diff --git a/gameblocks/modules/behavior/GridPathPlanner.js b/gameblocks/modules/behavior/GridPathPlanner.js
index c352caf..4280d74 100644
--- a/gameblocks/modules/behavior/GridPathPlanner.js
+++ b/gameblocks/modules/behavior/GridPathPlanner.js
@@ -24,7 +24,11 @@ export function normalizeBlockedCells(blocked = []) {
 }
 
 function wrapDelta(delta, size) {
-  return Math.min(Math.abs(delta), size - Math.abs(delta));
+  if (!Number.isFinite(size) || size <= 0) {
+    return Math.abs(delta);
+  }
+  const span = ((Math.abs(delta) % size) + size) % size;
+  return Math.min(span, size - span);
 }
 
 function priorityInsert(open, entry) {
diff --git a/gameblocks/modules/behavior/GridPathPlanner.test.js b/gameblocks/modules/behavior/GridPathPlanner.test.js
new file mode 100644
index 0000000..3422600
--- /dev/null
+++ b/gameblocks/modules/behavior/GridPathPlanner.test.js
@@ -0,0 +1,26 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { GridPathPlanner } from './GridPathPlanner.js';
+
+const navigation = {
+  vectors: {
+    north: { right: 0, forward: 1 },
+    east: { right: 1, forward: 0 },
+    south: { right: 0, forward: -1 },
+    west: { right: -1, forward: 0 },
+  },
+  neighborOrder: ['north', 'east', 'south', 'west'],
+};
+
+test('wrap heuristic stays non-negative when cells differ by more than the board size', () => {
+  const planner = new GridPathPlanner({ navigation, columns: 8, rows: 8, wrap: true });
+  const h = planner.heuristic({ right: 0, forward: 0 }, { right: 10, forward: 0 });
+  assert.equal(h, 2);
+  assert.ok(h >= 0);
+});
+
+test('wrap heuristic is the shorter torus arc', () => {
+  const planner = new GridPathPlanner({ navigation, columns: 10, rows: 10, wrap: true });
+  assert.equal(planner.heuristic({ right: 0, forward: 0 }, { right: 9, forward: 0 }), 1);
+  assert.equal(planner.heuristic({ right: 0, forward: 0 }, { right: 1, forward: 0 }), 1);
+});