diff --git a/src/__tests__/cookieless.test.ts b/src/__tests__/cookieless.test.ts
index 95f056620..da1f55ba2 100644
--- a/src/__tests__/cookieless.test.ts
+++ b/src/__tests__/cookieless.test.ts
@@ -27,9 +27,9 @@ describe('cookieless', () => {
         expect(event.properties.distinct_id).toBe('$posthog_cklsh')
         expect(event.properties.$anon_distinct_id).toBe(undefined)
         expect(event.properties.$device_id).toBe(null)
-        expect(event.properties.$session_id).toBe(null)
-        expect(event.properties.$window_id).toBe(null)
-        expect(event.properties.$cklsh).toEqual(true)
+        expect(event.properties.$session_id).toBe(undefined)
+        expect(event.properties.$window_id).toBe(undefined)
+        expect(event.properties.$cklsh_mode).toEqual(true)
         expect(document.cookie).toBe('')
 
         // simulate user giving cookie consent
@@ -42,9 +42,9 @@ describe('cookieless', () => {
         expect(event.properties.distinct_id).toBe('$posthog_cklsh')
         expect(event.properties.$anon_distinct_id).toBe(undefined)
         expect(event.properties.$device_id).toBe(null)
-        expect(event.properties.$session_id).toBe(null)
-        expect(event.properties.$window_id).toBe(null)
-        expect(event.properties.$cklsh).toEqual(true)
+        expect(event.properties.$session_id).toBe(undefined)
+        expect(event.properties.$window_id).toBe(undefined)
+        expect(event.properties.$cklsh_mode).toEqual(true)
         expect(document.cookie).not.toBe('')
 
         // a user identifying
@@ -54,9 +54,9 @@ describe('cookieless', () => {
         expect(event.properties.distinct_id).toBe(identifiedDistinctId)
         expect(event.properties.$anon_distinct_id).toBe('$posthog_cklsh')
         expect(event.properties.$device_id).toBe(null)
-        expect(event.properties.$session_id).toBe(null)
-        expect(event.properties.$window_id).toBe(null)
-        expect(event.properties.$cklsh).toEqual(true)
+        expect(event.properties.$session_id).toBe(undefined)
+        expect(event.properties.$window_id).toBe(undefined)
+        expect(event.properties.$cklsh_mode).toEqual(true)
 
         // an event after identifying
         posthog.capture(eventName, eventProperties)
@@ -65,9 +65,9 @@ describe('cookieless', () => {
         expect(event.properties.distinct_id).toBe(identifiedDistinctId)
         expect(event.properties.$anon_distinct_id).toBe(undefined)
         expect(event.properties.$device_id).toBe(null)
-        expect(event.properties.$session_id).toBe(null)
-        expect(event.properties.$window_id).toBe(null)
-        expect(event.properties.$cklsh).toEqual(true)
+        expect(event.properties.$session_id).toBe(undefined)
+        expect(event.properties.$window_id).toBe(undefined)
+        expect(event.properties.$cklsh_mode).toEqual(true)
 
         // reset
         posthog.reset()
@@ -80,9 +80,9 @@ describe('cookieless', () => {
         expect(event.properties.distinct_id).toBe('$posthog_cklsh')
         expect(event.properties.$anon_distinct_id).toBe(undefined)
         expect(event.properties.$device_id).toBe(null)
-        expect(event.properties.$session_id).toBe(null)
-        expect(event.properties.$window_id).toBe(null)
-        expect(event.properties.$cklsh).toEqual(true)
+        expect(event.properties.$session_id).toBe(undefined)
+        expect(event.properties.$window_id).toBe(undefined)
+        expect(event.properties.$cklsh_mode).toEqual(true)
         expect(document.cookie).toBe('')
     })
 })
diff --git a/src/constants.ts b/src/constants.ts
index cbc360d23..34b7a8bad 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -54,6 +54,7 @@ export const TOOLBAR_CONTAINER_CLASS = 'toolbar-global-fade-container'
  * Sentinel value for distinct id, device id, session id. Signals that the server should generate the value
  * */
 export const COOKIELESS_SENTINEL_VALUE = '$posthog_cklsh'
+export const COOKIELESS_MODE_FLAG_PROPERTY = '$cklsh_mode'
 
 export const WEB_EXPERIMENTS = '$web_experiments'
 
diff --git a/src/extensions/replay/sessionrecording.ts b/src/extensions/replay/sessionrecording.ts
index 0200c6832..a94350534 100644
--- a/src/extensions/replay/sessionrecording.ts
+++ b/src/extensions/replay/sessionrecording.ts
@@ -97,8 +97,8 @@ type SessionRecordingStatus = 'disabled' | 'sampled' | 'active' | 'buffering' |
 export interface SnapshotBuffer {
     size: number
     data: any[]
-    sessionId: string | null
-    windowId: string | null
+    sessionId: string
+    windowId: string
 }
 
