diff --git a/app/game-api/src/router/inherit/index.ts b/app/game-api/src/router/inherit/index.ts
index 76a1e3ae..2bb0886e 100644
--- a/app/game-api/src/router/inherit/index.ts
+++ b/app/game-api/src/router/inherit/index.ts
@@ -38,6 +38,8 @@ const BUFF_KEYS: InheritBuffType[] = [
 const UNIQUE_ITEM_SLOT_ORDER: readonly ItemSlot[] = ['horse', 'weapon', 'book', 'item'];
 
 const POSTGRES_INTEGER_MAX = 2_147_483_647;
+const MIN_INHERIT_BUFF_LEVEL = 1;
+const MAX_INHERIT_BUFF_LEVEL = 5;
 
 const parseBuffRecord = (raw: unknown): Record<string, number> => {
     if (typeof raw === 'string') {
@@ -302,7 +304,7 @@ export const inheritRouter = router({
         .input(
             z.object({
                 type: z.enum(BUFF_KEYS),
-                level: z.number().int().min(1).max(5),
+                level: z.number().int(),
             })
         )
         .mutation(async ({ ctx, input }) => {
@@ -310,6 +312,12 @@ export const inheritRouter = router({
             if (!userId) {
                 throw new TRPCError({ code: 'UNAUTHORIZED' });
             }
+            if (input.level < MIN_INHERIT_BUFF_LEVEL || input.level > MAX_INHERIT_BUFF_LEVEL) {
+                throw new TRPCError({
+                    code: 'BAD_REQUEST',
+                    message: '유산 강화는 1단계부터 5단계까지만 구입할 수 있습니다.',
+                });
+            }
 
             const result = await requestInheritanceAction(ctx, userId, {
                 action: 'buyHiddenBuff',
diff --git a/app/game-api/test/inheritRouter.test.ts b/app/game-api/test/inheritRouter.test.ts
index ae9af24c..34a4e00c 100644
--- a/app/game-api/test/inheritRouter.test.ts
+++ b/app/game-api/test/inheritRouter.test.ts
@@ -285,6 +285,21 @@ describe('inherit router actor and permission boundaries', () => {
         });
     });
 
+    it('rejects an out-of-range inheritance buff level with a user-facing message', async () => {
+        const fixture = buildContext({});
+
+        await expect(
+            appRouter.createCaller(fixture.context).inherit.buyHiddenBuff({
+                type: 'warAvoidRatio',
+                level: 6,
+            })
+        ).rejects.toMatchObject({
+            code: 'BAD_REQUEST',
+            message: '유산 강화는 1단계부터 5단계까지만 구입할 수 있습니다.',
+        });
+        expect(fixture.requestCommand).not.toHaveBeenCalled();
+    });
+
     it('reports the Ref S100 stat-reset ban and maps the authoritative daemon rejection', async () => {
         const fixture = buildContext({
             configMap: { targetGeneralPool: 'SPoolUnderU100' },
diff --git a/tools/frontend-legacy-parity/inheritance-management.spec.ts b/tools/frontend-legacy-parity/inheritance-management.spec.ts
index 185ade7c..32d99348 100644
--- a/tools/frontend-legacy-parity/inheritance-management.spec.ts
+++ b/tools/frontend-legacy-parity/inheritance-management.spec.ts
@@ -9,6 +9,13 @@ const artifactRoot = process.env.FRONTEND_PARITY_ARTIFACT_DIR;
 const gameUrl = `http://127.0.0.1:${process.env.FRONTEND_PARITY_GAME_PORT ?? '15102'}/che/inherit`;
 
 const response = (data: unknown) => ({ result: { data } });
+const errorResponse = (path: string, message: string) => ({
+    error: {
+        message,
+        code: -32000,
+        data: { code: 'BAD_REQUEST', httpStatus: 400, path },
+    },
+});
 
 const operations = (route: Route): string[] => {
     const pathname = new URL(route.request().url()).pathname;
@@ -149,9 +156,11 @@ const installFixture = async (
         const requestBody: unknown = route.request().postData() ? route.request().postDataJSON() : null;
         if (options.failBuff && names.includes('inherit.buyHiddenBuff')) {
             await route.fulfill({
-                status: 500,
+                status: 400,
                 contentType: 'application/json',
-                body: JSON.stringify({ error: { message: '의도한 유산 구입 오류' } }),
+                body: JSON.stringify([
+                    errorResponse('inherit.buyHiddenBuff', '유산 강화는 1단계부터 5단계까지만 구입할 수 있습니다.'),
+                ]),
             });
             return;
         }
@@ -451,6 +460,9 @@ test.describe('inheritance management legacy parity', () => {
         await page.locator('#buff-warAvoidRatio').fill('1');
         await page.locator('#buff-warAvoidRatio').locator('xpath=../..').getByRole('button', { name: '구입' }).click();
         await expect(page.locator('[role="alert"]')).toBeVisible();
+        await expect(page.locator('[role="alert"]')).toContainText(
+            '유산 강화는 1단계부터 5단계까지만 구입할 수 있습니다.'
+        );
         await expect(page.locator('#buff-warAvoidRatio')).toHaveValue('1');
         await expect(page.locator('#buff-warAvoidRatio')).toBeEnabled();
     });