diff --git a/backend/internal/repository/scheduler_outbox_repo.go b/backend/internal/repository/scheduler_outbox_repo.go
index 9edde6945c..cea5d451f6 100644
--- a/backend/internal/repository/scheduler_outbox_repo.go
+++ b/backend/internal/repository/scheduler_outbox_repo.go
@@ -8,6 +8,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"strconv"
+	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
@@ -16,6 +17,12 @@ type schedulerOutboxRepository struct {
 	db *sql.DB
 }
 
+type schedulerOutboxCleanupLease struct {
+	conn *sql.Conn
+}
+
+const schedulerOutboxDefaultCleanSize = 5000
+
 func NewSchedulerOutboxRepository(db *sql.DB) service.SchedulerOutboxRepository {
 	return &schedulerOutboxRepository{db: db}
 }
@@ -94,6 +101,60 @@ func (r *schedulerOutboxRepository) MaxID(ctx context.Context) (int64, error) {
 	return maxID, nil
 }
 
+func (r *schedulerOutboxRepository) DeleteConsumedUpTo(ctx context.Context, watermark int64, limit int) (int64, error) {
+	if watermark <= 0 {
+		return 0, nil
+	}
+	if limit <= 0 {
+		limit = schedulerOutboxDefaultCleanSize
+	}
+	result, err := r.db.ExecContext(ctx, `
+		WITH doomed AS (
+			SELECT id
+			FROM scheduler_outbox
+			WHERE id <= $1
+			ORDER BY id ASC
+			LIMIT $2
+		)
+		DELETE FROM scheduler_outbox o
+		USING doomed d
+		WHERE o.id = d.id
+	`, watermark, limit)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected()
+}
+
+func (r *schedulerOutboxRepository) TryAcquireCleanupLock(ctx context.Context) (service.SchedulerOutboxCleanupLease, bool, error) {
+	conn, err := r.db.Conn(ctx)
+	if err != nil {
+		return nil, false, err
+	}
+
+	var acquired bool
+	if err := conn.QueryRowContext(ctx, "SELECT pg_try_advisory_lock(hashtext('scheduler_outbox_cleanup'))").Scan(&acquired); err != nil {
+		_ = conn.Close()
+		return nil, false, err
+	}
+	if !acquired {
+		_ = conn.Close()
+		return nil, false, nil
+	}
+	return &schedulerOutboxCleanupLease{conn: conn}, true, nil
+}
+
+func (l *schedulerOutboxCleanupLease) Release() {
+	if l == nil || l.conn == nil {
+		return
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	_, _ = l.conn.ExecContext(ctx, "SELECT pg_advisory_unlock(hashtext('scheduler_outbox_cleanup'))")
+	_ = l.conn.Close()
+	l.conn = nil
+}
+
 func enqueueSchedulerOutbox(ctx context.Context, exec sqlExecutor, eventType string, accountID *int64, groupID *int64, payload any) error {
 	if exec == nil {
 		return nil
diff --git a/backend/internal/repository/scheduler_outbox_repo_test.go b/backend/internal/repository/scheduler_outbox_repo_test.go
new file mode 100644
index 0000000000..ea91f6c73e
--- /dev/null
+++ b/backend/internal/repository/scheduler_outbox_repo_test.go
@@ -0,0 +1,91 @@
+package repository
+
+import (
+	"context"
+	"regexp"
+	"testing"
+
+	sqlmock "github.com/DATA-DOG/go-sqlmock"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSchedulerOutboxRepositoryDeleteConsumedUpToUsesBoundedCTE(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	repo := &schedulerOutboxRepository{db: db}
+	const expectedSQL = `
+		WITH doomed AS (
+			SELECT id
+			FROM scheduler_outbox
+			WHERE id <= $1
+			ORDER BY id ASC
+			LIMIT $2
+		)
+		DELETE FROM scheduler_outbox o
+		USING doomed d
+		WHERE o.id = d.id
+	`
+	mock.ExpectExec(regexp.QuoteMeta(expectedSQL)).
+		WithArgs(int64(42), 5000).
