diff --git a/core/domain/build.gradle.kts b/core/domain/build.gradle.kts
index da4e69e..f487d27 100644
--- a/core/domain/build.gradle.kts
+++ b/core/domain/build.gradle.kts
@@ -15,4 +15,6 @@ kotlin {
 dependencies {
     implementation(libs.javax.inject)
     api(libs.kotlinx.coroutines.core)
+    testImplementation(libs.junit)
+    testImplementation(libs.kotlinx.coroutines.test)
 }
diff --git a/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatAction.kt b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatAction.kt
new file mode 100644
index 0000000..0e91be0
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatAction.kt
@@ -0,0 +1,14 @@
+package com.kikidan.domain.model.chat
+
+import java.time.LocalDate
+
+data class ChatAction(
+    val type: ChatActionType,
+    val label: String,
+    val category: String,
+    val date: LocalDate?,
+)
+
+enum class ChatActionType {
+    CALENDAR_ADD,
+}
diff --git a/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatEntry.kt b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatEntry.kt
new file mode 100644
index 0000000..8bb438e
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatEntry.kt
@@ -0,0 +1,29 @@
+package com.kikidan.domain.model.chat
+
+data class ChatEntry(
+    val greeting: String,
+    val suggestions: List<ChatSuggestion>,
+    val quota: ChatQuota,
+)
+
+data class ChatSuggestion(
+    val emoji: String,
+    val label: String,
+    val seedPrompt: String,
+    val category: ChatCategory?,
+)
+
+data class ChatQuota(
+    val used: Int,
+    val limit: Int,
+) {
+    val remaining: Int get() = (limit - used).coerceAtLeast(0)
+}
+
+enum class ChatCategory {
+    RELATIONSHIP,
+    LOVE,
+    ACHIEVEMENT,
+    MONEY,
+    HEALTH,
+}
diff --git a/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatMessage.kt b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatMessage.kt
new file mode 100644
index 0000000..b3a137f
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatMessage.kt
@@ -0,0 +1,23 @@
+package com.kikidan.domain.model.chat
+
+import java.time.Instant
+
+data class ChatMessage(
+    val id: String,
+    val role: MessageRole,
+    val content: String,
+    val status: MessageStatus,
+    val action: ChatAction?,
+    val createdAt: Instant,
+)
+
+enum class MessageRole {
+    USER,
+    ASSISTANT,
+}
+
+enum class MessageStatus {
+    GENERATING,
+    COMPLETED,
+    FAILED,
+}
diff --git a/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatStreamEvent.kt b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatStreamEvent.kt
new file mode 100644
index 0000000..b959fc7
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/model/chat/ChatStreamEvent.kt
@@ -0,0 +1,27 @@
+package com.kikidan.domain.model.chat
+
+sealed interface ChatStreamEvent {
+    data class Start(
+        val conversationId: String,
+        val userMessageId: String,
+        val assistantMessageId: String,
+        val quota: ChatQuota,
+    ) : ChatStreamEvent
+
+    data class Delta(
+        val text: String,
+    ) : ChatStreamEvent
+
+    data class Action(
+        val action: ChatAction,
+    ) : ChatStreamEvent
+
+    data class Done(
+        val assistantMessageId: String,
+    ) : ChatStreamEvent
+
+    data class Error(
+        val code: String,
+        val message: String,
+    ) : ChatStreamEvent
+}
diff --git a/core/domain/src/main/java/com/kikidan/domain/model/chat/Conversation.kt b/core/domain/src/main/java/com/kikidan/domain/model/chat/Conversation.kt
new file mode 100644
index 0000000..128c5a2
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/model/chat/Conversation.kt
@@ -0,0 +1,16 @@
+package com.kikidan.domain.model.chat
+
+import java.time.Instant
+
+data class Conversation(
+    val id: String,
+    val title: String,
+    val messages: List<ChatMessage>,
+)
+
+data class ConversationSummary(
+    val id: String,
+    val title: String,
+    val lastMessageAt: Instant,
+    val unread: Boolean,
+)
diff --git a/core/domain/src/main/java/com/kikidan/domain/repository/ChatRepository.kt b/core/domain/src/main/java/com/kikidan/domain/repository/ChatRepository.kt
new file mode 100644
index 0000000..1e9a49d
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/repository/ChatRepository.kt
@@ -0,0 +1,23 @@
+package com.kikidan.domain.repository
+
+import com.kikidan.domain.model.chat.ChatEntry
+import com.kikidan.domain.model.chat.ChatStreamEvent
+import com.kikidan.domain.model.chat.Conversation
+import com.kikidan.domain.model.chat.ConversationSummary
+import kotlinx.coroutines.flow.Flow
+
+interface ChatRepository {
+    suspend fun getChatEntry(): Result<ChatEntry>
+
+    // conversationId가 null이면 새 대화를 시작한다. 새 대화의 id는 첫 Start 이벤트로 내려온다.
