diff --git a/app/src/main/java/com/bitchat/android/mesh/BluetoothConnectionManager.kt b/app/src/main/java/com/bitchat/android/mesh/BluetoothConnectionManager.kt
index 0cc7f967..19c103a3 100644
--- a/app/src/main/java/com/bitchat/android/mesh/BluetoothConnectionManager.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/BluetoothConnectionManager.kt
@@ -247,6 +247,19 @@ class BluetoothConnectionManager(
             serverManager.getCharacteristic()
         )
     }
+
+    /**
+     * Send a packet directly to a specific peer, without broadcasting to others.
+     */
+    fun sendPacketToPeer(peerID: String, packet: BitchatPacket): Boolean {
+        if (!isActive) return false
+        return packetBroadcaster.sendPacketToPeer(
+            RoutedPacket(packet),
+            peerID,
+            serverManager.getGattServer(),
+            serverManager.getCharacteristic()
+        )
+    }
     
 
     // Expose role controls for debug UI
diff --git a/app/src/main/java/com/bitchat/android/mesh/BluetoothMeshService.kt b/app/src/main/java/com/bitchat/android/mesh/BluetoothMeshService.kt
index f6e5ac64..8d6b357f 100644
--- a/app/src/main/java/com/bitchat/android/mesh/BluetoothMeshService.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/BluetoothMeshService.kt
@@ -10,6 +10,8 @@ import com.bitchat.android.model.IdentityAnnouncement
 import com.bitchat.android.protocol.BitchatPacket
 import com.bitchat.android.protocol.MessageType
 import com.bitchat.android.protocol.SpecialRecipients
+import com.bitchat.android.model.RequestSyncPacket
+import com.bitchat.android.sync.GossipSyncManager
 import com.bitchat.android.util.toHexString
 import kotlinx.coroutines.*
 import java.util.*
@@ -49,6 +51,7 @@ class BluetoothMeshService(private val context: Context) {
     private val messageHandler = MessageHandler(myPeerID)
     internal val connectionManager = BluetoothConnectionManager(context, myPeerID, fragmentManager) // Made internal for access
     private val packetProcessor = PacketProcessor(myPeerID)
+    private lateinit var gossipSyncManager: GossipSyncManager
     
     // Service state management
     private var isActive = false
@@ -63,6 +66,38 @@ class BluetoothMeshService(private val context: Context) {
         setupDelegates()
         messageHandler.packetProcessor = packetProcessor
         //startPeriodicDebugLogging()
+
+        // Initialize sync manager (needs serviceScope)
+        gossipSyncManager = GossipSyncManager(
+            myPeerID = myPeerID,
+            scope = serviceScope,
+            configProvider = object : GossipSyncManager.ConfigProvider {
+                override fun seenCapacity(): Int = try {
+                    com.bitchat.android.ui.debug.DebugPreferenceManager.getSeenPacketCapacity(500)
+                } catch (_: Exception) { 500 }
+
+                override fun gcsMaxBytes(): Int = try {
+                    com.bitchat.android.ui.debug.DebugPreferenceManager.getGcsMaxFilterBytes(400)
+                } catch (_: Exception) { 400 }
+
+                override fun gcsTargetFpr(): Double = try {
+                    com.bitchat.android.ui.debug.DebugPreferenceManager.getGcsFprPercent(1.0) / 100.0
+                } catch (_: Exception) { 0.01 }
+            }
+        )
+
+        // Wire sync manager delegate
+        gossipSyncManager.delegate = object : GossipSyncManager.Delegate {
+            override fun sendPacket(packet: BitchatPacket) {
+                connectionManager.broadcastPacket(RoutedPacket(packet))
+            }
+            override fun sendPacketToPeer(peerID: String, packet: BitchatPacket) {
+                connectionManager.sendPacketToPeer(peerID, packet)
+            }
+            override fun signPacketForBroadcast(packet: BitchatPacket): BitchatPacket {
+                return signPacketBeforeBroadcast(packet)
+            }
+        }
     }
     
     /**
@@ -113,6 +148,9 @@ class BluetoothMeshService(private val context: Context) {
             override fun onPeerListUpdated(peerIDs: List<String>) {
                 delegate?.didUpdatePeerList(peerIDs)
             }
+            override fun onPeerRemoved(peerID: String) {
+                try { gossipSyncManager.removeAnnouncementForPeer(peerID) } catch (_: Exception) { }
+            }
         }
         
         // SecurityManager delegate for key exchange notifications
@@ -380,13 +418,26 @@ class BluetoothMeshService(private val context: Context) {
                                     } catch (_: Exception) { }
                                 }
                             } catch (_: Exception) { }
+
+                            // Schedule initial sync for this new directly connected peer only
+                            try { gossipSyncManager.scheduleInitialSyncToPeer(pid, 1_000) } catch (_: Exception) { }
                         }
                     }
+                    // Track for sync
+                    try { gossipSyncManager.onPublicPacketSeen(routed.packet) } catch (_: Exception) { }
                 }
             }
             
             override fun handleMessage(routed: RoutedPacket) {
                 serviceScope.launch { messageHandler.handleMessage(routed) }
+                // Track broadcast messages for sync
+                try {
+                    val pkt = routed.packet
+                    val isBroadcast = (pkt.recipientID == null || pkt.recipientID.contentEquals(SpecialRecipients.BROADCAST))
+                    if (isBroadcast && pkt.type == MessageType.MESSAGE.value) {
+                        gossipSyncManager.onPublicPacketSeen(pkt)
+                    }
+                } catch (_: Exception) { }
             }
             
             override fun handleLeave(routed: RoutedPacket) {
@@ -408,6 +459,13 @@ class BluetoothMeshService(private val context: Context) {
             override fun relayPacket(routed: RoutedPacket) {
                 connectionManager.broadcastPacket(routed)
             }
+
+            override fun handleRequestSync(routed: RoutedPacket) {
+                // Decode request and respond with missing packets
+                val fromPeer = routed.peerID ?: return
+                val req = RequestSyncPacket.decode(routed.packet.payload) ?: return
+                gossipSyncManager.handleRequestSync(fromPeer, req)
+            }
         }
         
         // BluetoothConnectionManager delegates
@@ -480,6 +538,8 @@ class BluetoothMeshService(private val context: Context) {
             // Start periodic announcements for peer discovery and connectivity
             sendPeriodicBroadcastAnnounce()
             Log.d(TAG, "Started periodic broadcast announcements (every 30 seconds)")
+            // Start periodic syncs
+            gossipSyncManager.start()
         } else {
             Log.e(TAG, "Failed to start Bluetooth services")
         }
@@ -504,6 +564,7 @@ class BluetoothMeshService(private val context: Context) {
             delay(200) // Give leave message time to send
             
             // Stop all components
+            gossipSyncManager.stop()
             connectionManager.stopServices()
             peerManager.shutdown()
             fragmentManager.shutdown()
@@ -537,6 +598,8 @@ class BluetoothMeshService(private val context: Context) {
             // Sign the packet before broadcasting
             val signedPacket = signPacketBeforeBroadcast(packet)
             connectionManager.broadcastPacket(RoutedPacket(signedPacket))
+            // Track our own broadcast message for sync
+            try { gossipSyncManager.onPublicPacketSeen(signedPacket) } catch (_: Exception) { }
         }
     }
     
@@ -708,6 +771,8 @@ class BluetoothMeshService(private val context: Context) {
             
             connectionManager.broadcastPacket(RoutedPacket(signedPacket))
             Log.d(TAG, "Sent iOS-compatible signed TLV announce (${tlvPayload.size} bytes)")
+            // Track announce for sync
+            try { gossipSyncManager.onPublicPacketSeen(signedPacket) } catch (_: Exception) { }
         }
     }
     
@@ -756,6 +821,9 @@ class BluetoothMeshService(private val context: Context) {
         connectionManager.broadcastPacket(RoutedPacket(signedPacket))
         peerManager.markPeerAsAnnouncedTo(peerID)
         Log.d(TAG, "Sent iOS-compatible signed TLV peer announce to $peerID (${tlvPayload.size} bytes)")
+
+        // Track announce for sync
+        try { gossipSyncManager.onPublicPacketSeen(signedPacket) } catch (_: Exception) { }
     }
 
