diff --git a/nova_plan/contracts.py b/nova_plan/contracts.py
index 485d8f2..cedecac 100644
--- a/nova_plan/contracts.py
+++ b/nova_plan/contracts.py
@@ -123,6 +123,10 @@ class PlanDelta:
     sequence: Optional[int] = None
     nonce: Optional[int] = None
     signature: Optional[str] = None
+    # Delta versioning for improved replay-detection and ordering in GoC
+    # This is a lightweight addition to support protocol evolution while
+    # maintaining backward-compatibility for existing Delta instances.
+    delta_version: int = 1
 
     def to_json(self) -> str:
         """Serialize this PlanDelta to a deterministic JSON string.
@@ -140,6 +144,7 @@ class PlanDelta:
             "sequence": self.sequence,
             "nonce": self.nonce,
             "signature": self.signature,
+            "delta_version": self.delta_version,
         }
         return json.dumps(payload, sort_keys=True)