+    fun sendMessage(
+        conversationId: String?,
+        content: String,
+    ): Flow<Result<ChatStreamEvent>>
+
+    suspend fun getConversations(): Result<List<ConversationSummary>>
+
+    suspend fun getConversationDetail(conversationId: String): Result<Conversation>
+
+    suspend fun deleteConversation(conversationId: String): Result<Unit>
+}
diff --git a/core/domain/src/main/java/com/kikidan/domain/usecase/DeleteConversationUseCase.kt b/core/domain/src/main/java/com/kikidan/domain/usecase/DeleteConversationUseCase.kt
new file mode 100644
index 0000000..e3dd03a
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/usecase/DeleteConversationUseCase.kt
@@ -0,0 +1,13 @@
+package com.kikidan.domain.usecase
+
+import com.kikidan.domain.repository.ChatRepository
+import javax.inject.Inject
+
+class DeleteConversationUseCase
+    @Inject
+    constructor(
+        private val chatRepository: ChatRepository,
+    ) {
+        suspend operator fun invoke(conversationId: String): Result<Unit> =
+            chatRepository.deleteConversation(conversationId)
+    }
diff --git a/core/domain/src/main/java/com/kikidan/domain/usecase/GetChatEntryUseCase.kt b/core/domain/src/main/java/com/kikidan/domain/usecase/GetChatEntryUseCase.kt
new file mode 100644
index 0000000..9c5cea7
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/usecase/GetChatEntryUseCase.kt
@@ -0,0 +1,13 @@
+package com.kikidan.domain.usecase
+
+import com.kikidan.domain.model.chat.ChatEntry
+import com.kikidan.domain.repository.ChatRepository
+import javax.inject.Inject
+
+class GetChatEntryUseCase
+    @Inject
+    constructor(
+        private val chatRepository: ChatRepository,
+    ) {
+        suspend operator fun invoke(): Result<ChatEntry> = chatRepository.getChatEntry()
+    }
diff --git a/core/domain/src/main/java/com/kikidan/domain/usecase/GetConversationDetailUseCase.kt b/core/domain/src/main/java/com/kikidan/domain/usecase/GetConversationDetailUseCase.kt
new file mode 100644
index 0000000..f1a9f3e
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/usecase/GetConversationDetailUseCase.kt
@@ -0,0 +1,14 @@
+package com.kikidan.domain.usecase
+
+import com.kikidan.domain.model.chat.Conversation
+import com.kikidan.domain.repository.ChatRepository
+import javax.inject.Inject
+
+class GetConversationDetailUseCase
+    @Inject
+    constructor(
+        private val chatRepository: ChatRepository,
+    ) {
+        suspend operator fun invoke(conversationId: String): Result<Conversation> =
+            chatRepository.getConversationDetail(conversationId)
+    }
diff --git a/core/domain/src/main/java/com/kikidan/domain/usecase/GetConversationsUseCase.kt b/core/domain/src/main/java/com/kikidan/domain/usecase/GetConversationsUseCase.kt
new file mode 100644
index 0000000..4878062
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/usecase/GetConversationsUseCase.kt
@@ -0,0 +1,13 @@
+package com.kikidan.domain.usecase
+
+import com.kikidan.domain.model.chat.ConversationSummary
+import com.kikidan.domain.repository.ChatRepository
+import javax.inject.Inject
+
+class GetConversationsUseCase
+    @Inject
+    constructor(
+        private val chatRepository: ChatRepository,
+    ) {
+        suspend operator fun invoke(): Result<List<ConversationSummary>> = chatRepository.getConversations()
+    }
diff --git a/core/domain/src/main/java/com/kikidan/domain/usecase/SendChatMessageUseCase.kt b/core/domain/src/main/java/com/kikidan/domain/usecase/SendChatMessageUseCase.kt
new file mode 100644
index 0000000..f9c41f6
--- /dev/null
+++ b/core/domain/src/main/java/com/kikidan/domain/usecase/SendChatMessageUseCase.kt
@@ -0,0 +1,33 @@
+package com.kikidan.domain.usecase
+
+import com.kikidan.domain.model.chat.ChatStreamEvent
+import com.kikidan.domain.repository.ChatRepository
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.flowOf
+import javax.inject.Inject
+
+class SendChatMessageUseCase
+    @Inject
+    constructor(
+        private val chatRepository: ChatRepository,
+    ) {
+        operator fun invoke(
+            conversationId: String?,
+            content: String,
+        ): Flow<Result<ChatStreamEvent>> {
+            val trimmed = content.trim()
+            if (trimmed.isEmpty() || trimmed.length > MAX_CONTENT_LENGTH) {
+                return flowOf(
+                    Result.failure(
+                        IllegalArgumentException("메시지는 1자 이상 ${MAX_CONTENT_LENGTH}자 이하여야 합니다."),
+                    ),
+                )
+            }
+            return chatRepository.sendMessage(conversationId, trimmed)
+        }
+
+        companion object {
+            // presentation의 입력창 maxLength도 이 상수를 참조한다.
