diff --git a/prisma/seed/credential-policy.ts b/prisma/seed/credential-policy.ts
index b5f14679..1597bd6b 100644
--- a/prisma/seed/credential-policy.ts
+++ b/prisma/seed/credential-policy.ts
@@ -3,16 +3,26 @@
  * 통과 못 하는 값으로 만들어 두면 시드는 성공하는데 로그인은 ValidationPipe에서 전부 거절된다.
  */
 import { IsStrongPasswordConstraint } from '@/common/validators/strong-password.validator';
-
-const USERNAME_PATTERN = /^[a-z0-9._-]{4,80}$/;
+import {
+  MAX_USERNAME_LENGTH,
+  MIN_USERNAME_LENGTH,
+  USERNAME_PATTERN,
+} from '@/features/auth/constants/auth-admin.constants';
 
 export function assertSeedCredential(args: {
   username: string;
   password: string;
 }): void {
-  if (!USERNAME_PATTERN.test(args.username)) {
+  const { username } = args;
+  if (
+    username.length < MIN_USERNAME_LENGTH ||
+    username.length > MAX_USERNAME_LENGTH ||
+    !USERNAME_PATTERN.test(username)
+  ) {
     // 값은 메시지에 싣지 않는다 — env에서 온 자격증명이 오류 로그로 새는 경로(CodeQL clear-text-logging)
-    throw new Error('시드 username 정책 위반: 4~80자, 소문자·숫자·._- 만 허용');
+    throw new Error(
+      `시드 username 정책 위반: ${MIN_USERNAME_LENGTH}~${MAX_USERNAME_LENGTH}자, 영문 대소문자·숫자·._- 만 허용`,
+    );
   }
   if (!new IsStrongPasswordConstraint().validate(args.password)) {
     throw new Error(
diff --git a/src/features/auth/auth-admin-account.graphql b/src/features/auth/auth-admin-account.graphql
index 91ef591c..dd09a877 100644
--- a/src/features/auth/auth-admin-account.graphql
+++ b/src/features/auth/auth-admin-account.graphql
@@ -85,7 +85,7 @@ type AdminAccountConnection {
 """
 input AdminCreateAdminInput {
   """
-  로그인 username. 4~80자, 소문자·숫자·`.`·`_`·`-`만 허용. 이미 쓰이고 있으면 BAD_USER_INPUT.
+  로그인 username. 4~80자, 영문 대소문자·숫자·`.`·`_`·`-`만 허용. 이미 쓰이고 있으면(대소문자만 달라도) BAD_USER_INPUT.
   """
   username: String!
   """
diff --git a/src/features/auth/constants/auth-admin.constants.ts b/src/features/auth/constants/auth-admin.constants.ts
index 21c36ed6..e31a8091 100644
--- a/src/features/auth/constants/auth-admin.constants.ts
+++ b/src/features/auth/constants/auth-admin.constants.ts
@@ -2,8 +2,8 @@
 
 export const MIN_USERNAME_LENGTH = 4;
 export const MAX_USERNAME_LENGTH = 80;
-/** 정책: 소문자·숫자·`.`·`_`·`-`만. 대소문자 혼용 username 충돌을 원천 차단한다. */
-export const USERNAME_PATTERN = /^[a-z0-9._-]+$/;
+/** 정책: 영문 대소문자·숫자·`.`·`_`·`-`만. 대소문자만 다른 username은 컬럼 정렬(ci)이 같은 값으로 봐 중복으로 막는다. */
+export const USERNAME_PATTERN = /^[A-Za-z0-9._-]+$/;
 
 export const MAX_EMAIL_LENGTH = 320;
 export const MAX_ACCOUNT_NAME_LENGTH = 100;
diff --git a/src/features/auth/dto/inputs/admin-create-admin.input.spec.ts b/src/features/auth/dto/inputs/admin-create-admin.input.spec.ts
index c6074f9f..0ab692e8 100644
--- a/src/features/auth/dto/inputs/admin-create-admin.input.spec.ts
+++ b/src/features/auth/dto/inputs/admin-create-admin.input.spec.ts
@@ -19,10 +19,15 @@ describe('AdminCreateAdminInput', () => {
     ).toHaveLength(0);
   });
 
