diff --git a/extension/src/app/api/importState.ts b/extension/src/app/api/importState.ts
index 74f8da51..bacd30b9 100644
--- a/extension/src/app/api/importState.ts
+++ b/extension/src/app/api/importState.ts
@@ -1,6 +1,11 @@
 import mapValues from 'lodash/mapValues';
 import jsan from 'jsan';
 import seralizeImmutable from '@redux-devtools/serialize/lib/immutable/serialize';
+import {
+  Config,
+  SerializeWithImmutable,
+} from '../../browser/extension/inject/pageScript';
+import Immutable from 'immutable';
 
 function deprecate(param: string) {
   // eslint-disable-next-line no-console
@@ -9,14 +14,34 @@ function deprecate(param: string) {
   );
 }
 
+interface SerializeWithRequiredImmutable extends SerializeWithImmutable {
+  readonly immutable: typeof Immutable;
+}
+
+function isSerializeWithImmutable(
+  serialize: boolean | SerializeWithImmutable
+): serialize is SerializeWithRequiredImmutable {
+  return !!(serialize as SerializeWithImmutable).immutable;
+}
+
+interface SerializeWithRequiredReviver extends SerializeWithImmutable {
+  readonly reviver: (key: string, value: unknown) => unknown;
+}
+
+function isSerializeWithReviver(
+  serialize: boolean | SerializeWithImmutable
+): serialize is SerializeWithRequiredReviver {
+  return !!(serialize as SerializeWithImmutable).immutable;
+}
+
 export default function importState(
-  state,
-  { deserializeState, deserializeAction, serialize }
+  state: string | undefined,
+  { deserializeState, deserializeAction, serialize }: Config
 ) {
   if (!state) return undefined;
   let parse = jsan.parse;
   if (serialize) {
-    if (serialize.immutable) {
+    if (isSerializeWithImmutable(serialize)) {
       parse = (v) =>
         jsan.parse(
           v,
@@ -27,7 +52,7 @@ export default function importState(
             serialize.reviver
           ).reviver
         );
-    } else if (serialize.reviver) {
+    } else if (isSerializeWithReviver(serialize)) {
       parse = (v) => jsan.parse(v, serialize.reviver);
     }
   }
diff --git a/extension/src/app/api/index.ts b/extension/src/app/api/index.ts
index a3a7d8ed..d10768f5 100644
--- a/extension/src/app/api/index.ts
+++ b/extension/src/app/api/index.ts
@@ -9,8 +9,14 @@ import { Config } from '../../browser/extension/inject/pageScript';
 import { Action } from 'redux';
 import { LiftedState, PerformAction } from '@redux-devtools/instrument';
 import { LibConfig } from '@redux-devtools/app/lib/actions';
+import { ContentScriptToPageScriptMessage } from '../../browser/extension/inject/contentScript';
+import { Position } from './openWindow';
 
-const listeners = {};
+const listeners: {
+  [instanceId: string]:
+    | ((message: ContentScriptToPageScriptMessage) => void)
+    | ((message: ContentScriptToPageScriptMessage) => void)[];
+} = {};
 export const source = '@devtools-page';
 
 function windowReplacer(key: string, value: unknown) {
@@ -129,15 +135,15 @@ interface InitMessage<S, A extends Action<unknown>> {
   libConfig?: LibConfig;
 }
 
-interface SerializedPartialLiftedState<S, A extends Action<unknown>> {
+interface SerializedPartialLiftedState {
   readonly stagedActionIds: readonly number[];
   readonly currentStateIndex: number;
   readonly nextActionId: number;
 }
 