+		WillReturnResult(sqlmock.NewResult(0, 17))
+
+	deleted, err := repo.DeleteConsumedUpTo(context.Background(), 42, 5000)
+
+	require.NoError(t, err)
+	require.EqualValues(t, 17, deleted)
+	require.NoError(t, mock.ExpectationsWereMet())
+}
+
+func TestSchedulerOutboxRepositoryDeleteConsumedUpToSkipsNonPositiveWatermark(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	repo := &schedulerOutboxRepository{db: db}
+
+	deleted, err := repo.DeleteConsumedUpTo(context.Background(), 0, 5000)
+
+	require.NoError(t, err)
+	require.EqualValues(t, 0, deleted)
+	require.NoError(t, mock.ExpectationsWereMet())
+}
+
+func TestSchedulerOutboxRepositoryTryAcquireCleanupLock(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	repo := &schedulerOutboxRepository{db: db}
+	mock.ExpectQuery(regexp.QuoteMeta("SELECT pg_try_advisory_lock(hashtext('scheduler_outbox_cleanup'))")).
+		WillReturnRows(sqlmock.NewRows([]string{"pg_try_advisory_lock"}).AddRow(true))
+	mock.ExpectExec(regexp.QuoteMeta("SELECT pg_advisory_unlock(hashtext('scheduler_outbox_cleanup'))")).
+		WillReturnResult(sqlmock.NewResult(0, 1))
+
+	lease, acquired, err := repo.TryAcquireCleanupLock(context.Background())
+	require.NoError(t, err)
+	require.True(t, acquired)
+	require.NotNil(t, lease)
+
+	lease.Release()
+
+	require.NoError(t, mock.ExpectationsWereMet())
+}
+
+func TestSchedulerOutboxRepositoryTryAcquireCleanupLockUnavailable(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	require.NoError(t, err)
+	defer func() { _ = db.Close() }()
+
+	repo := &schedulerOutboxRepository{db: db}
+	mock.ExpectQuery(regexp.QuoteMeta("SELECT pg_try_advisory_lock(hashtext('scheduler_outbox_cleanup'))")).
+		WillReturnRows(sqlmock.NewRows([]string{"pg_try_advisory_lock"}).AddRow(false))
+
+	lease, acquired, err := repo.TryAcquireCleanupLock(context.Background())
+	require.NoError(t, err)
+	require.False(t, acquired)
+	require.Nil(t, lease)
+
+	require.NoError(t, mock.ExpectationsWereMet())
+}
diff --git a/backend/internal/service/scheduler_outbox.go b/backend/internal/service/scheduler_outbox.go
index c138b7e5a5..2b7665ad78 100644
--- a/backend/internal/service/scheduler_outbox.go
+++ b/backend/internal/service/scheduler_outbox.go
@@ -18,4 +18,12 @@ type SchedulerOutboxEvent struct {
 type SchedulerOutboxRepository interface {
 	ListAfterAndReleaseDedup(ctx context.Context, afterID int64, limit int) ([]SchedulerOutboxEvent, error)
 	MaxID(ctx context.Context) (int64, error)
+	DeleteConsumedUpTo(ctx context.Context, watermark int64, limit int) (int64, error)
+	TryAcquireCleanupLock(ctx context.Context) (SchedulerOutboxCleanupLease, bool, error)
+}
+
+// SchedulerOutboxCleanupLease holds the PostgreSQL advisory lock used by
+// scheduler outbox cleanup.
