diff --git a/src/energiamesh/core.py b/src/energiamesh/core.py
index fe22c14..b001051 100644
--- a/src/energiamesh/core.py
+++ b/src/energiamesh/core.py
@@ -93,6 +93,8 @@ class PlanDelta:
     nonce: str | None = None
     # Optional source attribution for auditing and routing
     source: str | None = None
+    # Optional replay trace for offline/delta-sync recovery (lightweight)
+    replay_log: list[str] = field(default_factory=list)
 
 
 @dataclass
diff --git a/src/energiamesh/sync.py b/src/energiamesh/sync.py
index c06aef1..6881145 100644
--- a/src/energiamesh/sync.py
+++ b/src/energiamesh/sync.py
@@ -30,3 +30,27 @@ def compute_delta(lp: LocalProblem, sv: SharedVariables) -> PlanDelta:
 
     delta = PlanDelta(delta=payload, delta_id=f"d-{int(time.time())}")
     return delta
+
+
+class DeltaStore:
+    """Simple, in-process store for PlanDelta objects per site.
+
+    This enables offline-first delta synchronization by persisting deltas
+    locally and replaying them in order when connectivity is restored.
+    """
+
+    def __init__(self) -> None:
+        # Mapping: site_id -> list of PlanDelta (in order of arrival)
+        self._store: Dict[str, list[PlanDelta]] = {}
+
+    def add_delta(self, site_id: str, delta: PlanDelta) -> None:
+        if site_id not in self._store:
+            self._store[site_id] = []
+        self._store[site_id].append(delta)
+
+    def get_deltas(self, site_id: str) -> list[PlanDelta]:
+        return list(self._store.get(site_id, []))
+
+    def replay(self, site_id: str) -> list[PlanDelta]:
+        """Return a copy of the deltas for replay in the original order."""
+        return self.get_deltas(site_id)
diff --git a/tests/test_delta_store.py b/tests/test_delta_store.py
new file mode 100644
index 0000000..c9c4622
--- /dev/null
+++ b/tests/test_delta_store.py
@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import time
+from energiamesh.core import LocalProblem, SharedVariables, PlanDelta
+from energiamesh.sync import compute_delta, DeltaStore
+
+
+def test_delta_store_basic_store_and_replay():
+    # Prepare sample problem/state
+    lp = LocalProblem(site_id="site-1", problem_id="p1", description="test")
+    sv = SharedVariables(signals={"load_forecast": 123.0})
+
+    # Create two deltas to simulate incremental updates
+    d1 = compute_delta(lp, sv)
+    time.sleep(0.001)
+    sv.update("load_forecast", 120.0)
+    d2 = compute_delta(lp, sv)
+
+    store = DeltaStore()
+    store.add_delta(lp.site_id, d1)
+    store.add_delta(lp.site_id, d2)
+
+    # Replay should yield deltas in order
+    deltas = store.replay(lp.site_id)
+    assert len(deltas) == 2
+    assert isinstance(deltas[0], PlanDelta)
+    assert deltas[0].delta == d1.delta
+    assert deltas[1].delta == d2.delta