diff --git a/packages/console/src/pages/JwtClaims/SettingsSection/TestTab.tsx b/packages/console/src/pages/JwtClaims/SettingsSection/TestTab.tsx
index cdcf138af..79d3ad1aa 100644
--- a/packages/console/src/pages/JwtClaims/SettingsSection/TestTab.tsx
+++ b/packages/console/src/pages/JwtClaims/SettingsSection/TestTab.tsx
@@ -1,8 +1,10 @@
-import { type JsonObject, LogtoJwtTokenPath } from '@logto/schemas';
+import { type JsonObject, LogtoJwtTokenPath, type RequestErrorBody } from '@logto/schemas';
 import classNames from 'classnames';
+import { HTTPError } from 'ky';
 import { useCallback, useEffect, useMemo, useState } from 'react';
 import { useFormContext, Controller, type ControllerRenderProps } from 'react-hook-form';
 import { useTranslation } from 'react-i18next';
+import { z } from 'zod';
 
 import Button from '@/ds-components/Button';
 import Card from '@/ds-components/Card';
@@ -27,6 +29,7 @@ type Props = {
 const accessTokenModelSettings = [accessTokenPayloadTestModel, userContextTestModel];
 const clientCredentialsModelSettings = [clientCredentialsPayloadTestModel];
 const testEndpointPath = 'api/configs/jwt-customizer/test';
+const jwtCustomizerGeneralErrorCode = 'jwt_customizer.general';
 
 function TestTab({ isActive }: Props) {
   const { t } = useTranslation(undefined, { keyPrefix: 'admin_console.jwt_claims' });
@@ -62,7 +65,21 @@ function TestTab({ isActive }: Props) {
         json: formatFormDataToTestRequestPayload(payload),
       })
       .json<JsonObject>()
-      .catch((error: unknown) => {
+      .catch(async (error: unknown) => {
+        if (error instanceof HTTPError) {
+          const { response } = error;
+          const metadata = await response.clone().json<RequestErrorBody>();
+          if (metadata.code === jwtCustomizerGeneralErrorCode) {
+            const result = z.object({ message: z.string() }).safeParse(metadata.data);
+            if (result.success) {
+              setTestResult({
+                error: result.data.message,
+              });
+              return;
+            }
+          }
+        }
+
         setTestResult({
           error: error instanceof Error ? error.message : String(error),
         });
diff --git a/packages/core/src/routes/logto-config.ts b/packages/core/src/routes/logto-config.ts
index 5590d7876..9d059bae2 100644
--- a/packages/core/src/routes/logto-config.ts
+++ b/packages/core/src/routes/logto-config.ts
@@ -19,6 +19,7 @@ import {
   jsonObjectGuard,
 } from '@logto/schemas';
 import { adminTenantId } from '@logto/schemas';
+import { ResponseError } from '@withtyped/client';
 import { z } from 'zod';
 
 import { EnvSet } from '#src/env-set/index.js';
@@ -340,13 +341,27 @@ export default function logtoConfigRoutes<T extends AuthedRouter>(
 
       const client = await cloudConnection.getClient();
 