     /**
diff --git a/app/src/main/java/com/bitchat/android/mesh/BluetoothPacketBroadcaster.kt b/app/src/main/java/com/bitchat/android/mesh/BluetoothPacketBroadcaster.kt
index 1fc52bb4..188e9d2e 100644
--- a/app/src/main/java/com/bitchat/android/mesh/BluetoothPacketBroadcaster.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/BluetoothPacketBroadcaster.kt
@@ -139,6 +139,47 @@ class BluetoothPacketBroadcaster(
         broadcastSinglePacket(routed, gattServer, characteristic)
     }
 
+    /**
+     * Send a packet to a specific peer only, without broadcasting.
+     * Returns true if a direct path was found and used.
+     */
+    fun sendPacketToPeer(
+        routed: RoutedPacket,
+        targetPeerID: String,
+        gattServer: BluetoothGattServer?,
+        characteristic: BluetoothGattCharacteristic?
+    ): Boolean {
+        val packet = routed.packet
+        val data = packet.toBinaryData() ?: return false
+        val typeName = MessageType.fromValue(packet.type)?.name ?: packet.type.toString()
+        val incomingAddr = routed.relayAddress
+        val incomingPeer = incomingAddr?.let { connectionTracker.addressPeerMap[it] }
+        val senderPeerID = routed.peerID ?: packet.senderID.toHexString()
+        val senderNick = senderPeerID.let { pid -> nicknameResolver?.invoke(pid) }
+
+        // Prefer server-side subscriptions
+        val serverTarget = connectionTracker.getSubscribedDevices()
+            .firstOrNull { connectionTracker.addressPeerMap[it.address] == targetPeerID }
+        if (serverTarget != null) {
+            if (notifyDevice(serverTarget, data, gattServer, characteristic)) {
+                logPacketRelay(typeName, senderPeerID, senderNick, incomingPeer, incomingAddr, targetPeerID, serverTarget.address, packet.ttl)
+                return true
+            }
+        }
+
+        // Then client connections
+        val clientTarget = connectionTracker.getConnectedDevices().values
+            .firstOrNull { connectionTracker.addressPeerMap[it.device.address] == targetPeerID }
+        if (clientTarget != null) {
+            if (writeToDeviceConn(clientTarget, data)) {
+                logPacketRelay(typeName, senderPeerID, senderNick, incomingPeer, incomingAddr, targetPeerID, clientTarget.device.address, packet.ttl)
+                return true
+            }
+        }
+
+        return false
+    }
+
     
     /**
      * Public entry point for broadcasting - submits request to actor for serialization
diff --git a/app/src/main/java/com/bitchat/android/mesh/PacketProcessor.kt b/app/src/main/java/com/bitchat/android/mesh/PacketProcessor.kt
index 54ecffbe..fca35ca1 100644
--- a/app/src/main/java/com/bitchat/android/mesh/PacketProcessor.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/PacketProcessor.kt
@@ -146,6 +146,7 @@ class PacketProcessor(private val myPeerID: String) {
             MessageType.MESSAGE -> handleMessage(routed)
             MessageType.LEAVE -> handleLeave(routed)
             MessageType.FRAGMENT -> handleFragment(routed)
+            MessageType.REQUEST_SYNC -> handleRequestSync(routed)
             else -> {
                 // Handle private packet types (address check required)
                 if (packetRelayManager.isPacketAddressedToMe(packet)) {
@@ -232,6 +233,15 @@ class PacketProcessor(private val myPeerID: String) {
         
         // Fragment relay is now handled by centralized PacketRelayManager
     }
+
+    /**
+     * Handle REQUEST_SYNC packets (public, TTL=1)
+     */
+    private suspend fun handleRequestSync(routed: RoutedPacket) {
+        val peerID = routed.peerID ?: "unknown"
+        Log.d(TAG, "Processing REQUEST_SYNC from ${formatPeerForLog(peerID)}")
+        delegate?.handleRequestSync(routed)
+    }
     
     /**
      * Handle delivery acknowledgment
@@ -305,6 +315,7 @@ interface PacketProcessorDelegate {
     fun handleMessage(routed: RoutedPacket)
     fun handleLeave(routed: RoutedPacket)
     fun handleFragment(packet: BitchatPacket): BitchatPacket?
+    fun handleRequestSync(routed: RoutedPacket)
     
     // Communication
     fun sendAnnouncementToPeer(peerID: String)
diff --git a/app/src/main/java/com/bitchat/android/mesh/PacketRelayManager.kt b/app/src/main/java/com/bitchat/android/mesh/PacketRelayManager.kt
index f292534b..bc401daa 100644
--- a/app/src/main/java/com/bitchat/android/mesh/PacketRelayManager.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/PacketRelayManager.kt
@@ -41,7 +41,7 @@ class PacketRelayManager(private val myPeerID: String) {
         val packet = routed.packet
         val peerID = routed.peerID ?: "unknown"
         
-        Log.d(TAG, "Evaluating relay for packet type ${'$'}{packet.type} from ${'$'}peerID (TTL: ${'$'}{packet.ttl})")
+        Log.d(TAG, "Evaluating relay for packet type ${packet.type} from ${peerID} (TTL: ${packet.ttl})")
         
         // Double-check this packet isn't addressed to us
         if (isPacketAddressedToMe(packet)) {
@@ -63,7 +63,7 @@ class PacketRelayManager(private val myPeerID: String) {
         
         // Decrement TTL by 1
         val relayPacket = packet.copy(ttl = (packet.ttl - 1u).toUByte())
-        Log.d(TAG, "Decremented TTL from ${'$'}{packet.ttl} to ${'$'}{relayPacket.ttl}")
+        Log.d(TAG, "Decremented TTL from ${packet.ttl} to ${relayPacket.ttl}")
         
         // Apply relay logic based on packet type and debug switch
         val shouldRelay = isRelayEnabled() && shouldRelayPacket(relayPacket, peerID)
@@ -71,7 +71,7 @@ class PacketRelayManager(private val myPeerID: String) {
         if (shouldRelay) {
             relayPacket(RoutedPacket(relayPacket, peerID, routed.relayAddress))
         } else {
-            Log.d(TAG, "Relay decision: NOT relaying packet type ${'$'}{packet.type}")
+            Log.d(TAG, "Relay decision: NOT relaying packet type ${packet.type}")
         }
     }
     
@@ -103,7 +103,7 @@ class PacketRelayManager(private val myPeerID: String) {
     private fun shouldRelayPacket(packet: BitchatPacket, fromPeerID: String): Boolean {
         // Always relay if TTL is high enough (indicates important message)
         if (packet.ttl >= 4u) {
-            Log.d(TAG, "High TTL (${ '$' }{packet.ttl}), relaying")
+            Log.d(TAG, "High TTL (${packet.ttl}), relaying")
             return true
         }
         
@@ -112,7 +112,7 @@ class PacketRelayManager(private val myPeerID: String) {
         
         // Small networks always relay to ensure connectivity
         if (networkSize <= 3) {
-            Log.d(TAG, "Small network (${ '$' }networkSize peers), relaying")
+            Log.d(TAG, "Small network (${networkSize} peers), relaying")
             return true
         }
         
@@ -126,52 +126,16 @@ class PacketRelayManager(private val myPeerID: String) {
         }
         
         val shouldRelay = Random.nextDouble() < relayProb
-        Log.d(TAG, "Network size: ${'$'}networkSize, Relay probability: ${'$'}relayProb, Decision: ${'$'}shouldRelay")
+        Log.d(TAG, "Network size: ${networkSize}, Relay probability: ${relayProb}, Decision: ${shouldRelay}")
         
         return shouldRelay
     }
     
-    /**
-     * Relay message with adaptive probability and timing (same as iOS)
-     * Moved from MessageHandler.kt
-     */
-    suspend fun relayMessage(routed: RoutedPacket) {
-        val packet = routed.packet
-        
-        if (packet.ttl == 0u.toUByte()) {
-            Log.d(TAG, "TTL expired, not relaying message")
-            return
-        }
-        
-        val relayPacket = packet.copy(ttl = (packet.ttl - 1u).toUByte())
-        
-        // Check network size and apply adaptive relay probability
-        val networkSize = delegate?.getNetworkSize() ?: 1
-        val relayProb = when {
-            networkSize <= 10 -> 1.0
-            networkSize <= 30 -> 0.85
-            networkSize <= 50 -> 0.7
-            networkSize <= 100 -> 0.55
-            else -> 0.4
-        }
-        
-        val shouldRelay = relayPacket.ttl >= 4u || networkSize <= 3 || Random.nextDouble() < relayProb
-        
-        if (shouldRelay) {
-            val delay = Random.nextLong(50, 500) // Random delay like iOS
-            Log.d(TAG, "Relaying message after ${'$'}delay ms delay")
-            delay(delay)
-            relayPacket(routed.copy(packet = relayPacket))
-        } else {
-            Log.d(TAG, "Relay decision: NOT relaying message (network size: ${'$'}networkSize, prob: ${'$'}relayProb)")
-        }
-    }
-    
     /**
      * Actually broadcast the packet for relay
      */
     private fun relayPacket(routed: RoutedPacket) {
-        Log.d(TAG, "🔄 Relaying packet type ${'$'}{routed.packet.type} with TTL ${'$'}{routed.packet.ttl}")
+        Log.d(TAG, "🔄 Relaying packet type ${routed.packet.type} with TTL ${routed.packet.ttl}")
         delegate?.broadcastPacket(routed)
     }
     
