diff --git a/apps/web/src/apis/chat/api.ts b/apps/web/src/apis/chat/api.ts
index 1fa093d0..f9ddd7d0 100644
--- a/apps/web/src/apis/chat/api.ts
+++ b/apps/web/src/apis/chat/api.ts
@@ -1,6 +1,14 @@
 import type { AxiosResponse } from "axios";
 import type { ChatMessage, ChatPartner, ChatRoom } from "@/types/chat";
 import { axiosInstance } from "@/utils/axiosInstance";
+import {
+  normalizeChatMessage,
+  normalizeChatPartner,
+  normalizeChatRoom,
+  type RawChatMessage,
+  type RawChatPartner,
+  type RawChatRoom,
+} from "./normalize";
 
 // QueryKeys for chat domain
 export const ChatQueryKeys = {
@@ -27,20 +35,34 @@ interface GetChatHistoriesParams {
   page?: number;
 }
 
+interface RawChatHistoriesResponse {
+  nextPageNumber: number;
+  content: RawChatMessage[];
+}
+
+interface RawChatRoomListResponse {
+  chatRooms: RawChatRoom[];
+}
+
 export const chatApi = {
   getChatHistories: async ({ roomId, size = 20, page = 0 }: GetChatHistoriesParams): Promise<ChatHistoriesResponse> => {
-    const res = await axiosInstance.get<ChatHistoriesResponse>(`/chats/rooms/${roomId}`, {
+    const res = await axiosInstance.get<RawChatHistoriesResponse>(`/chats/rooms/${roomId}`, {
       params: {
         size,
         page,
       },
     });
-    return res.data;
+    return {
+      nextPageNumber: res.data.nextPageNumber,
+      content: (res.data.content ?? []).map(normalizeChatMessage),
+    };
   },
 
   getChatRooms: async (): Promise<ChatRoomListResponse> => {
-    const res = await axiosInstance.get<ChatRoomListResponse>("/chats/rooms");
-    return res.data;
+    const res = await axiosInstance.get<RawChatRoomListResponse>("/chats/rooms");
+    return {
+      chatRooms: (res.data.chatRooms ?? []).map(normalizeChatRoom),
+    };
   },
 
   putReadChatRoom: async (roomId: number): Promise<void> => {
@@ -49,7 +71,7 @@ export const chatApi = {
   },
 
