diff --git a/core/domain/src/test/java/com/afternote/core/domain/usecase/auth/LoginUseCaseTest.kt b/core/domain/src/test/java/com/afternote/core/domain/usecase/auth/LoginUseCaseTest.kt
new file mode 100644
index 00000000..ef08bfef
--- /dev/null
+++ b/core/domain/src/test/java/com/afternote/core/domain/usecase/auth/LoginUseCaseTest.kt
@@ -0,0 +1,176 @@
+package com.afternote.core.domain.usecase.auth
+
+import com.afternote.core.domain.repository.auth.AuthRepository
+import com.afternote.core.model.Session
+import com.afternote.core.model.TokenBundle
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.flowOf
+import kotlinx.coroutines.runBlocking
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertSame
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * [LoginUseCase] 비즈니스 로직 회귀 가드.
+ *
+ * 검증 핵심:
+ * 1. 로그인 타입(Email/Kakao/Google)에 맞는 [AuthRepository] 메서드로 분기하는지
+ * 2. 로그인 성공 시 받은 세션 토큰을 그대로 [AuthRepository.saveSession]에 전달하는지
+ * 3. 로그인 실패면 **saveSession을 호출하지 않고** 그 실패를 그대로 반환(short-circuit)하는지
+ * 4. saveSession 실패면 그 실패를 반환하는지
+ * 5. 반환값(신규 가입자 여부) — 소셜 `newUser=true` 만 true, 이메일·기존(false)·null 은 false
+ *
+ * 외부 라이브러리(mockk 등) 없이 호출 인자/횟수를 기록하는 직접 작성 fake를 사용한다.
+ */
+class LoginUseCaseTest {
+    @Test
+    fun `Email 로그인 성공 - defaultLogin 호출 후 세션 토큰으로 saveSession`() {
+        val repo =
+            FakeAuthRepository().apply {
+                defaultLoginResult = Result.success(Session.DefaultSession(accessToken = "AT", refreshToken = "RT"))
+            }
+        val result = runBlocking { LoginUseCase(repo)(LoginType.Email(email = "a@b.com", password = "pw")) }
+
+        assertTrue(result.isSuccess)
+        assertFalse(result.getOrThrow()) // 이메일 로그인 = 기존 유저(false)
+        assertEquals("a@b.com" to "pw", repo.defaultLoginArgs)
+        assertEquals("AT" to "RT", repo.saveSessionArgs)
+        assertEquals(1, repo.saveSessionCallCount)
+    }
+
+    @Test
+    fun `Kakao 로그인 성공 - kakaoLogin 호출 + saveSession`() {
+        val repo =
+            FakeAuthRepository().apply {
+                kakaoResult =
+                    Result.success(Session.SocialSession(accessToken = "KAT", refreshToken = "KRT", isNewUser = true))
+            }
+        val result = runBlocking { LoginUseCase(repo)(LoginType.Kakao(oauthToken = "kakao-token")) }
+
+        assertTrue(result.isSuccess)
+        assertTrue(result.getOrThrow()) // isNewUser=true → 신규
+        assertEquals("kakao-token", repo.kakaoArg)
+        assertEquals("KAT" to "KRT", repo.saveSessionArgs)
+    }
+
+    @Test
+    fun `Google 로그인 성공 - googleLogin 호출 + saveSession`() {
+        val repo =
+            FakeAuthRepository().apply {
+                googleResult =
+                    Result.success(Session.SocialSession(accessToken = "GAT", refreshToken = "GRT", isNewUser = false))
+            }
+        val result = runBlocking { LoginUseCase(repo)(LoginType.Google(idToken = "google-id-token")) }
+
+        assertTrue(result.isSuccess)
+        assertFalse(result.getOrThrow()) // isNewUser=false → 기존
+        assertEquals("google-id-token", repo.googleArg)
+        assertEquals("GAT" to "GRT", repo.saveSessionArgs)
+    }
+
+    @Test
+    fun `소셜 로그인 - isNewUser null 이면 false (기존 유저 취급)`() {
+        val repo =
+            FakeAuthRepository().apply {
+                kakaoResult =
+                    Result.success(Session.SocialSession(accessToken = "KAT", refreshToken = "KRT", isNewUser = null))
+            }
+        val result = runBlocking { LoginUseCase(repo)(LoginType.Kakao(oauthToken = "kakao-token")) }
+
+        assertTrue(result.isSuccess)