@@ -181,9 +145,9 @@ class PacketRelayManager(private val myPeerID: String) {
     fun getDebugInfo(): String {
         return buildString {
             appendLine("=== Packet Relay Manager Debug Info ===")
-            appendLine("Relay Scope Active: ${'$'}{relayScope.isActive}")
-            appendLine("My Peer ID: ${'$'}myPeerID")
-            appendLine("Network Size: ${'$'}{delegate?.getNetworkSize() ?: \"unknown\"}")
+            appendLine("Relay Scope Active: ${relayScope.isActive}")
+            appendLine("My Peer ID: ${myPeerID}")
+            appendLine("Network Size: ${delegate?.getNetworkSize() ?: "unknown"}")
         }
     }
     
diff --git a/app/src/main/java/com/bitchat/android/mesh/PeerManager.kt b/app/src/main/java/com/bitchat/android/mesh/PeerManager.kt
index 4b8a16dc..68a539eb 100644
--- a/app/src/main/java/com/bitchat/android/mesh/PeerManager.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/PeerManager.kt
@@ -262,6 +262,8 @@ class PeerManager {
         fingerprintManager.removePeer(peerID)
         
         if (notifyDelegate && removed != null) {
+            // Notify specific removal event then list update
+            try { delegate?.onPeerRemoved(peerID) } catch (_: Exception) {}
             notifyPeerListUpdate()
         }
     }
@@ -529,4 +531,5 @@ class PeerManager {
  */
 interface PeerManagerDelegate {
     fun onPeerListUpdated(peerIDs: List<String>)
+    fun onPeerRemoved(peerID: String)
 }
diff --git a/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt b/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt
index 89f0197c..f763424a 100644
--- a/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt
+++ b/app/src/main/java/com/bitchat/android/mesh/SecurityManager.kt
@@ -50,12 +50,6 @@ class SecurityManager(private val encryptionService: EncryptionService, private
             return false
         }
         
-        // TTL check
-        if (packet.ttl == 0u.toUByte()) {
-            Log.d(TAG, "Dropping packet with TTL 0")
-            return false
-        }
-        
         // Validate packet payload
         if (packet.payload.isEmpty()) {
             Log.d(TAG, "Dropping packet with empty payload")
@@ -67,11 +61,11 @@ class SecurityManager(private val encryptionService: EncryptionService, private
         val packetTime = packet.timestamp.toLong()
         val timeDiff = kotlin.math.abs(currentTime - packetTime)
         
-        if (timeDiff > MESSAGE_TIMEOUT) {
-            Log.d(TAG, "Dropping old packet from $peerID, time diff: ${timeDiff/1000}s")
-            return false
-        }
-        
+//        if (timeDiff > MESSAGE_TIMEOUT) {
+//            Log.d(TAG, "Dropping old packet from $peerID, time diff: ${timeDiff/1000}s")
+//            return false
+//        }
+
         // Duplicate detection
         val messageID = generateMessageID(packet, peerID)
         if (processedMessages.contains(messageID)) {
diff --git a/app/src/main/java/com/bitchat/android/model/RequestSyncPacket.kt b/app/src/main/java/com/bitchat/android/model/RequestSyncPacket.kt
new file mode 100644
index 00000000..ab52f071
--- /dev/null
+++ b/app/src/main/java/com/bitchat/android/model/RequestSyncPacket.kt
@@ -0,0 +1,82 @@
+package com.bitchat.android.model
+
+import com.bitchat.android.sync.SyncDefaults
+
+/**
+ * REQUEST_SYNC payload using GCS (Golomb-Coded Set) parameters.
+ * TLV (type, length16, value), types:
+ *  - 0x01: P (uint8) — Golomb-Rice parameter
+ *  - 0x02: M (uint32, big-endian) — hash range (N * 2^P)
+ *  - 0x03: data (opaque) — GR bitstream bytes
+ */
+data class RequestSyncPacket(
+    val p: Int,
+    val m: Long,
+    val data: ByteArray
+) {
+    fun encode(): ByteArray {
+        val out = ArrayList<Byte>()
+        fun putTLV(t: Int, v: ByteArray) {
+            out.add(t.toByte())
+            val len = v.size
+            out.add(((len ushr 8) and 0xFF).toByte())
+            out.add((len and 0xFF).toByte())
+            out.addAll(v.toList())
+        }
+        // P
+        putTLV(0x01, byteArrayOf(p.toByte()))
+        // M (uint32)
+        val m32 = m.coerceAtMost(0xffff_ffffL)
+        putTLV(
+            0x02,
+            byteArrayOf(
+                ((m32 ushr 24) and 0xFF).toByte(),
+                ((m32 ushr 16) and 0xFF).toByte(),
+                ((m32 ushr 8) and 0xFF).toByte(),
+                (m32 and 0xFF).toByte()
+            )
+        )
+        // data
+        putTLV(0x03, data)
+        return out.toByteArray()
+    }
+
+    companion object {
+        // Receiver-side safety limit (configurable constant)
+        const val MAX_ACCEPT_FILTER_BYTES: Int = SyncDefaults.MAX_ACCEPT_FILTER_BYTES
+
+        fun decode(data: ByteArray): RequestSyncPacket? {
+            var off = 0
+            var p: Int? = null
+            var m: Long? = null
+            var payload: ByteArray? = null
+
+            while (off + 3 <= data.size) {
+                val t = (data[off].toInt() and 0xFF); off += 1
+                val len = ((data[off].toInt() and 0xFF) shl 8) or (data[off+1].toInt() and 0xFF); off += 2
+                if (off + len > data.size) return null
+                val v = data.copyOfRange(off, off + len); off += len
+                when (t) {
+                    0x01 -> if (len == 1) p = (v[0].toInt() and 0xFF)
+                    0x02 -> if (len == 4) {
+                        val mm = ((v[0].toLong() and 0xFF) shl 24) or
+                                 ((v[1].toLong() and 0xFF) shl 16) or
+                                 ((v[2].toLong() and 0xFF) shl 8) or
+                                 (v[3].toLong() and 0xFF)
+                        m = mm
+                    }
+                    0x03 -> {
+                        if (v.size > MAX_ACCEPT_FILTER_BYTES) return null
+                        payload = v
+                    }
+                }
+            }
+
+            val pp = p ?: return null
+            val mm = m ?: return null
+            val dd = payload ?: return null
+            if (pp < 1 || mm <= 0L) return null
+            return RequestSyncPacket(pp, mm, dd)
+        }
+    }
+}
diff --git a/app/src/main/java/com/bitchat/android/protocol/BinaryProtocol.kt b/app/src/main/java/com/bitchat/android/protocol/BinaryProtocol.kt
index 3dcc47a2..7724ca04 100644
--- a/app/src/main/java/com/bitchat/android/protocol/BinaryProtocol.kt
+++ b/app/src/main/java/com/bitchat/android/protocol/BinaryProtocol.kt
@@ -15,7 +15,8 @@ enum class MessageType(val value: UByte) {
     LEAVE(0x03u),
     NOISE_HANDSHAKE(0x10u),  // Noise handshake
     NOISE_ENCRYPTED(0x11u),  // Noise encrypted transport message
-    FRAGMENT(0x20u); // Fragmentation for large packets
+    FRAGMENT(0x20u), // Fragmentation for large packets
+    REQUEST_SYNC(0x21u); // GCS-based sync request
 
