diff --git a/signalvault/replay.py b/signalvault/replay.py
index 2bf067a..1696b79 100644
--- a/signalvault/replay.py
+++ b/signalvault/replay.py
@@ -13,13 +13,20 @@ def _hash(obj: object) -> str:
 class DeterministicReplayEngine:
     """A tiny deterministic replay engine that applies deltas to an in-memory state.
 
-    State is a dict with keys:
-      - nodes: dict[id, SignalNode]
-      - edges: list[Edge]
-      - hedges: dict[id, HedgePlan]
+    This engine supports two usage modes to satisfy multiple test surfaces:
+    - State-based delta application (two-argument form): apply_delta(state, delta)
+      Returns a new state dict with the delta applied. This is used by core tests
+      that exercise deterministic state evolution with simple keys like
+      "signals" and "hedges" along with a version field.
+    - Traditional delta-based application (one-argument form): apply_delta(delta)
+      Returns a hash of the applied delta together with the current graph state, for
+      compatibility with the MVP tests that rely on a hash-based replay platform.
+      The internal state keeps legacy graph primitives (nodes/edges/hedges) for
+      MVP-style tests.
     """
 
     def __init__(self) -> None:
+        # Internal mutable state used by the MVP delta path
         self.nodes = {}
         self.edges = []
         self.hedges = {}
@@ -30,24 +37,63 @@ class DeterministicReplayEngine:
         self._node_counter += 1
         return f"n{self._node_counter}"
 
-    def apply_delta(self, delta):
-        # delta can contain: add_nodes, add_edges, add_hedges
-        if isinstance(delta, dict) and "add_nodes" in delta and delta["add_nodes"]:
-            for n in delta["add_nodes"]:
+    def apply_delta(self, base_or_delta, delta=None):
+        # Dual-API support:
+        # 1) If delta is provided, treat base_or_delta as the base state and apply delta
+        #    to produce and return a new state (state-based delta application).
+        # 2) If delta is None, treat base_or_delta as a delta payload and apply it
+        #    to the internal state, returning a hash (legacy MVP path).
+        if delta is not None:
+            # State-based delta application
+            base_state = dict(base_or_delta) if base_or_delta is not None else {}
+            # Normalize to lists for mutating safely
+            signals = list(base_state.get("signals", []))
+            hedges = list(base_state.get("hedges", []))
+            version = base_state.get("version")
+
+            d = delta
+            if isinstance(d, dict):
+                if "signals" in d:
+                    signals.extend(d["signals"])
+                if "hedges" in d:
+                    hedges.extend(d["hedges"])
+                if "version" in d:
+                    version = d["version"]
+
+            new_state: Dict[str, object] = {
+                "signals": signals,
+                "hedges": hedges,
+            }
+            if version is not None:
+                new_state["version"] = version
+            else:
+                new_state["version"] = 0 if new_state.get("version") is None else new_state["version"]
+            return new_state
+
+        # Legacy MVP path: apply delta to internal state and return a hash
+        delta_payload = base_or_delta
+        # Proliferate nodes
+        if isinstance(delta_payload, dict) and "add_nodes" in delta_payload and delta_payload["add_nodes"]:
+            for n in delta_payload["add_nodes"]:
                 if getattr(n, "id", None) is None:
                     self._node_counter += 1
-                    n = SignalNode(asset=n.asset, venue=n.venue, signal_type=n.signal_type, timestamp=n.timestamp, quality=n.quality, id=f"n{self._node_counter}")
+                    # Instantiate a canonical id if missing
+                    n = type(n)(asset=getattr(n, "asset", None), venue=getattr(n, "venue", None),
+                                 signal_type=getattr(n, "signal_type", None), timestamp=getattr(n, "timestamp", None),
+                                 quality=getattr(n, "quality", 1.0), id=f"n{self._node_counter}")
                 self.nodes[n.id] = n
 
-        if isinstance(delta, dict) and "add_edges" in delta and delta["add_edges"]:
-            for e in delta["add_edges"]:
+        # Edges
+        if isinstance(delta_payload, dict) and "add_edges" in delta_payload and delta_payload["add_edges"]:
+            for e in delta_payload["add_edges"]:
                 self.edges.append(e)
 
-        if isinstance(delta, dict) and "add_hedges" in delta and delta["add_hedges"]:
-            for h in delta["add_hedges"]:
+        # Hedge plans
+        if isinstance(delta_payload, dict) and "add_hedges" in delta_payload and delta_payload["add_hedges"]:
+            for h in delta_payload["add_hedges"]:
                 self.hedges[h.id] = h
 
-        h = _hash((delta, self.nodes, self.edges, self.hedges))
+        h = _hash((delta_payload, self.nodes, self.edges, self.hedges))
         self._applied_hashes.append(h)
         return h