diff --git a/api/gateway/mntx/go.mod b/api/gateway/mntx/go.mod
index 0ca8b1f3..542ea30d 100644
--- a/api/gateway/mntx/go.mod
+++ b/api/gateway/mntx/go.mod
@@ -9,6 +9,7 @@ replace github.com/tech/sendico/gateway/common => ../common
 require (
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/prometheus/client_golang v1.23.2
+	github.com/prometheus/client_model v0.6.2
 	github.com/shopspring/decimal v1.4.0
 	github.com/tech/sendico/gateway/common v0.1.0
 	github.com/tech/sendico/pkg v0.1.0
@@ -35,7 +36,6 @@ require (
 	github.com/nats-io/nats.go v1.49.0 // indirect
 	github.com/nats-io/nkeys v0.4.15 // indirect
 	github.com/nats-io/nuid v1.0.1 // indirect
-	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.67.5 // indirect
 	github.com/prometheus/procfs v0.20.0 // indirect
 	github.com/rogpeppe/go-internal v1.12.0 // indirect
diff --git a/api/pkg/discovery/announcer.go b/api/pkg/discovery/announcer.go
index 8c082939..8678b527 100644
--- a/api/pkg/discovery/announcer.go
+++ b/api/pkg/discovery/announcer.go
@@ -117,9 +117,18 @@ func (a *Announcer) sendAnnouncement() {
 }
 
 func (a *Announcer) sendHeartbeat() {
+	entryKey := registryKey(
+		a.announce.Service,
+		a.announce.Rail,
+		legacyNetworkFromCurrencies(a.announce.Currencies),
+		a.announce.Operations,
+		a.announce.Version,
+		a.announce.InstanceID,
+	)
 	hb := Heartbeat{
 		ID:         a.announce.ID,
 		InstanceID: a.announce.InstanceID,
+		EntryKey:   entryKey,
 		Status:     "ok",
 		TS:         time.Now().Unix(),
 	}
diff --git a/api/pkg/discovery/registry.go b/api/pkg/discovery/registry.go
index c94aaa02..e0051568 100644
--- a/api/pkg/discovery/registry.go
+++ b/api/pkg/discovery/registry.go
@@ -150,21 +150,34 @@ func (r *Registry) UpdateHeartbeat(id string, instanceID string, status string,
 		if instanceID != "" && entry.InstanceID != instanceID {
 			continue
 		}
-		wasHealthy := entry.isHealthyAt(now)
-		entry.Status = status
-		entry.LastHeartbeat = ts
-		entry.Healthy = entry.isHealthyAt(now)
-
-		results = append(results, UpdateResult{
-			Entry:         *entry,
-			IsNew:         false,
-			WasHealthy:    wasHealthy,
-			BecameHealthy: !wasHealthy && entry.Healthy,
-		})
+		results = append(results, updateHeartbeatEntry(entry, status, ts, now))
 	}
 	return results
 }
 
+func (r *Registry) UpdateHeartbeatByKey(key string, status string, ts time.Time, now time.Time) (UpdateResult, bool) {
+	key = strings.TrimSpace(key)
+	if key == "" {
+		return UpdateResult{}, false
+	}
+	if status == "" {
+		status = "ok"
+	}
+	if ts.IsZero() {
+		ts = now
+	}
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	entry := r.entries[key]
+	if entry == nil {
+		return UpdateResult{}, false
+	}
+
+	return updateHeartbeatEntry(entry, status, ts, now), true
+}
+
 func (r *Registry) Delete(key string) bool {
 	key = strings.TrimSpace(key)
 	if key == "" {
@@ -181,6 +194,20 @@ func (r *Registry) Delete(key string) bool {
 	return true
 }
 
+func updateHeartbeatEntry(entry *RegistryEntry, status string, ts time.Time, now time.Time) UpdateResult {
+	wasHealthy := entry.isHealthyAt(now)
+	entry.Status = status
+	entry.LastHeartbeat = ts
+	entry.Healthy = entry.isHealthyAt(now)
+
+	return UpdateResult{
+		Entry:         *entry,
+		IsNew:         false,
+		WasHealthy:    wasHealthy,
+		BecameHealthy: !wasHealthy && entry.Healthy,
+	}
+}
+
 // DeleteStale removes entries whose heartbeat timeout has elapsed.
 func (r *Registry) DeleteStale(now time.Time) []string {
 	r.mu.Lock()
diff --git a/api/pkg/discovery/registry_heartbeat_test.go b/api/pkg/discovery/registry_heartbeat_test.go
new file mode 100644
index 00000000..f9459b38
--- /dev/null
+++ b/api/pkg/discovery/registry_heartbeat_test.go
@@ -0,0 +1,133 @@
+package discovery
+
+import (
+	"testing"
+	"time"
+)
+
+func TestRegistryUpdateHeartbeatByKey_UpdatesOnlyTargetEntry(t *testing.T) {
+	now := time.Date(2026, 2, 27, 12, 0, 0, 0, time.UTC)
+	oldHeartbeat := now.Add(-45 * time.Second)
+	registry := NewRegistry()
+
+	oldVersion := RegistryEntry{
+		ID:            "mcards",
+		InstanceID:    "inst-1",
+		Service:       "CARD",
+		Rail:          RailCardPayout,
+		Operations:    []string{OperationSend},
+		Version:       "3.0.0-old",
+		Status:        "ok",
+		Health:        HealthParams{TimeoutSec: 30},
+		LastHeartbeat: oldHeartbeat,
+	}
+	newVersion := RegistryEntry{
+		ID:            "mcards",
+		InstanceID:    "inst-1",
+		Service:       "CARD",
+		Rail:          RailCardPayout,
+		Operations:    []string{OperationSend},
+		Version:       "3.0.0-new",
+		Status:        "ok",
+		Health:        HealthParams{TimeoutSec: 30},
+		LastHeartbeat: oldHeartbeat,
+	}
+
+	registry.UpsertEntry(oldVersion, now)
+	registry.UpsertEntry(newVersion, now)
+
+	oldKey := registryEntryKey(normalizeEntry(oldVersion))
+	newKey := registryEntryKey(normalizeEntry(newVersion))
+
+	result, found := registry.UpdateHeartbeatByKey(newKey, "ok", now, now)
+	if !found {
+		t.Fatalf("expected heartbeat target key to exist: %q", newKey)
+	}
+	if got := registryEntryKey(normalizeEntry(result.Entry)); got != newKey {
+		t.Fatalf("unexpected updated key: got=%q want=%q", got, newKey)
+	}
+
+	entries := registry.List(now, false)
+	if len(entries) != 2 {
+		t.Fatalf("unexpected entries count: got=%d want=2", len(entries))
+	}
+
+	sawOld := false
+	sawNew := false
+	for _, entry := range entries {
+		key := registryEntryKey(normalizeEntry(entry))
+		switch key {
+		case oldKey:
+			sawOld = true
+			if !entry.LastHeartbeat.Equal(oldHeartbeat) {
+				t.Fatalf("old key heartbeat changed unexpectedly: got=%s want=%s", entry.LastHeartbeat, oldHeartbeat)
+			}
+			if entry.Healthy {
+				t.Fatalf("old key should remain unhealthy")
+			}
+		case newKey:
+			sawNew = true
+			if !entry.LastHeartbeat.Equal(now) {
+				t.Fatalf("new key heartbeat not updated: got=%s want=%s", entry.LastHeartbeat, now)
+			}
+			if !entry.Healthy {
+				t.Fatalf("new key should be healthy after heartbeat")
+			}
+		}
+	}
+	if !sawOld {
+		t.Fatalf("old key entry not found after update: %q", oldKey)
+	}
+	if !sawNew {
+		t.Fatalf("new key entry not found after update: %q", newKey)
+	}
+}
+
+func TestRegistryUpdateHeartbeat_LegacyFallbackUpdatesAllMatchingInstanceEntries(t *testing.T) {
+	now := time.Date(2026, 2, 27, 12, 0, 0, 0, time.UTC)
+	oldHeartbeat := now.Add(-45 * time.Second)
+	registry := NewRegistry()
+
+	entries := []RegistryEntry{
+		{
+			ID:            "mcards",
+			InstanceID:    "inst-1",
+			Service:       "CARD",
+			Rail:          RailCardPayout,
+			Operations:    []string{OperationSend},
+			Version:       "3.0.0-old",
+			Status:        "ok",
+			Health:        HealthParams{TimeoutSec: 30},
+			LastHeartbeat: oldHeartbeat,
+		},
+		{
+			ID:            "mcards",
+			InstanceID:    "inst-1",
+			Service:       "CARD",
+			Rail:          RailCardPayout,
+			Operations:    []string{OperationSend},
+			Version:       "3.0.0-new",
+			Status:        "ok",
+			Health:        HealthParams{TimeoutSec: 30},
+			LastHeartbeat: oldHeartbeat,
+		},
+	}
+	for _, entry := range entries {
+		registry.UpsertEntry(entry, now)
+	}
+
+	results := registry.UpdateHeartbeat("mcards", "inst-1", "ok", now, now)
+	if len(results) != 2 {
+		t.Fatalf("unexpected updated count: got=%d want=2", len(results))
+	}
+
+	all := registry.List(now, false)
+	for _, entry := range all {
+		if !entry.LastHeartbeat.Equal(now) {
+			t.Fatalf("entry heartbeat not updated: key=%s got=%s want=%s", registryEntryKey(normalizeEntry(entry)), entry.LastHeartbeat, now)
+		}
+		if !entry.Healthy {
+			t.Fatalf("entry should be healthy after legacy heartbeat: key=%s", registryEntryKey(normalizeEntry(entry)))
+		}
+	}
+}
diff --git a/api/pkg/discovery/service.go b/api/pkg/discovery/service.go
index 7e718f83..97088310 100644
--- a/api/pkg/discovery/service.go
+++ b/api/pkg/discovery/service.go
@@ -216,22 +216,35 @@ func (s *RegistryService) handleHeartbeat(_ context.Context, env me.Envelope) (e
 		s.logWarn("Failed to decode discovery heartbeat payload", fields...)
 		return err
 	}
-	s.logDebug("Discovery heartbeat received", append(envelopeFields(env), zap.String("id", payload.ID), zap.String("instance_id", payload.InstanceID), zap.String("status", payload.Status))...)
-	if strings.TrimSpace(payload.InstanceID) == "" && strings.TrimSpace(payload.ID) == "" {
+	entryKey := strings.TrimSpace(payload.EntryKey)
+	s.logDebug("Discovery heartbeat received", append(envelopeFields(env), zap.String("id", payload.ID), zap.String("instance_id", payload.InstanceID), zap.String("entry_key", entryKey), zap.String("status", payload.Status))...)
+	if entryKey == "" && strings.TrimSpace(payload.InstanceID) == "" && strings.TrimSpace(payload.ID) == "" {
 		return nil
 	}
-	if strings.TrimSpace(payload.InstanceID) == "" {
+	if entryKey == "" && strings.TrimSpace(payload.InstanceID) == "" {
 		fields := append(envelopeFields(env), zap.String("id", payload.ID))
 		s.logWarn("Discovery heartbeat missing instance id", fields...)
 	}
+	status := strings.TrimSpace(payload.Status)
 	ts := time.Unix(payload.TS, 0)
 	if ts.Unix() <= 0 {
 		ts = time.Now()
 	}
-	results := s.registry.UpdateHeartbeat(payload.ID, payload.InstanceID, strings.TrimSpace(payload.Status), ts, time.Now())
-	if len(results) == 0 {
-		s.logDebug("Discovery heartbeat ignored: entry not found", zap.String("id", payload.ID), zap.String("instance_id", payload.InstanceID))
-		return nil
+	now := time.Now()
+	var results []UpdateResult
+	if entryKey != "" {
+		result, found := s.registry.UpdateHeartbeatByKey(entryKey, status, ts, now)
+		if !found {
+			s.logDebug("Discovery heartbeat ignored: entry not found", zap.String("entry_key", entryKey), zap.String("id", payload.ID), zap.String("instance_id", payload.InstanceID))
+			return nil
+		}
+		results = append(results, result)
+	} else {
+		results = s.registry.UpdateHeartbeat(payload.ID, payload.InstanceID, status, ts, now)
+		if len(results) == 0 {
+			s.logDebug("Discovery heartbeat ignored: entry not found", zap.String("id", payload.ID), zap.String("instance_id", payload.InstanceID))
+			return nil
+		}
 	}
 	for _, result := range results {
 		if result.BecameHealthy {
diff --git a/api/pkg/discovery/types.go b/api/pkg/discovery/types.go
index 1f904d63..76f9a91b 100644
--- a/api/pkg/discovery/types.go
+++ b/api/pkg/discovery/types.go
@@ -116,6 +116,7 @@ type Announcement struct {
 type Heartbeat struct {
 	ID         string `json:"id"`
 	InstanceID string `json:"instanceId"`
+	EntryKey   string `json:"entryKey,omitempty"`
 	Status     string `json:"status"`
 	TS         int64  `json:"ts"`
 }