     companion object {
         fun fromValue(value: UByte): MessageType? {
diff --git a/app/src/main/java/com/bitchat/android/sync/GCSFilter.kt b/app/src/main/java/com/bitchat/android/sync/GCSFilter.kt
new file mode 100644
index 00000000..3cc64c58
--- /dev/null
+++ b/app/src/main/java/com/bitchat/android/sync/GCSFilter.kt
@@ -0,0 +1,191 @@
+package com.bitchat.android.sync
+
+import java.security.MessageDigest
+import kotlin.math.ceil
+import kotlin.math.ln
+
+/**
+ * Golomb-Coded Set (GCS) filter implementation for sync.
+ *
+ * Hashing:
+ * - h64(id) = first 8 bytes of SHA-256 over the 16-byte PacketId (big-endian unsigned)
+ * - Map to range [0, M) via (h64 % M)
+ *
+ * Encoding (v1):
+ * - Sort mapped values ascending; encode deltas (first is v0, then vi - v{i-1}) as positive integers
+ * - For each delta x >= 1, write Golomb-Rice code with parameter P:
+ *   q = (x - 1) >> P (unary q ones followed by a zero), then P low bits r = (x - 1) & ((1<<P)-1)
+ * - Bitstream is packed MSB-first in each byte.
+ */
+object GCSFilter {
+    data class Params(
+        val p: Int,         // Golomb-Rice parameter (>= 1)
+        val m: Long,        // Range M = N * 2^P
+        val data: ByteArray // Encoded GR bitstream
+    )
+
+    // Derive P from target FPR; FPR ~= 1 / 2^P
+    fun deriveP(targetFpr: Double): Int {
+        val f = targetFpr.coerceIn(0.000001, 0.25)
+        return ceil(ln(1.0 / f) / ln(2.0)).toInt().coerceAtLeast(1)
+    }
+
+    // Rough capacity estimate: expected bits per element ~= P + 2 (quotient unary ~ around 2 bits)
+    fun estimateMaxElementsForSize(bytes: Int, p: Int): Int {
+        val bits = (bytes * 8).coerceAtLeast(8)
+        val per = (p + 2).coerceAtLeast(3)
+        return (bits / per).coerceAtLeast(1)
+    }
+
+    fun buildFilter(
+        ids: List<ByteArray>, // 16-byte PacketId bytes
+        maxBytes: Int,
+        targetFpr: Double
+    ): Params {
+        val p = deriveP(targetFpr)
+        var nCap = estimateMaxElementsForSize(maxBytes, p)
+        val n = ids.size.coerceAtMost(nCap)
+        val selected = ids.take(n)
+        // Map to [0, M)
+        val m = (n.toLong() shl p)
+        val mapped = selected.map { id -> (h64(id) % m) }.sorted()
+        var encoded = encode(mapped, p)
+        // If estimate was too optimistic, trim until it fits
+        var trimmedN = n
+        while (encoded.size > maxBytes && trimmedN > 0) {
+            trimmedN = (trimmedN * 9) / 10 // drop 10%
+            val mapped2 = mapped.take(trimmedN)
+            encoded = encode(mapped2, p)
+        }
+        val finalM = (trimmedN.toLong() shl p)
+        return Params(p = p, m = finalM, data = encoded)
+    }
+
+    fun decodeToSortedSet(p: Int, m: Long, data: ByteArray): LongArray {
+        val values = ArrayList<Long>()
+        val reader = BitReader(data)
+        var acc = 0L
+        val mask = (1L shl p) - 1L
+        while (!reader.eof()) {
+            // Read unary quotient (q ones terminated by zero)
+            var q = 0L
+            while (true) {
+                val b = reader.readBit() ?: break
+                if (b == 1) q++ else break
+            }
+            if (reader.lastWasEOF) break
+            // Read remainder
+            val r = reader.readBits(p) ?: break
+            val x = (q shl p) + r + 1
+            acc += x
+            if (acc >= m) break // out of range safeguard
+            values.add(acc)
+        }
+        return values.toLongArray()
+    }
+
+    fun contains(sortedValues: LongArray, candidate: Long): Boolean {
+        var lo = 0
+        var hi = sortedValues.size - 1
+        while (lo <= hi) {
+            val mid = (lo + hi) ushr 1
+            val v = sortedValues[mid]
+            if (v == candidate) return true
+            if (v < candidate) lo = mid + 1 else hi = mid - 1
+        }
+        return false
+    }
+
+    private fun h64(id16: ByteArray): Long {
+        val md = MessageDigest.getInstance("SHA-256")
+        md.update(id16)
+        val d = md.digest()
+        var x = 0L
+        for (i in 0 until 8) {
+            x = (x shl 8) or ((d[i].toLong() and 0xFF))
+        }
+        return x and 0x7fff_ffff_ffff_ffffL // positive
+    }
+
+    private fun encode(sorted: List<Long>, p: Int): ByteArray {
+        val bw = BitWriter()
+        var prev = 0L
+        val mask = (1L shl p) - 1L
+        for (v in sorted) {
+            val delta = v - prev
+            prev = v
+            val x = delta
+            val q = (x - 1) ushr p
+            val r = (x - 1) and mask
+            // unary q ones then a zero
+            repeat(q.toInt()) { bw.writeBit(1) }
+            bw.writeBit(0)
+            // then P bits of r (MSB-first)
+            bw.writeBits(r, p)
+        }
+        return bw.toByteArray()
+    }
+
+    // Simple MSB-first bit writer
+    private class BitWriter {
+        private val buf = ArrayList<Byte>()
+        private var cur = 0
+        private var nbits = 0
+        fun writeBit(bit: Int) {
+            cur = (cur shl 1) or (bit and 1)
+            nbits++
+            if (nbits == 8) {
+                buf.add(cur.toByte())
+                cur = 0; nbits = 0
+            }
+        }
+        fun writeBits(value: Long, count: Int) {
+            if (count <= 0) return
+            for (i in count - 1 downTo 0) {
+                val bit = ((value ushr i) and 1L).toInt()
+                writeBit(bit)
+            }
+        }
+        fun toByteArray(): ByteArray {
+            if (nbits > 0) {
+                val rem = cur shl (8 - nbits)
+                buf.add(rem.toByte())
+                cur = 0; nbits = 0
+            }
+            return buf.toByteArray()
+        }
+    }
+
+    // Simple MSB-first bit reader
+    private class BitReader(private val data: ByteArray) {
+        private var i = 0
+        private var nleft = 8
+        private var cur = if (data.isNotEmpty()) (data[0].toInt() and 0xFF) else 0
+        var lastWasEOF: Boolean = false
+            private set
+        fun eof() = i >= data.size
+        fun readBit(): Int? {
+            if (i >= data.size) { lastWasEOF = true; return null }
+            val bit = (cur ushr 7) and 1
+            cur = (cur shl 1) and 0xFF
+            nleft--
+            if (nleft == 0) {
+                i++
+                if (i < data.size) {
+                    cur = data[i].toInt() and 0xFF
+                    nleft = 8
+                }
+            }
+            return bit
+        }
+        fun readBits(count: Int): Long? {
+            var v = 0L
+            for (k in 0 until count) {
+                val b = readBit() ?: return null
+                v = (v shl 1) or b.toLong()
+            }
+            return v
+        }
+    }
+}
+
diff --git a/app/src/main/java/com/bitchat/android/sync/GossipSyncManager.kt b/app/src/main/java/com/bitchat/android/sync/GossipSyncManager.kt
new file mode 100644
index 00000000..38d23a91
--- /dev/null
+++ b/app/src/main/java/com/bitchat/android/sync/GossipSyncManager.kt