+        assertFalse(result.getOrThrow()) // null → false (`== true` 가 null 흡수)
+    }
+
+    @Test
+    fun `로그인 실패면 saveSession 호출하지 않고 실패를 그대로 반환`() {
+        val loginError = IllegalStateException("login failed")
+        val repo =
+            FakeAuthRepository().apply {
+                defaultLoginResult = Result.failure(loginError)
+            }
+        val result = runBlocking { LoginUseCase(repo)(LoginType.Email(email = "a@b.com", password = "pw")) }
+
+        assertTrue(result.isFailure)
+        assertSame(loginError, result.exceptionOrNull())
+        assertEquals(0, repo.saveSessionCallCount)
+        assertNull(repo.saveSessionArgs)
+    }
+
+    @Test
+    fun `saveSession 실패면 그 실패를 반환`() {
+        val saveError = IllegalStateException("save failed")
+        val repo =
+            FakeAuthRepository().apply {
+                defaultLoginResult = Result.success(Session.DefaultSession(accessToken = "AT", refreshToken = "RT"))
+                saveSessionResult = Result.failure(saveError)
+            }
+        val result = runBlocking { LoginUseCase(repo)(LoginType.Email(email = "a@b.com", password = "pw")) }
+
+        assertFalse(result.isSuccess)
+        assertSame(saveError, result.exceptionOrNull())
+        assertEquals(1, repo.saveSessionCallCount)
+    }
+
+    private class FakeAuthRepository : AuthRepository {
+        var defaultLoginArgs: Pair<String, String>? = null
+        var kakaoArg: String? = null
+        var googleArg: String? = null
+        var saveSessionArgs: Pair<String, String>? = null
+        var saveSessionCallCount = 0
+
+        var defaultLoginResult: Result<Session.DefaultSession> =
+            Result.success(Session.DefaultSession(accessToken = "at", refreshToken = "rt"))
+        var kakaoResult: Result<Session.SocialSession> =
+            Result.success(Session.SocialSession(accessToken = "at", refreshToken = "rt", isNewUser = false))
+        var googleResult: Result<Session.SocialSession> =
+            Result.success(Session.SocialSession(accessToken = "at", refreshToken = "rt", isNewUser = false))
+        var saveSessionResult: Result<Unit> = Result.success(Unit)
+
+        override val isLoggedIn: Flow<Boolean> = flowOf(false)
+
+        override suspend fun saveSession(
+            accessToken: String,
+            refreshToken: String,
+        ): Result<Unit> {
+            saveSessionCallCount++
+            saveSessionArgs = accessToken to refreshToken
+            return saveSessionResult
+        }
+
+        override suspend fun updateTokens(
+            accessToken: String,
+            refreshToken: String,
+        ): Result<Unit> = Result.success(Unit)
+
+        override suspend fun clearSession(): Result<Unit> = Result.success(Unit)
+
+        override suspend fun getAccessToken(): Result<String?> = Result.success(null)
+
+        override suspend fun getRefreshToken(): Result<String?> = Result.success(null)
+
+        override suspend fun defaultLogin(
+            email: String,
+            password: String,
+        ): Result<Session.DefaultSession> {
+            defaultLoginArgs = email to password
+            return defaultLoginResult
+        }
+
+        override suspend fun kakaoLogin(oauthToken: String): Result<Session.SocialSession> {
+            kakaoArg = oauthToken
+            return kakaoResult
+        }
+
+        override suspend fun googleLogin(idToken: String): Result<Session.SocialSession> {
+            googleArg = idToken
+            return googleResult
+        }
+
+        override suspend fun rotateToken(): Result<TokenBundle> = Result.success(TokenBundle(accessToken = "at", refreshToken = "rt"))
+
+        override suspend fun logout(): Result<Unit> = Result.success(Unit)
+    }
+}
diff --git a/feature/afternote/domain/src/test/java/com/afternote/feature/afternote/domain/usecase/editor/ResolveMemorialMediaForSaveUseCaseTest.kt b/feature/afternote/domain/src/test/java/com/afternote/feature/afternote/domain/usecase/editor/ResolveMemorialMediaForSaveUseCaseTest.kt
new file mode 100644
index 00000000..306a07cf
--- /dev/null
+++ b/feature/afternote/domain/src/test/java/com/afternote/feature/afternote/domain/usecase/editor/ResolveMemorialMediaForSaveUseCaseTest.kt
@@ -0,0 +1,149 @@
+package com.afternote.feature.afternote.domain.usecase.editor
+
+import com.afternote.feature.afternote.domain.repository.author.MediaInput
+import com.afternote.feature.afternote.domain.repository.author.MemorialPhotoUploadRepository
+import com.afternote.feature.afternote.domain.repository.author.MemorialVideoUploadRepository
+import com.afternote.feature.afternote.domain.repository.author.PhotoUploadOutcome
+import com.afternote.feature.afternote.domain.repository.author.VideoUploadOutcome
+import kotlinx.coroutines.runBlocking
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertSame
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * [ResolveMemorialMediaForSaveUseCase] 비즈니스 로직 회귀 가드.
+ *
+ * 검증 핵심:
+ * 1. 입력 [MediaInput] 을 각 Repository 에 그대로 전달하는지 (로컬/원격 확정은 호출부 책임)
+ * 2. sealed [VideoUploadOutcome]/[PhotoUploadOutcome] 분기를 저장 페이로드 URL 로 매핑하는지
+ *    (Empty→null, Existing→url, FreshlyUploaded→url)
+ * 3. 영상 resolve 실패면 [MemorialVideoSaveException] 으로 wrap 하고 **사진 Repository 는 호출하지 않은 채**
+ *    short-circuit 하는지 (non-local return)
+ * 4. 사진 resolve 실패면 [MemorialPhotoSaveException] 으로 wrap 하고, 두 경우 모두 원본 예외를 cause 로 보존하는지
+ *
+ * 외부 라이브러리(mockk 등) 없이 호출 인자/횟수를 기록하는 직접 작성 fake를 사용한다.
+ */
+class ResolveMemorialMediaForSaveUseCaseTest {
+    @Test
+    fun `영상-사진 모두 성공 - FreshlyUploaded 영상과 Existing 사진을 각 URL 로 매핑`() {
+        val videoRepo = FakeVideoUploadRepository().apply { result = Result.success(VideoUploadOutcome.FreshlyUploaded("v-url")) }
+        val photoRepo = FakePhotoUploadRepository().apply { result = Result.success(PhotoUploadOutcome.Existing("p-url")) }
+
+        val result =
+            runBlocking {
+                ResolveMemorialMediaForSaveUseCase(videoRepo, photoRepo)(
+                    video = MediaInput.Local("content://video"),
+                    photo = MediaInput.Remote("https://cdn/photo.jpg"),
+                )
+            }
+
+        assertTrue(result.isSuccess)
+        assertEquals("v-url", result.getOrThrow().resolvedVideoUrl)
+        assertEquals("p-url", result.getOrThrow().resolvedMemorialPhotoUrl)
+    }
+
+    @Test
+    fun `Empty Outcome 은 null 로 매핑 - 미첨부`() {
+        val videoRepo = FakeVideoUploadRepository().apply { result = Result.success(VideoUploadOutcome.Empty) }
+        val photoRepo = FakePhotoUploadRepository().apply { result = Result.success(PhotoUploadOutcome.Empty) }
+
+        val resolved =
+            runBlocking {
+                ResolveMemorialMediaForSaveUseCase(videoRepo, photoRepo)(MediaInput.None, MediaInput.None)
+            }.getOrThrow()
+
+        assertNull(resolved.resolvedVideoUrl)
+        assertNull(resolved.resolvedMemorialPhotoUrl)
+    }
+
+    @Test
+    fun `Existing 영상과 FreshlyUploaded 사진도 각 URL 로 매핑`() {
+        val videoRepo = FakeVideoUploadRepository().apply { result = Result.success(VideoUploadOutcome.Existing("existing-v")) }