+type SchedulerOutboxCleanupLease interface {
+	Release()
 }
diff --git a/backend/internal/service/scheduler_snapshot_outbox_cleanup_test.go b/backend/internal/service/scheduler_snapshot_outbox_cleanup_test.go
new file mode 100644
index 0000000000..535f8d54e2
--- /dev/null
+++ b/backend/internal/service/scheduler_snapshot_outbox_cleanup_test.go
@@ -0,0 +1,269 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"reflect"
+	"testing"
+	"time"
+)
+
+type outboxCleanupCache struct {
+	watermark     int64
+	setWatermarks []int64
+	updateErr     error
+}
+
+func (c *outboxCleanupCache) GetSnapshot(ctx context.Context, bucket SchedulerBucket) ([]*Account, bool, error) {
+	return nil, false, nil
+}
+
+func (c *outboxCleanupCache) SetSnapshot(ctx context.Context, bucket SchedulerBucket, accounts []Account) error {
+	return nil
+}
+
+func (c *outboxCleanupCache) GetAccount(ctx context.Context, accountID int64) (*Account, error) {
+	return nil, nil
+}
+
+func (c *outboxCleanupCache) SetAccount(ctx context.Context, account *Account) error {
+	return nil
+}
+
+func (c *outboxCleanupCache) DeleteAccount(ctx context.Context, accountID int64) error {
+	return nil
+}
+
+func (c *outboxCleanupCache) UpdateLastUsed(ctx context.Context, updates map[int64]time.Time) error {
+	return c.updateErr
+}
+
+func (c *outboxCleanupCache) TryLockBucket(ctx context.Context, bucket SchedulerBucket, ttl time.Duration) (bool, error) {
+	return true, nil
+}
+
+func (c *outboxCleanupCache) UnlockBucket(ctx context.Context, bucket SchedulerBucket) error {
+	return nil
+}
+
+func (c *outboxCleanupCache) ListBuckets(ctx context.Context) ([]SchedulerBucket, error) {
+	return nil, nil
+}
+
+func (c *outboxCleanupCache) GetOutboxWatermark(ctx context.Context) (int64, error) {
+	return c.watermark, nil
+}
+
+func (c *outboxCleanupCache) SetOutboxWatermark(ctx context.Context, id int64) error {
+	c.watermark = id
+	c.setWatermarks = append(c.setWatermarks, id)
+	return nil
+}
+
+type outboxCleanupDeleteCall struct {
+	watermark int64
+	limit     int
+}
+
+type outboxCleanupRepo struct {
+	events       []SchedulerOutboxEvent
+	rows         []int64
+	lockAcquired bool
+	lockAttempts int
+	releaseCount int
+	deleteCalls  []outboxCleanupDeleteCall
+}
+
+func (r *outboxCleanupRepo) ListAfterAndReleaseDedup(ctx context.Context, afterID int64, limit int) ([]SchedulerOutboxEvent, error) {
+	events := make([]SchedulerOutboxEvent, 0, len(r.events))
+	for _, event := range r.events {
+		if event.ID <= afterID {
+			continue
+		}
+		events = append(events, event)
+		if limit > 0 && len(events) >= limit {
+			break
+		}
+	}
+	return events, nil
+}
+
+func (r *outboxCleanupRepo) MaxID(ctx context.Context) (int64, error) {
+	var maxID int64
+	for _, id := range r.rows {
+		if id > maxID {
+			maxID = id
+		}
+	}
+	return maxID, nil
+}
+
+func (r *outboxCleanupRepo) DeleteConsumedUpTo(ctx context.Context, watermark int64, limit int) (int64, error) {
+	r.deleteCalls = append(r.deleteCalls, outboxCleanupDeleteCall{
+		watermark: watermark,
+		limit:     limit,
+	})
+	if watermark <= 0 || limit <= 0 {
+		return 0, nil
+	}
+
+	deleted := int64(0)
+	kept := make([]int64, 0, len(r.rows))
+	for _, id := range r.rows {
+		if id <= watermark && deleted < int64(limit) {
+			deleted++
+			continue
+		}
+		kept = append(kept, id)