@@ -0,0 +1,263 @@
+package com.bitchat.android.sync
+
+import android.util.Log
+import com.bitchat.android.mesh.BluetoothPacketBroadcaster
+import com.bitchat.android.model.RequestSyncPacket
+import com.bitchat.android.protocol.BitchatPacket
+import com.bitchat.android.protocol.MessageType
+import com.bitchat.android.protocol.SpecialRecipients
+import kotlinx.coroutines.*
+import java.util.concurrent.ConcurrentHashMap
+
+/**
+ * Gossip-based synchronization manager using on-demand GCS filters.
+ * Tracks seen public packets (ANNOUNCE, broadcast MESSAGE) and periodically requests sync
+ * from neighbors. Responds to REQUEST_SYNC by sending missing packets.
+ */
+class GossipSyncManager(
+    private val myPeerID: String,
+    private val scope: CoroutineScope,
+    private val configProvider: ConfigProvider
+) {
+    interface Delegate {
+        fun sendPacket(packet: BitchatPacket)
+        fun sendPacketToPeer(peerID: String, packet: BitchatPacket)
+        fun signPacketForBroadcast(packet: BitchatPacket): BitchatPacket
+    }
+
+    interface ConfigProvider {
+        fun seenCapacity(): Int // max packets we sync per request (cap across types)
+        fun gcsMaxBytes(): Int
+        fun gcsTargetFpr(): Double // percent -> 0.0..1.0
+    }
+
+    companion object { private const val TAG = "GossipSyncManager" }
+
+    var delegate: Delegate? = null
+
+    // Defaults (configurable constants)
+    private val defaultMaxBytes = SyncDefaults.DEFAULT_FILTER_BYTES
+    private val defaultFpr = SyncDefaults.DEFAULT_FPR_PERCENT
+
+    // Stored packets for sync:
+    // - broadcast messages: keep up to seenCapacity() most recent, keyed by packetId
+    private val messages = LinkedHashMap<String, BitchatPacket>()
+    // - announcements: only keep latest per sender peerID
+    private val latestAnnouncementByPeer = ConcurrentHashMap<String, Pair<String, BitchatPacket>>()
+
+    private var periodicJob: Job? = null
+    fun start() {
+        periodicJob?.cancel()
+        periodicJob = scope.launch(Dispatchers.IO) {
+            while (isActive) {
+                try {
+                    delay(30_000)
+                    sendRequestSync()
+                } catch (e: CancellationException) { throw e }
+                catch (e: Exception) { Log.e(TAG, "Periodic sync error: ${e.message}") }
+            }
+        }
+    }
+
+    fun stop() {
+        periodicJob?.cancel(); periodicJob = null
+    }
+
+    fun scheduleInitialSync(delayMs: Long = 5_000L) {
+        scope.launch(Dispatchers.IO) {
+            delay(delayMs)
+            sendRequestSync()
+        }
+    }
+
+    fun scheduleInitialSyncToPeer(peerID: String, delayMs: Long = 5_000L) {
+        scope.launch(Dispatchers.IO) {
+            delay(delayMs)
+            sendRequestSyncToPeer(peerID)
+        }
+    }
+
+    fun onPublicPacketSeen(packet: BitchatPacket) {
+        // Only ANNOUNCE or broadcast MESSAGE
+        val mt = MessageType.fromValue(packet.type)
+        val isBroadcastMessage = (mt == MessageType.MESSAGE && (packet.recipientID == null || packet.recipientID.contentEquals(SpecialRecipients.BROADCAST)))
+        val isAnnouncement = (mt == MessageType.ANNOUNCE)
+        if (!isBroadcastMessage && !isAnnouncement) return
+
+        val idBytes = PacketIdUtil.computeIdBytes(packet)
+        val id = idBytes.joinToString("") { b -> "%02x".format(b) }
+
+        if (isBroadcastMessage) {
+            synchronized(messages) {
+                messages[id] = packet
+                // Enforce capacity (remove oldest when exceeded)
+                val cap = configProvider.seenCapacity().coerceAtLeast(1)
+                while (messages.size > cap) {
+                    val it = messages.entries.iterator()
+                    if (it.hasNext()) { it.next(); it.remove() } else break
+                }
+            }
+        } else if (isAnnouncement) {
+            // senderID is fixed-size 8 bytes; map to hex string for key
+            val sender = packet.senderID.joinToString("") { b -> "%02x".format(b) }
+            latestAnnouncementByPeer[sender] = id to packet
+            // Enforce capacity (remove oldest when exceeded)
+            val cap = configProvider.seenCapacity().coerceAtLeast(1)
+            while (latestAnnouncementByPeer.size > cap) {
+                val it = latestAnnouncementByPeer.entries.iterator()
+                if (it.hasNext()) { it.next(); it.remove() } else break
+            }
+        }
+    }
+
+    private fun sendRequestSync() {
+        val payload = buildGcsPayload()
+
+        val packet = BitchatPacket(
+            type = MessageType.REQUEST_SYNC.value,
+            senderID = hexStringToByteArray(myPeerID),
+            timestamp = System.currentTimeMillis().toULong(),
+            payload = payload,
+            ttl = 0u // neighbors only
+        )
+        // Sign and broadcast
+        val signed = delegate?.signPacketForBroadcast(packet) ?: packet
+        delegate?.sendPacket(signed)
+    }
+
+    private fun sendRequestSyncToPeer(peerID: String) {
+        val payload = buildGcsPayload()
+
+        val packet = BitchatPacket(
+            type = MessageType.REQUEST_SYNC.value,
+            senderID = hexStringToByteArray(myPeerID),
+            recipientID = hexStringToByteArray(peerID),
+            timestamp = System.currentTimeMillis().toULong(),
+            payload = payload,
+            ttl = 0u // neighbor only
+        )
+        Log.d(TAG, "Sending sync request to $peerID (${payload.size} bytes)")
+        // Sign and send directly to peer
+        val signed = delegate?.signPacketForBroadcast(packet) ?: packet
+        delegate?.sendPacketToPeer(peerID, signed)
+    }
+
+    fun handleRequestSync(fromPeerID: String, request: RequestSyncPacket) {
+        // Decode GCS into sorted set for membership checks
+        val sorted = GCSFilter.decodeToSortedSet(request.p, request.m, request.data)
+        fun mightContain(id: ByteArray): Boolean {
+            val v = (GCSFilter.run {
+                // reuse hashing method from GCSFilter
+                val md = java.security.MessageDigest.getInstance("SHA-256");
+                md.update(id); val d = md.digest();
+                var x = 0L; for (i in 0 until 8) { x = (x shl 8) or (d[i].toLong() and 0xFF) }
+                (x and 0x7fff_ffff_ffff_ffffL) % request.m
+            })
+            return GCSFilter.contains(sorted, v)
+        }
+
+        // 1) Announcements: send latest per peerID if remote doesn't have them
+        for ((_, pair) in latestAnnouncementByPeer.entries) {
+            val (id, pkt) = pair
+            val idBytes = hexToBytes(id)
+            if (!mightContain(idBytes)) {
+                // Send original packet unchanged to requester only (keep local TTL)
+                val toSend = pkt.copy(ttl = 0u)
+                delegate?.sendPacketToPeer(fromPeerID, toSend)
+                Log.d(TAG, "Sent sync announce: Type ${toSend.type} from ${toSend.senderID.toHexString()} to $fromPeerID packet id ${idBytes.toHexString()}")
