diff --git a/internal/chat/service_conversation.go b/internal/chat/service_conversation.go
new file mode 100644
index 0000000..dbe7d51
--- /dev/null
+++ b/internal/chat/service_conversation.go
@@ -0,0 +1,203 @@
+package chat
+
+import (
+	"context"
+	"log/slog"
+	"strings"
+	"time"
+
+	"github.com/google/uuid"
+
+	"github.com/yan1h/agent-coding-workflow/internal/audit"
+	"github.com/yan1h/agent-coding-workflow/internal/infra/errs"
+	"github.com/yan1h/agent-coding-workflow/internal/infra/llm"
+)
+
+// compile-time assertion
+var _ ConversationService = (*conversationService)(nil)
+
+type conversationService struct {
+	repo     Repository
+	registry *llm.Registry
+	auditor  audit.Recorder
+	log      *slog.Logger
+}
+
+// NewConversationService constructs a ConversationService.
+func NewConversationService(repo Repository, reg *llm.Registry, a audit.Recorder, log *slog.Logger) ConversationService {
+	return &conversationService{repo: repo, registry: reg, auditor: a, log: log}
+}
+
+// Create validates the model and optional template, then inserts a new conversation.
+func (s *conversationService) Create(ctx context.Context, userID uuid.UUID, in CreateConversationInput) (*Conversation, error) {
+	// Validate model exists and is enabled.
+	model, err := s.repo.GetLLMModel(ctx, in.ModelID)
+	if err != nil {
+		return nil, err
+	}
+	if !model.Enabled {
+		return nil, errs.New(errs.CodeChatModelNotFound, "model is disabled")
+	}
+
+	// Resolve system prompt: custom override > template content > empty string.
+	var systemPrompt string
+	if in.SystemPrompt != nil {
+		systemPrompt = *in.SystemPrompt
+	} else if in.TemplateID != nil {
+		tpl, err := s.repo.GetPromptTemplate(ctx, *in.TemplateID)
+		if err != nil {
+			return nil, err
+		}
+		// Check user-scoped template ownership.
+		if tpl.Scope == TemplateScopeUser && (tpl.OwnerID == nil || *tpl.OwnerID != userID) {
+			return nil, errs.New(errs.CodeForbidden, "template not accessible")
+		}
+		systemPrompt = tpl.Content
+	}
+
+	id, err := uuid.NewV7()
+	if err != nil {
+		return nil, errs.Wrap(err, errs.CodeInternal, "generate conversation id")
+	}
+
+	c, err := s.repo.InsertConversation(ctx, InsertConversationParams{
+		ID:           id,
+		UserID:       userID,
+		ProjectID:    in.ProjectID,
+		WorkspaceID:  in.WorkspaceID,
+		IssueID:      in.IssueID,
+		ModelID:      in.ModelID,
+		SystemPrompt: systemPrompt,
+		Title:        nil,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	s.tryAudit(ctx, audit.Entry{
+		UserID:     &userID,
+		Action:     "chat.conversation.created",
+		TargetType: "conversation",
+		TargetID:   c.ID.String(),
+	})
+	return c, nil
+}
+
+// Get returns a conversation and its most recent messages, enforcing ownership.
+func (s *conversationService) Get(ctx context.Context, userID, id uuid.UUID) (*Conversation, []Message, error) {
+	c, err := s.repo.GetConversation(ctx, id)
+	if err != nil {
+		return nil, nil, err
+	}
+	if c.UserID != userID {
+		return nil, nil, errs.New(errs.CodeForbidden, "not conversation owner")
+	}
+	msgs, err := s.repo.ListMessagesByConversation(ctx, id, nil, 50)
+	if err != nil {
+		return nil, nil, err
+	}
+	return c, msgs, nil
+}
+
+// List returns conversations for the given user, defaulting limit to 50.
+func (s *conversationService) List(ctx context.Context, userID uuid.UUID, f ConversationFilter) ([]Conversation, error) {
+	if f.Limit == 0 {
+		f.Limit = 50
+	}
+	return s.repo.ListConversationsByUser(ctx, userID, f)
+}
+
+// UpdateTitle manually sets a conversation title, enforcing ownership.
+func (s *conversationService) UpdateTitle(ctx context.Context, userID, id uuid.UUID, title string) error {
+	c, err := s.repo.GetConversation(ctx, id)
+	if err != nil {
+		return err
+	}
+	if c.UserID != userID {
+		return errs.New(errs.CodeForbidden, "not conversation owner")
+	}
+	return s.repo.SetConversationTitleManual(ctx, id, &title)
+}
+
+// SoftDelete marks a conversation as deleted, enforcing ownership.
