diff --git a/internal/chat/service_usage.go b/internal/chat/service_usage.go
new file mode 100644
index 0000000..884c018
--- /dev/null
+++ b/internal/chat/service_usage.go
@@ -0,0 +1,35 @@
+package chat
+
+import (
+	"context"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type usageService struct{ repo Repository }
+
+var _ UsageService = (*usageService)(nil)
+
+// NewUsageService 构造 llm_usage 写入与汇总查询服务。
+func NewUsageService(repo Repository) UsageService { return &usageService{repo: repo} }
+
+func (s *usageService) Insert(ctx context.Context, in UsageRecord) error {
+	return s.repo.InsertUsage(ctx, in)
+}
+
+func (s *usageService) SummarizeByUser(ctx context.Context, from, to time.Time) ([]UserUsageRow, error) {
+	return s.repo.SummarizeUsageByUser(ctx, from, to)
+}
+
+func (s *usageService) SummarizeByModel(ctx context.Context, from, to time.Time) ([]ModelUsageRow, error) {
+	return s.repo.SummarizeUsageByModel(ctx, from, to)
+}
+
+func (s *usageService) SummarizeByDay(ctx context.Context, from, to time.Time) ([]DayUsageRow, error) {
+	return s.repo.SummarizeUsageByDay(ctx, from, to)
+}
+
+func (s *usageService) UserDaily(ctx context.Context, userID uuid.UUID, from, to time.Time) ([]DayUsageRow, error) {
+	return s.repo.SummarizeUserDaily(ctx, userID, from, to)
+}
diff --git a/internal/chat/service_usage_test.go b/internal/chat/service_usage_test.go
new file mode 100644
index 0000000..af09f17
--- /dev/null
+++ b/internal/chat/service_usage_test.go
@@ -0,0 +1,98 @@
+package chat
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/require"
+)
+
+// usageRepo is a tiny stub that records what was called and returns canned values.
+// It satisfies only the UsageService-relevant subset of Repository; unused methods
+// inherit panic stubs from fakeRepo so we don't need to declare them here.
+type usageRepo struct {
+	*fakeRepo
+
+	insertedUsage []UsageRecord
+
+	byUserCalls  []timeRange
+	byModelCalls []timeRange
+	byDayCalls   []timeRange
+	dailyCalls   []userTimeRange
+
+	usersOut  []UserUsageRow
+	modelsOut []ModelUsageRow
+	daysOut   []DayUsageRow
+}
+
+type timeRange struct{ from, to time.Time }
+type userTimeRange struct {
+	user uuid.UUID
+	from time.Time
+	to   time.Time
+}
+
+func (r *usageRepo) InsertUsage(_ context.Context, rec UsageRecord) error {
+	r.insertedUsage = append(r.insertedUsage, rec)
+	return nil
+}
+func (r *usageRepo) SummarizeUsageByUser(_ context.Context, from, to time.Time) ([]UserUsageRow, error) {
+	r.byUserCalls = append(r.byUserCalls, timeRange{from, to})
+	return r.usersOut, nil
+}
+func (r *usageRepo) SummarizeUsageByModel(_ context.Context, from, to time.Time) ([]ModelUsageRow, error) {
+	r.byModelCalls = append(r.byModelCalls, timeRange{from, to})
+	return r.modelsOut, nil
+}
+func (r *usageRepo) SummarizeUsageByDay(_ context.Context, from, to time.Time) ([]DayUsageRow, error) {
+	r.byDayCalls = append(r.byDayCalls, timeRange{from, to})
+	return r.daysOut, nil
+}
+func (r *usageRepo) SummarizeUserDaily(_ context.Context, uid uuid.UUID, from, to time.Time) ([]DayUsageRow, error) {
+	r.dailyCalls = append(r.dailyCalls, userTimeRange{uid, from, to})
+	return r.daysOut, nil
+}
+
+func TestUsageService_DelegatesToRepo(t *testing.T) {
+	repo := &usageRepo{fakeRepo: newFakeRepo()}
+	svc := NewUsageService(repo)
+	from := time.Date(2026, 5, 1, 0, 0, 0, 0, time.UTC)
+	to := from.Add(24 * time.Hour)
+	uid := uuid.Must(uuid.NewV7())
+
+	rec := UsageRecord{
+		ConversationID: uuid.Must(uuid.NewV7()),
+		MessageID:      42,
+		UserID:         uid,
+		PromptTokens:   100, CompletionTokens: 50, ThinkingTokens: 0,
+		CostUSD: 0.001,
+	}
+	require.NoError(t, svc.Insert(context.Background(), rec))
+	require.Len(t, repo.insertedUsage, 1)
+	require.Equal(t, rec, repo.insertedUsage[0])
+
+	repo.usersOut = []UserUsageRow{{UserID: uid, PromptTokens: 100}}
+	users, err := svc.SummarizeByUser(context.Background(), from, to)
+	require.NoError(t, err)
+	require.Equal(t, repo.usersOut, users)
+	require.Equal(t, []timeRange{{from, to}}, repo.byUserCalls)
+
+	repo.modelsOut = []ModelUsageRow{{ModelID: uuid.Must(uuid.NewV7()), CostUSD: 1.5}}
+	models, err := svc.SummarizeByModel(context.Background(), from, to)
+	require.NoError(t, err)
+	require.Equal(t, repo.modelsOut, models)
+	require.Equal(t, []timeRange{{from, to}}, repo.byModelCalls)
+
+	repo.daysOut = []DayUsageRow{{Day: from, PromptTokens: 200}}
+	days, err := svc.SummarizeByDay(context.Background(), from, to)
+	require.NoError(t, err)
+	require.Equal(t, repo.daysOut, days)
+	require.Equal(t, []timeRange{{from, to}}, repo.byDayCalls)
+
+	dailyOut, err := svc.UserDaily(context.Background(), uid, from, to)
+	require.NoError(t, err)
+	require.Equal(t, repo.daysOut, dailyOut)
+	require.Equal(t, []userTimeRange{{uid, from, to}}, repo.dailyCalls)
+}