+            const val MAX_CONTENT_LENGTH = 500
+        }
+    }
diff --git a/core/domain/src/test/java/com/kikidan/domain/fake/FakeChatRepository.kt b/core/domain/src/test/java/com/kikidan/domain/fake/FakeChatRepository.kt
new file mode 100644
index 0000000..d443c6c
--- /dev/null
+++ b/core/domain/src/test/java/com/kikidan/domain/fake/FakeChatRepository.kt
@@ -0,0 +1,36 @@
+package com.kikidan.domain.fake
+
+import com.kikidan.domain.model.chat.ChatEntry
+import com.kikidan.domain.model.chat.ChatStreamEvent
+import com.kikidan.domain.model.chat.Conversation
+import com.kikidan.domain.model.chat.ConversationSummary
+import com.kikidan.domain.repository.ChatRepository
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.asFlow
+
+class FakeChatRepository : ChatRepository {
+    var lastSentConversationId: String? = null
+    var lastSentContent: String? = null
+    var sendMessageCallCount: Int = 0
+    var streamEvents: List<Result<ChatStreamEvent>> = emptyList()
+
+    override suspend fun getChatEntry(): Result<ChatEntry> = Result.failure(NotImplementedError())
+
+    override fun sendMessage(
+        conversationId: String?,
+        content: String,
+    ): Flow<Result<ChatStreamEvent>> {
+        lastSentConversationId = conversationId
+        lastSentContent = content
+        sendMessageCallCount++
+        return streamEvents.asFlow()
+    }
+
+    override suspend fun getConversations(): Result<List<ConversationSummary>> = Result.failure(NotImplementedError())
+
+    override suspend fun getConversationDetail(conversationId: String): Result<Conversation> =
+        Result.failure(NotImplementedError())
+
+    override suspend fun deleteConversation(conversationId: String): Result<Unit> =
+        Result.failure(NotImplementedError())
+}
diff --git a/core/domain/src/test/java/com/kikidan/domain/usecase/SendChatMessageUseCaseTest.kt b/core/domain/src/test/java/com/kikidan/domain/usecase/SendChatMessageUseCaseTest.kt
new file mode 100644
index 0000000..e933c7b
--- /dev/null
+++ b/core/domain/src/test/java/com/kikidan/domain/usecase/SendChatMessageUseCaseTest.kt
@@ -0,0 +1,76 @@
+package com.kikidan.domain.usecase
+
+import com.kikidan.domain.fake.FakeChatRepository
+import kotlinx.coroutines.flow.toList
+import kotlinx.coroutines.test.runTest
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Before
+import org.junit.Test
+
+class SendChatMessageUseCaseTest {
+    private lateinit var fakeRepository: FakeChatRepository
+    private lateinit var useCase: SendChatMessageUseCase
+
+    @Before
+    fun setUp() {
+        fakeRepository = FakeChatRepository()
+        useCase = SendChatMessageUseCase(fakeRepository)
+    }
+
+    @Test
+    fun `정상 content 전송 시 Repository Flow가 그대로 전달된다`() =
+        runTest {
+            val results = useCase(conversationId = null, content = "안녕하세요").toList()
+            assertEquals(1, fakeRepository.sendMessageCallCount)
+            assertEquals("안녕하세요", fakeRepository.lastSentContent)
+        }
+
+    @Test
+    fun `content가 빈 문자열이면 Repository 호출 없이 failure를 방출한다`() =
+        runTest {
+            val results = useCase(conversationId = null, content = "").toList()
+            assertEquals(0, fakeRepository.sendMessageCallCount)
+            assertEquals(1, results.size)
+            assertTrue(results[0].isFailure)
+            assertTrue(results[0].exceptionOrNull() is IllegalArgumentException)
+        }
+
+    @Test
+    fun `content가 공백만이면 Repository 호출 없이 failure를 방출한다`() =
+        runTest {
+            val results = useCase(conversationId = null, content = "   ").toList()
+            assertEquals(0, fakeRepository.sendMessageCallCount)
+            assertEquals(1, results.size)
+            assertTrue(results[0].isFailure)
+            assertTrue(results[0].exceptionOrNull() is IllegalArgumentException)
+        }
+
+    @Test
+    fun `content가 501자이면 Repository 호출 없이 failure를 방출한다`() =
+        runTest {
+            val content = "a".repeat(SendChatMessageUseCase.MAX_CONTENT_LENGTH + 1)
+            val results = useCase(conversationId = null, content = content).toList()
+            assertEquals(0, fakeRepository.sendMessageCallCount)
+            assertEquals(1, results.size)
+            assertTrue(results[0].isFailure)
+            assertTrue(results[0].exceptionOrNull() is IllegalArgumentException)
+        }
+
+    @Test
+    fun `content 앞뒤 공백은 trim되어 Repository에 전달된다`() =
+        runTest {
+            useCase(conversationId = null, content = " 안녕 ").toList()
+            assertEquals(1, fakeRepository.sendMessageCallCount)
+            assertEquals("안녕", fakeRepository.lastSentContent)
+        }
+
+    @Test
+    fun `content가 정확히 500자이면 정상 통과한다`() =
+        runTest {
+            val content = "a".repeat(SendChatMessageUseCase.MAX_CONTENT_LENGTH)
+            useCase(conversationId = null, content = content).toList()
+            assertEquals(1, fakeRepository.sendMessageCallCount)
+            assertEquals(content, fakeRepository.lastSentContent)
+        }
+}