+func (s *conversationService) SoftDelete(ctx context.Context, userID, id uuid.UUID) error {
+	c, err := s.repo.GetConversation(ctx, id)
+	if err != nil {
+		return err
+	}
+	if c.UserID != userID {
+		return errs.New(errs.CodeForbidden, "not conversation owner")
+	}
+	if err := s.repo.SoftDeleteConversation(ctx, id); err != nil {
+		return err
+	}
+	s.tryAudit(ctx, audit.Entry{
+		UserID:     &userID,
+		Action:     "chat.conversation.deleted",
+		TargetType: "conversation",
+		TargetID:   id.String(),
+	})
+	return nil
+}
+
+// GenerateTitle is NOT on the ConversationService interface.
+// It is called by the streamer (T14) via type assertion on the concrete type.
+// It runs in a fire-and-forget goroutine; errors are silently swallowed.
+func (s *conversationService) GenerateTitle(ctx context.Context, conversationID uuid.UUID) {
+	tCtx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	titleModel, err := s.repo.GetTitleGeneratorModel(tCtx)
+	if err != nil {
+		s.log.Warn("GenerateTitle: no title model", "err", err)
+		return
+	}
+
+	first, err := s.repo.GetFirstUserMessage(tCtx, conversationID)
+	if err != nil {
+		s.log.Warn("GenerateTitle: no first user message", "conversation_id", conversationID, "err", err)
+		return
+	}
+
+	client, err := s.registry.GetClient(tCtx, titleModel.EndpointID)
+	if err != nil {
+		s.log.Warn("GenerateTitle: get client failed", "endpoint_id", titleModel.EndpointID, "err", err)
+		return
+	}
+
+	req := llm.Request{
+		SystemPrompt: "用 ≤20 字概括下面对话主题，仅输出标题：",
+		Messages: []llm.Message{{
+			Role:   llm.RoleUser,
+			Blocks: []llm.ContentBlock{{Type: "text", Text: first.Content}},
+		}},
+		MaxOutputTokens: 64,
+	}
+
+	ch, err := client.Stream(tCtx, titleModel.ModelID, req)
+	if err != nil {
+		s.log.Warn("GenerateTitle: stream failed", "err", err)
+		return
+	}
+
+	var title string
+	for ev := range ch {
+		if ev.Type == llm.EventTextDelta {
+			title += ev.Text
+		}
+	}
+	title = strings.TrimSpace(title)
+	if title == "" {
+		return
+	}
+
+	if err := s.repo.UpdateConversationTitle(tCtx, conversationID, &title); err != nil {
+		s.log.Warn("GenerateTitle: update title failed", "conversation_id", conversationID, "err", err)
+	}
+}
+
+func (s *conversationService) tryAudit(ctx context.Context, e audit.Entry) {
+	if err := s.auditor.Record(ctx, e); err != nil {
+		s.log.Warn("audit record failed", "action", e.Action, "err", err)
+	}
+}
diff --git a/internal/chat/service_conversation_test.go b/internal/chat/service_conversation_test.go
new file mode 100644
index 0000000..764d6cc
--- /dev/null
+++ b/internal/chat/service_conversation_test.go
@@ -0,0 +1,433 @@
+package chat
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/yan1h/agent-coding-workflow/internal/audit"
+	"github.com/yan1h/agent-coding-workflow/internal/infra/errs"
+)
+
+// ===== conversation fake repo extension =====
+// convFakeRepo embeds fakeRepo and overrides conversation/template/model methods.
