diff --git a/core/include/vf/ChunkSync.h b/core/include/vf/ChunkSync.h
new file mode 100644
index 0000000..7036558
--- /dev/null
+++ b/core/include/vf/ChunkSync.h
@@ -0,0 +1,46 @@
+#pragma once
+
+#include <cstddef>
+#include <set>
+#include <unordered_map>
+#include "Chunk.h"
+#include "Math.h"
+#include "Protocol.h"
+#include "World.h"
+
+namespace vf {
+
+// Server side of chunk replication: records which block slots in each chunk
+// changed since the last flush and turns them into wire packets.
+class ServerChunkSync {
+public:
+    void markDirty(const Vec3i& chunkCoord, int x, int y, int z);
+    void markDirtyIndex(const Vec3i& chunkCoord, int localIndex);
+
+    std::size_t dirtyCount(const Vec3i& chunkCoord) const;
+    bool anyDirty() const;
+    void clear(const Vec3i& chunkCoord);
+    void clearAll() { dirty.clear(); }
+
+    // A full chunk snapshot for the initial send; false when the chunk is absent.
+    bool buildFull(const World& world, const Vec3i& chunkCoord, ChunkData& out) const;
+
+    // An incremental update from the dirty set; false when nothing is dirty.
+    bool buildDelta(const World& world, const Vec3i& chunkCoord, MultiBlockChange& out) const;
+
+    // Whether a full resend is cheaper than a per-block delta at this dirty size.
+    bool shouldResendFull(const Vec3i& chunkCoord, std::size_t threshold) const;
+
+private:
+    std::unordered_map<Vec3i, std::set<int>, Vec3iHash> dirty;
+};
+
+// Client side: applies received packets into a local world mirror.
+class ClientChunkSync {
+public:
+    void applyFull(const ChunkData& packet, World& world);
+    void applyDelta(const MultiBlockChange& packet, World& world);
+    void applyBlockChange(const BlockChange& packet, World& world);
+};
+
+} // namespace vf
diff --git a/core/src/ChunkSync.cpp b/core/src/ChunkSync.cpp
new file mode 100644
index 0000000..2036012
--- /dev/null
+++ b/core/src/ChunkSync.cpp
@@ -0,0 +1,71 @@
+#include "vf/ChunkSync.h"
+
+namespace vf {
+
+void ServerChunkSync::markDirty(const Vec3i& chunkCoord, int x, int y, int z) {
+    dirty[chunkCoord].insert(localIndex(x, y, z));
+}
+
+void ServerChunkSync::markDirtyIndex(const Vec3i& chunkCoord, int localIndex) {
+    dirty[chunkCoord].insert(localIndex);
+}
+
+std::size_t ServerChunkSync::dirtyCount(const Vec3i& chunkCoord) const {
+    auto it = dirty.find(chunkCoord);
+    return it == dirty.end() ? 0 : it->second.size();
+}
+
+bool ServerChunkSync::anyDirty() const {
+    for (const auto& entry : dirty)
+        if (!entry.second.empty()) return true;
+    return false;
+}
+
+void ServerChunkSync::clear(const Vec3i& chunkCoord) { dirty.erase(chunkCoord); }
+
+bool ServerChunkSync::buildFull(const World& world, const Vec3i& chunkCoord, ChunkData& out) const {
+    const Chunk* chunk = world.chunkAt(chunkCoord);
+    if (!chunk) return false;
+    out.setFromChunk(chunkCoord.x, chunkCoord.y, chunkCoord.z, *chunk);
+    return true;
+}
+
+bool ServerChunkSync::buildDelta(const World& world, const Vec3i& chunkCoord,
+                                 MultiBlockChange& out) const {
+    auto it = dirty.find(chunkCoord);
+    if (it == dirty.end() || it->second.empty()) return false;
+    const Chunk* chunk = world.chunkAt(chunkCoord);
+    if (!chunk) return false;
+
+    out.chunkX = chunkCoord.x;
+    out.chunkY = chunkCoord.y;
+    out.chunkZ = chunkCoord.z;
+    out.changes.clear();
+    out.changes.reserve(it->second.size());
+    const std::array<Block, chunkVolume>& blocks = chunk->raw();
+    for (int index : it->second)
+        out.changes.push_back({static_cast<uint16_t>(index), blocks[static_cast<std::size_t>(index)]});
+    return true;
+}
+
+bool ServerChunkSync::shouldResendFull(const Vec3i& chunkCoord, std::size_t threshold) const {
+    return dirtyCount(chunkCoord) >= threshold;
+}
+
+void ClientChunkSync::applyFull(const ChunkData& packet, World& world) {
+    Chunk& chunk = world.ensureChunk({packet.chunkX, packet.chunkY, packet.chunkZ});
+    packet.toChunk(chunk);
+}
+
+void ClientChunkSync::applyDelta(const MultiBlockChange& packet, World& world) {
+    Chunk& chunk = world.ensureChunk({packet.chunkX, packet.chunkY, packet.chunkZ});
+    std::array<Block, chunkVolume>& blocks = chunk.raw();
+    for (const MultiBlockChange::Change& change : packet.changes)
+        blocks[change.localIndex] = change.block;
+}
+
+void ClientChunkSync::applyBlockChange(const BlockChange& packet, World& world) {
+    world.setBlock(packet.x, packet.y, packet.z, packet.block);
+}
+
+} // namespace vf