-interface SerializedPartialStateMessage<S, A extends Action<unknown>> {
+interface SerializedPartialStateMessage {
   readonly type: 'PARTIAL_STATE';
-  readonly payload: SerializedPartialLiftedState<S, A>;
+  readonly payload: SerializedPartialLiftedState;
   readonly source: typeof source;
   readonly instanceId: number;
   readonly maxAge: number;
@@ -177,27 +183,41 @@ interface SerializedStateMessage<S, A extends Action<unknown>> {
   readonly computedStates: string;
   readonly committedState: boolean;
 }
-export type PageScriptToContentScriptMessageWithoutDisconnectOrInitInstance<
+
+interface OpenMessage {
+  readonly source: typeof source;
+  readonly type: 'OPEN';
+  readonly position: Position;
+}
+
+export type PageScriptToContentScriptMessageForwardedToMonitors<
   S,
   A extends Action<unknown>
 > =
   | InitMessage<S, A>
   | LiftedMessage
-  | SerializedPartialStateMessage<S, A>
+  | SerializedPartialStateMessage
   | SerializedExportMessage
   | SerializedActionMessage
-  | SerializedStateMessage<S, A>
+  | SerializedStateMessage<S, A>;
+
+export type PageScriptToContentScriptMessageWithoutDisconnectOrInitInstance<
+  S,
+  A extends Action<unknown>
+> =
+  | PageScriptToContentScriptMessageForwardedToMonitors<S, A>
   | ErrorMessage
-  | InitInstanceMessage
   | GetReportMessage
-  | StopMessage;
+  | StopMessage
+  | OpenMessage;
 
 export type PageScriptToContentScriptMessageWithoutDisconnect<
   S,
   A extends Action<unknown>
 > =
   | PageScriptToContentScriptMessageWithoutDisconnectOrInitInstance<S, A>
-  | InitInstancePageScriptToContentScriptMessage;
+  | InitInstancePageScriptToContentScriptMessage
+  | InitInstanceMessage;
 
 export type PageScriptToContentScriptMessage<S, A extends Action<unknown>> =
   | PageScriptToContentScriptMessageWithoutDisconnect<S, A>
@@ -262,7 +282,7 @@ function amendActionType(
   return { action, timestamp, stack };
 }
 
-export interface LiftedMessage {
+interface LiftedMessage {
   readonly type: 'LIFTED';
   readonly liftedState: { readonly isPaused: boolean | undefined };
   readonly instanceId: number;
@@ -303,7 +323,7 @@ interface StateMessage<S, A extends Action<unknown>> {
   readonly libConfig?: LibConfig;
 }
 
-interface ErrorMessage {
+export interface ErrorMessage {
   readonly type: 'ERROR';
   readonly payload: unknown;
   readonly source: typeof source;
@@ -412,7 +432,7 @@ export function sendMessage(
   toContentScript(message, config.serialize, config.serialize);
 }
 
-function handleMessages(event) {
+function handleMessages(event: MessageEvent<ContentScriptToPageScriptMessage>) {
   if (process.env.BABEL_ENV !== 'test' && (!event || event.source !== window)) {
     return;
   }
@@ -420,33 +440,38 @@ function handleMessages(event) {
   if (!message || message.source !== '@devtools-extension') return;
   Object.keys(listeners).forEach((id) => {
     if (message.id && id !== message.id) return;
-    if (typeof listeners[id] === 'function') listeners[id](message);
+    const listenersForId = listeners[id];
+    if (typeof listenersForId === 'function') listenersForId(message);
     else {
-      listeners[id].forEach((fn) => {
+      listenersForId.forEach((fn) => {
         fn(message);
       });
     }
   });
 }
 
-export function setListener(onMessage, instanceId) {
+export function setListener(
+  onMessage: (message: ContentScriptToPageScriptMessage) => void,
+  instanceId: number
+) {
   listeners[instanceId] = onMessage;
   window.addEventListener('message', handleMessages, false);
 }
 
