diff --git a/mock/e2e/phy.spec.ts b/mock/e2e/phy.spec.ts
index f151337..91fcf5b 100644
--- a/mock/e2e/phy.spec.ts
+++ b/mock/e2e/phy.spec.ts
@@ -603,3 +603,17 @@ test("「その他」の部品はpartGroupを設定せず、選択後は追加
   await expect(select).toHaveValue("");
   await expect(page.locator('[data-node-type="component"] .part-label').last()).toHaveText("part");
 });
+
+test("クラスター内でキーを足すと、足したキーを選ぶ", async ({ page }) => {
+  const cluster = await makeCluster(page);
+  const clusterKeys = cluster.locator('[data-node-type="key"]');
+  const before = await clusterKeys.evaluateAll((es) => es.map((e) => e.getAttribute("data-node-id")));
+  await page.getByText("キー・部品 2件", { exact: true }).click();
+  await page.locator(`[data-tree-id="${before[0]}"]`).first().click();
+  await page.getByRole("button", { name: "キーを足す" }).click();
+  await expect(clusterKeys).toHaveCount(3);
+  const after = await clusterKeys.evaluateAll((es) => es.map((e) => e.getAttribute("data-node-id")));
+  const added = after.find((id) => !before.includes(id));
+  await expect(page.locator(`[data-sel-id="${added}"]`)).toHaveCount(1);
+  await expect(page.locator("[data-sel-id]")).toHaveCount(1);
+});
diff --git a/mock/src/ui/phy/usePhyCommands.ts b/mock/src/ui/phy/usePhyCommands.ts
index 98603b6..f3ac57c 100644
--- a/mock/src/ui/phy/usePhyCommands.ts
+++ b/mock/src/ui/phy/usePhyCommands.ts
@@ -120,6 +120,12 @@ export function usePhyCommands(options: Options) {
     options.popover.value = null;
   }
 
+  /** The node id present in `after` but not in `before`; the tree order does not tell where it went. */
+  function addedId(before: ProjectFile["phy"], after: ProjectFile["phy"]): string {
+    const existing = new Set(options.allNodes(before.root).map((node) => node.id));
+    return options.allNodes(after.root).find((node) => !existing.has(node.id))?.id || "";
+  }
+
   function keyAdd() {
     const chosen = options.node(options.selected.value.at(-1) || "");
     const parent = chosen ? options.selectedContainer(chosen.id) : insertionParent();
@@ -137,7 +143,7 @@ export function usePhyCommands(options: Options) {
     let id = "";
     options.dispatch("キー追加", (project) => {
       const phy = addKeyToParent(project.phy, parent, { x, y, rotation: 0 }, { w: 1, h: 1 });
-      id = options.allNodes(phy.root).at(-1)?.id || "";
+      id = addedId(project.phy, phy);
       return { ...project, phy, idx: { ...project.idx, order: [...project.idx.order, id] } };
     });
     if (id) options.selected.value = [id];
@@ -171,7 +177,7 @@ export function usePhyCommands(options: Options) {
         { x, y, rotation: 0 },
         group === "その他" ? undefined : group,
       );
-      id = options.allNodes(phy.root).at(-1)?.id || "";
+      id = addedId(project.phy, phy);
       return { ...project, phy };
     });
     if (id) options.selected.value = [id];