diff --git a/server/cmd/server/main.go b/server/cmd/server/main.go
index 7f5bf1f2..13fd3092 100644
--- a/server/cmd/server/main.go
+++ b/server/cmd/server/main.go
@@ -65,6 +65,10 @@ func main() {
 		Handler: r,
 	}
 
+	// Start background sweeper to mark stale runtimes as offline.
+	sweepCtx, sweepCancel := context.WithCancel(context.Background())
+	go runRuntimeSweeper(sweepCtx, queries, bus)
+
 	// Graceful shutdown
 	go func() {
 		slog.Info("server starting", "port", port)
@@ -79,6 +83,7 @@ func main() {
 	<-quit
 
 	slog.Info("shutting down server")
+	sweepCancel()
 	shutdownCtx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 
diff --git a/server/cmd/server/runtime_sweeper.go b/server/cmd/server/runtime_sweeper.go
new file mode 100644
index 00000000..e1d47a53
--- /dev/null
+++ b/server/cmd/server/runtime_sweeper.go
@@ -0,0 +1,65 @@
+package main
+
+import (
+	"context"
+	"log/slog"
+	"time"
+
+	"github.com/multica-ai/multica/server/internal/events"
+	"github.com/multica-ai/multica/server/internal/util"
+	db "github.com/multica-ai/multica/server/pkg/db/generated"
+	"github.com/multica-ai/multica/server/pkg/protocol"
+)
+
+const (
+	// sweepInterval is how often we check for stale runtimes.
+	sweepInterval = 30 * time.Second
+	// staleThresholdSeconds marks runtimes offline if no heartbeat for this long.
+	// The daemon heartbeat interval is 15s, so 45s = 3 missed heartbeats.
+	staleThresholdSeconds = 45.0
+)
+
+// runRuntimeSweeper periodically marks runtimes as offline if their
+// last_seen_at exceeds the stale threshold. This handles cases where the
+// daemon crashes or is killed without calling the deregister endpoint.
+func runRuntimeSweeper(ctx context.Context, queries *db.Queries, bus *events.Bus) {
+	ticker := time.NewTicker(sweepInterval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			staleRows, err := queries.MarkStaleRuntimesOffline(ctx, staleThresholdSeconds)
+			if err != nil {
+				slog.Warn("runtime sweeper: failed to mark stale runtimes offline", "error", err)
+				continue
+			}
+			if len(staleRows) == 0 {
+				continue
+			}
+
+			// Collect unique workspace IDs to notify.
+			workspaces := make(map[string]bool)
+			for _, row := range staleRows {
+				wsID := util.UUIDToString(row.WorkspaceID)
+				workspaces[wsID] = true
+			}
+
+			slog.Info("runtime sweeper: marked stale runtimes offline", "count", len(staleRows), "workspaces", len(workspaces))
+
+			// Notify frontend clients so they re-fetch runtime list.
+			for wsID := range workspaces {
+				bus.Publish(events.Event{
+					Type:        protocol.EventDaemonRegister,
+					WorkspaceID: wsID,
+					ActorType:   "system",
+					Payload: map[string]any{
+						"action": "stale_sweep",
+					},
+				})
+			}
+		}
+	}
+}
diff --git a/server/pkg/db/generated/runtime.sql.go b/server/pkg/db/generated/runtime.sql.go
index 1946ec33..d871d72d 100644
--- a/server/pkg/db/generated/runtime.sql.go
+++ b/server/pkg/db/generated/runtime.sql.go
@@ -105,6 +105,39 @@ func (q *Queries) ListAgentRuntimes(ctx context.Context, workspaceID pgtype.UUID
 	return items, nil
 }
 
+const markStaleRuntimesOffline = `-- name: MarkStaleRuntimesOffline :many
+UPDATE agent_runtime
+SET status = 'offline', updated_at = now()
+WHERE status = 'online'
+  AND last_seen_at < now() - make_interval(secs => $1::double precision)
+RETURNING id, workspace_id
+`
+
+type MarkStaleRuntimesOfflineRow struct {
+	ID          pgtype.UUID `json:"id"`
+	WorkspaceID pgtype.UUID `json:"workspace_id"`
+}
+
+func (q *Queries) MarkStaleRuntimesOffline(ctx context.Context, staleSeconds float64) ([]MarkStaleRuntimesOfflineRow, error) {
+	rows, err := q.db.Query(ctx, markStaleRuntimesOffline, staleSeconds)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	items := []MarkStaleRuntimesOfflineRow{}
+	for rows.Next() {
+		var i MarkStaleRuntimesOfflineRow
+		if err := rows.Scan(&i.ID, &i.WorkspaceID); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const setAgentRuntimeOffline = `-- name: SetAgentRuntimeOffline :exec
 UPDATE agent_runtime
 SET status = 'offline', updated_at = now()
diff --git a/server/pkg/db/queries/runtime.sql b/server/pkg/db/queries/runtime.sql
index 5a40ee8a..6aabb657 100644
--- a/server/pkg/db/queries/runtime.sql
+++ b/server/pkg/db/queries/runtime.sql
@@ -44,3 +44,10 @@ RETURNING *;
 UPDATE agent_runtime
 SET status = 'offline', updated_at = now()
 WHERE id = $1;
+
+-- name: MarkStaleRuntimesOffline :many
+UPDATE agent_runtime
+SET status = 'offline', updated_at = now()
+WHERE status = 'online'
+  AND last_seen_at < now() - make_interval(secs => @stale_seconds::double precision)
+RETURNING id, workspace_id;