diff --git a/backend/internal/repository/openai_oauth_service.go b/backend/internal/repository/openai_oauth_service.go
index dca0b612..acb270a3 100644
--- a/backend/internal/repository/openai_oauth_service.go
+++ b/backend/internal/repository/openai_oauth_service.go
@@ -2,6 +2,7 @@ package repository
 
 import (
 	"context"
+	"errors"
 	"net/http"
 	"net/url"
 	"strings"
@@ -53,6 +54,9 @@ func (s *openaiOAuthService) ExchangeCode(ctx context.Context, code, codeVerifie
 		Post(s.tokenURL)
 
 	if err != nil {
+		if shouldReturnOpenAINoProxyHint(ctx, proxyURL, err) {
+			return nil, newOpenAINoProxyHintError(err)
+		}
 		return nil, infraerrors.Newf(http.StatusBadGateway, "OPENAI_OAUTH_REQUEST_FAILED", "request failed: %v", err)
 	}
 
@@ -98,6 +102,9 @@ func (s *openaiOAuthService) refreshTokenWithClientID(ctx context.Context, refre
 		Post(s.tokenURL)
 
 	if err != nil {
+		if shouldReturnOpenAINoProxyHint(ctx, proxyURL, err) {
+			return nil, newOpenAINoProxyHintError(err)
+		}
 		return nil, infraerrors.Newf(http.StatusBadGateway, "OPENAI_OAUTH_REQUEST_FAILED", "request failed: %v", err)
 	}
 
@@ -114,3 +121,21 @@ func createOpenAIReqClient(proxyURL string) (*req.Client, error) {
 		Timeout:  120 * time.Second,
 	})
 }
+
+func shouldReturnOpenAINoProxyHint(ctx context.Context, proxyURL string, err error) bool {
+	if strings.TrimSpace(proxyURL) != "" || err == nil {
+		return false
+	}
+	if ctx != nil && ctx.Err() != nil {
+		return false
+	}
+	return !errors.Is(err, context.Canceled)
+}
+
+func newOpenAINoProxyHintError(cause error) error {
+	return infraerrors.New(
+		http.StatusBadGateway,
+		"OPENAI_OAUTH_PROXY_REQUIRED",
+		"OpenAI OAuth request failed: no proxy is configured and this server could not reach OpenAI directly. Select a proxy that can access OpenAI, then retry; if the authorization code has expired, regenerate the authorization URL.",
+	).WithCause(cause)
+}
diff --git a/backend/internal/repository/openai_oauth_service_test.go b/backend/internal/repository/openai_oauth_service_test.go
index c1901d71..b43e2b52 100644
--- a/backend/internal/repository/openai_oauth_service_test.go
+++ b/backend/internal/repository/openai_oauth_service_test.go
@@ -8,6 +8,7 @@ import (
 	"net/url"
 	"testing"
 
+	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/openai"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
@@ -204,6 +205,17 @@ func (s *OpenAIOAuthServiceSuite) TestRequestError_ClosedServer() {
 	require.ErrorContains(s.T(), err, "request failed")
 }
 
+func (s *OpenAIOAuthServiceSuite) TestExchangeCode_RequestErrorWithoutProxyReturnsProxyHint() {
+	s.setupServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
+	s.srv.Close()
+
+	_, err := s.svc.ExchangeCode(s.ctx, "code", "ver", openai.DefaultRedirectURI, "", "")
+
+	require.Error(s.T(), err)
+	require.Equal(s.T(), "OPENAI_OAUTH_PROXY_REQUIRED", infraerrors.Reason(err))
+	require.Contains(s.T(), infraerrors.Message(err), "no proxy is configured")
+}
+
 func (s *OpenAIOAuthServiceSuite) TestContextCancel() {
 	started := make(chan struct{})
 	block := make(chan struct{})
diff --git a/frontend/src/composables/__tests__/useOpenAIOAuth.spec.ts b/frontend/src/composables/__tests__/useOpenAIOAuth.spec.ts
index 3058819c..cf67aca4 100644
--- a/frontend/src/composables/__tests__/useOpenAIOAuth.spec.ts
+++ b/frontend/src/composables/__tests__/useOpenAIOAuth.spec.ts
@@ -6,6 +6,19 @@ vi.mock('@/stores/app', () => ({
   })
 }))
 
+vi.mock('vue-i18n', () => ({
+  useI18n: () => ({
+    t: (key: string) => {
+      const messages: Record<string, string> = {
+        'admin.accounts.oauth.openai.failedToExchangeCode': 'OpenAI 授权码兑换失败',
+        'admin.accounts.oauth.openai.errors.OPENAI_OAUTH_PROXY_REQUIRED':
+          '未设置代理，当前服务器无法直连 OpenAI，导致 OpenAI OAuth 请求失败。请先选择可访问 OpenAI 的代理后重试；如果授权码已失效，请重新生成授权链接。'
+      }
+      return messages[key] ?? key
+    }
+  })
+}))
+
 vi.mock('@/api/admin', () => ({
   adminAPI: {
     accounts: {
@@ -17,6 +30,7 @@ vi.mock('@/api/admin', () => ({
 }))
 
 import { useOpenAIOAuth } from '@/composables/useOpenAIOAuth'
+import { adminAPI } from '@/api/admin'
 
 describe('useOpenAIOAuth.buildCredentials', () => {
   it('should keep client_id when token response contains it', () => {
@@ -46,3 +60,21 @@ describe('useOpenAIOAuth.buildCredentials', () => {
     expect(creds.refresh_token).toBe('rt')
   })
 })
+
+describe('useOpenAIOAuth.exchangeAuthCode', () => {
+  it('shows a clear proxy hint when code exchange fails without a proxy', async () => {
+    vi.mocked(adminAPI.accounts.exchangeCode).mockRejectedValueOnce({
+      status: 502,
+      reason: 'OPENAI_OAUTH_PROXY_REQUIRED',
+      message: 'OpenAI OAuth token exchange failed: no proxy is configured.'
+    })
+    const oauth = useOpenAIOAuth()
+
+    const tokenInfo = await oauth.exchangeAuthCode('code', 'session-id', 'state')
+
+    expect(tokenInfo).toBeNull()
+    expect(oauth.error.value).toBe(
+      '未设置代理，当前服务器无法直连 OpenAI，导致 OpenAI OAuth 请求失败。请先选择可访问 OpenAI 的代理后重试；如果授权码已失效，请重新生成授权链接。'
+    )
+  })
+})
diff --git a/frontend/src/composables/useOpenAIOAuth.ts b/frontend/src/composables/useOpenAIOAuth.ts
index 060ddbd2..4483e4ad 100644
--- a/frontend/src/composables/useOpenAIOAuth.ts
+++ b/frontend/src/composables/useOpenAIOAuth.ts
@@ -1,6 +1,8 @@
 import { ref } from 'vue'
+import { useI18n } from 'vue-i18n'
 import { useAppStore } from '@/stores/app'
 import { adminAPI } from '@/api/admin'
+import { extractApiErrorMessage, extractI18nErrorMessage } from '@/utils/apiError'
 
 export interface OpenAITokenInfo {
   access_token?: string
@@ -26,6 +28,7 @@ export type OpenAIOAuthPlatform = 'openai'
 
 export function useOpenAIOAuth() {
   const appStore = useAppStore()
+  const { t } = useI18n()
   const endpointPrefix = '/admin/openai'
 
   // State
@@ -78,7 +81,7 @@ export function useOpenAIOAuth() {
       }
       return true
     } catch (err: any) {
-      error.value = err.response?.data?.detail || 'Failed to generate OpenAI auth URL'
+      error.value = extractApiErrorMessage(err, t('admin.accounts.oauth.openai.failedToGenerateUrl'))
       appStore.showError(error.value)
       return false
     } finally {
@@ -114,7 +117,12 @@ export function useOpenAIOAuth() {
       const tokenInfo = await adminAPI.accounts.exchangeCode(`${endpointPrefix}/exchange-code`, payload)
       return tokenInfo as OpenAITokenInfo
     } catch (err: any) {
-      error.value = err.response?.data?.detail || 'Failed to exchange OpenAI auth code'
+      error.value = extractI18nErrorMessage(
+        err,
+        t,
+        'admin.accounts.oauth.openai.errors',
+        t('admin.accounts.oauth.openai.failedToExchangeCode')
+      )
       appStore.showError(error.value)
       return null
     } finally {
@@ -147,7 +155,12 @@ export function useOpenAIOAuth() {
       )
       return tokenInfo as OpenAITokenInfo
     } catch (err: any) {
-      error.value = err.response?.data?.detail || err.message || 'Failed to validate refresh token'
+      error.value = extractI18nErrorMessage(
+        err,
+        t,
+        'admin.accounts.oauth.openai.errors',
+        t('admin.accounts.oauth.openai.failedToValidateRT')
+      )
       appStore.showError(error.value)
       return null
     } finally {
diff --git a/frontend/src/i18n/locales/en.ts b/frontend/src/i18n/locales/en.ts
index bab6cbe4..63a50fbe 100644
--- a/frontend/src/i18n/locales/en.ts
+++ b/frontend/src/i18n/locales/en.ts
@@ -2794,6 +2794,13 @@ export default {
             'Option 1: Copy the complete URL\n(http://localhost:xxx/auth/callback?code=...)\nOption 2: Copy only the code parameter value',
           authCodeHint:
             'You can copy the entire URL or just the code parameter value, the system will auto-detect',
+          failedToGenerateUrl: 'Failed to generate OpenAI auth URL',
+          failedToExchangeCode: 'Failed to exchange OpenAI auth code',
+          failedToValidateRT: 'Failed to validate refresh token',
+          errors: {
+            OPENAI_OAUTH_PROXY_REQUIRED:
+              'No proxy is configured and this server could not reach OpenAI directly, so the OpenAI OAuth request failed. Select a proxy that can access OpenAI and retry; if the authorization code has expired, regenerate the authorization URL.'
+          },
           // Refresh Token auth
           refreshTokenAuth: 'Manual RT Input',
           refreshTokenDesc: 'Enter your existing OpenAI Refresh Token(s). Supports batch input (one per line). The system will automatically validate and create accounts.',
diff --git a/frontend/src/i18n/locales/zh.ts b/frontend/src/i18n/locales/zh.ts
index 1996dbd6..697058ec 100644
--- a/frontend/src/i18n/locales/zh.ts
+++ b/frontend/src/i18n/locales/zh.ts
@@ -2929,6 +2929,13 @@ export default {
           authCodePlaceholder:
             '方式1：复制完整的链接\n(http://localhost:xxx/auth/callback?code=...)\n方式2：仅复制 code 参数的值',
           authCodeHint: '您可以直接复制整个链接或仅复制 code 参数值，系统会自动识别',
+          failedToGenerateUrl: '生成 OpenAI 授权链接失败',
+          failedToExchangeCode: 'OpenAI 授权码兑换失败',
+          failedToValidateRT: '验证 Refresh Token 失败',
+          errors: {
+            OPENAI_OAUTH_PROXY_REQUIRED:
+              '未设置代理，当前服务器无法直连 OpenAI，导致 OpenAI OAuth 请求失败。请先选择可访问 OpenAI 的代理后重试；如果授权码已失效，请重新生成授权链接。'
+          },
           // Refresh Token auth
           refreshTokenAuth: '手动输入 RT',
           refreshTokenDesc: '输入您已有的 OpenAI Refresh Token，支持批量输入（每行一个），系统将自动验证并创建账号。',