diff --git a/A00_Common/b_page_state.ts b/A00_Common/b_page_state.ts
index e6cd74bd1..71c7ea6f0 100644
--- a/A00_Common/b_page_state.ts
+++ b/A00_Common/b_page_state.ts
@@ -438,6 +438,20 @@ export function onStateRemoved(fn: (key: string) => void): () => void {
   return () => removedListeners.delete(fn);
 }
 
+/** 세션 칸을 골라 지움(401 떠남) — 칸마다 지움 알림(설계 가게가 미룬 쓰기를 거둠). */
+export function clearSessionKeys(match: (key: string) => boolean): void {
+  const doomed: string[] = [];
+  try {
+    for (let index = 0; index < window.sessionStorage.length; index += 1) {
+      const key = window.sessionStorage.key(index);
+      if (key !== null && match(key)) doomed.push(key);
+    }
+  } catch {
+    return; // 저장소가 막힌 브라우저 — 지울 것이 없다.
+  }
+  doomed.forEach((key) => writeRaw(key, null));
+}
+
 function writeRaw(key: string, value: string | null): void {
   if (value === null) removedListeners.forEach((fn) => fn(key));
   try {
diff --git a/A00_Common/b_project_switch.ts b/A00_Common/b_project_switch.ts
index 9e1df05b5..5e300651d 100644
--- a/A00_Common/b_project_switch.ts
+++ b/A00_Common/b_project_switch.ts
@@ -11,7 +11,7 @@
  * ========================================================================== */
 
 import { CURRENT_PROJECT_ID_KEY } from "../config/config_frontend";
-import { STATE_REGISTRY, type StateEntry } from "./b_page_state";
+import { clearSessionKeys, STATE_REGISTRY, type StateEntry } from "./b_page_state";
 
 /** 이 탭이 마지막으로 들어온 프로젝트. */
 const ENTERED_KEY = "aislo:entered-project";
@@ -212,14 +212,10 @@ export async function wipeIfOwnerChanged(user: string | number): Promise<boolean
  */
 export async function leaveProject(projectId: string): Promise<void> {
   if (!projectId) return;
-  try {
-    // 프로젝트 칸 열쇠는 모두 프로젝트 번호(uuid)를 품음 — 등록표 · 옛 키 · 페이지 칸을 한 줄로.
-    Object.keys(sessionEntries())
-      .filter((key) => key.includes(projectId))
-      .forEach((key) => window.sessionStorage.removeItem(key));
-  } catch {
-    /* 저장소가 막힌 브라우저 — 지울 것이 없다. */
-  }
+  // 프로젝트 칸 열쇠는 모두 프로젝트 번호(uuid)를 품음 — 등록표 · 옛 키 · 페이지 칸을 한 줄로.
+  // 등록표 창구로 지움 — 지움 알림을 받은 설계 가게가 미뤄 둔 쓰기(1 s)를 거둠(떠난 뒤 되살아남 막음).
+  // ponytail: 아직 한 번도 안 쓴 칸의 미룬 쓰기는 못 거둠 — 생기면 가게에 프로젝트 단위 거둠을 더함.
+  clearSessionKeys((key) => key.includes(projectId));
   const { purgeProject } = await import("./b_asset_cache");
   await purgeProject(projectId);
 }
diff --git a/A00_Common/main.ts b/A00_Common/main.ts
index cd89db516..58e4bbe11 100644
--- a/A00_Common/main.ts
+++ b/A00_Common/main.ts
@@ -28,9 +28,11 @@ import {
   ensureInitialHash,
   setLeaveGuard,
   draftRoute,
+  isProjectRoute,
   showWithinPage,
   takeLeaveGuard,
 } from "./router";
+import { currentProjectId, leaveProject } from "./b_project_switch";
 