+type convFakeRepo struct {
+	*fakeRepo
+	mu            sync.Mutex
+	conversations map[uuid.UUID]*Conversation
+	messages      map[uuid.UUID][]Message
+	templates     map[uuid.UUID]*PromptTemplate
+	titleUpdated  map[uuid.UUID]*string
+	softDeleted   []uuid.UUID
+}
+
+func newConvFakeRepo() *convFakeRepo {
+	return &convFakeRepo{
+		fakeRepo:      newFakeRepo(),
+		conversations: make(map[uuid.UUID]*Conversation),
+		messages:      make(map[uuid.UUID][]Message),
+		templates:     make(map[uuid.UUID]*PromptTemplate),
+		titleUpdated:  make(map[uuid.UUID]*string),
+	}
+}
+
+func (r *convFakeRepo) InsertConversation(_ context.Context, p InsertConversationParams) (*Conversation, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	c := &Conversation{
+		ID:           p.ID,
+		UserID:       p.UserID,
+		ProjectID:    p.ProjectID,
+		WorkspaceID:  p.WorkspaceID,
+		IssueID:      p.IssueID,
+		ModelID:      p.ModelID,
+		SystemPrompt: p.SystemPrompt,
+		Title:        p.Title,
+		CreatedAt:    time.Now(),
+		UpdatedAt:    time.Now(),
+	}
+	r.conversations[p.ID] = c
+	return c, nil
+}
+
+func (r *convFakeRepo) GetConversation(_ context.Context, id uuid.UUID) (*Conversation, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	c, ok := r.conversations[id]
+	if !ok {
+		return nil, errs.New(errs.CodeChatConversationNotFound, "conversation not found")
+	}
+	return c, nil
+}
+
+func (r *convFakeRepo) ListConversationsByUser(_ context.Context, userID uuid.UUID, f ConversationFilter) ([]Conversation, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	var out []Conversation
+	for _, c := range r.conversations {
+		if c.UserID == userID && c.DeletedAt == nil {
+			out = append(out, *c)
+		}
+	}
+	limit := f.Limit
+	if limit > 0 && len(out) > limit {
+		out = out[:limit]
+	}
+	return out, nil
+}
+
+func (r *convFakeRepo) SetConversationTitleManual(_ context.Context, id uuid.UUID, title *string) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	c, ok := r.conversations[id]
+	if !ok {
+		return errs.New(errs.CodeChatConversationNotFound, "conversation not found")
+	}
+	c.Title = title
+	r.titleUpdated[id] = title
+	return nil
+}
+
+func (r *convFakeRepo) UpdateConversationTitle(_ context.Context, id uuid.UUID, title *string) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	c, ok := r.conversations[id]
+	if !ok {
+		return errs.New(errs.CodeChatConversationNotFound, "conversation not found")
+	}
+	c.Title = title
+	r.titleUpdated[id] = title
+	return nil
+}
+
+func (r *convFakeRepo) SoftDeleteConversation(_ context.Context, id uuid.UUID) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	c, ok := r.conversations[id]
+	if !ok {
+		return errs.New(errs.CodeChatConversationNotFound, "conversation not found")
+	}
+	now := time.Now()
+	c.DeletedAt = &now
+	r.softDeleted = append(r.softDeleted, id)
+	return nil
+}
+
+func (r *convFakeRepo) ListMessagesByConversation(_ context.Context, convID uuid.UUID, _ *int64, limit int) ([]Message, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	msgs := r.messages[convID]
+	if limit > 0 && len(msgs) > limit {
+		msgs = msgs[:limit]
+	}
+	return msgs, nil
+}
+
+func (r *convFakeRepo) GetLLMModel(_ context.Context, id uuid.UUID) (*LLMModel, error) {
+	for _, m := range r.fakeRepo.models {
+		if m.ID == id {
+			cp := m
+			return &cp, nil
+		}
+	}
+	return nil, errs.New(errs.CodeChatModelNotFound, "model not found")
+}
+
+func (r *convFakeRepo) GetPromptTemplate(_ context.Context, id uuid.UUID) (*PromptTemplate, error) {
+	t, ok := r.templates[id]
+	if !ok {
+		return nil, errs.New(errs.CodeChatTemplateNotFound, "template not found")
+	}
+	return t, nil
+}
+
+// ===== recording auditor =====
+
+type recordingAuditor struct {
+	mu      sync.Mutex
+	entries []audit.Entry
+}
+
+func (a *recordingAuditor) Record(_ context.Context, e audit.Entry) error {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	a.entries = append(a.entries, e)
