diff --git a/packages/core/client.d.ts b/packages/core/client.d.ts
index d869ee50b..b4fc339f9 100644
--- a/packages/core/client.d.ts
+++ b/packages/core/client.d.ts
@@ -24,7 +24,11 @@ interface SessionDeliveryPayload {
   notifier?: Notifier
   device?: Device
   app?: App
-  sessions?: Session[]
+  sessions?: Array<{
+    id: string
+    startedAt: Date
+    user?: User
+  }>
 }
 
 interface Delivery {
@@ -32,6 +36,12 @@ interface Delivery {
   sendSession(session: SessionDeliveryPayload, cb: (err?: Error | null) => void): void
 }
 
+export interface SessionDelegate {
+  startSession: (client: ClientWithInternals, session: Session) => ClientWithInternals
+  pauseSession: (client: ClientWithInternals) => void
+  resumeSession: (client: ClientWithInternals) => ClientWithInternals
+}
+
 /**
  * Extend the public type definitions with internal declarations.
  *
@@ -58,11 +68,7 @@ export default class ClientWithInternals<T extends Config = Config> extends Clie
   _session: Session | null
   _pausedSession: Session | null
 
-  _sessionDelegate: {
-    startSession: (client: ClientWithInternals, session: Session) => ClientWithInternals
-    pauseSession: (client: ClientWithInternals) => void
-    resumeSession: (client: ClientWithInternals) => ClientWithInternals
-  }
+  _sessionDelegate: SessionDelegate
 
   _addOnSessionPayload: (cb: (sessionPayload: Session) => void) => void
 
diff --git a/packages/plugin-browser-session/src/session.ts b/packages/plugin-browser-session/src/session.ts
index 8036e84d4..8dab60296 100644
--- a/packages/plugin-browser-session/src/session.ts
+++ b/packages/plugin-browser-session/src/session.ts
@@ -1,30 +1,14 @@
-import { Client, Logger, Plugin, Session } from '@bugsnag/core'
+import { Plugin, Session } from '@bugsnag/core'
+import ClientWithInternals, { Notifier } from '@bugsnag/core/client'
 import includes from '@bugsnag/core/lib/es-utils/includes'
 
-interface SessionDelegate {
-  startSession: (client: InternalClient, session: Session) => InternalClient
-  resumeSession: (client: { _session: any, _pausedSession: null, startSession: () => any}) => any
-  pauseSession: (client: { _pausedSession: any, _session: null }) => void
-}
-
-interface InternalClient extends Client {
-  _config: {
-    enabledReleaseStages: string[] | null
-    releaseStage: string
-    releaseStages: string[]
-  }
-  _delivery: any
-  _logger: Logger
-  _notifier: any
-  _session: Session
-  _sessionDelegate: SessionDelegate
-  _pausedSession: any
+interface InternalClient extends ClientWithInternals {
+  _notifier?: Notifier
 }
 
 const plugin: Plugin = {
   load: client => {
-    const internalClient = client as InternalClient
-    internalClient._sessionDelegate = sessionDelegate
+    (client as InternalClient)._sessionDelegate = sessionDelegate
   }
 }
 
@@ -35,7 +19,7 @@ const sessionDelegate = {
     sessionClient._pausedSession = null
 
     // exit early if the current releaseStage is not enabled
-    if (sessionClient._config.enabledReleaseStages !== null && !includes(sessionClient._config.enabledReleaseStages, sessionClient._config.releaseStage)) {
+    if (sessionClient._config.enabledReleaseStages && !includes(sessionClient._config.enabledReleaseStages, sessionClient._config.releaseStage)) {
       sessionClient._logger.warn('Session not sent due to releaseStage/enabledReleaseStages configuration')
       return sessionClient
     }
@@ -51,10 +35,10 @@ const sessionDelegate = {
           user: session._user
         }
       ]
-    })
+    }, () => {})
     return sessionClient
   },
-  resumeSession: (client: { _session: any, _pausedSession: null, startSession: () => any }) => {
+  resumeSession: (client: InternalClient) => {
     // Do nothing if there's already an active session
     if (client._session) {
       return client
@@ -71,7 +55,7 @@ const sessionDelegate = {
     // Otherwise start a new session
     return client.startSession()
   },
-  pauseSession: (client: { _pausedSession: any, _session: null }) => {
+  pauseSession: (client: InternalClient) => {
     client._pausedSession = client._session
     client._session = null
   }