 const FOOTERLESS_ROUTES: readonly RoutePath[] = [
   ROUTES.B01_ACCOUNT,
@@ -76,6 +78,10 @@ function bootstrap(): void {
       "is-bm-route",
       NATIVE_MENU_ROUTE.test(currentRoute()),
     ); // 전역 글 선택 막음(81-1)
+    // 프로젝트를 떠남(401) — 떠남 물음을 지난 뒤 · 그 프로젝트 세션 칸 · 3D 보관함 몫 비움
+    if (isProjectRoute(shown) && !isProjectRoute(currentRoute())) {
+      void leaveProject(currentProjectId());
+    }
     const within = showWithinPage(currentRoute()); // B05 ↔ B06 — 보기만 바꿈(24-3a)
     shown = currentRoute();
     shownHash = window.location.hash;
diff --git a/resources/tester/helper_401_project_isolation.cjs b/resources/tester/helper_401_project_isolation.cjs
index 38da3bbd3..a086d0226 100644
--- a/resources/tester/helper_401_project_isolation.cjs
+++ b/resources/tester/helper_401_project_isolation.cjs
@@ -74,7 +74,7 @@ function openTab() {
     currentRoute: () => router.route,
     setLeaveGuard: (fn) => ((router.guard = fn), (router.sets += 1)),
   });
-  return { win, sw };
+  return { win, sw, ps: load("A00_Common/b_page_state.ts") };
 }
 
 const X = "11111111-aaaa-4aaa-8aaa-000000000001";
@@ -147,8 +147,12 @@ const keys = (win) => Object.keys(win.sessionStorage.dump()).sort();
   ]) {
     shared.idb.data.set(`${pid}|${url}`, { key: `${pid}|${url}`, projectId: pid, url });
   }
+  const removed = [];
+  const stop = A.ps.onStateRemoved((key) => removed.push(key)); // 설계 가게가 미룬 쓰기를 거두는 알림
   await A.sw.leaveProject(X);
+  stop();
   out.leave = {
+    notified: removed.length,
     keys: keys(A.win),
     expected: [...keep].sort(),
     entered: A.win.sessionStorage.getItem("aislo:entered-project"),
diff --git a/resources/tester/test_401_project_isolation.py b/resources/tester/test_401_project_isolation.py
index 619496f48..757f14daa 100644
--- a/resources/tester/test_401_project_isolation.py
+++ b/resources/tester/test_401_project_isolation.py
@@ -52,6 +52,7 @@ def test_떠남은_그_프로젝트_몫만_지움(out):
     assert leave["local"] == ["1", "1"]  # 취향 · CAD 복구본
     assert leave["idb"] == [f"{Y}|u1"]  # 보관함 = X 줄만 지움
     assert leave["otherTabKept"] == "1"  # 다른 탭 세션은 안 건드림
+    assert leave["notified"] == 13  # 지운 칸마다 지움 알림 — 설계 가게가 미룬 쓰기(1 s)를 거둠
 
 
 def test_떠남_물음_한_번(out):
@@ -96,3 +97,12 @@ def test_떠남_취소_뒤_단계_단추_다시_맞춤():
     assert "window.dispatchEvent(new Event(ROUTE_RESTORED_EVENT))" in main
     help_ts = (ROOT / "ui_template/ui_template_help.ts").read_text(encoding="utf-8")
     assert "window.addEventListener(ROUTE_RESTORED_EVENT, onHash);" in help_ts
+
+
+def test_떠남은_물음_뒤_한_자리에서_비움():
+    # 572-4 — hashchange 가드(물음)를 지난 renderAll 첫머리 · 프로젝트 화면 → 밖 일 때만
+    main = (ROOT / "A00_Common/main.ts").read_text(encoding="utf-8")
+    render = main[main.index("const renderAll") : main.index("const within")]
+    assert "isProjectRoute(shown) && !isProjectRoute(currentRoute())" in render
+    assert "leaveProject(currentProjectId())" in render
+    assert main.index("takeLeaveGuard()") > main.index("const renderAll")  # 물음은 renderAll 앞에서