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/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/);
+  });
+});