-      ctx.body = await client.post(`/api/services/custom-jwt`, {
-        body: {
-          ...rest,
-          token: tokenSample,
-          context: contextSample,
-        },
-      });
+      try {
+        ctx.body = await client.post(`/api/services/custom-jwt`, {
+          body: {
+            ...rest,
+            token: tokenSample,
+            context: contextSample,
+          },
+        });
+      } catch (error: unknown) {
+        /**
+         * `ResponseError` comes from `@withtyped/client` and all `logto/core` API returns error in the
+         * format of `RequestError`, we manually transform it here to keep the error format consistent.
+         */
+        if (error instanceof ResponseError) {
+          const { message } = z.object({ message: z.string() }).parse(await error.response.json());
+          throw new RequestError({ code: 'jwt_customizer.general', status: 422 }, { message });
+        }
+
+        throw error;
+      }
+
       return next();
     }
   );
diff --git a/packages/phrases/src/locales/de/errors/index.ts b/packages/phrases/src/locales/de/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/de/errors/index.ts
+++ b/packages/phrases/src/locales/de/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/de/errors/jwt-customizer.ts b/packages/phrases/src/locales/de/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/de/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/en/errors/index.ts b/packages/phrases/src/locales/en/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/en/errors/index.ts
+++ b/packages/phrases/src/locales/en/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/en/errors/jwt-customizer.ts b/packages/phrases/src/locales/en/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/en/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/es/errors/index.ts b/packages/phrases/src/locales/es/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/es/errors/index.ts
+++ b/packages/phrases/src/locales/es/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/es/errors/jwt-customizer.ts b/packages/phrases/src/locales/es/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/es/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/fr/errors/index.ts b/packages/phrases/src/locales/fr/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/fr/errors/index.ts
+++ b/packages/phrases/src/locales/fr/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/fr/errors/jwt-customizer.ts b/packages/phrases/src/locales/fr/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/fr/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/it/errors/index.ts b/packages/phrases/src/locales/it/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/it/errors/index.ts
+++ b/packages/phrases/src/locales/it/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/it/errors/jwt-customizer.ts b/packages/phrases/src/locales/it/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/it/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/ja/errors/index.ts b/packages/phrases/src/locales/ja/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/ja/errors/index.ts
+++ b/packages/phrases/src/locales/ja/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/ja/errors/jwt-customizer.ts b/packages/phrases/src/locales/ja/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/ja/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/ko/errors/index.ts b/packages/phrases/src/locales/ko/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/ko/errors/index.ts
+++ b/packages/phrases/src/locales/ko/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/ko/errors/jwt-customizer.ts b/packages/phrases/src/locales/ko/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/ko/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/pl-pl/errors/index.ts b/packages/phrases/src/locales/pl-pl/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/pl-pl/errors/index.ts
+++ b/packages/phrases/src/locales/pl-pl/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/pl-pl/errors/jwt-customizer.ts b/packages/phrases/src/locales/pl-pl/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/pl-pl/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/pt-br/errors/index.ts b/packages/phrases/src/locales/pt-br/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/pt-br/errors/index.ts
+++ b/packages/phrases/src/locales/pt-br/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/pt-br/errors/jwt-customizer.ts b/packages/phrases/src/locales/pt-br/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/pt-br/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/pt-pt/errors/index.ts b/packages/phrases/src/locales/pt-pt/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/pt-pt/errors/index.ts
+++ b/packages/phrases/src/locales/pt-pt/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/pt-pt/errors/jwt-customizer.ts b/packages/phrases/src/locales/pt-pt/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/pt-pt/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/ru/errors/index.ts b/packages/phrases/src/locales/ru/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/ru/errors/index.ts
+++ b/packages/phrases/src/locales/ru/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/ru/errors/jwt-customizer.ts b/packages/phrases/src/locales/ru/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/ru/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/tr-tr/errors/index.ts b/packages/phrases/src/locales/tr-tr/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/tr-tr/errors/index.ts
+++ b/packages/phrases/src/locales/tr-tr/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/tr-tr/errors/jwt-customizer.ts b/packages/phrases/src/locales/tr-tr/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/tr-tr/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/zh-cn/errors/index.ts b/packages/phrases/src/locales/zh-cn/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/zh-cn/errors/index.ts
+++ b/packages/phrases/src/locales/zh-cn/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/zh-cn/errors/jwt-customizer.ts b/packages/phrases/src/locales/zh-cn/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/zh-cn/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/zh-hk/errors/index.ts b/packages/phrases/src/locales/zh-hk/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/zh-hk/errors/index.ts
+++ b/packages/phrases/src/locales/zh-hk/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/zh-hk/errors/jwt-customizer.ts b/packages/phrases/src/locales/zh-hk/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/zh-hk/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);
diff --git a/packages/phrases/src/locales/zh-tw/errors/index.ts b/packages/phrases/src/locales/zh-tw/errors/index.ts
index 74995792d..40df6998e 100644
--- a/packages/phrases/src/locales/zh-tw/errors/index.ts
+++ b/packages/phrases/src/locales/zh-tw/errors/index.ts
@@ -5,6 +5,7 @@ import domain from './domain.js';
 import entity from './entity.js';
 import guard from './guard.js';
 import hook from './hook.js';
+import jwt_customizer from './jwt-customizer.js';
 import localization from './localization.js';
 import log from './log.js';
 import oidc from './oidc.js';
@@ -34,6 +35,7 @@ const errors = {
   connector,
   verification_code,
   sign_in_experiences,
+  jwt_customizer,
   localization,
   swagger,
   entity,
diff --git a/packages/phrases/src/locales/zh-tw/errors/jwt-customizer.ts b/packages/phrases/src/locales/zh-tw/errors/jwt-customizer.ts
new file mode 100644
index 000000000..f2ca29554
--- /dev/null
+++ b/packages/phrases/src/locales/zh-tw/errors/jwt-customizer.ts
@@ -0,0 +1,6 @@
+const jwt_customizer = {
+  /** UNTRANSLATED */
+  general: 'An error occurred while customizing the JWT token. Please try again later.',
+};
+
+export default Object.freeze(jwt_customizer);