diff --git a/packages/dev-middleware/src/inspector-proxy/Device.js b/packages/dev-middleware/src/inspector-proxy/Device.js
index 9666fa222ed927..fba8ecd13ce622 100644
--- a/packages/dev-middleware/src/inspector-proxy/Device.js
+++ b/packages/dev-middleware/src/inspector-proxy/Device.js
@@ -69,7 +69,7 @@ export default class Device {
   _deviceSocket: WS;
 
   // Stores last list of device's pages.
-  _pages: Array<Page>;
+  _pages: $ReadOnlyArray<Page>;
 
   // Stores information about currently connected debugger (if any).
   _debuggerConnection: ?DebuggerInfo = null;
@@ -152,7 +152,7 @@ export default class Device {
     return this._app;
   }
 
-  getPagesList(): Array<Page> {
+  getPagesList(): $ReadOnlyArray<Page> {
     if (this._lastConnectedReactNativePage) {
       const reactNativeReloadablePage = {
         id: REACT_NATIVE_RELOADABLE_PAGE_ID,
@@ -216,18 +216,18 @@ export default class Device {
         pageId: this._debuggerConnection?.pageId ?? null,
         frontendUserAgent: metadata.userAgent,
       });
-      const handled = this._interceptMessageFromDebugger(
+      const processedReq = this._interceptMessageFromDebugger(
         debuggerRequest,
         debuggerInfo,
         socket,
       );
 
-      if (!handled) {
+      if (processedReq) {
         this._sendMessageToDevice({
           event: 'wrappedEvent',
           payload: {
             pageId: this._mapToDevicePageId(pageId),
-            wrappedEvent: JSON.stringify(debuggerRequest),
+            wrappedEvent: JSON.stringify(processedReq),
           },
         });
       }
@@ -545,46 +545,51 @@ export default class Device {
     req: DebuggerRequest,
     debuggerInfo: DebuggerInfo,
     socket: WS,
-  ): boolean {
+  ): ?DebuggerRequest {
     if (req.method === 'Debugger.setBreakpointByUrl') {
-      this._processDebuggerSetBreakpointByUrl(req, debuggerInfo);
+      return this._processDebuggerSetBreakpointByUrl(req, debuggerInfo);
     } else if (req.method === 'Debugger.getScriptSource') {
       this._processDebuggerGetScriptSource(req, socket);
-      return true;
+      return null;
     }
-    return false;
+    return req;
   }
 
   _processDebuggerSetBreakpointByUrl(
     req: SetBreakpointByUrlRequest,
     debuggerInfo: DebuggerInfo,
-  ) {
+  ): SetBreakpointByUrlRequest {
     // If we replaced Android emulator's address to localhost we need to change it back.
     if (debuggerInfo.originalSourceURLAddress != null) {
-      if (req.params.url != null) {
-        req.params.url = req.params.url.replace(
+      const processedReq = {...req, params: {...req.params}};
+      if (processedReq.params.url != null) {
+        processedReq.params.url = processedReq.params.url.replace(
           'localhost',
           debuggerInfo.originalSourceURLAddress,
         );
 
         if (
-          req.params.url &&
-          req.params.url.startsWith(FILE_PREFIX) &&
+          processedReq.params.url &&
+          processedReq.params.url.startsWith(FILE_PREFIX) &&
           debuggerInfo.prependedFilePrefix
         ) {
           // Remove fake URL prefix if we modified URL in _processMessageFromDevice.
           // $FlowFixMe[incompatible-use]
-          req.params.url = req.params.url.slice(FILE_PREFIX.length);
+          processedReq.params.url = processedReq.params.url.slice(
+            FILE_PREFIX.length,
+          );
         }
       }
-      if (req.params.urlRegex != null) {
-        req.params.urlRegex = req.params.urlRegex.replace(
+      if (processedReq.params.urlRegex != null) {
+        processedReq.params.urlRegex = processedReq.params.urlRegex.replace(
           /localhost/g,
           // $FlowFixMe[incompatible-call]
           debuggerInfo.originalSourceURLAddress,
         );
       }
+      return processedReq;
     }
+    return req;
   }
 