+	return nil
+}
+
+func (a *recordingAuditor) lastEntry() (audit.Entry, bool) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if len(a.entries) == 0 {
+		return audit.Entry{}, false
+	}
+	return a.entries[len(a.entries)-1], true
+}
+
+// ===== helpers =====
+
+func newConvSvc(repo *convFakeRepo, aud audit.Recorder) ConversationService {
+	return NewConversationService(repo, nil, aud, discardLogger())
+}
+
+func addModel(repo *convFakeRepo, enabled bool) LLMModel {
+	m := LLMModel{
+		ID:      uuid.Must(uuid.NewV7()),
+		ModelID: "test-model",
+		Enabled: enabled,
+	}
+	repo.fakeRepo.models = append(repo.fakeRepo.models, m)
+	return m
+}
+
+func addTemplate(repo *convFakeRepo, scope TemplateScope, ownerID *uuid.UUID) *PromptTemplate {
+	t := &PromptTemplate{
+		ID:      uuid.Must(uuid.NewV7()),
+		Name:    "test-tpl",
+		Content: "system prompt from template",
+		Scope:   scope,
+		OwnerID: ownerID,
+	}
+	repo.templates[t.ID] = t
+	return t
+}
+
+func addConversation(repo *convFakeRepo, userID uuid.UUID) *Conversation {
+	c := &Conversation{
+		ID:        uuid.Must(uuid.NewV7()),
+		UserID:    userID,
+		CreatedAt: time.Now(),
+		UpdatedAt: time.Now(),
+	}
+	repo.conversations[c.ID] = c
+	return c
+}
+
+// ===== tests =====
+
+func TestConversationService_Create_HappyPath(t *testing.T) {
+	t.Helper()
+	repo := newConvFakeRepo()
+	aud := &recordingAuditor{}
+	svc := newConvSvc(repo, aud)
+	userID := uuid.Must(uuid.NewV7())
+	m := addModel(repo, true)
+
+	c, err := svc.Create(context.Background(), userID, CreateConversationInput{ModelID: m.ID})
+	require.NoError(t, err)
+	require.NotNil(t, c)
+	assert.Equal(t, userID, c.UserID)
+	assert.Equal(t, m.ID, c.ModelID)
+	assert.Empty(t, c.SystemPrompt)
+
+	entry, ok := aud.lastEntry()
+	require.True(t, ok)
+	assert.Equal(t, "chat.conversation.created", entry.Action)
+	assert.Equal(t, &userID, entry.UserID)
+}
+
+func TestConversationService_Create_WithSystemTemplate(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	m := addModel(repo, true)
+	tpl := addTemplate(repo, TemplateScopeSystem, nil)
+
+	c, err := svc.Create(context.Background(), userID, CreateConversationInput{
+		ModelID:    m.ID,
+		TemplateID: &tpl.ID,
+	})
+	require.NoError(t, err)
+	assert.Equal(t, "system prompt from template", c.SystemPrompt)
+}
+
+func TestConversationService_Create_TemplateUserScopeOwnerMismatch(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	otherID := uuid.Must(uuid.NewV7())
+	m := addModel(repo, true)
+	tpl := addTemplate(repo, TemplateScopeUser, &otherID)
+
+	_, err := svc.Create(context.Background(), userID, CreateConversationInput{
+		ModelID:    m.ID,
+		TemplateID: &tpl.ID,
+	})
+	require.Error(t, err)
+	var appErr *errs.AppError
+	require.ErrorAs(t, err, &appErr)
+	assert.Equal(t, errs.CodeForbidden, appErr.Code)
+}
+
+func TestConversationService_Create_WithSystemPromptOverride(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	m := addModel(repo, true)
+	tpl := addTemplate(repo, TemplateScopeSystem, nil)
+	override := "custom system prompt"
+
+	c, err := svc.Create(context.Background(), userID, CreateConversationInput{
+		ModelID:      m.ID,
+		TemplateID:   &tpl.ID,
+		SystemPrompt: &override,
+	})
+	require.NoError(t, err)
+	// override takes precedence; template content ignored
+	assert.Equal(t, override, c.SystemPrompt)
+}
+
+func TestConversationService_Create_ModelNotFound(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+
+	_, err := svc.Create(context.Background(), userID, CreateConversationInput{
+		ModelID: uuid.Must(uuid.NewV7()),
+	})
+	require.Error(t, err)
+	var appErr *errs.AppError
+	require.ErrorAs(t, err, &appErr)
+	assert.Equal(t, errs.CodeChatModelNotFound, appErr.Code)
+}
+
+func TestConversationService_Create_ModelDisabled(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	m := addModel(repo, false)