-const liftListener = (listener, config) => (message) => {
-  let data = {};
-  if (message.type === 'IMPORT') {
-    data.type = 'DISPATCH';
-    data.payload = {
-      type: 'IMPORT_STATE',
-      ...importState(message.state, config),
-    };
-  } else {
-    data = message;
-  }
-  listener(data);
-};
+const liftListener =
+  (listener, config: Config) => (message: ContentScriptToPageScriptMessage) => {
+    let data = {};
+    if (message.type === 'IMPORT') {
+      data.type = 'DISPATCH';
+      data.payload = {
+        type: 'IMPORT_STATE',
+        ...importState(message.state, config),
+      };
+    } else {
+      data = message;
+    }
+    listener(data);
+  };
 
 export function disconnect() {
   window.removeEventListener('message', handleMessages);
@@ -471,7 +496,7 @@ export function connect(preConfig: Config) {
   let delayedActions = [];
   let delayedStates = [];
 
-  const rootListener = (action) => {
+  const rootListener = (action: ContentScriptToPageScriptMessage) => {
     if (autoPause) {
       if (action.type === 'START') isPaused = false;
       else if (action.type === 'STOP') isPaused = true;
@@ -495,11 +520,14 @@ export function connect(preConfig: Config) {
   const subscribe = (listener) => {
     if (!listener) return undefined;
     const liftedListener = liftListener(listener, config);
-    listeners[id].push(liftedListener);
+    const listenersForId = listeners[id] as ((
+      message: ContentScriptToPageScriptMessage
+    ) => void)[];
+    listenersForId.push(liftedListener);
 
     return function unsubscribe() {
-      const index = listeners[id].indexOf(liftedListener);
-      listeners[id].splice(index, 1);
+      const index = listenersForId.indexOf(liftedListener);
+      listenersForId.splice(index, 1);
     };
   };
 
diff --git a/extension/src/app/api/notifyErrors.ts b/extension/src/app/api/notifyErrors.ts
index 8b226945..569cbb3a 100644
--- a/extension/src/app/api/notifyErrors.ts
+++ b/extension/src/app/api/notifyErrors.ts
@@ -1,4 +1,4 @@
-let handleError: () => boolean;
+let handleError: (() => boolean) | undefined;
 let lastTime = 0;
 
 function createExpBackoffTimer(step: number) {
@@ -42,7 +42,7 @@ function catchErrors(e: ErrorEvent) {
   postError(e.message);
 }
 
-export default function notifyErrors(onError: () => boolean) {
+export default function notifyErrors(onError?: () => boolean) {
   handleError = onError;
   window.addEventListener('error', catchErrors, false);
 }
diff --git a/extension/src/app/api/openWindow.ts b/extension/src/app/api/openWindow.ts
index c997d104..51f9d145 100644
--- a/extension/src/app/api/openWindow.ts
+++ b/extension/src/app/api/openWindow.ts
@@ -1,12 +1,18 @@
+import { Action } from 'redux';
+import { PageScriptToContentScriptMessage } from './index';
+
 export type Position = 'left' | 'right' | 'bottom' | 'panel' | 'remote';
 
-export default function openWindow(position?: Position) {
-  window.postMessage(
-    {
-      source: '@devtools-page',
-      type: 'OPEN',
-      position: position || 'right',
-    },
-    '*'
-  );
+function post<S, A extends Action<unknown>>(
+  message: PageScriptToContentScriptMessage<S, A>
+) {
+  window.postMessage(message, '*');
+}
+
+export default function openWindow(position?: Position) {
+  post({
+    source: '@devtools-page',
+    type: 'OPEN',
+    position: position || 'right',
+  });
 }
diff --git a/extension/src/app/middlewares/api.ts b/extension/src/app/middlewares/api.ts
index 7625c8d9..7e605729 100644
--- a/extension/src/app/middlewares/api.ts
+++ b/extension/src/app/middlewares/api.ts
@@ -15,11 +15,21 @@ import { getReport } from '../../browser/extension/background/logging';
 import {
   CustomAction,
   DispatchAction as AppDispatchAction,
+  LibConfig,
   LiftedActionAction,
   StoreAction,
 } from '@redux-devtools/app/lib/actions';
 import { Action, Dispatch } from 'redux';
-import { ContentScriptToBackgroundMessage } from '../../browser/extension/inject/contentScript';
+import {
+  ContentScriptToBackgroundMessage,
+  SplitMessage,
+} from '../../browser/extension/inject/contentScript';
+import {
+  ErrorMessage,
+  PageScriptToContentScriptMessageForwardedToMonitors,
+  PageScriptToContentScriptMessageWithoutDisconnectOrInitInstance,
+} from '../api';
+import { LiftedState } from '@redux-devtools/instrument';
 