 interface QueuedRRWebEvent {
@@ -252,8 +252,8 @@ export class SessionRecording {
 
     private _linkedFlagSeen: boolean = false
     private _lastActivityTimestamp: number = Date.now()
-    private windowId: string | null
-    private sessionId: string | null
+    private windowId: string
+    private sessionId: string
     private _linkedFlag: string | FlagVariant | null = null
 
     private _fullSnapshotTimer?: ReturnType<typeof setInterval>
@@ -452,6 +452,9 @@ export class SessionRecording {
             logger.error('started without valid sessionManager')
             throw new Error(LOGGER_PREFIX + ' started without valid sessionManager. This is a bug.')
         }
+        if (this.instance.config.__preview_experimental_cookieless_mode) {
+            throw new Error(LOGGER_PREFIX + ' cannot be used with __preview_experimental_cookieless_mode.')
+        }
 
         // we know there's a sessionManager, so don't need to start without a session id
         const { sessionId, windowId } = this.sessionManager.checkAndGetSessionAndWindowId()
diff --git a/src/posthog-core.ts b/src/posthog-core.ts
index 3bcb564b5..84e42334d 100644
--- a/src/posthog-core.ts
+++ b/src/posthog-core.ts
@@ -20,6 +20,7 @@ import {
     USER_STATE,
     ENABLE_PERSON_PROCESSING,
     COOKIELESS_SENTINEL_VALUE,
+    COOKIELESS_MODE_FLAG_PROPERTY,
 } from './constants'
 import { SessionRecording } from './extensions/replay/sessionrecording'
 import { Decide } from './decide'
@@ -430,16 +431,20 @@ export class PostHog {
         this._retryQueue = new RetryQueue(this)
         this.__request_queue = []
 
-        this.sessionManager = new SessionIdManager(this)
-        this.sessionPropsManager = new SessionPropsManager(this.sessionManager, this.persistence)
+        if (!this.config.__preview_experimental_cookieless_mode) {
+            this.sessionManager = new SessionIdManager(this)
+            this.sessionPropsManager = new SessionPropsManager(this.sessionManager, this.persistence)
+        }
 
         new TracingHeaders(this).startIfEnabledOrStop()
 
         this.siteApps = new SiteApps(this)
         this.siteApps?.init()
 
-        this.sessionRecording = new SessionRecording(this)
-        this.sessionRecording.startIfEnabledOrStop()
+        if (!this.config.__preview_experimental_cookieless_mode) {
+            this.sessionRecording = new SessionRecording(this)
+            this.sessionRecording.startIfEnabledOrStop()
+        }
 
         if (!this.config.disable_scroll_properties) {
             this.scrollManager.startMeasuringScrollPosition()
@@ -926,9 +931,7 @@ export class PostHog {
 
         if (this.config.__preview_experimental_cookieless_mode) {
             // Set a flag to tell the plugin server to use cookieless server hash mode
-            // on naming: $cklsh = cookieless server hash
-            // I didn't call it $cookieless, as it's possible to use cookies in this mode (after consent is given)
-            properties['$cklsh'] = true
+            properties[COOKIELESS_MODE_FLAG_PROPERTY] = true
         }
 
         if (event_name === '$snapshot') {
@@ -946,7 +949,7 @@ export class PostHog {
 
         const infoProperties = Info.properties()
 
-        if (this.sessionManager) {
+        if (this.sessionManager && !this.config.__preview_experimental_cookieless_mode) {
             const { sessionId, windowId } = this.sessionManager.checkAndGetSessionAndWindowId()
             properties['$session_id'] = sessionId
             properties['$window_id'] = windowId
diff --git a/src/sessionid.ts b/src/sessionid.ts
index 0ef93b2b8..43480c4e3 100644
--- a/src/sessionid.ts
+++ b/src/sessionid.ts
@@ -216,12 +216,9 @@ export class SessionIdManager {
      */
     checkAndGetSessionAndWindowId(readOnly = false, _timestamp: number | null = null) {
         if (this.config.__preview_experimental_cookieless_mode) {
-            return {
-                sessionId: null,
-                windowId: null,
-                sessionStartTimestamp: null,
-                lastActivityTimestamp: null,
-            }
+            throw new Error(
+                'checkAndGetSessionAndWindowId should not be called in __preview_experimental_cookieless_mode'
+            )
         }
         const timestamp = _timestamp || new Date().getTime()