diff --git a/amethyst/src/main/java/com/vitorpamplona/amethyst/service/relays/EOSE.kt b/amethyst/src/main/java/com/vitorpamplona/amethyst/service/relays/EOSE.kt
index 403fcaf39f..f6032b8800 100644
--- a/amethyst/src/main/java/com/vitorpamplona/amethyst/service/relays/EOSE.kt
+++ b/amethyst/src/main/java/com/vitorpamplona/amethyst/service/relays/EOSE.kt
@@ -28,6 +28,7 @@ import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
 typealias EOSERelayList = com.vitorpamplona.amethyst.commons.relays.EOSERelayList
 typealias SincePerRelayMap = com.vitorpamplona.amethyst.commons.relays.SincePerRelayMap
 typealias MutableTime = com.vitorpamplona.amethyst.commons.relays.MutableTime
+typealias EOSEAccountFast<T> = com.vitorpamplona.amethyst.commons.relays.EOSEAccountFast<T>
 
 open class EOSEByKey<U : Any>(
     cacheSize: Int = 200,
@@ -113,60 +114,3 @@ open class EOSEAccountKey<U : Any>(
         time: Long,
     ) = addOrUpdate(user, listCode, relayUrl, time)
 }
-
-class EOSEAccountFast<T : Any>(
-    cacheSize: Int = 20,
-) {
-    private val users: LruCache<T, EOSERelayList> = LruCache(cacheSize)
-    private val lock = Any()
-
-    fun addOrUpdate(
-        user: T,
-        relayUrl: NormalizedRelayUrl,
-        time: Long,
-    ) {
-        synchronized(lock) {
-            val relayList = users[user]
-            if (relayList == null) {
-                val newList = EOSERelayList()
-                users.put(user, newList)
-
-                newList.addOrUpdate(relayUrl, time)
-            } else {
-                relayList.addOrUpdate(relayUrl, time)
-            }
-        }
-    }
-
-    fun removeEveryoneBut(list: Set<T>) {
-        synchronized(lock) {
-            users.snapshot().forEach {
-                if (it.key !in list) {
-                    users.remove(it.key)
-                }
-            }
-        }
-    }
-
-    fun removeDataFor(user: T) {
-        synchronized(lock) {
-            users.remove(user)
-        }
-    }
-
-    fun since(key: T): SincePerRelayMap? =
-        synchronized(lock) {
-            users[key]?.relayList?.toMutableMap()
-        }
-
-    fun sinceRelaySet(key: T): Set<NormalizedRelayUrl>? =
-        synchronized(lock) {
-            users[key]?.relayList?.keys?.toSet()
-        }
-
-    fun newEose(
-        user: T,
-        relayUrl: NormalizedRelayUrl,
-        time: Long,
-    ) = addOrUpdate(user, relayUrl, time)
-}
diff --git a/commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/relays/EOSEAccountFast.kt b/commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/relays/EOSEAccountFast.kt
new file mode 100644
index 0000000000..84d5e90773
--- /dev/null
+++ b/commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/relays/EOSEAccountFast.kt
@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.commons.relays
+
+import androidx.collection.LruCache
+import com.vitorpamplona.amethyst.commons.util.KmpLock
+import com.vitorpamplona.amethyst.commons.util.withLock
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
+
+/**
+ * Per-key EOSE tracker keyed by an arbitrary [T] (a `User`, a pubkey, …), used
+ * by the relay-subscription assemblers to remember which relays already EOSE'd
+ * for a given key so the next filter assembly can add a `since` and avoid a full
+ * re-download.
+ *
+ * KMP-pure: uses [KmpLock] instead of `synchronized(...)` so it compiles for the
+ * iOS targets `commons` builds for. Moved out of `amethyst.service.relays` so the
+ * shared user/event finder assemblers can live in `commonMain`. The old location
+ * keeps a `typealias` for source compatibility.
+ */
+class EOSEAccountFast<T : Any>(
+    cacheSize: Int = 20,
+) {
+    private val users: LruCache<T, EOSERelayList> = LruCache(cacheSize)
+    private val lock = KmpLock()
+
+    fun addOrUpdate(
+        user: T,
+        relayUrl: NormalizedRelayUrl,
+        time: Long,
+    ) {
+        lock.withLock {
+            val relayList = users[user]
+            if (relayList == null) {
+                val newList = EOSERelayList()
+                users.put(user, newList)
+
+                newList.addOrUpdate(relayUrl, time)
+            } else {
+                relayList.addOrUpdate(relayUrl, time)
+            }
+        }
+    }
+
+    fun removeEveryoneBut(list: Set<T>) {
+        lock.withLock {
+            users.snapshot().forEach {
+                if (it.key !in list) {
+                    users.remove(it.key)
+                }
+            }
+        }
+    }
+
+    fun removeDataFor(user: T) {
+        lock.withLock {
+            users.remove(user)
+        }
+    }
+
+    fun since(key: T): SincePerRelayMap? =
+        lock.withLock {
+            users[key]?.relayList?.toMutableMap()
+        }
+
+    fun sinceRelaySet(key: T): Set<NormalizedRelayUrl>? =
+        lock.withLock {
+            users[key]?.relayList?.keys?.toSet()
+        }
+
+    fun newEose(
+        user: T,
+        relayUrl: NormalizedRelayUrl,
+        time: Long,
+    ) = addOrUpdate(user, relayUrl, time)
+}