+        val photoRepo = FakePhotoUploadRepository().apply { result = Result.success(PhotoUploadOutcome.FreshlyUploaded("fresh-p")) }
+
+        val resolved =
+            runBlocking {
+                ResolveMemorialMediaForSaveUseCase(videoRepo, photoRepo)(
+                    video = MediaInput.Remote("https://cdn/existing-v.mp4"),
+                    photo = MediaInput.Local("content://fresh-photo"),
+                )
+            }.getOrThrow()
+
+        assertEquals("existing-v", resolved.resolvedVideoUrl)
+        assertEquals("fresh-p", resolved.resolvedMemorialPhotoUrl)
+    }
+
+    @Test
+    fun `입력 MediaInput 을 각 Repository 에 그대로 전달`() {
+        val videoRepo = FakeVideoUploadRepository()
+        val photoRepo = FakePhotoUploadRepository()
+
+        val videoInput = MediaInput.Local("content://fv")
+        val photoInput = MediaInput.Remote("https://cdn/mp.jpg")
+        runBlocking {
+            ResolveMemorialMediaForSaveUseCase(videoRepo, photoRepo)(videoInput, photoInput)
+        }
+
+        assertEquals(videoInput, videoRepo.resolveVideoArg)
+        assertEquals(photoInput, photoRepo.resolvePhotoArg)
+    }
+
+    @Test
+    fun `영상 resolve 실패면 MemorialVideoSaveException 으로 wrap 하고 사진 Repository 는 호출하지 않음`() {
+        val videoError = IllegalStateException("video resolve failed")
+        val videoRepo = FakeVideoUploadRepository().apply { result = Result.failure(videoError) }
+        val photoRepo = FakePhotoUploadRepository()
+
+        val result =
+            runBlocking {
+                ResolveMemorialMediaForSaveUseCase(videoRepo, photoRepo)(MediaInput.Local("content://v"), MediaInput.None)
+            }
+
+        assertTrue(result.isFailure)
+        assertTrue(result.exceptionOrNull() is MemorialVideoSaveException)
+        assertSame(videoError, result.exceptionOrNull()?.cause) // 원본 예외를 cause 로 보존
+        assertEquals(0, photoRepo.callCount) // short-circuit — 사진 Repository 미호출
+    }
+
+    @Test
+    fun `사진 resolve 실패면 MemorialPhotoSaveException 으로 wrap`() {
+        val photoError = IllegalStateException("photo resolve failed")
+        val videoRepo = FakeVideoUploadRepository().apply { result = Result.success(VideoUploadOutcome.Empty) }
+        val photoRepo = FakePhotoUploadRepository().apply { result = Result.failure(photoError) }
+
+        val result =
+            runBlocking {
+                ResolveMemorialMediaForSaveUseCase(videoRepo, photoRepo)(MediaInput.None, MediaInput.Remote("https://cdn/p.jpg"))
+            }
+
+        assertTrue(result.isFailure)
+        assertTrue(result.exceptionOrNull() is MemorialPhotoSaveException)
+        assertSame(photoError, result.exceptionOrNull()?.cause)
+    }
+
+    private class FakeVideoUploadRepository : MemorialVideoUploadRepository {
+        var resolveVideoArg: MediaInput? = null
+        var callCount = 0
+        var result: Result<VideoUploadOutcome> = Result.success(VideoUploadOutcome.Empty)
+
+        override suspend fun resolveVideo(input: MediaInput): Result<VideoUploadOutcome> {
+            callCount++
+            resolveVideoArg = input
+            return result
+        }
+    }
+
+    private class FakePhotoUploadRepository : MemorialPhotoUploadRepository {
+        var resolvePhotoArg: MediaInput? = null
+        var callCount = 0
+        var result: Result<PhotoUploadOutcome> = Result.success(PhotoUploadOutcome.Empty)
+
+        override suspend fun resolvePhoto(input: MediaInput): Result<PhotoUploadOutcome> {
+            callCount++
+            resolvePhotoArg = input
+            return result
+        }
+    }
+}