diff --git a/src-tauri/src/services/proxy.rs b/src-tauri/src/services/proxy.rs
index cefa5b107..682e8edb5 100644
--- a/src-tauri/src/services/proxy.rs
+++ b/src-tauri/src/services/proxy.rs
@@ -2560,6 +2560,11 @@ mod tests {
     fn codex_custom_provider_live_write_preserves_oauth_auth_json() {
         let _home = TempHome::new();
         crate::settings::reload_settings().expect("reload settings");
+        crate::settings::update_settings(crate::settings::AppSettings {
+            preserve_codex_official_auth_on_switch: true,
+            ..Default::default()
+        })
+        .expect("enable Codex official auth preservation");
 
         let db = Arc::new(Database::memory().expect("init db"));
         let service = ProxyService::new(db);
@@ -2635,6 +2640,9 @@ wire_api = "responses"
             live_config.contains(PROXY_TOKEN_PLACEHOLDER),
             "live config should carry the proxy placeholder token"
         );
+
+        crate::settings::update_settings(crate::settings::AppSettings::default())
+            .expect("reset settings");
     }
 
     #[test]
diff --git a/src-tauri/src/settings.rs b/src-tauri/src/settings.rs
index 4f2a90945..02276ce60 100644
--- a/src-tauri/src/settings.rs
+++ b/src-tauri/src/settings.rs
@@ -253,8 +253,9 @@ pub struct AppSettings {
     /// Whether to show the failover toggle independently on the main page
     #[serde(default)]
     pub enable_failover_toggle: bool,
-    /// Keep Codex ChatGPT login material in auth.json when switching to third-party providers
-    #[serde(default = "default_true")]
+    /// Keep Codex ChatGPT login material in auth.json when switching to third-party providers.
+    /// Opt-in: defaults to false so third-party switches cleanly overwrite auth.json.
+    #[serde(default)]
     pub preserve_codex_official_auth_on_switch: bool,
     /// User has confirmed the failover toggle first-run notice
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -369,7 +370,7 @@ impl Default for AppSettings {
             usage_confirmed: None,
             stream_check_confirmed: None,
             enable_failover_toggle: false,
-            preserve_codex_official_auth_on_switch: true,
+            preserve_codex_official_auth_on_switch: false,
             failover_confirmed: None,
             first_run_notice_confirmed: None,
             common_config_confirmed: None,
diff --git a/src/components/settings/CodexAuthSettings.tsx b/src/components/settings/CodexAuthSettings.tsx
index 047fd7318..0472acfdc 100644
--- a/src/components/settings/CodexAuthSettings.tsx
+++ b/src/components/settings/CodexAuthSettings.tsx
@@ -25,7 +25,7 @@ export function CodexAuthSettings({
         icon={<KeyRound className="h-4 w-4 text-emerald-500" />}
         title={t("settings.preserveCodexOfficialAuthOnSwitch")}
         description={t("settings.preserveCodexOfficialAuthOnSwitchDescription")}
-        checked={settings.preserveCodexOfficialAuthOnSwitch ?? true}
+        checked={settings.preserveCodexOfficialAuthOnSwitch ?? false}
         onCheckedChange={(value) =>
           onChange({ preserveCodexOfficialAuthOnSwitch: value })
         }
diff --git a/src/hooks/useSettingsForm.ts b/src/hooks/useSettingsForm.ts
index 3a73fd9f2..245b7a66d 100644
--- a/src/hooks/useSettingsForm.ts
+++ b/src/hooks/useSettingsForm.ts
@@ -117,7 +117,7 @@ export function useSettingsForm(): UseSettingsFormResult {
       silentStartup: data.silentStartup ?? false,
       skipClaudeOnboarding: data.skipClaudeOnboarding ?? false,
       preserveCodexOfficialAuthOnSwitch:
-        data.preserveCodexOfficialAuthOnSwitch ?? true,
+        data.preserveCodexOfficialAuthOnSwitch ?? false,
       claudeConfigDir: sanitizeDir(data.claudeConfigDir),
       codexConfigDir: sanitizeDir(data.codexConfigDir),
       geminiConfigDir: sanitizeDir(data.geminiConfigDir),
@@ -142,7 +142,7 @@ export function useSettingsForm(): UseSettingsFormResult {
             useAppWindowControls: false,
             enableClaudePluginIntegration: false,
             skipClaudeOnboarding: false,
-            preserveCodexOfficialAuthOnSwitch: true,
+            preserveCodexOfficialAuthOnSwitch: false,
             language: readPersistedLanguage(),
           } as SettingsFormState);
 
@@ -181,7 +181,7 @@ export function useSettingsForm(): UseSettingsFormResult {
         silentStartup: serverData.silentStartup ?? false,
         skipClaudeOnboarding: serverData.skipClaudeOnboarding ?? false,
         preserveCodexOfficialAuthOnSwitch:
-          serverData.preserveCodexOfficialAuthOnSwitch ?? true,
+          serverData.preserveCodexOfficialAuthOnSwitch ?? false,
         claudeConfigDir: sanitizeDir(serverData.claudeConfigDir),
         codexConfigDir: sanitizeDir(serverData.codexConfigDir),
         geminiConfigDir: sanitizeDir(serverData.geminiConfigDir),