+	}
+	r.rows = kept
+	return deleted, nil
+}
+
+func (r *outboxCleanupRepo) TryAcquireCleanupLock(ctx context.Context) (SchedulerOutboxCleanupLease, bool, error) {
+	r.lockAttempts++
+	if !r.lockAcquired {
+		return nil, false, nil
+	}
+	return outboxCleanupLease{release: func() {
+		r.releaseCount++
+	}}, true, nil
+}
+
+type outboxCleanupLease struct {
+	release func()
+}
+
+func (l outboxCleanupLease) Release() {
+	if l.release != nil {
+		l.release()
+	}
+}
+
+func TestSchedulerSnapshotServicePollOutboxCleansConsumedRowsAfterWatermark(t *testing.T) {
+	cache := &outboxCleanupCache{}
+	repo := &outboxCleanupRepo{
+		events: []SchedulerOutboxEvent{
+			{ID: 10000, EventType: SchedulerOutboxEventAccountLastUsed},
+		},
+		rows:         int64Range(1, 10003),
+		lockAcquired: true,
+	}
+	svc := NewSchedulerSnapshotService(cache, repo, nil, nil, nil)
+
+	svc.pollOutbox()
+
+	if cache.watermark != 10000 {
+		t.Fatalf("expected watermark 10000, got %d", cache.watermark)
+	}
+	if !reflect.DeepEqual(cache.setWatermarks, []int64{10000}) {
+		t.Fatalf("unexpected watermark writes: %#v", cache.setWatermarks)
+	}
+	if !reflect.DeepEqual(repo.rows, []int64{10001, 10002, 10003}) {
+		t.Fatalf("expected rows above watermark to remain, got %#v", repo.rows)
+	}
+	if repo.lockAttempts != 1 || repo.releaseCount != 1 {
+		t.Fatalf("expected one lock acquire/release, got acquire=%d release=%d", repo.lockAttempts, repo.releaseCount)
+	}
+	if len(repo.deleteCalls) != 3 {
+		t.Fatalf("expected cleanup to loop until a short batch, got %d calls", len(repo.deleteCalls))
+	}
+	for _, call := range repo.deleteCalls {
+		if call.watermark != 10000 || call.limit != schedulerOutboxCleanupBatch {
+			t.Fatalf("unexpected cleanup call: %#v", call)
+		}
+	}
+}
+
+func TestSchedulerSnapshotServicePollOutboxSkipsCleanupWhenLockUnavailable(t *testing.T) {
+	cache := &outboxCleanupCache{}
+	repo := &outboxCleanupRepo{
+		events: []SchedulerOutboxEvent{
+			{ID: 3, EventType: SchedulerOutboxEventAccountLastUsed},
+		},
+		rows:         []int64{1, 2, 3, 4},
+		lockAcquired: false,
+	}
+	svc := NewSchedulerSnapshotService(cache, repo, nil, nil, nil)
+
+	svc.pollOutbox()
+
+	if cache.watermark != 3 {
+		t.Fatalf("expected watermark 3, got %d", cache.watermark)
+	}
+	if !reflect.DeepEqual(repo.rows, []int64{1, 2, 3, 4}) {
+		t.Fatalf("expected cleanup to skip all rows, got %#v", repo.rows)
+	}
+	if repo.lockAttempts != 1 {
+		t.Fatalf("expected one lock attempt, got %d", repo.lockAttempts)
+	}
+	if len(repo.deleteCalls) != 0 {
+		t.Fatalf("expected no delete calls, got %#v", repo.deleteCalls)
+	}
+	if repo.releaseCount != 0 {
+		t.Fatalf("expected no release without lock, got %d", repo.releaseCount)
+	}
+}
+
+func TestSchedulerSnapshotServicePollOutboxDoesNotCleanupOnHandleFailure(t *testing.T) {
+	cache := &outboxCleanupCache{
+		updateErr: errors.New("cache update failed"),
+	}
+	repo := &outboxCleanupRepo{
+		events: []SchedulerOutboxEvent{
+			{
+				ID:        5,
+				EventType: SchedulerOutboxEventAccountLastUsed,
+				Payload: map[string]any{
+					"last_used": map[string]any{"101": float64(123)},
+				},
+			},
+		},
+		rows:         []int64{1, 2, 3, 4, 5, 6},
+		lockAcquired: true,
+	}
+	svc := NewSchedulerSnapshotService(cache, repo, nil, nil, nil)
+
+	svc.pollOutbox()
+
+	if len(cache.setWatermarks) != 0 {