 interface TabMessageBase {
   readonly type: string;
@@ -72,8 +82,85 @@ interface NAAction {
   readonly id: string;
 }
 
-interface UpdateStateAction {
+interface InitMessage<S, A extends Action<unknown>> {
+  readonly type: 'INIT';
+  readonly payload: string;
+  readonly instanceId: string;
+  readonly source: '@devtools-page';
+  action?: string;
+  name?: string | undefined;
+  liftedState?: LiftedState<S, A, unknown>;
+  libConfig?: LibConfig;
+}
+
+interface LiftedMessage {
+  readonly type: 'LIFTED';
+  readonly liftedState: { readonly isPaused: boolean | undefined };
+  readonly instanceId: string;
+  readonly source: '@devtools-page';
+}
+
+interface SerializedPartialLiftedState {
+  readonly stagedActionIds: readonly number[];
+  readonly currentStateIndex: number;
+  readonly nextActionId: number;
+}
+
+interface SerializedPartialStateMessage {
+  readonly type: 'PARTIAL_STATE';
+  readonly payload: SerializedPartialLiftedState;
+  readonly source: '@devtools-page';
+  readonly instanceId: string;
+  readonly maxAge: number;
+  readonly actionsById: string;
+  readonly computedStates: string;
+  readonly committedState: boolean;
+}
+
+interface SerializedExportMessage {
+  readonly type: 'EXPORT';
+  readonly payload: string;
+  readonly committedState: string | undefined;
+  readonly source: '@devtools-page';
+  readonly instanceId: string;
+}
+
+interface SerializedActionMessage {
+  readonly type: 'ACTION';
+  readonly payload: string;
+  readonly source: '@devtools-page';
+  readonly instanceId: string;
+  readonly action: string;
+  readonly maxAge: number;
+  readonly nextActionId: number;
+}
+
+interface SerializedStateMessage<S, A extends Action<unknown>> {
+  readonly type: 'STATE';
+  readonly payload: Omit<
+    LiftedState<S, A, unknown>,
+    'actionsById' | 'computedStates' | 'committedState'
+  >;
+  readonly source: '@devtools-page';
+  readonly instanceId: string;
+  readonly libConfig?: LibConfig;
+  readonly actionsById: string;
+  readonly computedStates: string;
+  readonly committedState: boolean;
+}
+
+type UpdateStateRequest<S, A extends Action<unknown>> =
+  | InitMessage<S, A>
+  | LiftedMessage
+  | SerializedPartialStateMessage
+  | SerializedExportMessage
+  | SerializedActionMessage
+  | SerializedStateMessage<S, A>;
+
+interface UpdateStateAction<S, A extends Action<unknown>> {
   readonly type: typeof UPDATE_STATE;
+  readonly request?: UpdateStateRequest<S, A>;
+  readonly id?: string | number;
 }
 
 export type TabMessage =
@@ -84,17 +171,27 @@ export type TabMessage =
   | ImportAction
   | ActionAction
   | ExportAction;
-type PanelMessage = NAAction;
-type MonitorMessage = UpdateStateAction;
+type PanelMessage<S, A extends Action<unknown>> =
+  | NAAction
+  | ErrorMessage
+  | UpdateStateAction<S, A>;
+type MonitorMessage<S, A extends Action<unknown>> =
+  | NAAction
+  | ErrorMessage
+  | UpdateStateAction<S, A>;
 
