diff --git a/verifiers/v1/trace.py b/verifiers/v1/trace.py
index 9944483a9..1e3d44dbb 100644
--- a/verifiers/v1/trace.py
+++ b/verifiers/v1/trace.py
@@ -227,6 +227,8 @@ class Trace(StrictBaseModel, Generic[TaskT, StateT]):
     _head_index: dict = PrivateAttr(default_factory=dict)
     """`(parent, msg_hash) -> node_id` for the graph builder (`graph.prepare_turn` / `commit`);
     rebuilt lazily from `nodes` after deserialization."""
+    _num_turns_cache: tuple[int, int] = PrivateAttr(default=(0, 0))
+    """`(counted nodes, sampled turns)` for the append-only message graph."""
 
     @property
     def reward(self) -> float:
@@ -300,7 +302,19 @@ def num_branches(self) -> int:
     def num_turns(self) -> int:
         """Total model turns (sampled responses) across all branches — prompt-supplied
         assistant messages don't count."""
-        return sum(1 for n in self.nodes if n.sampled)
+        counted_nodes, num_turns = self._num_turns_cache
+        node_count = len(self.nodes)
+        # Graph commits append nodes, so count only the unseen suffix between reads.
+        # If a caller shrinks the list, rebuild the count from the remaining nodes.
+        if node_count == counted_nodes:
+            return num_turns
+        if node_count < counted_nodes:
+            counted_nodes, num_turns = 0, 0
+        while counted_nodes < node_count:
+            num_turns += self.nodes[counted_nodes].sampled
+            counted_nodes += 1
+        self._num_turns_cache = (node_count, num_turns)
+        return num_turns
 
     @property
     def is_truncated(self) -> bool: