diff --git a/src/tracksdata/graph/_rustworkx_graph.py b/src/tracksdata/graph/_rustworkx_graph.py
index 5f6a2645..8e172f97 100644
--- a/src/tracksdata/graph/_rustworkx_graph.py
+++ b/src/tracksdata/graph/_rustworkx_graph.py
@@ -1317,21 +1317,26 @@ def update_edge_attrs(
             edge_ids = self.edge_ids()
 
         size = len(edge_ids)
+        # broadcast scalars into a local copy so the caller's dict is not mutated
+        broadcast_attrs: dict[str, Any] = {}
         for key, value in attrs.items():
             if key not in self.edge_attr_keys():
                 raise ValueError(f"Edge attribute key '{key}' not found in graph. Expected '{self.edge_attr_keys()}'")
 
             if np.isscalar(value):
-                attrs[key] = [value] * size
+                broadcast_attrs[key] = [value] * size
 
-            elif len(attrs[key]) != size:
-                raise ValueError(f"Attribute '{key}' has wrong size. Expected {size}, got {len(attrs[key])}")
+            elif len(value) != size:
+                raise ValueError(f"Attribute '{key}' has wrong size. Expected {size}, got {len(value)}")
+
+            else:
+                broadcast_attrs[key] = value
 
         edge_map = self._graph.edge_index_map()
 
         for i, edge_id in enumerate(edge_ids):
             edge_attr = edge_map[edge_id][2]  # 0=source, 1=target, 2=attributes
-            for key, value in attrs.items():
+            for key, value in broadcast_attrs.items():
                 edge_attr[key] = value[i]
 
     def assign_tracklet_ids(
diff --git a/src/tracksdata/graph/_test/test_graph_backends.py b/src/tracksdata/graph/_test/test_graph_backends.py
index 641138e5..099e1ff8 100644
--- a/src/tracksdata/graph/_test/test_graph_backends.py
+++ b/src/tracksdata/graph/_test/test_graph_backends.py
@@ -607,6 +607,11 @@ def test_update_edge_attrs(graph_backend: BaseGraph) -> None:
     with pytest.raises(ValueError):
         graph_backend.update_edge_attrs(edge_ids=[edge_id], attrs={"weight": [1.0, 2.0]})
 
+    # the caller's attrs dict must not be mutated (e.g. scalar broadcast to a list)
+    user_attrs = {"weight": 2.0}
+    graph_backend.update_edge_attrs(edge_ids=[edge_id], attrs=user_attrs)
+    assert user_attrs == {"weight": 2.0}
+
 
 def test_num_edges(graph_backend: BaseGraph) -> None:
     """Test counting edges."""