diff --git a/src/features/authFiles/components/AuthFilesPrefixProxyEditorModal.tsx b/src/features/authFiles/components/AuthFilesPrefixProxyEditorModal.tsx
index 020192c..25ff267 100644
--- a/src/features/authFiles/components/AuthFilesPrefixProxyEditorModal.tsx
+++ b/src/features/authFiles/components/AuthFilesPrefixProxyEditorModal.tsx
@@ -34,6 +34,7 @@ export function AuthFilesPrefixProxyEditorModal(props: AuthFilesPrefixProxyEdito
     }
   };
   const previewText = formatJsonText(updatedText);
+  const invalidContentPreview = editor?.invalidContentPreview ?? '';
 
   return (
     <Modal
@@ -100,60 +101,70 @@ export function AuthFilesPrefixProxyEditorModal(props: AuthFilesPrefixProxyEdito
               </div>
               <div className={styles.prefixProxyJsonWrapper}>
                 <label className={styles.prefixProxyLabel}>
-                  {t('auth_files.prefix_proxy_source_label')}
+                  {editor.json
+                    ? t('auth_files.prefix_proxy_source_label')
+                    : t('auth_files.prefix_proxy_invalid_content_label')}
                 </label>
-                <textarea
-                  className={styles.prefixProxyTextarea}
-                  rows={10}
-                  readOnly
-                  value={previewText}
-                />
-              </div>
-              <div className={styles.prefixProxyFields}>
-                <Input
-                  label={t('auth_files.prefix_label')}
-                  value={editor.prefix}
-                  disabled={disableControls || editor.saving || !editor.json}
-                  onChange={(e) => onChange('prefix', e.target.value)}
-                />
-                <Input
-                  label={t('auth_files.proxy_url_label')}
-                  value={editor.proxyUrl}
-                  placeholder={t('auth_files.proxy_url_placeholder')}
-                  disabled={disableControls || editor.saving || !editor.json}
-                  onChange={(e) => onChange('proxyUrl', e.target.value)}
-                />
-                <Input
-                  label={t('auth_files.priority_label')}
-                  value={editor.priority}
-                  placeholder={t('auth_files.priority_placeholder')}
-                  hint={t('auth_files.priority_hint')}
-                  disabled={disableControls || editor.saving || !editor.json}
-                  onChange={(e) => onChange('priority', e.target.value)}
-                />
-                <div className="form-group">
-                  <label>{t('auth_files.headers_label')}</label>
+                {editor.json ? (
                   <textarea
-                    className={`input ${editor.headersError ? styles.prefixProxyTextareaInvalid : ''}`}
-                    value={editor.headersText}
-                    placeholder={t('auth_files.headers_placeholder')}
-                    rows={4}
-                    aria-invalid={Boolean(editor.headersError)}
-                    disabled={disableControls || editor.saving || !editor.json}
-                    onChange={(e) => onChange('headersText', e.target.value)}
+                    className={styles.prefixProxyTextarea}
+                    rows={10}
+                    readOnly
+                    value={previewText}
                   />
-                  {editor.headersError && <div className="error-box">{editor.headersError}</div>}
-                  <div className="hint">{t('auth_files.headers_hint')}</div>
-                </div>
-                <Input
-                  label={t('auth_files.note_label')}
-                  value={editor.note}
-                  placeholder={t('auth_files.note_placeholder')}
-                  hint={t('auth_files.note_hint')}
-                  disabled={disableControls || editor.saving || !editor.json}
-                  onChange={(e) => onChange('note', e.target.value)}
-                />
+                ) : (
+                  <pre className={styles.prefixProxyInvalidContentPreview}>
+                    {invalidContentPreview}
+                  </pre>
+                )}
               </div>
+              {editor.json && (
+                <div className={styles.prefixProxyFields}>
+                  <Input
+                    label={t('auth_files.prefix_label')}
+                    value={editor.prefix}
+                    disabled={disableControls || editor.saving || !editor.json}
+                    onChange={(e) => onChange('prefix', e.target.value)}
+                  />
+                  <Input
+                    label={t('auth_files.proxy_url_label')}
+                    value={editor.proxyUrl}
+                    placeholder={t('auth_files.proxy_url_placeholder')}
+                    disabled={disableControls || editor.saving || !editor.json}
+                    onChange={(e) => onChange('proxyUrl', e.target.value)}
+                  />
+                  <Input
+                    label={t('auth_files.priority_label')}
+                    value={editor.priority}
+                    placeholder={t('auth_files.priority_placeholder')}
+                    hint={t('auth_files.priority_hint')}
+                    disabled={disableControls || editor.saving || !editor.json}
+                    onChange={(e) => onChange('priority', e.target.value)}
+                  />
+                  <div className="form-group">
+                    <label>{t('auth_files.headers_label')}</label>
+                    <textarea
+                      className={`input ${editor.headersError ? styles.prefixProxyTextareaInvalid : ''}`}
+                      value={editor.headersText}
+                      placeholder={t('auth_files.headers_placeholder')}
+                      rows={4}
+                      aria-invalid={Boolean(editor.headersError)}
+                      disabled={disableControls || editor.saving || !editor.json}
+                      onChange={(e) => onChange('headersText', e.target.value)}
+                    />
+                    {editor.headersError && <div className="error-box">{editor.headersError}</div>}
+                    <div className="hint">{t('auth_files.headers_hint')}</div>
+                  </div>
+                  <Input
+                    label={t('auth_files.note_label')}
+                    value={editor.note}
+                    placeholder={t('auth_files.note_placeholder')}
+                    hint={t('auth_files.note_hint')}
+                    disabled={disableControls || editor.saving || !editor.json}
+                    onChange={(e) => onChange('note', e.target.value)}
+                  />
+                </div>
+              )}
             </>
           )}
         </div>
diff --git a/src/features/authFiles/hooks/useAuthFilesPrefixProxyEditor.ts b/src/features/authFiles/hooks/useAuthFilesPrefixProxyEditor.ts
index c4d685d..6e7c797 100644
--- a/src/features/authFiles/hooks/useAuthFilesPrefixProxyEditor.ts
+++ b/src/features/authFiles/hooks/useAuthFilesPrefixProxyEditor.ts
@@ -10,6 +10,9 @@ type AuthFileHeadersErrorKey =
   | 'auth_files.headers_invalid_json'
   | 'auth_files.headers_invalid_object'
   | 'auth_files.headers_invalid_value';
+type AuthFileContentErrorKey =
+  | 'auth_files.prefix_proxy_invalid_json'
+  | 'auth_files.prefix_proxy_html_challenge';
 
 export type PrefixProxyEditorField = 'prefix' | 'proxyUrl' | 'priority' | 'note' | 'headersText';
 
@@ -23,6 +26,7 @@ export type PrefixProxyEditorState = {
   error: string | null;
   originalText: string;
   rawText: string;
+  invalidContentPreview: string;
   json: Record<string, unknown> | null;
   prefix: string;
   proxyUrl: string;
@@ -90,6 +94,47 @@ const parseHeadersText = (
 const normalizeTextField = (value: unknown): string =>
   typeof value === 'string' ? value.trim() : '';
 
+const INVALID_CONTENT_PREVIEW_LIMIT = 1000;
+
+const buildInvalidContentPreview = (text: string): string => {
+  const trimmed = text.trim();
+  if (!trimmed) return '';
+  if (trimmed.length <= INVALID_CONTENT_PREVIEW_LIMIT) return trimmed;
+  return `${trimmed.slice(0, INVALID_CONTENT_PREVIEW_LIMIT)}\n...`;
+};
+
+const buildInvalidAuthFileContentState = (
+  text: string,
+  resolveError: (key: AuthFileContentErrorKey) => string
+): Pick<
+  PrefixProxyEditorState,
+  'loading' | 'error' | 'rawText' | 'originalText' | 'invalidContentPreview'
+> => ({
+  loading: false,
+  error: resolveError(getAuthFileContentErrorKey(text)),
+  rawText: text,
+  originalText: text,
+  invalidContentPreview: buildInvalidContentPreview(text),
+});
+
+const getAuthFileContentErrorKey = (text: string): AuthFileContentErrorKey => {
+  const head = text.trimStart().slice(0, 4096).toLowerCase();
+  const looksLikeHtml =
+    head.startsWith('<!doctype html') ||
+    head.startsWith('<html') ||
+    head.includes('<head') ||
+    head.includes('<body');
+  const looksLikeChallenge =
+    head.includes('cf_chl') ||
+    head.includes('__cf_chl_tk') ||
+    head.includes('challenge-platform') ||
+    head.includes('cloudflare');
+
+  return looksLikeHtml || looksLikeChallenge
+    ? 'auth_files.prefix_proxy_html_challenge'
+    : 'auth_files.prefix_proxy_invalid_json';
+};
+
 const hasKeys = (value: Record<string, unknown> | AuthFileFieldsPatch | null): boolean =>
   Boolean(value && Object.keys(value).length > 0);
 
@@ -270,7 +315,7 @@ export function useAuthFilesPrefixProxyEditor(
     prefixProxyEditor?.headersTouched && prefixProxyEditor.headersError
   );
   const prefixProxyUpdatedText =
-    prefixProxyEditor?.json && !hasBlockingValidationError
+    prefixProxyEditor && !hasBlockingValidationError
       ? buildPrefixProxyUpdatedText(prefixProxyEditor, (key) => t(key))
       : '';
 
@@ -302,6 +347,7 @@ export function useAuthFilesPrefixProxyEditor(
       error: null,
       originalText: '',
       rawText: '',
+      invalidContentPreview: '',
       json: null,
       prefix: '',
       proxyUrl: '',
@@ -325,10 +371,7 @@ export function useAuthFilesPrefixProxyEditor(
           if (!prev || prev.fileName !== name) return prev;
           return {
             ...prev,
-            loading: false,
-            error: t('auth_files.prefix_proxy_invalid_json'),
-            rawText: trimmed,
-            originalText: trimmed,
+            ...buildInvalidAuthFileContentState(rawText, (key) => t(key)),
           };
         });
         return;
@@ -339,10 +382,7 @@ export function useAuthFilesPrefixProxyEditor(
           if (!prev || prev.fileName !== name) return prev;
           return {
             ...prev,
-            loading: false,
-            error: t('auth_files.prefix_proxy_invalid_json'),
-            rawText: trimmed,
-            originalText: trimmed,
+            ...buildInvalidAuthFileContentState(rawText, (key) => t(key)),
           };
         });
         return;
@@ -370,6 +410,7 @@ export function useAuthFilesPrefixProxyEditor(
           loading: false,
           originalText,
           rawText: originalText,
+          invalidContentPreview: '',
           json,
           prefix,
           proxyUrl,
diff --git a/src/i18n/locales/en.json b/src/i18n/locales/en.json
index ebb8bd2..95d5449 100644
--- a/src/i18n/locales/en.json
+++ b/src/i18n/locales/en.json
@@ -591,6 +591,7 @@
     "prefix_proxy_loading": "Loading auth file...",
     "prefix_proxy_info_label": "Auth file info (info)",
     "prefix_proxy_source_label": "Auth file JSON (preview)",
+    "prefix_proxy_invalid_content_label": "Downloaded content (truncated)",
     "prefix_label": "Prefix (prefix)",
     "proxy_url_label": "Proxy URL (proxy_url)",
     "prefix_placeholder": "",
@@ -615,6 +616,7 @@
     "headers_invalid_object": "Custom headers must be a JSON object.",
     "headers_invalid_value": "Each custom header value must be a string.",
     "prefix_proxy_invalid_json": "This auth file is not a JSON object, so fields cannot be edited.",
+    "prefix_proxy_html_challenge": "Downloaded content is an HTML challenge page, not an auth JSON object. Re-authenticate or replace this auth file before editing fields.",
     "prefix_proxy_saved_success": "Updated auth file \"{{name}}\" successfully",
     "quota_refresh_success": "Quota refreshed for \"{{name}}\"",
     "quota_refresh_failed": "Failed to refresh quota for \"{{name}}\": {{message}}"
diff --git a/src/i18n/locales/ru.json b/src/i18n/locales/ru.json
index 067a4db..6cf6082 100644
--- a/src/i18n/locales/ru.json
+++ b/src/i18n/locales/ru.json
@@ -591,6 +591,7 @@
     "prefix_proxy_loading": "Загрузка файла авторизации...",
     "prefix_proxy_info_label": "Информация о файле авторизации (info)",
     "prefix_proxy_source_label": "JSON файла авторизации (предпросмотр)",
+    "prefix_proxy_invalid_content_label": "Скачанное содержимое (сокращено)",
