diff --git a/apps/client/src/features/page-history/utils/resolve-compare-pair.test.ts b/apps/client/src/features/page-history/utils/resolve-compare-pair.test.ts
new file mode 100644
index 000000000..449188f02
--- /dev/null
+++ b/apps/client/src/features/page-history/utils/resolve-compare-pair.test.ts
@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import { resolveComparePair } from "./resolve-compare-pair";
+
+// list is newest-first, matching usePageHistoryListQuery order
+const items = [{ id: "v3" }, { id: "v2" }, { id: "v1" }];
+
+describe("resolveComparePair", () => {
+  it("orders newer before older regardless of selection order", () => {
+    expect(resolveComparePair(items, ["v1", "v3"])).toEqual({
+      newerId: "v3",
+      olderId: "v1",
+    });
+    expect(resolveComparePair(items, ["v3", "v1"])).toEqual({
+      newerId: "v3",
+      olderId: "v1",
+    });
+  });
+
+  it("returns null unless exactly two versions are selected", () => {
+    expect(resolveComparePair(items, [])).toBeNull();
+    expect(resolveComparePair(items, ["v1"])).toBeNull();
+    expect(resolveComparePair(items, ["v1", "v2", "v3"])).toBeNull();
+  });
+
+  it("returns null when a selected id is not in the list", () => {
+    expect(resolveComparePair(items, ["v1", "missing"])).toBeNull();
+  });
+
+  it("returns null when the same id is selected twice", () => {
+    expect(resolveComparePair(items, ["v2", "v2"])).toBeNull();
+  });
+});
diff --git a/apps/client/src/features/page-history/utils/resolve-compare-pair.ts b/apps/client/src/features/page-history/utils/resolve-compare-pair.ts
new file mode 100644
index 000000000..d1d914346
--- /dev/null
+++ b/apps/client/src/features/page-history/utils/resolve-compare-pair.ts
@@ -0,0 +1,18 @@
+import { ComparePair } from "@/features/page-history/atoms/history-atoms";
+
+/**
+ * Resolves which of the two selected versions is newer using their position
+ * in the history list (list is newest-first: lower index = newer).
+ */
+export function resolveComparePair(
+  historyItems: { id: string }[],
+  selection: string[],
+): ComparePair | null {
+  if (selection.length !== 2) return null;
+  const indexA = historyItems.findIndex((item) => item.id === selection[0]);
+  const indexB = historyItems.findIndex((item) => item.id === selection[1]);
+  if (indexA === -1 || indexB === -1 || indexA === indexB) return null;
+  return indexA < indexB
+    ? { newerId: selection[0], olderId: selection[1] }
+    : { newerId: selection[1], olderId: selection[0] };
+}