   _processDebuggerGetScriptSource(req: GetScriptSourceRequest, socket: WS) {
diff --git a/packages/dev-middleware/src/inspector-proxy/types.js b/packages/dev-middleware/src/inspector-proxy/types.js
index 2d326ded6d44b0..d1374fd634e891 100644
--- a/packages/dev-middleware/src/inspector-proxy/types.js
+++ b/packages/dev-middleware/src/inspector-proxy/types.js
@@ -12,49 +12,43 @@
 // Page information received from the device. New page is created for
 // each new instance of VM and can appear when user reloads React Native
 // application.
-export type Page = {
+export type Page = $ReadOnly<{
   id: string,
   title: string,
   vm: string,
   app: string,
-  ...
-};
+}>;
 
 // Chrome Debugger Protocol message/event passed between device and debugger.
-export type WrappedEvent = {
+export type WrappedEvent = $ReadOnly<{
   event: 'wrappedEvent',
-  payload: {
+  payload: $ReadOnly<{
     pageId: string,
     wrappedEvent: string,
-    ...
-  },
-  ...
-};
+  }>,
+}>;
 
 // Request sent from Inspector Proxy to Device when new debugger is connected
 // to particular page.
-export type ConnectRequest = {
+export type ConnectRequest = $ReadOnly<{
   event: 'connect',
-  payload: {pageId: string, ...},
-  ...
-};
+  payload: $ReadOnly<{pageId: string}>,
+}>;
 
 // Request sent from Inspector Proxy to Device to notify that debugger is
 // disconnected.
-export type DisconnectRequest = {
+export type DisconnectRequest = $ReadOnly<{
   event: 'disconnect',
-  payload: {pageId: string, ...},
-  ...
-};
+  payload: $ReadOnly<{pageId: string}>,
+}>;
 
 // Request sent from Inspector Proxy to Device to get a list of pages.
-export type GetPagesRequest = {event: 'getPages', ...};
+export type GetPagesRequest = {event: 'getPages'};
 
 // Response to GetPagesRequest containing a list of page infos.
 export type GetPagesResponse = {
   event: 'getPages',
-  payload: Array<Page>,
-  ...
+  payload: $ReadOnlyArray<Page>,
 };
 
 // Union type for all possible messages sent from device to Inspector Proxy.
@@ -71,7 +65,7 @@ export type MessageToDevice =
   | DisconnectRequest;
 
 // Page description object that is sent in response to /json HTTP request from debugger.
-export type PageDescription = {
+export type PageDescription = $ReadOnly<{
   id: string,
   description: string,
   title: string,
@@ -79,60 +73,61 @@ export type PageDescription = {
   devtoolsFrontendUrl: string,
   type: string,
   webSocketDebuggerUrl: string,
+  deviceName: string,
+  vm: string,
   // Metadata specific to React Native
-  reactNative: {
+  reactNative: $ReadOnly<{
     logicalDeviceId: string,
-  },
-  ...
-};
-export type JsonPagesListResponse = Array<PageDescription>;
+  }>,
+}>;
+
+export type JsonPagesListResponse = $ReadOnlyArray<PageDescription>;
 
 // Response to /json/version HTTP request from the debugger specifying browser type and
 // Chrome protocol version.
-export type JsonVersionResponse = {
+export type JsonVersionResponse = $ReadOnly<{
   Browser: string,
   'Protocol-Version': string,
-  ...
-};
+}>;
 
 /**
  * Types were exported from https://github.com/ChromeDevTools/devtools-protocol/blob/master/types/protocol.d.ts
  */
 
-export type SetBreakpointByUrlRequest = {
+export type SetBreakpointByUrlRequest = $ReadOnly<{
   id: number,
   method: 'Debugger.setBreakpointByUrl',
-  params: {
+  params: $ReadOnly<{
     lineNumber: number,
     url?: string,
     urlRegex?: string,
     scriptHash?: string,
     columnNumber?: number,
     condition?: string,
-  },
-};
+  }>,
+}>;
 
-export type GetScriptSourceRequest = {
+export type GetScriptSourceRequest = $ReadOnly<{
   id: number,
   method: 'Debugger.getScriptSource',
   params: {
     scriptId: string,
   },
-};
+}>;
 
-export type GetScriptSourceResponse = {
+export type GetScriptSourceResponse = $ReadOnly<{
   scriptSource: string,
   /**
    * Wasm bytecode.
    */
   bytecode?: string,
-};
+}>;
 
-export type ErrorResponse = {
-  error: {
+export type ErrorResponse = $ReadOnly<{
+  error: $ReadOnly<{
     message: string,
-  },
-};
+  }>,
+}>;
 
 export type DebuggerRequest =
   | SetBreakpointByUrlRequest