+            }
+        }
+
+        // 2) Broadcast messages: send all they lack
+        val toSendMsgs = synchronized(messages) { messages.values.toList() }
+        for (pkt in toSendMsgs) {
+            val idBytes = PacketIdUtil.computeIdBytes(pkt)
+            if (!mightContain(idBytes)) {
+                val toSend = pkt.copy(ttl = 0u)
+                delegate?.sendPacketToPeer(fromPeerID, toSend)
+                Log.d(TAG, "Sent sync message: Type ${toSend.type} to $fromPeerID packet id ${idBytes.toHexString()}")
+            }
+        }
+    }
+
+    private fun hexStringToByteArray(hexString: String): ByteArray {
+        val result = ByteArray(8) { 0 }
+        var tempID = hexString
+        var index = 0
+        while (tempID.length >= 2 && index < 8) {
+            val hexByte = tempID.substring(0, 2)
+            val byte = hexByte.toIntOrNull(16)?.toByte()
+            if (byte != null) result[index] = byte
+            tempID = tempID.substring(2)
+            index++
+        }
+        return result
+    }
+
+    private fun hexToBytes(hex: String): ByteArray {
+        val clean = if (hex.length % 2 == 0) hex else "0$hex"
+        val out = ByteArray(clean.length / 2)
+        var i = 0
+        while (i < clean.length) {
+            out[i/2] = clean.substring(i, i+2).toInt(16).toByte()
+            i += 2
+        }
+        return out
+    }
+
+    private fun buildGcsPayload(): ByteArray {
+        // Collect candidates: latest announcement per peer + recent broadcast messages
+        val list = ArrayList<BitchatPacket>()
+        // announcements
+        for ((_, pair) in latestAnnouncementByPeer) {
+            list.add(pair.second)
+        }
+        // messages
+        synchronized(messages) {
+            list.addAll(messages.values)
+        }
+        // sort by timestamp desc, then take up to min(seenCapacity, fit capacity)
+        list.sortByDescending { it.timestamp.toLong() }
+
+        val maxBytes = try { configProvider.gcsMaxBytes() } catch (_: Exception) { defaultMaxBytes }
+        val fpr = try { configProvider.gcsTargetFpr() } catch (_: Exception) { defaultFpr }
+        val p = GCSFilter.deriveP(fpr)
+        val nMax = GCSFilter.estimateMaxElementsForSize(maxBytes, p)
+        val cap = configProvider.seenCapacity().coerceAtLeast(1)
+        val takeN = minOf(nMax, cap, list.size)
+        if (takeN <= 0) {
+            val p0 = GCSFilter.deriveP(fpr)
+            return RequestSyncPacket(p = p0, m = 1, data = ByteArray(0)).encode()
+        }
+        val ids = list.take(takeN).map { pkt -> PacketIdUtil.computeIdBytes(pkt) }
+        val params = GCSFilter.buildFilter(ids, maxBytes, fpr)
+        val mVal = if (params.m <= 0L) 1 else params.m
+        return RequestSyncPacket(p = params.p, m = mVal, data = params.data).encode()
+    }
+
+    // Explicitly remove stored announcement for a given peer (hex ID)
+    fun removeAnnouncementForPeer(peerID: String) {
+        val key = peerID.lowercase()
+        if (latestAnnouncementByPeer.remove(key) != null) {
+            Log.d(TAG, "Removed stored announcement for peer $peerID")
+        }
+
+        // Collect IDs to remove first to avoid modifying collection while iterating
+        val idsToRemove = mutableListOf<String>()
+        for ((id, message) in messages) {
+            val sender = message.senderID.joinToString("") { b -> "%02x".format(b) }
+            if (sender == key) {
+                idsToRemove.add(id)
+            }
+        }
+        
+        // Now remove the collected IDs
+        for (id in idsToRemove) {
+            messages.remove(id)
+        }
+        
+        if (idsToRemove.isNotEmpty()) {
+            Log.d(TAG, "Pruned ${idsToRemove.size} messages with senders without announcements")
+        }
+    }
+}
diff --git a/app/src/main/java/com/bitchat/android/sync/PacketIdUtil.kt b/app/src/main/java/com/bitchat/android/sync/PacketIdUtil.kt
new file mode 100644
index 00000000..4396ff5a
--- /dev/null
+++ b/app/src/main/java/com/bitchat/android/sync/PacketIdUtil.kt
@@ -0,0 +1,31 @@
+package com.bitchat.android.sync
+
+import com.bitchat.android.protocol.BitchatPacket
+import java.security.MessageDigest
+
+/**
+ * Deterministic packet ID helper for sync purposes.
+ * Uses SHA-256 over a canonical subset of packet fields:
+ * [type | senderID | timestamp | payload] to generate a stable ID.
+ * Returns a 16-byte (128-bit) truncated hash for compactness.
+ */
+object PacketIdUtil {
+    fun computeIdBytes(packet: BitchatPacket): ByteArray {
+        val md = MessageDigest.getInstance("SHA-256")
+        md.update(packet.type.toByte())
+        md.update(packet.senderID)
+        // Timestamp as 8 bytes big-endian
+        val ts = packet.timestamp.toLong()
+        for (i in 7 downTo 0) {
+            md.update(((ts ushr (i * 8)) and 0xFF).toByte())
+        }
+        md.update(packet.payload)
+        val digest = md.digest()
+        return digest.copyOf(16) // 128-bit ID
+    }
+
+    fun computeIdHex(packet: BitchatPacket): String {
+        return computeIdBytes(packet).joinToString("") { b -> "%02x".format(b) }
+    }
+}
+
diff --git a/app/src/main/java/com/bitchat/android/sync/SyncDefaults.kt b/app/src/main/java/com/bitchat/android/sync/SyncDefaults.kt
new file mode 100644
index 00000000..970c8afb
--- /dev/null
+++ b/app/src/main/java/com/bitchat/android/sync/SyncDefaults.kt
@@ -0,0 +1,11 @@
+package com.bitchat.android.sync
+
+object SyncDefaults {
+    // Default values used when debug prefs are unavailable
+    const val DEFAULT_FILTER_BYTES: Int = 256
+    const val DEFAULT_FPR_PERCENT: Double = 1.0
+
+    // Receiver-side hard cap to avoid DoS (also enforced in RequestSyncPacket)
+    const val MAX_ACCEPT_FILTER_BYTES: Int = 1024
+}
+
diff --git a/app/src/main/java/com/bitchat/android/ui/GeohashViewModel.kt b/app/src/main/java/com/bitchat/android/ui/GeohashViewModel.kt
index 08865c1e..9ecb94f5 100644
--- a/app/src/main/java/com/bitchat/android/ui/GeohashViewModel.kt
+++ b/app/src/main/java/com/bitchat/android/ui/GeohashViewModel.kt
@@ -153,7 +153,7 @@ class GeohashViewModel(
                 isRelay = false
             )
     fun startGeohashDM(pubkeyHex: String, onStartPrivateChat: (String) -> Unit) {
-        val convKey = "nostr_${'$'}{pubkeyHex.take(16)}"
+        val convKey = "nostr_${pubkeyHex.take(16)}"
         repo.putNostrKeyMapping(convKey, pubkeyHex)
         // Record the conversation's geohash using the currently selected location channel (if any)
         val current = state.selectedLocationChannel.value
@@ -163,7 +163,7 @@ class GeohashViewModel(
             com.bitchat.android.nostr.GeohashConversationRegistry.set(convKey, gh)
         }
         onStartPrivateChat(convKey)
-        Log.d(TAG, "🗨️ Started geohash DM with ${'$'}pubkeyHex -> ${'$'}convKey (geohash=${'$'}gh)")
+        Log.d(TAG, "🗨️ Started geohash DM with ${pubkeyHex} -> ${convKey} (geohash=${gh})")
     }
 