+  it('username에 영문 대문자를 허용한다', async () => {
+    expect(
+      await validate(build({ ...valid, username: 'Ops.Admin_1' })),
+    ).toHaveLength(0);
+  });
+
   it.each([
     ['길이 4 미만', 'abc'],
     ['길이 80 초과', 'a'.repeat(81)],
-    ['대문자 포함', 'Admin1'],
     ['공백 포함', 'ad min'],
     ['허용 외 문자(@)', 'ad@min'],
     ['한글', '관리자계정'],
diff --git a/src/features/auth/repositories/account-credential.repository.spec.ts b/src/features/auth/repositories/account-credential.repository.spec.ts
index 4f42f0ef..ab0994b9 100644
--- a/src/features/auth/repositories/account-credential.repository.spec.ts
+++ b/src/features/auth/repositories/account-credential.repository.spec.ts
@@ -77,6 +77,18 @@ describe('AccountCredentialRepository (real DB)', () => {
       const found = await repo.findCredentialByUsername('nonexistent');
       expect(found).toBeNull();
     });
+
+    // 로그인은 대소문자를 구분하지 않는다 — username 컬럼 정렬(ci)이 보장하는 계약
+    it.each(['ops.admin', 'OPS.ADMIN', 'Ops.Admin'])(
+      '대소문자가 달라도 같은 자격증명을 찾는다: %s',
+      async (input) => {
+        await createAccountCredential(prisma, { username: 'Ops.Admin' });
+
+        const found = await repo.findCredentialByUsername(input);
+
+        expect(found!.username).toBe('Ops.Admin');
+      },
+    );
   });
 
   describe('findCredentialByAccountId', () => {
diff --git a/src/features/auth/services/auth-admin-account.service.spec.ts b/src/features/auth/services/auth-admin-account.service.spec.ts
index 4a0a868b..86ccc215 100644
--- a/src/features/auth/services/auth-admin-account.service.spec.ts
+++ b/src/features/auth/services/auth-admin-account.service.spec.ts
@@ -270,6 +270,23 @@ describe('AdminAccountService (real DB)', () => {
       ).rejects.toThrowDomain(400);
     });
 
+    it('대문자 username은 입력 그대로 저장하고, 대소문자만 다른 username은 충돌한다', async () => {
+      const actor = await makeAdmin();
+
+      const created = await service.adminCreateAdmin(actor, {
+        ...validInput,
+        username: 'Ops.Admin',
+      });
+      expect(created.username).toBe('Ops.Admin');
+
+      await expect(
+        service.adminCreateAdmin(actor, {
+          ...validInput,
+          username: 'ops.admin',
+        }),
+      ).rejects.toThrowDomain('USERNAME_TAKEN');
+    });
+
     it('판매자가 쓰는 username도 충돌한다(자격증명 테이블 공용)', async () => {
       const actor = await makeAdmin();
       await createAccountCredential(prisma, {
diff --git a/src/features/store/dto/inputs/admin-create-seller.input.spec.ts b/src/features/store/dto/inputs/admin-create-seller.input.spec.ts
index b0f9c0d6..2ba89754 100644
--- a/src/features/store/dto/inputs/admin-create-seller.input.spec.ts
+++ b/src/features/store/dto/inputs/admin-create-seller.input.spec.ts
@@ -27,6 +27,12 @@ describe('AdminCreateSellerInput', () => {
     expect(await validate(build(valid))).toHaveLength(0);
   });
 
+  it('username에 영문 대문자를 허용한다', async () => {
+    expect(
+      await validate(build({ ...valid, username: 'Shop.Owner' })),
+    ).toHaveLength(0);
+  });
+
   it('store 누락 거절', async () => {
     const { store: _store, ...withoutStore } = valid;
     const errors = await validate(build(withoutStore));
@@ -69,7 +75,7 @@ describe('AdminCreateSellerInput', () => {
   });
 
   it.each([
-    ['username 대문자', { username: 'Shop' }],
+    ['username 허용 외 문자', { username: 'shop@owner' }],
     ['email 형식', { email: 'nope' }],
     ['businessName 누락', { businessName: undefined }],
   ])('%s 거절', async (_label, overrides) => {
diff --git a/src/features/store/services/store-admin-seller.service.spec.ts b/src/features/store/services/store-admin-seller.service.spec.ts
index 8942deea..cb65e9c5 100644
--- a/src/features/store/services/store-admin-seller.service.spec.ts
+++ b/src/features/store/services/store-admin-seller.service.spec.ts
@@ -360,6 +360,21 @@ describe('AdminSellerService (real DB)', () => {
       ).rejects.toThrowDomain(400);
     });
 
