diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Contracts/Protos/localdb_sync.proto b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Contracts/Protos/localdb_sync.proto
index 562e030..72b9465 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Contracts/Protos/localdb_sync.proto
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Contracts/Protos/localdb_sync.proto
@@ -1,9 +1,52 @@
 syntax = "proto3";
-
 package localdb_sync.v1;
-
 option csharp_namespace = "ZB.MOM.WW.LocalDb.Contracts";
 
-// Placeholder service. The real replication contract is defined in a later task.
+// One long-lived bidirectional stream carries BOTH directions of replication.
 service LocalDbSync {
+  rpc Sync (stream SyncMessage) returns (stream SyncMessage);
 }
+
+message SyncMessage {
+  oneof msg {
+    Handshake handshake = 1;          // first message each direction
+    HandshakeAck handshake_ack = 2;
+    DeltaBatch delta_batch = 3;
+    DeltaAck delta_ack = 4;
+    SnapshotBegin snapshot_begin = 5; // sender switches to snapshot mode
+    SnapshotBatch snapshot_batch = 6;
+    SnapshotComplete snapshot_complete = 7;
+  }
+}
+
+message Handshake {
+  string node_id = 1;
+  uint32 lib_schema_version = 2;
+  repeated TableDigest tables = 3;    // fail-closed on any mismatch
+  int64 last_applied_remote_seq = 4;  // highest of YOUR oplog seqs I have applied
+}
+message TableDigest { string table_name = 1; string digest = 2; }
+message HandshakeAck { string node_id = 1; bool snapshot_required = 2; }
+
+message OplogEntry {
+  int64 seq = 1;
+  string table_name = 2;
+  string pk_json = 3;
+  optional string row_json = 4;       // absent => tombstone
+  int64 hlc = 5;                      // UTC-based HLC (physical ms << 16 | counter)
+  string node_id = 6;
+  bool is_tombstone = 7;
+}
+message DeltaBatch { repeated OplogEntry entries = 1; }
+message DeltaAck { int64 applied_thru_seq = 1; }
+
+message SnapshotBegin { int64 as_of_seq = 1; }
+message SnapshotBatch { string table_name = 1; repeated SnapshotRow rows = 2; }
+message SnapshotRow {
+  string pk_json = 1;
+  optional string row_json = 2;
+  int64 hlc = 3;
+  string node_id = 4;
+  bool is_tombstone = 5;
+}
+message SnapshotComplete { int64 as_of_seq = 1; }