diff --git a/AGENTS.md b/AGENTS.md
index 1c755aa..a3d9e39 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -79,9 +79,16 @@ Shared axios clients live in `src/shared/api/axiosInstance.ts`.
 - `apiInstance`: main API. In development its base URL is empty so Vite proxy handles `/api`; in production it uses `VITE_API_URL`.
 - `chatInstance`: chat/interview API, based on `VITE_CHAT_URL`.
 - All clients use credentials and sync access tokens from responses.
-- 401 responses try `/api/v1/auth/refresh`; if refresh fails, the token is cleared and the user is sent to `/login`.
+- 401 responses try `/api/v1/auth/refresh`; if refresh fails, the token is cleared and the user is sent to `/login`. Concurrent requests share one refresh call.
+- `apiInstance` and `chatInstance` require an access token. When refresh cannot supply one, the request fails locally with `로그인이 필요합니다.` instead of being sent without an `Authorization` header.
 - `apiInstance` removes `Content-Type` automatically for `FormData`.
 
+Backend auth contract:
+
+- A missing token and an expired token both return `401 { "message": ... }`. There is no 400 auth error anymore.
+- Ownership checks on answer/question queries return `403 { "message": ... }` where a 404 used to be expected. A 403 must not clear the session.
+- Read error text through `extractErrorMessage` in `src/shared/api/errorMessage.ts`; it falls back to status-specific Korean copy for 401 and 403.
+
 Relevant environment variables:
 
 - `VITE_AUTH_URL`
diff --git a/src/features/interview-page/interview/api/prepare-interview/index.ts b/src/features/interview-page/interview/api/prepare-interview/index.ts
index c1c63a1..1f1dc5b 100644
--- a/src/features/interview-page/interview/api/prepare-interview/index.ts
+++ b/src/features/interview-page/interview/api/prepare-interview/index.ts
@@ -1,4 +1,12 @@
-import { API_URL, ensureAccessToken } from "@/shared/api/axiosInstance";
+import {
+  API_URL,
+  ensureAccessToken,
+  handleAuthenticationFailure,
+} from "@/shared/api/axiosInstance";
+import {
+  AUTH_REQUIRED_MESSAGE,
+  FORBIDDEN_MESSAGE,
+} from "@/shared/api/errorMessage";
 
 import {
   getCurrentUserId,
@@ -43,6 +51,37 @@ const getSsePayload = (data: string) => {
   }
 };
 
+// axios 인터셉터를 타지 않는 요청이라 401/403 응답을 여기서 직접 정리한다.
+const getResponseErrorMessage = async (response: Response, fallback: string) => {
+  try {
+    const payload = getRecord(await response.json());
+
+    return getTrimmedString(payload?.message) ?? fallback;
+  } catch {
+    return fallback;
+  }
+};
+
+const throwSubscribeResponseError = async (response: Response) => {
+  if (response.status === 401) {
+    const errorMessage = await getResponseErrorMessage(
+      response,
+      AUTH_REQUIRED_MESSAGE,
+    );
+
+    handleAuthenticationFailure();
+    throw new Error(errorMessage);
+  }
+
+  if (response.status === 403) {
+    throw new Error(
+      await getResponseErrorMessage(response, FORBIDDEN_MESSAGE),
+    );
+  }
+
+  throw new Error(`AI SSE request failed: ${response.status}`);
+};
+
 const getSseErrorMessage = (data: string, fallback: string) => {
   const payload = getSsePayload(data);
 
@@ -90,7 +129,7 @@ export const waitForInterviewReady = async (
     });
 
     if (!response.ok) {
-      throw new Error(`AI SSE request failed: ${response.status}`);
+      await throwSubscribeResponseError(response);
     }
 
     if (!response.body) {
diff --git a/src/shared/api/axiosInstance.ts b/src/shared/api/axiosInstance.ts
index bdf0f4d..e145da8 100644
--- a/src/shared/api/axiosInstance.ts
+++ b/src/shared/api/axiosInstance.ts
@@ -10,6 +10,7 @@ import {
   getAccessToken,
   syncAccessTokenFromResponse,
 } from "./accessToken";
+import { AUTH_REQUIRED_MESSAGE } from "./errorMessage";
 
 const resolveServerUrl = (url?: string) => {
   if (!url) return "";
@@ -55,11 +56,24 @@ interface RetryableRequestConfig extends InternalAxiosRequestConfig {
   _retry?: boolean;
 }
 
+const LOGIN_PATH = "/login";
+const AUTH_PAGE_PATHS = [LOGIN_PATH, "/signup"];
+
 const goToLoginPage = () => {
-  window.location.href = "/login";
+  if (AUTH_PAGE_PATHS.includes(window.location.pathname)) {
+    return;
+  }
+
+  window.location.href = LOGIN_PATH;
+};
+
+// 토큰 누락과 만료가 모두 401로 통일됐으므로 두 경우를 같은 흐름으로 정리한다.
+export const handleAuthenticationFailure = () => {
+  clearAccessToken();
+  goToLoginPage();
 };
 
-const tryRefreshSession = async () => {
+const requestRefreshSession = async () => {
   try {
     const response = await refreshInstance.post("/api/v1/auth/refresh");
     const refreshedAccessToken = syncAccessTokenFromResponse({
@@ -72,6 +86,20 @@ const tryRefreshSession = async () => {
   }
 };
 