             messageManager.addMessage(sysMsg)
diff --git a/app/src/main/java/com/bitchat/android/ui/debug/DebugPreferenceManager.kt b/app/src/main/java/com/bitchat/android/ui/debug/DebugPreferenceManager.kt
index 84b624f7..e2f9d966 100644
--- a/app/src/main/java/com/bitchat/android/ui/debug/DebugPreferenceManager.kt
+++ b/app/src/main/java/com/bitchat/android/ui/debug/DebugPreferenceManager.kt
@@ -16,6 +16,10 @@ object DebugPreferenceManager {
     private const val KEY_MAX_CONN_OVERALL = "max_connections_overall"
     private const val KEY_MAX_CONN_SERVER = "max_connections_server"
     private const val KEY_MAX_CONN_CLIENT = "max_connections_client"
+    private const val KEY_SEEN_PACKET_CAP = "seen_packet_capacity"
+    // GCS keys (no migration/back-compat)
+    private const val KEY_GCS_MAX_BYTES = "gcs_max_filter_bytes"
+    private const val KEY_GCS_FPR = "gcs_filter_fpr_percent"
 
     private lateinit var prefs: SharedPreferences
 
@@ -74,4 +78,26 @@ object DebugPreferenceManager {
     fun setMaxConnectionsClient(value: Int) {
         if (ready()) prefs.edit().putInt(KEY_MAX_CONN_CLIENT, value).apply()
     }
+
+    // Sync/GCS settings
+    fun getSeenPacketCapacity(default: Int = 500): Int =
+        if (ready()) prefs.getInt(KEY_SEEN_PACKET_CAP, default) else default
+
+    fun setSeenPacketCapacity(value: Int) {
+        if (ready()) prefs.edit().putInt(KEY_SEEN_PACKET_CAP, value).apply()
+    }
+
+    fun getGcsMaxFilterBytes(default: Int = 400): Int =
+        if (ready()) prefs.getInt(KEY_GCS_MAX_BYTES, default) else default
+
+    fun setGcsMaxFilterBytes(value: Int) {
+        if (ready()) prefs.edit().putInt(KEY_GCS_MAX_BYTES, value).apply()
+    }
+
+    fun getGcsFprPercent(default: Double = 1.0): Double =
+        if (ready()) java.lang.Double.longBitsToDouble(prefs.getLong(KEY_GCS_FPR, java.lang.Double.doubleToRawLongBits(default))) else default
+
+    fun setGcsFprPercent(value: Double) {
+        if (ready()) prefs.edit().putLong(KEY_GCS_FPR, java.lang.Double.doubleToRawLongBits(value)).apply()
+    }
 }
diff --git a/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsManager.kt b/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsManager.kt
index 81538058..9095d765 100644
--- a/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsManager.kt
+++ b/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsManager.kt
@@ -201,6 +201,37 @@ class DebugSettingsManager private constructor() {
     fun updateRelayStats(stats: PacketRelayStats) {
         _relayStats.value = stats
     }
+
+    // Sync/GCS settings (UI-configurable)
+    private val _seenPacketCapacity = MutableStateFlow(DebugPreferenceManager.getSeenPacketCapacity(500))
+    val seenPacketCapacity: StateFlow<Int> = _seenPacketCapacity.asStateFlow()
+
+    private val _gcsMaxBytes = MutableStateFlow(DebugPreferenceManager.getGcsMaxFilterBytes(400))
+    val gcsMaxBytes: StateFlow<Int> = _gcsMaxBytes.asStateFlow()
+
+    private val _gcsFprPercent = MutableStateFlow(DebugPreferenceManager.getGcsFprPercent(1.0))
+    val gcsFprPercent: StateFlow<Double> = _gcsFprPercent.asStateFlow()
+
+    fun setSeenPacketCapacity(value: Int) {
+        val clamped = value.coerceIn(10, 1000)
+        DebugPreferenceManager.setSeenPacketCapacity(clamped)
+        _seenPacketCapacity.value = clamped
+        addDebugMessage(DebugMessage.SystemMessage("🧩 max packets per sync set to $clamped"))
+    }
+
+    fun setGcsMaxBytes(value: Int) {
+        val clamped = value.coerceIn(128, 1024)
+        DebugPreferenceManager.setGcsMaxFilterBytes(clamped)
+        _gcsMaxBytes.value = clamped
+        addDebugMessage(DebugMessage.SystemMessage("🌸 max GCS filter size set to $clamped bytes"))
+    }
+
+    fun setGcsFprPercent(value: Double) {
+        val clamped = value.coerceIn(0.1, 5.0)
+        DebugPreferenceManager.setGcsFprPercent(clamped)
+        _gcsFprPercent.value = clamped
+        addDebugMessage(DebugMessage.SystemMessage("🎯 GCS FPR set to ${String.format("%.2f", clamped)}%"))
+    }
     
     // MARK: - Debug Message Creation Helpers
     
diff --git a/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsSheet.kt b/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsSheet.kt
index 03453aaf..23bfc524 100644
--- a/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsSheet.kt
+++ b/app/src/main/java/com/bitchat/android/ui/debug/DebugSettingsSheet.kt
@@ -48,6 +48,9 @@ fun DebugSettingsSheet(
     val scanResults by manager.scanResults.collectAsState()
     val connectedDevices by manager.connectedDevices.collectAsState()
     val relayStats by manager.relayStats.collectAsState()
+    val seenCapacity by manager.seenPacketCapacity.collectAsState()
+    val gcsMaxBytes by manager.gcsMaxBytes.collectAsState()
+    val gcsFpr by manager.gcsFprPercent.collectAsState()
 
     // Push live connected devices from mesh service whenever sheet is visible
     LaunchedEffect(isPresented) {
@@ -284,6 +287,27 @@ fun DebugSettingsSheet(
                 }
             }
 
