diff --git a/internal/master/model/model.go b/internal/master/model/model.go
index 7ca11b7..73af03f 100644
--- a/internal/master/model/model.go
+++ b/internal/master/model/model.go
@@ -27,9 +27,12 @@ type Agent struct {
 	Hostname     string
 	AgentVersion string
 	SwarmManager bool
-	LastSeenAt   *time.Time
-	RevokedAt    *time.Time
-	CreatedAt    time.Time
+	// SwarmClusterID is the Swarm cluster the agent's daemon was in when it last
+	// connected; "" outside a swarm.
+	SwarmClusterID string
+	LastSeenAt     *time.Time
+	RevokedAt      *time.Time
+	CreatedAt      time.Time
 }
 
 // Service is one service as last reported by an agent.
diff --git a/internal/master/store/agents.go b/internal/master/store/agents.go
index 79ff91f..4f9708f 100644
--- a/internal/master/store/agents.go
+++ b/internal/master/store/agents.go
@@ -13,11 +13,11 @@ import (
 	"github.com/shcizo/package-updater/internal/protocol"
 )
 
-const agentColumns = `id, name, hostname, agent_version, swarm_manager, last_seen_at, revoked_at, created_at`
+const agentColumns = `id, name, hostname, agent_version, swarm_manager, swarm_cluster_id, last_seen_at, revoked_at, created_at`
 
 func scanAgent(row pgx.Row) (model.Agent, error) {
 	var a model.Agent
-	err := row.Scan(&a.ID, &a.Name, &a.Hostname, &a.AgentVersion, &a.SwarmManager, &a.LastSeenAt, &a.RevokedAt, &a.CreatedAt)
+	err := row.Scan(&a.ID, &a.Name, &a.Hostname, &a.AgentVersion, &a.SwarmManager, &a.SwarmClusterID, &a.LastSeenAt, &a.RevokedAt, &a.CreatedAt)
 	if errors.Is(err, pgx.ErrNoRows) {
 		return model.Agent{}, model.ErrNotFound
 	}
@@ -101,8 +101,8 @@ func (s *Store) RevokeAgent(ctx context.Context, id int64, at time.Time) ([]int6
 // RecordHello stores what the agent reported about itself on connect.
 func (s *Store) RecordHello(ctx context.Context, id int64, h protocol.Hello, at time.Time) error {
 	if _, err := s.pool.Exec(ctx,
-		`UPDATE agents SET hostname = $2, agent_version = $3, swarm_manager = $4, last_seen_at = $5 WHERE id = $1`,
-		id, h.Hostname, h.AgentVersion, h.SwarmManager, at); err != nil {
+		`UPDATE agents SET hostname = $2, agent_version = $3, swarm_manager = $4, swarm_cluster_id = $6, last_seen_at = $5 WHERE id = $1`,
+		id, h.Hostname, h.AgentVersion, h.SwarmManager, at, h.SwarmClusterID); err != nil {
 		return fmt.Errorf("record hello: %w", err)
 	}
 	return nil
diff --git a/internal/master/store/agents_integration_test.go b/internal/master/store/agents_integration_test.go
index 9cc22b9..93f4383 100644
--- a/internal/master/store/agents_integration_test.go
+++ b/internal/master/store/agents_integration_test.go
@@ -80,6 +80,7 @@ func TestAgents_RecordHelloAndMarkSeen(t *testing.T) {
 	assert.Equal(t, "swarm-mgr", agents[0].Hostname)
 	assert.Equal(t, "v1", agents[0].AgentVersion)
 	assert.True(t, agents[0].SwarmManager)
+	assert.Equal(t, "c1", agents[0].SwarmClusterID)
 	require.NotNil(t, agents[0].LastSeenAt)
 	assert.True(t, agents[0].LastSeenAt.Equal(at.Add(time.Minute)))
 }
diff --git a/internal/master/store/migrations/0005_agent_cluster.sql b/internal/master/store/migrations/0005_agent_cluster.sql
new file mode 100644
index 0000000..14dba3b
--- /dev/null
+++ b/internal/master/store/migrations/0005_agent_cluster.sql
@@ -0,0 +1 @@
+ALTER TABLE agents ADD COLUMN swarm_cluster_id text NOT NULL DEFAULT '';