diff --git a/quartz/src/appleMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.apple.kt b/quartz/src/appleMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.apple.kt
index 9d29a6e2b..c64e575b3 100644
--- a/quartz/src/appleMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.apple.kt
+++ b/quartz/src/appleMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.apple.kt
@@ -85,7 +85,10 @@ actual class LargeCache<K, V> : ICacheOperations<K, V> {
     actual override fun size(): Int = concurrentMap.size
 
     actual override fun forEach(consumer: ICacheBiConsumer<K, V>) {
-        concurrentMap.forEach { consumer.accept(it.key, it.value) }
+        // Take a snapshot of entries to avoid ConcurrentModificationException
+        // when the map is modified during iteration (e.g., NostrClient.syncFilters
+        // iterating while subscriptions are added from another coroutine).
+        concurrentMap.entries.toList().forEach { consumer.accept(it.key, it.value) }
     }
 
     actual override fun filter(consumer: CacheCollectors.BiFilter<K, V>): List<V> =
diff --git a/quartz/src/linuxMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.linux.kt b/quartz/src/linuxMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.linux.kt
index 74861078f..2e49f22a5 100644
--- a/quartz/src/linuxMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.linux.kt
+++ b/quartz/src/linuxMain/kotlin/com/vitorpamplona/quartz/utils/cache/LargeCache.linux.kt
@@ -88,7 +88,8 @@ actual class LargeCache<K, V> : ICacheOperations<K, V> {
     actual override fun size(): Int = withMap { it.size }
 
     actual override fun forEach(consumer: ICacheBiConsumer<K, V>) {
-        withMap { map -> map.forEach { consumer.accept(it.key, it.value) } }
+        // Snapshot entries to avoid ConcurrentModificationException
+        withMap { map -> map.entries.toList() }.forEach { consumer.accept(it.key, it.value) }
     }
 
     actual override fun filter(consumer: CacheCollectors.BiFilter<K, V>): List<V> = withMap { map -> map.filter { consumer.filter(it.key, it.value) }.values.toList() }