+    it('대소문자만 다른 username도 충돌한다', async () => {
+      const actor = await admin();
+      await createAccountCredential(prisma, {
+        account_type: 'SELLER',
+        username: 'Cake.Shop',
+      });
+
+      await expect(
+        service.adminCreateSeller(actor, {
+          ...validInput,
+          username: 'cake.shop',
+        }),
+      ).rejects.toThrowDomain('USERNAME_TAKEN');
+    });
+
     it('감사 기록이 실패하면 계정·매장도 롤백된다(같은 트랜잭션)', async () => {
       const actor = await admin();
       const auditLogs = service['auditLogs'];
diff --git a/src/features/store/store-admin-seller.graphql b/src/features/store/store-admin-seller.graphql
index d91699c2..f2de9291 100644
--- a/src/features/store/store-admin-seller.graphql
+++ b/src/features/store/store-admin-seller.graphql
@@ -158,7 +158,7 @@ input AdminSellerListInput {
 """
 input AdminCreateSellerInput {
   """
-  로그인 username. 4~80자, 소문자·숫자·`.`·`_`·`-`만 허용. 이미 쓰이고 있으면 BAD_USER_INPUT.
+  로그인 username. 4~80자, 영문 대소문자·숫자·`.`·`_`·`-`만 허용. 이미 쓰이고 있으면(대소문자만 달라도) BAD_USER_INPUT.
   """
   username: String!
   """
diff --git a/src/test/seed-credential-policy.spec.ts b/src/test/seed-credential-policy.spec.ts
new file mode 100644
index 00000000..e6428f12
--- /dev/null
+++ b/src/test/seed-credential-policy.spec.ts
@@ -0,0 +1,61 @@
+import 'reflect-metadata';
+
+import { plainToInstance } from 'class-transformer';
+import { validate } from 'class-validator';
+
+import { assertSeedCredential } from '../../prisma/seed/credential-policy';
+
+import { AdminCreateAdminInput } from '@/features/auth/dto/inputs/admin-create-admin.input';
+
+const PASSWORD = 'Strong!Pass1';
+
+function seedAccepts(username: string): boolean {
+  try {
+    assertSeedCredential({ username, password: PASSWORD });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+async function dtoAccepts(username: string): Promise<boolean> {
+  const input = plainToInstance(AdminCreateAdminInput, {
+    username,
+    password: PASSWORD,
+  });
+  return (await validate(input)).length === 0;
+}
+
+// 시드가 만든 계정이 관리자 생성 경로와 같은 username 정책을 따르는지 — 두 판정이 어긋나면 안 된다
+describe('시드 자격증명 username 정책', () => {
+  it.each([
+    ['소문자', 'ops.admin', true],
+    ['대문자 혼용', 'Ops.Admin_1', true],
+    ['전부 대문자', 'OPS-ADMIN', true],
+    ['4자', 'abcd', true],
+    ['80자', 'a'.repeat(80), true],
+    ['3자', 'abc', false],
+    ['81자', 'a'.repeat(81), false],
+    ['공백', 'ops admin', false],
+    ['허용 외 문자(@)', 'ops@admin', false],
+    ['한글', '관리자계정', false],
+  ])('%s: 생성 DTO와 같은 판정(%s → %s)', async (_label, username, ok) => {
+    expect(seedAccepts(username)).toBe(ok);
+    expect(await dtoAccepts(username)).toBe(ok);
+  });
+
+  it('위반 메시지에 username 값을 싣지 않는다', () => {
+    expect(() =>
+      assertSeedCredential({ username: 'bad@name', password: PASSWORD }),
+    ).toThrow(/^시드 username 정책 위반: 4~80자/);
+    expect(() =>
+      assertSeedCredential({ username: 'bad@name', password: PASSWORD }),
+    ).not.toThrow(/bad@name/);
+  });
+
+  it('password 정책 위반은 별도 메시지로 거절한다', () => {
+    expect(() =>
+      assertSeedCredential({ username: 'ops.admin', password: 'weak' }),
+    ).toThrow(/^시드 password 정책 위반/);
+  });
+});