 type TabPort = Omit<chrome.runtime.Port, 'postMessage'> & {
   postMessage: (message: TabMessage) => void;
 };
 type PanelPort = Omit<chrome.runtime.Port, 'postMessage'> & {
-  postMessage: (message: PanelMessage) => void;
+  postMessage: <S, A extends Action<unknown>>(
+    message: PanelMessage<S, A>
+  ) => void;
 };
 type MonitorPort = Omit<chrome.runtime.Port, 'postMessage'> & {
-  postMessage: (message: MonitorMessage) => void;
+  postMessage: <S, A extends Action<unknown>>(
+    message: MonitorMessage<S, A>
+  ) => void;
 };
 
 const CONNECTED = 'socket/CONNECTED';
@@ -108,17 +205,25 @@ const connections: {
   panel: {},
   monitor: {},
 };
-const chunks = {};
+const chunks: {
+  [instanceId: string]: PageScriptToContentScriptMessageForwardedToMonitors<
+    unknown,
+    Action<unknown>
+  >;
+} = {};
 let monitors = 0;
 let isMonitored = false;
 
 const getId = (sender: chrome.runtime.MessageSender, name?: string) =>
   sender.tab ? sender.tab.id! : name || sender.id!;
 
-type MonitorAction = NAAction;
+type MonitorAction<S, A extends Action<unknown>> =
+  | NAAction
+  | ErrorMessage
+  | UpdateStateAction<S, A>;
 
-function toMonitors(
-  action: MonitorAction,
+function toMonitors<S, A extends Action<unknown>>(
+  action: MonitorAction<S, A>,
   tabId?: string | number,
   verbose?: boolean
 ) {
@@ -195,12 +300,14 @@ function togglePersist() {
   }
 }
 
-type BackgroundStoreMessage = unknown;
+type BackgroundStoreMessage<S, A extends Action<unknown>> =
+  | PageScriptToContentScriptMessageWithoutDisconnectOrInitInstance<S, A>
+  | SplitMessage;
 type BackgroundStoreResponse = { readonly options: Options };
 
 // Receive messages from content scripts
-function messaging(
-  request: BackgroundStoreMessage,
+function messaging<S, A extends Action<unknown>>(
+  request: BackgroundStoreMessage<S, A>,
   sender: chrome.runtime.MessageSender,
   sendResponse?: (response?: BackgroundStoreResponse) => void
 ) {
@@ -259,9 +366,13 @@ function messaging(
     return;
   }
 
-  const action = { type: UPDATE_STATE, request, id: tabId };
+  const action: UpdateStateAction<S, A> = {
+    type: UPDATE_STATE,
+    request,
+    id: tabId,
+  };
   const instanceId = `${tabId}/${request.instanceId}`;
-  if (request.split) {
+  if ('split' in request) {
     if (request.split === 'start') {
       chunks[instanceId] = request;
       return;
diff --git a/extension/src/browser/extension/background/logging.ts b/extension/src/browser/extension/background/logging.ts
index bc5a231c..38735253 100644
--- a/extension/src/browser/extension/background/logging.ts
+++ b/extension/src/browser/extension/background/logging.ts
@@ -1,6 +1,10 @@
 import { LIFTED_ACTION } from '@redux-devtools/app/lib/constants/actionTypes';
 
-export function getReport(reportId, tabId, instanceId) {
+export function getReport(
+  reportId: string,
+  tabId: string | number,
+  instanceId: number
+) {
   chrome.storage.local.get(['s:hostname', 's:port', 's:secure'], (options) => {
     if (!options['s:hostname'] || !options['s:port']) return;
     const url = `${options['s:secure'] ? 'https' : 'http'}://${
diff --git a/extension/src/browser/extension/devpanel/index.tsx b/extension/src/browser/extension/devpanel/index.tsx
index 5b58b884..acf68e36 100644
--- a/extension/src/browser/extension/devpanel/index.tsx
+++ b/extension/src/browser/extension/devpanel/index.tsx
@@ -18,7 +18,7 @@ const messageStyle: CSSProperties = {
   textAlign: 'center',
 };
 
