diff --git a/src/test/kotlin/kr/co/vividnext/sodalive/admin/partner/agent/assignment/AdminAgentCreatorServiceTest.kt b/src/test/kotlin/kr/co/vividnext/sodalive/admin/partner/agent/assignment/AdminAgentCreatorServiceTest.kt
index ab178c81..2881cbeb 100644
--- a/src/test/kotlin/kr/co/vividnext/sodalive/admin/partner/agent/assignment/AdminAgentCreatorServiceTest.kt
+++ b/src/test/kotlin/kr/co/vividnext/sodalive/admin/partner/agent/assignment/AdminAgentCreatorServiceTest.kt
@@ -59,6 +59,24 @@ class AdminAgentCreatorServiceTest {
         assertEquals(null, relationCaptor.value.unassignedAt)
     }
 
+    @Test
+    @DisplayName("동일한 회원을 에이전트와 크리에이터로 동시에 소속 지정할 수 없다")
+    fun shouldThrowWhenAgentAndCreatorAreSameMember() {
+        val request = AssignAgentCreatorRequest(
+            agentId = 11L,
+            creatorId = 11L,
+            assignedAt = LocalDateTime.of(2026, 4, 9, 10, 0)
+        )
+
+        val exception = assertThrows(SodaException::class.java) {
+            service.assignCreator(request)
+        }
+
+        assertEquals("partner.agent.assignment.invalid_relation", exception.messageKey)
+        Mockito.verifyNoInteractions(memberRepository)
+        Mockito.verifyNoInteractions(relationRepository)
+    }
+
     @Test
     @DisplayName("관리자 소속 지정은 creator row를 잠근 뒤 저장한다")
     fun shouldLockCreatorBeforeAssigning() {