+
+	_, err := svc.Create(context.Background(), userID, CreateConversationInput{ModelID: m.ID})
+	require.Error(t, err)
+	var appErr *errs.AppError
+	require.ErrorAs(t, err, &appErr)
+	assert.Equal(t, errs.CodeChatModelNotFound, appErr.Code)
+}
+
+func TestConversationService_Create_TemplateNotFound(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	m := addModel(repo, true)
+	missingID := uuid.Must(uuid.NewV7())
+
+	_, err := svc.Create(context.Background(), userID, CreateConversationInput{
+		ModelID:    m.ID,
+		TemplateID: &missingID,
+	})
+	require.Error(t, err)
+	var appErr *errs.AppError
+	require.ErrorAs(t, err, &appErr)
+	assert.Equal(t, errs.CodeChatTemplateNotFound, appErr.Code)
+}
+
+func TestConversationService_Get_HappyPath(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	conv := addConversation(repo, userID)
+	repo.messages[conv.ID] = []Message{
+		{ID: 1, ConversationID: conv.ID, Role: RoleUser, Content: "hello"},
+	}
+
+	got, msgs, err := svc.Get(context.Background(), userID, conv.ID)
+	require.NoError(t, err)
+	require.NotNil(t, got)
+	assert.Equal(t, conv.ID, got.ID)
+	require.Len(t, msgs, 1)
+	assert.Equal(t, "hello", msgs[0].Content)
+}
+
+func TestConversationService_Get_NotOwner(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	ownerID := uuid.Must(uuid.NewV7())
+	callerID := uuid.Must(uuid.NewV7())
+	conv := addConversation(repo, ownerID)
+
+	_, _, err := svc.Get(context.Background(), callerID, conv.ID)
+	require.Error(t, err)
+	var appErr *errs.AppError
+	require.ErrorAs(t, err, &appErr)
+	assert.Equal(t, errs.CodeForbidden, appErr.Code)
+}
+
+func TestConversationService_List_DefaultLimit(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	// Add 3 conversations
+	for i := 0; i < 3; i++ {
+		addConversation(repo, userID)
+	}
+
+	// Pass Limit=0 — should default to 50 internally (all 3 returned)
+	convs, err := svc.List(context.Background(), userID, ConversationFilter{Limit: 0})
+	require.NoError(t, err)
+	assert.Len(t, convs, 3)
+}
+
+func TestConversationService_UpdateTitle_HappyPath(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	userID := uuid.Must(uuid.NewV7())
+	conv := addConversation(repo, userID)
+
+	err := svc.UpdateTitle(context.Background(), userID, conv.ID, "new title")
+	require.NoError(t, err)
+	// Verify title was written
+	title, ok := repo.titleUpdated[conv.ID]
+	require.True(t, ok)
+	require.NotNil(t, title)
+	assert.Equal(t, "new title", *title)
+}
+
+func TestConversationService_UpdateTitle_NotOwner(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	ownerID := uuid.Must(uuid.NewV7())
+	callerID := uuid.Must(uuid.NewV7())
+	conv := addConversation(repo, ownerID)
+
+	err := svc.UpdateTitle(context.Background(), callerID, conv.ID, "hack")
+	require.Error(t, err)
+	var appErr *errs.AppError
+	require.ErrorAs(t, err, &appErr)
+	assert.Equal(t, errs.CodeForbidden, appErr.Code)
+}
+
+func TestConversationService_SoftDelete_HappyPath(t *testing.T) {
+	repo := newConvFakeRepo()
+	aud := &recordingAuditor{}
+	svc := newConvSvc(repo, aud)
+	userID := uuid.Must(uuid.NewV7())
+	conv := addConversation(repo, userID)
+
+	err := svc.SoftDelete(context.Background(), userID, conv.ID)
+	require.NoError(t, err)
+	require.Contains(t, repo.softDeleted, conv.ID)
+
+	entry, ok := aud.lastEntry()
+	require.True(t, ok)
+	assert.Equal(t, "chat.conversation.deleted", entry.Action)
+	assert.Equal(t, &userID, entry.UserID)
+}
+
+func TestConversationService_SoftDelete_NotOwner(t *testing.T) {
+	repo := newConvFakeRepo()
+	svc := newConvSvc(repo, noopAuditor{})
+	ownerID := uuid.Must(uuid.NewV7())
+	callerID := uuid.Must(uuid.NewV7())
+	conv := addConversation(repo, ownerID)
+
+	err := svc.SoftDelete(context.Background(), callerID, conv.ID)
+	require.Error(t, err)
+	var appErr *errs.AppError
+	require.ErrorAs(t, err, &appErr)
+	assert.Equal(t, errs.CodeForbidden, appErr.Code)
+}