diff --git a/frontend/src/pages/groups/GroupsPage.tsx b/frontend/src/pages/groups/GroupsPage.tsx
index f39b5ae62..94141d8ac 100644
--- a/frontend/src/pages/groups/GroupsPage.tsx
+++ b/frontend/src/pages/groups/GroupsPage.tsx
@@ -18,6 +18,7 @@ import {
 } from '@tabler/icons-react';
 import { groupApi } from '../../services/groupApi';
 import { fetchJson } from '../../services/api';
+import { getErrorMessage } from '../../services/apiError';
 import {
     compareCursor,
     type GroupActivity,
@@ -634,8 +635,8 @@ export default function GroupsPage() {
             await queryClient.invalidateQueries({ queryKey: ['group-sessions', targetGroupId] });
             setExpandedGroups((current) => new Set(current).add(targetGroupId));
             navigate(`/groups/${targetGroupId}/${session.id}`);
-        } catch (error: any) {
-            toast.error(error?.message ?? t('groups.createSessionFailed', '创建会话失败'));
+        } catch (error: unknown) {
+            toast.error(getErrorMessage(error, t('groups.createSessionFailed', '创建会话失败')));
         }
     };
 
diff --git a/frontend/src/services/apiError.ts b/frontend/src/services/apiError.ts
index 8dcdd8370..a9cc3ce95 100644
--- a/frontend/src/services/apiError.ts
+++ b/frontend/src/services/apiError.ts
@@ -99,17 +99,17 @@ function validationMessage(items: unknown[]): string | undefined {
     return messages.length ? messages.join('; ') : undefined;
 }
 
-function messageFromValue(value: unknown): string | undefined {
+function messageFromValue(value: unknown, depth = 0): string | undefined {
+    if (depth > 4) return undefined;
     if (typeof value === 'string') return optionalString(value);
     if (Array.isArray(value)) return validationMessage(value);
     if (!isRecord(value)) return value == null ? undefined : String(value);
 
-    const direct = optionalString(value.message) ?? optionalString(value.detail);
+    const direct = messageFromValue(value.message, depth + 1)
+        ?? messageFromValue(value.detail, depth + 1);
     if (direct) return direct;
-    if (isRecord(value.error)) {
-        const nested = optionalString(value.error.message) ?? optionalString(value.error.detail);
-        if (nested) return nested;
-    }
+    const nested = messageFromValue(value.error, depth + 1);
+    if (nested) return nested;
     return stableStringify(value);
 }
 
@@ -180,14 +180,18 @@ export async function parseHttpErrorResponse(response: Response): Promise<ApiErr
     });
 }
 
+export function getErrorMessage(error: unknown, fallback: string): string {
+    const value = error instanceof Error ? error.message : error;
+    const message = messageFromValue(value);
+    return message && message !== '[object Object]' ? message : fallback;
+}
+
 export function normalizeUnknownError(
     error: unknown,
     context: Partial<Omit<AppErrorContext, 'message'>> = {},
 ): AppError {
     if (error instanceof AppError) return error;
-    const message = error instanceof Error
-        ? error.message
-        : messageFromValue(error) ?? 'Unknown error';
+    const message = getErrorMessage(error, 'Unknown error');
     return new AppError({
         message,
         code: context.code ?? 'unknown_error',
diff --git a/frontend/tests/apiError.test.mjs b/frontend/tests/apiError.test.mjs
index 69e47689e..cfdc7407d 100644
--- a/frontend/tests/apiError.test.mjs
+++ b/frontend/tests/apiError.test.mjs
@@ -5,6 +5,7 @@ import test from 'node:test';
 import {
   ApiError,
   AppError,
+  getErrorMessage,
   normalizeUnknownError,
   parseHttpError,
   parseHttpErrorResponse,
@@ -104,6 +105,18 @@ test('plain text and empty responses receive useful messages', () => {
   assert.equal(parseHttpError({ status: 404, statusText: 'Not Found', bodyText: '' }).message, 'HTTP 404 Not Found');
 });
 
+test('user-facing errors unwrap nested messages and reject object coercion', () => {
+  assert.equal(
+    getErrorMessage({ message: { error: { message: 'Session backend unavailable' } } }, 'Could not create session'),
+    'Session backend unavailable',
+  );
+
+  const malformed = new Error('placeholder');
+  Object.defineProperty(malformed, 'message', { value: { detail: 'Session creation denied' } });
+  assert.equal(getErrorMessage(malformed, 'Could not create session'), 'Session creation denied');
+  assert.equal(getErrorMessage(new Error('[object Object]'), 'Could not create session'), 'Could not create session');
+});
+
 test('unknown thrown values normalize to typed AppError instances', () => {
   const native = normalizeUnknownError(new Error('connection reset'), {
     code: 'network_error',
diff --git a/frontend/tests/groupInteractionContract.test.mjs b/frontend/tests/groupInteractionContract.test.mjs
index be9a1556c..21819a2cb 100644
--- a/frontend/tests/groupInteractionContract.test.mjs
+++ b/frontend/tests/groupInteractionContract.test.mjs
@@ -31,6 +31,14 @@ const groupStyles = readFileSync(
   'utf8',
 );
 
+test('session creation displays a normalized string error instead of object coercion', () => {
+  assert.match(groupsPage, /import \{ getErrorMessage \} from '\.\.\/\.\.\/services\/apiError'/);
+  assert.match(
+    groupsPage,
+    /const createSession = async[\s\S]*?catch \(error: unknown\) \{[\s\S]*?toast\.error\(getErrorMessage\(error, t\('groups\.createSessionFailed'/,
+  );
+});
+
 test('new group sessions may use the backend default title while group names stay required', () => {
   assert.match(promptModal, /allowEmpty\?: boolean/);
   assert.match(promptModal, /allowEmpty \|\| Boolean\(value\.trim\(\)\)/);