   getChatPartner: async (roomId: number): Promise<ChatPartner> => {
-    const res = await axiosInstance.get<ChatPartner>(`/chats/rooms/${roomId}/partner`);
-    return res.data;
+    const res = await axiosInstance.get<RawChatPartner>(`/chats/rooms/${roomId}/partner`);
+    return normalizeChatPartner(res.data);
   },
 };
diff --git a/apps/web/src/apis/chat/normalize.ts b/apps/web/src/apis/chat/normalize.ts
new file mode 100644
index 00000000..8850d708
--- /dev/null
+++ b/apps/web/src/apis/chat/normalize.ts
@@ -0,0 +1,80 @@
+import type { ChatAttachment, ChatMessage, ChatPartner, ChatRoom } from "@/types/chat";
+
+type NumericLike = number | string | null | undefined;
+
+interface RawChatAttachment {
+  id?: NumericLike;
+  isImage: boolean;
+  url: string;
+  thumbnailUrl?: string | null;
+  createdAt: string;
+}
+
+export interface RawChatMessage {
+  id?: NumericLike;
+  content: string;
+  senderId?: NumericLike;
+  siteUserId?: NumericLike;
+  createdAt: string;
+  attachments?: RawChatAttachment[];
+}
+
+export interface RawChatPartner {
+  partnerId?: NumericLike;
+  siteUserId?: NumericLike;
+  nickname: string;
+  profileUrl?: string | null;
+  university?: string | null;
+}
+
+export interface RawChatRoom {
+  id: number;
+  lastChatMessage: string;
+  lastReceivedTime: string;
+  partner: RawChatPartner;
+  unReadCount: number;
+}
+
+const toNumber = (value: NumericLike): number => {
+  if (typeof value === "number" && Number.isFinite(value)) {
+    return value;
+  }
+
+  if (typeof value === "string" && value.trim() !== "") {
+    const parsed = Number(value);
+    return Number.isFinite(parsed) ? parsed : 0;
+  }
+
+  return 0;
+};
+
+const normalizeAttachment = (attachment: RawChatAttachment): ChatAttachment => ({
+  id: toNumber(attachment.id),
+  isImage: attachment.isImage,
+  url: attachment.url,
+  thumbnailUrl: attachment.thumbnailUrl ?? "",
+  createdAt: attachment.createdAt,
+});
+
+export const normalizeChatMessage = (message: RawChatMessage): ChatMessage => ({
+  id: toNumber(message.id),
+  content: message.content,
+  senderId: toNumber(message.senderId ?? message.siteUserId),
+  createdAt: message.createdAt,
+  attachments: (message.attachments ?? []).map(normalizeAttachment),
+});
+
+export const normalizeChatPartner = (partner: RawChatPartner): ChatPartner => ({
+  partnerId: toNumber(partner.partnerId ?? partner.siteUserId),
+  nickname: partner.nickname,
+  profileUrl: partner.profileUrl ?? null,
+  university: partner.university ?? null,
+});
+
+export const normalizeChatRoom = (room: RawChatRoom): ChatRoom => ({
+  id: room.id,
+  lastChatMessage: room.lastChatMessage,
+  lastReceivedTime: room.lastReceivedTime,
+  partner: normalizeChatPartner(room.partner),
+  unReadCount: room.unReadCount,
+});
diff --git a/apps/web/src/app/mentor/chat/[chatId]/_ui/ChatContent/index.tsx b/apps/web/src/app/mentor/chat/[chatId]/_ui/ChatContent/index.tsx
index 8fb606ad..6a51670a 100644
--- a/apps/web/src/app/mentor/chat/[chatId]/_ui/ChatContent/index.tsx
+++ b/apps/web/src/app/mentor/chat/[chatId]/_ui/ChatContent/index.tsx
@@ -24,7 +24,7 @@ interface ChatContentProps {
 const ChatContent = ({ chatId }: ChatContentProps) => {
   const { accessToken } = useAuthStore();
   const parsedData = tokenParse(accessToken);
-  const userId = parsedData?.sub ?? 0;
+  const userId = Number(parsedData?.sub ?? 0) || 0;
 
   const isMentor = parsedData?.role === UserRole.MENTOR || parsedData?.role === UserRole.ADMIN;
 
diff --git a/apps/web/src/lib/web-socket/useConnectWebSocket.ts b/apps/web/src/lib/web-socket/useConnectWebSocket.ts
index 11532820..cc831805 100644
--- a/apps/web/src/lib/web-socket/useConnectWebSocket.ts
+++ b/apps/web/src/lib/web-socket/useConnectWebSocket.ts
@@ -2,6 +2,7 @@ import { Client } from "@stomp/stompjs";
 import type { MutableRefObject } from "react";
 import { useEffect, useState } from "react";
 import SockJS from "sockjs-client";
+import { normalizeChatMessage, type RawChatMessage } from "@/apis/chat/normalize";
 
 import { type ChatMessage, ConnectionStatus } from "@/types/chat";
 import useAuthStore from "../zustand/useAuthStore";
@@ -54,7 +55,7 @@ const useConnectWebSocket = ({ roomId, clientRef }: UseConnectWebSocketProps): U
           setConnectionStatus(ConnectionStatus.Connected);
           client.subscribe(`/topic/chat/${roomId}`, (message) => {
             try {
-              const receivedMessage = JSON.parse(message.body) as ChatMessage;
+              const receivedMessage = normalizeChatMessage(JSON.parse(message.body) as RawChatMessage);
 
               if (!receivedMessage.createdAt || Number.isNaN(new Date(receivedMessage.createdAt).getTime())) {
                 receivedMessage.createdAt = new Date().toISOString();
diff --git a/apps/web/src/utils/jwtUtils.ts b/apps/web/src/utils/jwtUtils.ts
index be8f9d8f..b7955405 100644
--- a/apps/web/src/utils/jwtUtils.ts
+++ b/apps/web/src/utils/jwtUtils.ts
@@ -1,5 +1,5 @@
 interface JwtPayload {
-  sub: number;
+  sub: number | string;
   role: string;
   iat: number;
   exp: number;