+// 모든 요청이 토큰을 요구하게 되면서 재발급이 동시에 여러 번 호출될 수 있어,
+// 진행 중인 요청 하나를 공유한다.
+let refreshSessionPromise: Promise<boolean> | null = null;
+
+const tryRefreshSession = () => {
+  if (!refreshSessionPromise) {
+    refreshSessionPromise = requestRefreshSession().finally(() => {
+      refreshSessionPromise = null;
+    });
+  }
+
+  return refreshSessionPromise;
+};
+
 const getReadyAccessToken = async (shouldRefreshBeforeRequest: boolean) => {
   const currentAccessToken = getAccessToken();
   if (currentAccessToken || !shouldRefreshBeforeRequest) {
@@ -86,7 +114,8 @@ export const ensureAccessToken = async () => {
   const authorizationHeader = await getReadyAccessToken(true);
 
   if (!authorizationHeader) {
-    throw new Error("로그인 토큰을 찾지 못했습니다. 다시 로그인해 주세요.");
+    handleAuthenticationFailure();
+    throw new Error(AUTH_REQUIRED_MESSAGE);
   }
 
   return authorizationHeader;
@@ -94,13 +123,20 @@ export const ensureAccessToken = async () => {
 
 const addAuthorizationInterceptor = (
   instance: AxiosInstance,
-  options: { refreshBeforeRequest?: boolean } = {},
+  options: { refreshBeforeRequest?: boolean; requireAccessToken?: boolean } = {},
 ) => {
   instance.interceptors.request.use(async (config) => {
     const authorizationHeader = await getReadyAccessToken(
       options.refreshBeforeRequest ?? false,
     );
-    if (!authorizationHeader) return config;
+
+    // 서버가 모든 엔드포인트에서 토큰을 요구하므로, 헤더 없이 보내면 401만 돌아온다.
+    if (!authorizationHeader) {
+      if (!options.requireAccessToken) return config;
+
+      handleAuthenticationFailure();
+      throw new Error(AUTH_REQUIRED_MESSAGE);
+    }
 
     const nextHeaders = axios.AxiosHeaders.from(config.headers) as AxiosHeaders;
     if (!nextHeaders.has("Authorization")) {
@@ -180,8 +216,7 @@ const addRefreshInterceptor = (instance: AxiosInstance) => {
       }
 
       if (error.response?.status === 401 && !isSessionlessRequest) {
-        clearAccessToken();
-        goToLoginPage();
+        handleAuthenticationFailure();
       }
       return Promise.reject(error);
     },
@@ -196,5 +231,11 @@ addRefreshInterceptor(apiInstance);
 addRefreshInterceptor(chatInstance);
 addFormDataInterceptor(apiInstance);
 addAuthorizationInterceptor(authInstance);
-addAuthorizationInterceptor(apiInstance, { refreshBeforeRequest: true });
-addAuthorizationInterceptor(chatInstance, { refreshBeforeRequest: true });
+addAuthorizationInterceptor(apiInstance, {
+  refreshBeforeRequest: true,
+  requireAccessToken: true,
+});
+addAuthorizationInterceptor(chatInstance, {
+  refreshBeforeRequest: true,
+  requireAccessToken: true,
+});
diff --git a/src/shared/api/errorMessage.ts b/src/shared/api/errorMessage.ts
index c2bf095..b009f2d 100644
--- a/src/shared/api/errorMessage.ts
+++ b/src/shared/api/errorMessage.ts
@@ -89,14 +89,29 @@ const getMessageFromValue = (value: unknown, depth = 0): string | null => {
   return getTrimmedString(record.code);
 };
 
+// 백엔드 인증 계약: 토큰 누락과 만료가 모두 401 { message } 로 통일됐고,
+// 소유권 검사에 걸린 조회는 404가 아니라 403 { message } 로 내려온다.
+export const AUTH_REQUIRED_MESSAGE = "로그인이 필요합니다.";
+export const FORBIDDEN_MESSAGE = "접근 권한이 없습니다.";
+
+const FALLBACK_MESSAGE_BY_STATUS: Record<number, string> = {
+  401: AUTH_REQUIRED_MESSAGE,
+  403: FORBIDDEN_MESSAGE,
+};
+
+const getFallbackMessage = (status: number, fallback: string) =>
+  FALLBACK_MESSAGE_BY_STATUS[status] ?? fallback;
+
 export const extractErrorMessage = (error: unknown, fallback: string) => {
   if (axios.isAxiosError(error)) {
     if (!error.response) {
       return "서버에 연결할 수 없습니다. 잠시 후 다시 시도해주세요.";
     }
 
+    const statusFallback = getFallbackMessage(error.response.status, fallback);
+
     if (getContentType(error.response.headers).includes("text/html")) {
-      return fallback;
+      return statusFallback;
     }
 
     const responseMessage = getMessageFromValue(error.response.data);
@@ -105,7 +120,7 @@ export const extractErrorMessage = (error: unknown, fallback: string) => {
       return responseMessage;
     }
 
-    return fallback;
+    return statusFallback;
   }
 
   const nestedMessage = getMessageFromValue(error);