-let rendered: boolean;
+let rendered: boolean | undefined;
 let store: Store<StoreState, StoreAction> | undefined;
 let bgConnection: chrome.runtime.Port;
 let naTimeout: NodeJS.Timeout;
diff --git a/extension/src/browser/extension/inject/contentScript.ts b/extension/src/browser/extension/inject/contentScript.ts
index 671a7ae9..f163ddff 100644
--- a/extension/src/browser/extension/inject/contentScript.ts
+++ b/extension/src/browser/extension/inject/contentScript.ts
@@ -158,6 +158,8 @@ interface SplitMessageBase {
 }
 
 interface SplitMessageStart extends SplitMessageBase {
+  readonly instanceId: number;
+  readonly source: typeof pageSource;
   readonly split: 'start';
 }
 
@@ -174,7 +176,10 @@ interface SplitMessageEnd extends SplitMessageBase {
   readonly split: 'end';
 }
 
-type SplitMessage = SplitMessageStart | SplitMessageChunk | SplitMessageEnd;
+export type SplitMessage =
+  | SplitMessageStart
+  | SplitMessageChunk
+  | SplitMessageEnd;
 
 function tryCatch<S, A extends Action<unknown>>(
   fn: (
diff --git a/extension/src/browser/extension/inject/pageScript.ts b/extension/src/browser/extension/inject/pageScript.ts
index 3fbf7c5c..930f655e 100644
--- a/extension/src/browser/extension/inject/pageScript.ts
+++ b/extension/src/browser/extension/inject/pageScript.ts
@@ -36,11 +36,7 @@ import {
   getSerializeParameter,
   Serialize,
 } from '../../../app/api';
-import {
-  LiftedAction,
-  LiftedState,
-  PerformAction,
-} from '@redux-devtools/instrument';
+import { LiftedAction, LiftedState } from '@redux-devtools/instrument';
 import {
   CustomAction,
   DispatchAction,
@@ -63,7 +59,7 @@ function deprecateParam(oldParam: string, newParam: string) {
   /* eslint-enable no-console */
 }
 
-interface SerializeWithImmutable extends Serialize {
+export interface SerializeWithImmutable extends Serialize {
   readonly immutable?: typeof Immutable;
   readonly refs?: (new (data: any) => unknown)[] | null;
 }
@@ -81,12 +77,12 @@ export interface ConfigWithExpandedMaxAge {
     | boolean
     | ((key: string, value: unknown) => unknown)
     | Serialize;
-  readonly statesFilter?: <S>(state: S, index: number) => S;
+  readonly statesFilter?: <S>(state: S, index?: number) => S;
   readonly actionsFilter?: <A extends Action<unknown>>(
     action: A,
     id: number
   ) => A;
-  readonly stateSanitizer?: <S>(state: S, index: number) => S;
+  readonly stateSanitizer?: <S>(state: S, index?: number) => S;
   readonly actionSanitizer?: <A extends Action<unknown>>(
     action: A,
     id: number
@@ -117,6 +113,7 @@ export interface ConfigWithExpandedMaxAge {
   name?: string;
   readonly autoPause?: boolean;
   readonly features?: Features;
+  readonly type?: string;
 }
 
 export interface Config extends ConfigWithExpandedMaxAge {
@@ -131,7 +128,7 @@ interface ReduxDevtoolsExtension {
   ): Store<S, A>;
   (config?: Config): StoreEnhancer;
   open: (position?: Position) => void;
-  notifyErrors: (onError: () => boolean) => void;
+  notifyErrors: (onError?: () => boolean) => void;
   disconnect: () => void;
 }