+		t.Fatalf("expected no watermark write on handle failure, got %#v", cache.setWatermarks)
+	}
+	if repo.lockAttempts != 0 {
+		t.Fatalf("expected cleanup lock not to be attempted, got %d", repo.lockAttempts)
+	}
+	if len(repo.deleteCalls) != 0 {
+		t.Fatalf("expected no delete calls, got %#v", repo.deleteCalls)
+	}
+	if !reflect.DeepEqual(repo.rows, []int64{1, 2, 3, 4, 5, 6}) {
+		t.Fatalf("expected rows unchanged, got %#v", repo.rows)
+	}
+}
+
+func TestSchedulerSnapshotServiceCleanupSkipsNonPositiveWatermark(t *testing.T) {
+	repo := &outboxCleanupRepo{
+		rows:         []int64{1, 2, 3},
+		lockAcquired: true,
+	}
+	svc := NewSchedulerSnapshotService(&outboxCleanupCache{}, repo, nil, nil, nil)
+
+	svc.cleanupConsumedOutbox(0)
+
+	if repo.lockAttempts != 0 {
+		t.Fatalf("expected no lock attempt for non-positive watermark, got %d", repo.lockAttempts)
+	}
+	if len(repo.deleteCalls) != 0 {
+		t.Fatalf("expected no delete calls, got %#v", repo.deleteCalls)
+	}
+	if !reflect.DeepEqual(repo.rows, []int64{1, 2, 3}) {
+		t.Fatalf("expected rows unchanged, got %#v", repo.rows)
+	}
+}
+
+func int64Range(start, end int64) []int64 {
+	values := make([]int64, 0, end-start+1)
+	for id := start; id <= end; id++ {
+		values = append(values, id)
+	}
+	return values
+}
diff --git a/backend/internal/service/scheduler_snapshot_service.go b/backend/internal/service/scheduler_snapshot_service.go
index 6b25c0430f..fd48da27af 100644
--- a/backend/internal/service/scheduler_snapshot_service.go
+++ b/backend/internal/service/scheduler_snapshot_service.go
@@ -18,7 +18,10 @@ var (
 	ErrSchedulerFallbackLimited = errors.New("scheduler db fallback limited")
 )
 
-const outboxEventTimeout = 2 * time.Minute
+const (
+	outboxEventTimeout          = 2 * time.Minute
+	schedulerOutboxCleanupBatch = 5000
+)
 
 // batchSeenKey tracks which (groupID, platform) bucket sets have already been
 // rebuilt within a single pollOutbox call, to avoid redundant work when multiple
@@ -280,11 +283,42 @@ func (s *SchedulerSnapshotService) pollOutbox() {
 		logger.LegacyPrintf("service.scheduler_snapshot", "[Scheduler] outbox watermark write failed: %v", wmErr)
 	} else {
 		watermarkForCheck = lastID
+		s.cleanupConsumedOutbox(lastID)
 	}
 
 	s.checkOutboxLag(ctx, events[0], watermarkForCheck)
 }
 
+func (s *SchedulerSnapshotService) cleanupConsumedOutbox(watermark int64) {
+	if s == nil || s.outboxRepo == nil || watermark <= 0 {
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	lease, acquired, err := s.outboxRepo.TryAcquireCleanupLock(ctx)
+	if err != nil {
+		logger.LegacyPrintf("service.scheduler_snapshot", "[Scheduler] outbox cleanup lock failed: %v", err)
+		return
+	}
+	if !acquired {
+		return
+	}
+	defer lease.Release()
+
+	for {
+		deleted, err := s.outboxRepo.DeleteConsumedUpTo(ctx, watermark, schedulerOutboxCleanupBatch)
+		if err != nil {
+			logger.LegacyPrintf("service.scheduler_snapshot", "[Scheduler] outbox cleanup failed: watermark=%d err=%v", watermark, err)
+			return
+		}
+		if deleted == 0 || deleted < schedulerOutboxCleanupBatch {
+			return
+		}
+	}
+}
+
 func (s *SchedulerSnapshotService) handleOutboxEvent(ctx context.Context, event SchedulerOutboxEvent, seen map[batchSeenKey]struct{}) error {
 	switch event.EventType {
 	case SchedulerOutboxEventAccountLastUsed: