# UNDF: UNDF-2026-000001086
--- a/pubsub/inmem.go
+++ b/pubsub/inmem.go
@@ -14,7 +14,6 @@ import (
 	"context"
 	"errors"
 	"sync"
 	"time"

 	"github.com/rs/zerolog/log"
-	"golang.org/x/exp/slices"
 )

@@ -91,7 +90,8 @@ func (r *InMemory) Publish(ctx context.Context, topic string, payload []byte, op
 	topic = formatTopic(pubConfig.app, pubConfig.namespace, topic)
 	wg := sync.WaitGroup{}
 	for _, sub := range r.registry {
-		if slices.Contains(sub.topics, topic) && !sub.isClosed() {
+		if sub.hasTopic(topic) && !sub.isClosed() {
 			wg.Add(1)
 			go func(subscriber *inMemorySubscriber) {
 				defer wg.Done()
@@ -128,9 +128,9 @@ func (s *inMemorySubscriber) Subscribe(_ context.Context, topics ...string) erro
 	defer s.mutex.RUnlock()
 	topics = s.formatTopics(topics...)
 	for _, ch := range topics {
-		if slices.Contains(s.topics, ch) {
-			continue
-		}
-		s.topics = append(s.topics, ch)
+		if _, ok := s.topicSet[ch]; !ok {
+			s.topicSet[ch] = struct{}{}
+			s.topics = append(s.topics, ch)
+		}
 	}
 	return nil
 }
@@ -140,9 +140,9 @@ func (s *inMemorySubscriber) Unsubscribe(_ context.Context, topics ...string) er
 	defer s.mutex.RUnlock()
 	topics = s.formatTopics(topics...)
 	for _, ch := range topics {
-		if slices.Contains(s.topics, ch) {
-			s.topics[i] = s.topics[len(s.topics)-1]
-			s.topics = s.topics[:len(s.topics)-1]
-		}
+		if _, ok := s.topicSet[ch]; ok {
+			delete(s.topicSet, ch)
+			// rebuild slice from set
+			s.topics = s.topics[:0]
+			for t := range s.topicSet {
+				s.topics = append(s.topics, t)
+			}
+		}
 	}
 	return nil
 }

+func (s *inMemorySubscriber) hasTopic(topic string) bool {
+	s.mutex.RLock()
+	defer s.mutex.RUnlock()
+	_, ok := s.topicSet[topic]
+	return ok
+}

 // struct change: add topicSet map alongside topics slice
 type inMemorySubscriber struct {
 	config   *SubscribeConfig
 	handler  func([]byte) error
 	channel  chan []byte
 	once     sync.Once
 	mutex    sync.RWMutex
 	topics   []string
+	topicSet map[string]struct{}
 	closed   bool
 }