diff --git a/replay-ui/src/__tests__/useStep.test.ts b/replay-ui/src/__tests__/useStep.test.ts
new file mode 100644
index 0000000..8fec0ea
--- /dev/null
+++ b/replay-ui/src/__tests__/useStep.test.ts
@@ -0,0 +1,24 @@
+import { describe, it, expect } from "bun:test";
+import { clampIndex } from "../hooks/useStep";
+
+// Bug class: a wrong boundary lets the URL address step 0 or a step past the
+// end, so the viewer requests a non-existent step and renders nothing. Steps
+// are 1-based and capped at maxIndex.
+describe("clampIndex", () => {
+  const cases: [number, number | undefined, number][] = [
+    [0, 5, 1],
+    [-3, 5, 1],
+    [1, 5, 1],
+    [3, 5, 3],
+    [5, 5, 5],
+    [6, 5, 5],
+    [100, 5, 5],
+    [3, undefined, 3],
+    [0, undefined, 1],
+  ];
+  for (const [index, max, expected] of cases) {
+    it(`clamps (${index}, ${max}) -> ${expected}`, () => {
+      expect(clampIndex(index, max)).toBe(expected);
+    });
+  }
+});