c848ca02f..a143a371a 100644
--- a/packages/browser/src/client/client.ts
+++ b/packages/browser/src/client/client.ts
@@ -6,6 +6,7 @@ import type { WebSocketBrowserEvents, WebSocketBrowserHandlers } from '../types'
 import type { IframeOrchestrator } from './orchestrator'
 import { createBirpc } from 'birpc'
 import { parse, stringify } from 'flatted'
+import { replaceUndefined, reviveUndefined } from '../shared/serialization'
 import { getBrowserState } from './utils'
 
 const PAGE_TYPE = getBrowserState().type
@@ -134,9 +135,9 @@ function createClient() {
               stack: v.stack,
             }
           }
-          return v
+          return replaceUndefined(v)
         }),
-      deserialize: parse,
+      deserialize: (e) => parse(e, reviveUndefined),
       timeout: -1, // createTesters can take a while
     },
   )
diff --git a/packages/browser/src/client/tester/tester-utils.ts b/packages/browser/src/client/tester/tester-utils.ts
index 61ec06afd..d207242b3 100644
--- a/packages/browser/src/client/tester/tester-utils.ts
+++ b/packages/browser/src/client/tester/tester-utils.ts
@@ -141,7 +141,6 @@ export class CommandsManager {
     const rpc = state.rpc as any as BrowserRPC
     const { sessionId, traces } = getBrowserState()
     const filepath = state.filepath || state.current?.file?.filepath
-    args = args.filter((arg) => arg !== undefined) // remove optional fields
 
     const actionTraceGroupName = ACTION_TRACE_COMMANDS.has(command)
       ? `vitest:${command.slice('__vitest_'.length)}`
@@ -179,7 +178,10 @@ export class CommandsManager {
         let status: BrowserTraceEntryStatus = 'pass'
         const traceRangeId = hasActiveTraceView ? createBrowserTraceRangeId() : undefined
         const element =
-          typeof args[0] === 'object' && 'selector' in args[0] && 'locator' in args[0]
+          typeof args[0] === 'object' &&
+          args[0] !== null &&
+          'selector' in args[0] &&
+          'locator' in args[0]
             ? args[0]
             : undefined
         if (hasActiveTraceView) {
diff --git a/packages/browser/src/node/rpc.ts b/packages/browser/src/node/rpc.ts
index a252a9819..7139d0da8 100644
--- a/packages/browser/src/node/rpc.ts
+++ b/packages/browser/src/node/rpc.ts
@@ -32,6 +32,7 @@ import {
   isValidApiRequest,
 } from 'vitest/node'
 import { WebSocketServer } from 'ws'
+import { replaceUndefined, reviveUndefined } from '../shared/serialization'
 import { slash } from './utils'
 
 const debug = createDebugger('vitest:browser:api')
@@ -538,7 +539,7 @@ export function setupBrowserRpc(
         on: (fn) => ws.on('message', fn),
         eventNames: ['onCancel', 'cdpEvent'],
         serialize: (data: any) => stringify(data, stringifyReplace),
-        deserialize: parse,
+        deserialize: (data) => parse(data, reviveUndefined),
         timeout: -1, // createTesters can take a long time
       },
     )
@@ -574,6 +575,6 @@ function stringifyReplace(key: string, value: any): any {
       ...cloned,
     }
   } else {
-    return value
+    return replaceUndefined(value)
   }
 }
diff --git a/packages/browser/src/shared/serialization.ts b/packages/browser/src/shared/serialization.ts
new file mode 100644
index 000000000..4046c846a
--- /dev/null
+++ b/packages/browser/src/shared/serialization.ts
@@ -0,0 +1,10 @@
+// JSON drops `undefined` in objects and turns it into `null` in arrays
+const UNDEFINED_VALUE = '__vitest_undefined__'
+
+export function replaceUndefined(value: unknown): unknown {
+  return value === undefined ? UNDEFINED_VALUE : value
+}
+
+export function reviveUndefined(_key: string, value: unknown): unknown {
+  return value === UNDEFINED_VALUE ? undefined : value
+}
diff --git a/test/browser/test/commands.test.ts b/test/browser/test/commands.test.ts
index dd323e12f..3484f1f10 100644
--- a/test/browser/test/commands.test.ts
+++ b/test/browser/test/commands.test.ts
@@ -41,22 +41,47 @@ it('can manipulate files', async () => {
 })
 
 it('can run custom commands', async () => {
-  const result = await myCustomCommand('arg1', 'arg2')
-  expect(result).toEqual({
-    testPath: expect.stringMatching('test/browser/test/commands.test.ts'),
-    arg1: 'arg1',
-    arg2: 'arg2',
-  })
+  {
+    const result = await myCustomCommand('arg1', 'arg2')
+    expect(result).toEqual({
+      testPath: expect.stringMatching('test/browser/test/commands.test.ts'),
+      arg1: 'arg1',
+      arg2: 'arg2',
+    })
+  }
+
+  {
+    const result = await myCustomCommand(undefined, 'arg2')
+    expect(result).toEqual({
+      testPath: expect.stringMatching('test/browser/test/commands.test.ts'),
+      arg1: undefined,
+      arg2: 'arg2',
+    })
+  }
+
+  {
+    const result = await myCustomCommand(null, 'arg2')
+    expect(result).toEqual({
+      testPath: expect.stringMatching('test/browser/test/commands.test.ts'),
+      arg1: null,
+      arg2: 'arg2',
+    })
+  }
+
+  {
+    const result = await myCustomCommand([undefined, { value: undefined }], 'arg2')
+    expect(result.arg1).toStrictEqual([undefined, { value: undefined }])
+  }
 })
 
 declare module 'vitest/browser' {
   interface BrowserCommands {
     myCustomCommand: (
-      arg1: string,
+      arg1: unknown,
       arg2: string,
     ) => Promise<{
       testPath: string
-      arg1: string
+      arg1: unknown
       arg2: string
     }>