diff --git a/pkg/service/app.go b/pkg/service/app.go
index 2f8a868..6717e1f 100644
--- a/pkg/service/app.go
+++ b/pkg/service/app.go
@@ -210,7 +210,13 @@ func (a *App) Build(node string) (*Job, error) {
 	if err := a.ensureReplacementReady(node); err != nil {
 		return nil, err
 	}
-	job := a.newJob("build", node, "", "")
+	if active := a.activeJobForNode(node); active != nil {
+		return nil, &activeNodeJobError{Node: node, Kind: active.Kind, JobID: active.ID}
+	}
+	job, err := a.reserveJob("build", node, "", "")
+	if err != nil {
+		return nil, err
+	}
 	go a.runBuild(job, false)
 	return job, nil
 }
@@ -223,10 +229,16 @@ func (a *App) Replace(node, host, device string) (*Job, error) {
 	if err := a.ensureReplacementReady(node); err != nil {
 		return nil, err
 	}
+	if active := a.activeJobForNode(node); active != nil {
+		return nil, &activeNodeJobError{Node: node, Kind: active.Kind, JobID: active.ID}
+	}
 	if _, err := a.ensureDevice(host, device); err != nil {
 		return nil, err
 	}
-	job := a.newJob("replace", node, host, device)
+	job, err := a.reserveJob("replace", node, host, device)
+	if err != nil {
+		return nil, err
+	}
 	go a.runBuild(job, true)
 	return job, nil
 }
diff --git a/pkg/service/app_helpers.go b/pkg/service/app_helpers.go
index 7081221..613e2a1 100644
--- a/pkg/service/app_helpers.go
+++ b/pkg/service/app_helpers.go
@@ -34,6 +34,76 @@ func (a *App) newJob(kind, node, host, device string) *Job {
 	return job
 }
 
+type activeNodeJobError struct {
+	Node  string
+	Kind  string
+	JobID string
+}
+
+func (e *activeNodeJobError) Error() string {
+	if e == nil {
+		return "node already has an active metis job"
+	}
+	return fmt.Sprintf("node %s already has an active %s job (%s)", e.Node, e.Kind, e.JobID)
+}
+
+func (a *App) activeJobForNode(node string) *Job {
+	a.mu.RLock()
+	defer a.mu.RUnlock()
+	return a.activeJobForNodeLocked(node)
+}
+
+func (a *App) activeJobForNodeLocked(node string) *Job {
+	node = strings.TrimSpace(node)
+	if node == "" {
+		return nil
+	}
+	var active *Job
+	for _, job := range a.jobs {
+		if job == nil || strings.TrimSpace(job.Node) != node {
+			continue
+		}
+		if job.Status != JobQueued && job.Status != JobRunning {
+			continue
+		}
+		switch job.Kind {
+		case "build", "replace":
+		default:
+			continue
+		}
+		if active == nil || job.StartedAt.Before(active.StartedAt) {
+			active = job
+		}
+	}
+	if active == nil {
+		return nil
+	}
+	copyJob := *active
+	return &copyJob
+}
+
+func (a *App) reserveJob(kind, node, host, device string) (*Job, error) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if active := a.activeJobForNodeLocked(node); active != nil {
+		return nil, &activeNodeJobError{Node: node, Kind: active.Kind, JobID: active.ID}
+	}
+	now := time.Now().UTC()
+	job := &Job{
+		ID:          fmt.Sprintf("%d", now.UnixNano()),
+		Kind:        kind,
+		Node:        node,
+		Host:        host,
+		Device:      device,
+		Status:      JobQueued,
+		ProgressPct: 0,
+		StartedAt:   now,
+		UpdatedAt:   now,
+	}
+	a.jobs[job.ID] = job
+	return job, nil
+}
+
 func (a *App) job(id string) *Job {
 	a.mu.RLock()
 	defer a.mu.RUnlock()
diff --git a/pkg/service/cluster.go b/pkg/service/cluster.go
index e1612b7..5561068 100644
--- a/pkg/service/cluster.go
+++ b/pkg/service/cluster.go
@@ -158,6 +158,54 @@ func clusterNodes() []clusterNode {
 	return nodes
 }
 
+func clusterActiveRemotePodLoads(namespace, run string) map[string]int {
+	kube, err := kubeClientFactory()
+	if err != nil {
+		return nil
+	}
+	ns := url.PathEscape(strings.TrimSpace(namespace))
+	if ns == "" {
+		return nil
+	}
+	selector := "app=metis-remote"
+	if value := strings.TrimSpace(run); value != "" {
+		selector += ",metis-run=" + value
+	}
+	path := fmt.Sprintf("/api/v1/namespaces/%s/pods?labelSelector=%s", ns, url.QueryEscape(selector))
+	var payload struct {
+		Items []struct {
+			Metadata struct {
+				Labels map[string]string `json:"labels"`
+			} `json:"metadata"`
+			Spec struct {
+				NodeName string `json:"nodeName"`
+			} `json:"spec"`
+			Status struct {
+				Phase string `json:"phase"`
+			} `json:"status"`
+		} `json:"items"`
+	}
+	if err := kube.jsonRequest(http.MethodGet, path, nil, &payload); err != nil {
+		return nil
+	}
+	loads := map[string]int{}
+	for _, item := range payload.Items {
+		phase := strings.TrimSpace(item.Status.Phase)
+		if phase == "Succeeded" || phase == "Failed" {
+			continue
+		}
+		if value := strings.TrimSpace(run); value != "" && strings.TrimSpace(item.Metadata.Labels["metis-run"]) != value {
+			continue
+		}
+		nodeName := strings.TrimSpace(item.Spec.NodeName)
+		if nodeName == "" {
+			continue
+		}
+		loads[nodeName]++
+	}
+	return loads
+}
+
 func (a *App) podImageForArch(arch string) string {
 	switch strings.TrimSpace(arch) {
 	case "arm64":
diff --git a/pkg/service/cluster_test.go b/pkg/service/cluster_test.go
index 0ca3c5d..b3e7ba6 100644
--- a/pkg/service/cluster_test.go
+++ b/pkg/service/cluster_test.go
@@ -121,3 +121,50 @@ func TestDeleteNodeObjectFallback(t *testing.T) {
 		t.Fatalf("deleteNodeObject fallback: %v", err)
 	}
 }
+
+func TestClusterActiveRemotePodLoadsCountsOnlyLivePods(t *testing.T) {
+	kube := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch {
+		case r.Method == http.MethodGet && r.URL.Path == "/api/v1/namespaces/maintenance/pods":
+			if got := r.URL.Query().Get("labelSelector"); got != "app=metis-remote,metis-run=build" {
+				t.Fatalf("unexpected labelSelector %q", got)
+			}
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"items": []any{
+					map[string]any{
+						"metadata": map[string]any{"labels": map[string]string{"app": "metis-remote", "metis-run": "build"}},
+						"spec":     map[string]any{"nodeName": "titan-04"},
+						"status":   map[string]any{"phase": "Running"},
+					},
+					map[string]any{
+						"metadata": map[string]any{"labels": map[string]string{"app": "metis-remote", "metis-run": "build"}},
+						"spec":     map[string]any{"nodeName": "titan-04"},
+						"status":   map[string]any{"phase": "Pending"},
+					},
+					map[string]any{
+						"metadata": map[string]any{"labels": map[string]string{"app": "metis-remote", "metis-run": "build"}},
+						"spec":     map[string]any{"nodeName": "titan-05"},
+						"status":   map[string]any{"phase": "Succeeded"},
+					},
+				},
+			})
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer kube.Close()
+
+	origFactory := kubeClientFactory
+	kubeClientFactory = func() (*kubeClient, error) {
+		return kubeClientFactoryForURL(kube.URL, kube.Client()), nil
+	}
+	t.Cleanup(func() { kubeClientFactory = origFactory })
+
+	loads := clusterActiveRemotePodLoads("maintenance", "build")
+	if loads["titan-04"] != 2 {
+		t.Fatalf("expected titan-04 load 2, got %#v", loads)
+	}
+	if _, ok := loads["titan-05"]; ok {
+		t.Fatalf("expected succeeded pod to be ignored, got %#v", loads)
+	}
+}
diff --git a/pkg/service/helpers_test.go b/pkg/service/helpers_test.go
index c62bf3f..a6e6a57 100644
--- a/pkg/service/helpers_test.go
+++ b/pkg/service/helpers_test.go
@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -125,6 +126,17 @@ func TestAppJobDeviceAndStateHelpers(t *testing.T) {
 	}
 }
 
+func TestBuildAndReplaceRejectDuplicateActiveNodeJobs(t *testing.T) {
+	app := newTestApp(t)
+	active := app.newJob("replace", "titan-15", "titan-22", "/dev/sdz")
+	if _, err := app.Build("titan-15"); err == nil || !strings.Contains(err.Error(), active.ID) {
+		t.Fatalf("expected build conflict mentioning %s, got %v", active.ID, err)
+	}
+	if _, err := app.Replace("titan-15", "titan-22", "/dev/sdz"); err == nil || !strings.Contains(err.Error(), "active replace job") {
+		t.Fatalf("expected replace conflict, got %v", err)
+	}
+}
+
 func TestAppPersistenceAndTargets(t *testing.T) {
 	dir := t.TempDir()
 	invPath := filepath.Join(dir, "inventory.yaml")
diff --git a/pkg/service/remote_helpers.go b/pkg/service/remote_helpers.go
index 6361408..c2e9e41 100644
--- a/pkg/service/remote_helpers.go
+++ b/pkg/service/remote_helpers.go
@@ -81,6 +81,8 @@ func (a *App) ensureDevice(host, path string) (*Device, error) {
 
 func (a *App) selectBuilderHost(arch, flashHost string) (clusterNode, error) {
 	nodes := clusterNodes()
+	activeBuilds := clusterActiveRemotePodLoads(a.settings.Namespace, "build")
+	activeRemotePods := clusterActiveRemotePodLoads(a.settings.Namespace, "")
 	storageNodes := map[string]struct{}{}
 	for _, node := range a.inventory.Nodes {
 		if len(node.LonghornDisks) > 0 {
@@ -119,6 +121,12 @@ func (a *App) selectBuilderHost(arch, flashHost string) (clusterNode, error) {
 		if flashHost != "" && node.Name == flashHost {
 			score += 5
 		}
+		if count := activeBuilds[node.Name]; count > 0 {
+			score -= 100 * count
+		}
+		if count := activeRemotePods[node.Name]; count > 0 {
+			score -= 15 * count
+		}
 		candidates = append(candidates, scored{node: node, score: score})
 	}
 	sort.Slice(candidates, func(i, j int) bool {
diff --git a/pkg/service/remote_helpers_test.go b/pkg/service/remote_helpers_test.go
index ceeb922..f7a9557 100644
--- a/pkg/service/remote_helpers_test.go
+++ b/pkg/service/remote_helpers_test.go
@@ -1,6 +1,9 @@
 package service
 
 import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 	"time"
@@ -66,3 +69,60 @@ func TestSelectBuilderHostErrorBranch(t *testing.T) {
 		t.Fatal("expected selectBuilderHost error")
 	}
 }
+
+func TestSelectBuilderHostAvoidsBusyBuilderWhenPeersAreFree(t *testing.T) {
+	kube := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch {
+		case r.Method == http.MethodGet && r.URL.Path == "/api/v1/nodes":
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"items": []any{
+					map[string]any{
+						"metadata": map[string]any{
+							"name": "titan-04",
+							"labels": map[string]string{
+								"kubernetes.io/arch":             "arm64",
+								"hardware":                       "rpi5",
+								"node-role.kubernetes.io/worker": "true",
+							},
+						},
+						"spec": map[string]any{"unschedulable": false},
+					},
+					map[string]any{
+						"metadata": map[string]any{
+							"name": "titan-05",
+							"labels": map[string]string{
+								"kubernetes.io/arch":             "arm64",
+								"hardware":                       "rpi5",
+								"node-role.kubernetes.io/worker": "true",
+							},
+						},
+						"spec": map[string]any{"unschedulable": false},
+					},
+				},
+			})
+		case r.Method == http.MethodGet && r.URL.Path == "/api/v1/namespaces/maintenance/pods":
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"items": []any{
+					map[string]any{
+						"metadata": map[string]any{"labels": map[string]string{"app": "metis-remote", "metis-run": "build"}},
+						"spec":     map[string]any{"nodeName": "titan-04"},
+						"status":   map[string]any{"phase": "Running"},
+					},
+				},
+			})
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer kube.Close()
+	installKubeFactory(t, kube)
+	app := newTestApp(t)
+	app.settings.Namespace = "maintenance"
+	node, err := app.selectBuilderHost("arm64", "")
+	if err != nil {
+		t.Fatalf("selectBuilderHost: %v", err)
+	}
+	if node.Name != "titan-05" {
+		t.Fatalf("expected titan-05 builder, got %s", node.Name)
+	}
+}
diff --git a/pkg/service/server.go b/pkg/service/server.go
index a8e335a..6f603f6 100644
--- a/pkg/service/server.go
+++ b/pkg/service/server.go
@@ -2,6 +2,7 @@ package service
 
 import (
 	"encoding/json"
+	"errors"
 	"html/template"
 	"net/http"
 	"strings"
@@ -99,7 +100,7 @@ func (a *App) handleBuild(w http.ResponseWriter, r *http.Request) {
 	node := values["node"]
 	job, err := a.Build(node)
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusBadRequest)
+		http.Error(w, err.Error(), statusForJobError(err))
 		return
 	}
 	writeJSON(w, http.StatusAccepted, job)
@@ -116,12 +117,20 @@ func (a *App) handleReplace(w http.ResponseWriter, r *http.Request) {
 	device := values["device"]
 	job, err := a.Replace(node, host, device)
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusBadRequest)
+		http.Error(w, err.Error(), statusForJobError(err))
 		return
 	}
 	writeJSON(w, http.StatusAccepted, job)
 }
 
+func statusForJobError(err error) int {
+	var conflict *activeNodeJobError
+	if errors.As(err, &conflict) {
+		return http.StatusConflict
+	}
+	return http.StatusBadRequest
+}
+
 func (a *App) handleWatch(w http.ResponseWriter, r *http.Request) {
 	if r.Method != http.MethodPost {
 		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
diff --git a/pkg/service/server_test.go b/pkg/service/server_test.go
index a14fb8e..edfbd59 100644
--- a/pkg/service/server_test.go
+++ b/pkg/service/server_test.go
@@ -229,6 +229,21 @@ func TestRequestValuesFormAndAuthHelpers(t *testing.T) {
 	}
 }
 
+func TestHandleBuildReturnsConflictForDuplicateActiveNodeJob(t *testing.T) {
+	app := newTestApp(t)
+	app.newJob("build", "titan-15", "", "")
+	handler := app.Handler()
+	req := httptest.NewRequest(http.MethodPost, "/api/jobs/build", strings.NewReader(`{"node":"titan-15"}`))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("X-Auth-Request-User", "brad")
+	req.Header.Set("X-Auth-Request-Groups", "admin")
+	resp := httptest.NewRecorder()
+	handler.ServeHTTP(resp, req)
+	if resp.Code != http.StatusConflict {
+		t.Fatalf("expected conflict, got %d: %s", resp.Code, resp.Body.String())
+	}
+}
+
 func TestHTTPHandlersExerciseErrorBranches(t *testing.T) {
 	kube := fakeKubeServer(t)
 	installKubeFactory(t, kube)
diff --git a/pkg/service/workflow_test.go b/pkg/service/workflow_test.go
index d2733b5..83dd4c4 100644
--- a/pkg/service/workflow_test.go
+++ b/pkg/service/workflow_test.go
@@ -166,6 +166,8 @@ func fakeKubeServer(t *testing.T) *httptest.Server {
 					},
 				},
 			})
+		case r.Method == http.MethodGet && r.URL.Path == "/api/v1/namespaces/maintenance/pods":
+			_ = json.NewEncoder(w).Encode(map[string]any{"items": []any{}})
 		case r.Method == http.MethodPost && strings.Contains(r.URL.Path, "/pods"):
 			w.WriteHeader(http.StatusCreated)
 		case r.Method == http.MethodDelete && strings.Contains(r.URL.Path, "/pods/"):