+            // Connected devices
+            item {
+                Surface(shape = RoundedCornerShape(12.dp), color = colorScheme.surfaceVariant.copy(alpha = 0.2f)) {
+                    Column(Modifier.padding(16.dp), verticalArrangement = Arrangement.spacedBy(10.dp)) {
+                        Row(verticalAlignment = Alignment.CenterVertically, horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                            Icon(Icons.Filled.SettingsEthernet, contentDescription = null, tint = Color(0xFF9C27B0))
+                            Text("sync settings", fontFamily = FontFamily.Monospace, fontSize = 14.sp, fontWeight = FontWeight.Medium)
+                        }
+                        Text("max packets per sync: $seenCapacity", fontFamily = FontFamily.Monospace, fontSize = 11.sp, color = colorScheme.onSurface.copy(alpha = 0.7f))
+                        Slider(value = seenCapacity.toFloat(), onValueChange = { manager.setSeenPacketCapacity(it.toInt()) }, valueRange = 10f..1000f, steps = 99)
+                        Text("max GCS filter size: $gcsMaxBytes bytes (128–1024)", fontFamily = FontFamily.Monospace, fontSize = 11.sp, color = colorScheme.onSurface.copy(alpha = 0.7f))
+                        Slider(value = gcsMaxBytes.toFloat(), onValueChange = { manager.setGcsMaxBytes(it.toInt()) }, valueRange = 128f..1024f, steps = 0)
+                        Text("target FPR: ${String.format("%.2f", gcsFpr)}%", fontFamily = FontFamily.Monospace, fontSize = 11.sp, color = colorScheme.onSurface.copy(alpha = 0.7f))
+                        Slider(value = gcsFpr.toFloat(), onValueChange = { manager.setGcsFprPercent(it.toDouble()) }, valueRange = 0.1f..5.0f, steps = 49)
+                        val p = remember(gcsFpr) { com.bitchat.android.sync.GCSFilter.deriveP(gcsFpr / 100.0) }
+                        val nmax = remember(gcsFpr, gcsMaxBytes) { com.bitchat.android.sync.GCSFilter.estimateMaxElementsForSize(gcsMaxBytes, p) }
+                        Text("derived P: $p • est. max elements: $nmax", fontFamily = FontFamily.Monospace, fontSize = 11.sp, color = colorScheme.onSurface.copy(alpha = 0.7f))
+                    }
+                }
+            }
+
             // Connected devices
             item {
                 Surface(shape = RoundedCornerShape(12.dp), color = colorScheme.surfaceVariant.copy(alpha = 0.2f)) {
diff --git a/docs/sync.md b/docs/sync.md
new file mode 100644
index 00000000..0bbd0735
--- /dev/null
+++ b/docs/sync.md
@@ -0,0 +1,149 @@
+# GCS Filter Sync (REQUEST_SYNC)
+
+This document specifies the gossip-based synchronization feature for BitChat, inspired by Plumtree. It ensures eventual consistency of public packets (ANNOUNCE and broadcast MESSAGE) across nodes via periodic sync requests containing a compact Golomb‑Coded Set (GCS) of recently seen packets.
+
+## Overview
+
+- Each node maintains a rolling set of public BitChat packets it has seen recently:
+  - Broadcast messages (MessageType.MESSAGE where recipient is broadcast)
+  - Identity announcements (MessageType.ANNOUNCE)
+  - Default retention is 100 recent packets (configurable in the debug sheet). This value is the maximum number of packets that are synchronized per request (across both types combined).
+- Nodes do not maintain a rolling Bloom filter. Instead, they compute a GCS filter on demand when sending a REQUEST_SYNC.
+- Every 30 seconds, a node sends a REQUEST_SYNC packet to all immediate neighbors (local only; not relayed).
+- Additionally, 5 seconds after the first announcement from a newly directly connected peer is detected, a node sends a REQUEST_SYNC only to that peer (unicast; local only).
+- The receiver checks which packets are not in the sender’s filter and sends those packets back. For announcements, only the latest announcement per peerID is sent; for broadcast messages, all missing ones are sent.
+
+This synchronization is strictly local (not relayed), ensuring only immediate neighbors participate and preventing wide-area flooding while converging content across the mesh.
+
+## Packet ID
+
+To compare packets across peers, a deterministic packet ID is used:
+
+- ID = first 16 bytes of SHA-256 over: [type | senderID | timestamp | payload]
+- This yields a 128-bit ID used in the filter.
+
+Implementation: `com.bitchat.android.sync.PacketIdUtil`.
+
+## GCS Filter (On-demand)
+
+Implementation: `com.bitchat.android.sync.GCSFilter`.
+
+- Parameters (configurable):
+  - size: 128–1024 bytes (default 256)
+  - target false positive rate (FPR): default 1% (range 0.1%–5%)
+- Derivations:
+  - P = ceil(log2(1/FPR))
+  - Maximum number of elements that fit into the filter is estimated as: N_max ≈ floor((8 * sizeBytes) / (P + 2))
+    - This estimate is used to cap the set; the actual encoder will trim further if needed to stay within the configured size.
+- What goes into the set:
+  - Combine the following and sort by packet timestamp (descending):
+    - Broadcast messages (MessageType 1)
+    - The most recent ANNOUNCE per peer
+  - Take at most `min(N_max, maxPacketsPerSync)` items from this ordered list.
+  - Compute the 16-byte Packet ID (see below), then for hashing use the first 8 bytes of SHA‑256 over the 16‑byte ID.
+  - Map each hash to [0, M) with M = N * 2^P; sort ascending and encode deltas with Golomb‑Rice parameter P.
+
+Hashing scheme (fixed for cross‑impl compatibility):
+- Packet ID: first 16 bytes of SHA‑256 over [type | senderID | timestamp | payload].
+- GCS hash: h64 = first 8 bytes of SHA‑256 over the 16‑byte Packet ID, interpreted as an unsigned 64‑bit integer. Value = h64 % M.
+
+## REQUEST_SYNC Packet
+
+MessageType: `REQUEST_SYNC (0x21)`
+
+- Header: normal BitChat header with TTL indicating “local-only” semantics. Implementations SHOULD set TTL=0 to prevent any relay; neighbors still receive the packet over the direct link-layer. For periodic sync, recipient is broadcast; for per-peer initial sync, recipient is the specific peer.
+- Payload: TLV with 16‑bit big‑endian length fields (type, length16, value)
+  - 0x01: P (uint8) — Golomb‑Rice parameter
+  - 0x02: M (uint32) — hash range N * 2^P
+  - 0x03: data (opaque) — GCS bitstream (MSB‑first bit packing)
+
+Notes:
+- The GCS bitstream uses MSB‑first packing (bit 7 is the first bit in each byte).
+- Receivers MUST reject filters with data length exceeding the local maximum (default 1024 bytes) to avoid DoS.
+
+Encode/Decode implementation: `com.bitchat.android.model.RequestSyncPacket`.
+
+## Behavior
+
+Sender behavior:
+- Periodic: every 30 seconds, send REQUEST_SYNC with a freshly computed GCS snapshot, broadcast to immediate neighbors, and mark as local‑only (TTL=0 recommended; do not relay).
+- Initial per-peer: upon receiving the first ANNOUNCE from a new directly connected peer, send a REQUEST_SYNC only to that peer after ~5 seconds (unicast; TTL=0 recommended; do not relay).
+
+Receiver behavior:
+- Decode the REQUEST_SYNC payload and reconstruct the sorted set of mapped values using the provided P, M, and bitstream.
+- For each locally stored public packet ID:
+  - Compute h64(ID) % M and check if it is in the reconstructed set; if NOT present, send the original packet back with `ttl=0` to the requester only.
+  - For announcements, send only the latest announcement per (sender peerID).
+  - For broadcast messages, send all missing ones.
+
+Announcement retention and pruning (consensus):
+- Store only the most recent announcement per peerID for sync purposes.
+- Age-out policy: announcements older than 60 seconds MUST be removed from the sync candidate set.
+- Pruning cadence: run pruning every 15 seconds to drop expired announcements.
+- LEAVE handling: upon receiving a LEAVE message from a peer, immediately remove that peer’s stored announcement from the sync candidate set.
+- Stale/offline peer handling: when a peer is considered stale/offline (e.g., last announcement older than 60 seconds), immediately remove that peer’s stored announcement from the sync candidate set.
+
+Important: original packets are sent unmodified to preserve original signatures (e.g., ANNOUNCE). They MUST NOT be relayed beyond immediate neighbors. Implementations SHOULD send these response packets with TTL=0 (local-only) and, when possible, route them only to the requesting peer without altering the original packet contents.
+
+## Scope and Types Included
+
+Included in sync:
+- Public broadcast messages: `MessageType.MESSAGE` with BROADCAST recipient (or null recipient).
+- Identity announcements: `MessageType.ANNOUNCE`.
+- Both packets produced by other peers and packets produced by the requester itself MUST be represented in the requester’s GCS; the responder MUST track and consider its own produced public packets as candidates to return when they are missing on the requester.
+- Announcements included in the GCS MUST be at most 60 seconds old at the time of filter construction; older announcements are excluded by pruning.
+
+Not included:
+- Private messages and any packets addressed to a non-broadcast recipient.
+
+## Configuration (Debug Sheet)
+
+Exposed under “sync settings” in the debug settings sheet:
+- Max packets per sync (default 100)
+- Max GCS filter size in bytes (default 256, min 128, max 1024)
+- GCS target FPR in percent (default 1%, 0.1%–5%)
+- Derived values (display only): P and the estimated maximum number of elements that fit into the filter.
+
+Backed by `DebugPreferenceManager` getters and setters:
+- `getSeenPacketCapacity` / `setSeenPacketCapacity`
+- `getGcsMaxFilterBytes` / `setGcsMaxFilterBytes`
+- `getGcsFprPercent` / `setGcsFprPercent`
+
+## Android Integration
+
+- New/updated types and classes:
+  - `MessageType.REQUEST_SYNC` (0x21) in `BinaryProtocol.kt`
+  - `RequestSyncPacket` in `model/RequestSyncPacket.kt`
+  - `GCSFilter` and `PacketIdUtil` in `sync/`
+  - `GossipSyncManager` in `sync/`
+- `BluetoothMeshService` wires and starts the sync manager, schedules per-peer initial (unicast) and periodic (broadcast) syncs, and forwards seen public packets (including our own) to the manager.
+- `PacketProcessor` handles REQUEST_SYNC and forwards to `BluetoothMeshService` which responds via the sync manager with responses targeted only to the requester.
+
+## Compatibility Notes
+
+- GCS hashing and TLV structures are fully specified above; other implementations should use the same hashing scheme and payload layout for interoperability.
+- REQUEST_SYNC and responses are local-only and MUST NOT be relayed. Implementations SHOULD use TTL=0 to prevent relaying. If an implementation requires TTL>0 for local delivery, it MUST still ensure that REQUEST_SYNC and responses are not relayed beyond direct neighbors (e.g., by special-casing these types in relay logic).
+
+## Consensus vs. Configurable
+
+The following items require consensus across all implementations to ensure interoperability:
+
+- Packet ID recipe: first 16 bytes of SHA‑256(type | senderID | timestamp | payload).
+- GCS hashing function and mapping to [0, M) as specified above (v1), and MSB‑first bit packing for the bitstream.
+- Payload encoding: TLV with 16‑bit big‑endian lengths; TLV types 0x01 = P (uint8), 0x02 = M (uint32), 0x03 = data (opaque).
+- Packet type and scope: REQUEST_SYNC = 0x21; local-only (not relayed); only ANNOUNCE and broadcast MESSAGE are synchronized; ANNOUNCE de‑dupe is “latest per sender peerID”.
+
+The following are requester‑defined and communicated or local policy (no global agreement required):
+
+- GCS parameters: P and M are carried in the REQUEST_SYNC and must be used by the receiver for membership tests. The sender chooses size and FPR; receivers MUST cap accepted data length for DoS protection.
+- Local storage policy: how many packets to consider and how you determine the “latest” announcement per peer.
+- Sync cadence: how often to send REQUEST_SYNC and initial delay after new neighbor connection; whether to use unicast for initial per-peer sync versus broadcast for periodic sync. The number of packets included is bounded by the debug setting and filter capacity.
+
+Validation and limits (recommended):
+
+- Reject malformed REQUEST_SYNC payloads (e.g., P < 1, M <= 0, or data length too large for local limits).
+- Practical bounds: data length in [0, 1024]; P in [1, 24]; M up to 2^32‑1.
+
+Versioning:
+
+- This document defines a fixed GCS hashing scheme (“v1”) with no explicit version field in the payload. Changing the hashing or ID recipe would require a new message or an additional TLV in a future revision; current deployments must adhere to the constants above.