     "prefix_label": "Префикс (prefix)",
     "proxy_url_label": "URL прокси (proxy_url)",
     "prefix_placeholder": "",
@@ -609,6 +610,7 @@
     "note_hint": "Необязательно. Используется для описания назначения или владельца учётных данных; оставьте пустым, чтобы не записывать.",
     "note_display": "Заметка",
     "prefix_proxy_invalid_json": "Этот файл авторизации не является JSON-объектом, поэтому поля нельзя редактировать.",
+    "prefix_proxy_html_challenge": "Скачанное содержимое является HTML-страницей проверки, а не JSON-объектом авторизации. Повторно авторизуйтесь или замените файл перед редактированием полей.",
     "prefix_proxy_saved_success": "Файл авторизации \"{{name}}\" успешно обновлён",
     "card_tools_title": "Инструменты",
     "quota_refresh_single": "Обновить квоту",
diff --git a/src/i18n/locales/zh-CN.json b/src/i18n/locales/zh-CN.json
index 688aa05..d7c8204 100644
--- a/src/i18n/locales/zh-CN.json
+++ b/src/i18n/locales/zh-CN.json
@@ -591,6 +591,7 @@
     "prefix_proxy_loading": "正在加载认证文件...",
     "prefix_proxy_info_label": "认证文件信息（info）",
     "prefix_proxy_source_label": "认证文件 JSON（预览）",
+    "prefix_proxy_invalid_content_label": "下载内容（已截断）",
     "prefix_label": "前缀（prefix）",
     "proxy_url_label": "代理 URL（proxy_url）",
     "prefix_placeholder": "",
@@ -615,6 +616,7 @@
     "headers_invalid_object": "自定义请求头必须是 JSON 对象。",
     "headers_invalid_value": "每个自定义请求头的值都必须是字符串。",
     "prefix_proxy_invalid_json": "该认证文件不是 JSON 对象，无法编辑字段。",
+    "prefix_proxy_html_challenge": "下载到的是 HTML 验证页面，不是认证 JSON 对象。请重新认证或替换该认证文件后再编辑字段。",
     "prefix_proxy_saved_success": "已更新认证文件 \"{{name}}\"",
     "quota_refresh_success": "已刷新 \"{{name}}\" 的额度",
     "quota_refresh_failed": "刷新 \"{{name}}\" 的额度失败：{{message}}"
diff --git a/src/i18n/locales/zh-TW.json b/src/i18n/locales/zh-TW.json
index 7990e71..ffdc69a 100644
--- a/src/i18n/locales/zh-TW.json
+++ b/src/i18n/locales/zh-TW.json
@@ -591,6 +591,7 @@
     "prefix_proxy_loading": "正在載入驗證檔案...",
     "prefix_proxy_info_label": "驗證檔案資訊（info）",
     "prefix_proxy_source_label": "驗證檔案 JSON（預覽）",
+    "prefix_proxy_invalid_content_label": "下載內容（已截斷）",
     "prefix_label": "前綴（prefix）",
     "proxy_url_label": "代理 URL（proxy_url）",
     "prefix_placeholder": "",
@@ -615,6 +616,7 @@
     "headers_invalid_object": "自訂請求標頭必須是 JSON 物件。",
     "headers_invalid_value": "每個自訂請求標頭的值都必須是字串。",
     "prefix_proxy_invalid_json": "該驗證檔案不是 JSON 物件，無法編輯欄位。",
+    "prefix_proxy_html_challenge": "下載到的是 HTML 驗證頁面，不是驗證 JSON 物件。請重新驗證或替換該驗證檔案後再編輯欄位。",
     "prefix_proxy_saved_success": "已更新驗證檔案「{{name}}」",
     "quota_refresh_success": "已重新整理「{{name}}」的配額",
     "quota_refresh_failed": "重新整理「{{name}}」的配額失敗：{{message}}"
diff --git a/src/pages/AuthFilesPage.module.scss b/src/pages/AuthFilesPage.module.scss
index 7ff7a57..4b62686 100644
--- a/src/pages/AuthFilesPage.module.scss
+++ b/src/pages/AuthFilesPage.module.scss
@@ -1425,6 +1425,23 @@
   }
 }
 
+.prefixProxyInvalidContentPreview {
+  width: 100%;
+  max-height: 220px;
+  margin: 0;
+  padding: $spacing-sm $spacing-md;
+  border: 1px solid var(--border-color);
+  border-radius: $radius-md;
+  background-color: var(--bg-secondary);
+  color: var(--text-secondary);
+  font-size: 12px;
+  font-family: monospace;
+  line-height: 1.5;
+  white-space: pre-wrap;
+  overflow: auto;
+  box-sizing: border-box;
+}
+
 .prefixProxyFields {
   display: grid;
   grid-template-columns: 1fr;