diff --git a/docs/deploy-checklist.md b/docs/deploy-checklist.md
index 1ac8994..03542c7 100644
--- a/docs/deploy-checklist.md
+++ b/docs/deploy-checklist.md
@@ -37,7 +37,7 @@ Phase 1 **A~C** 이후 실행 트랙. 작업 단위를 하나씩 처리한다.
 
 - 앱 코드는 클로즈드 베타 직전 수준
 - **`https://msn.iykyka.com` 라이브 + N3 완료 + Gemini 실초안 OK + Track A/B 완료**
-- **웹 고도화:** [`web-upgrade.md`](./web-upgrade.md) — W0~W3·W5~W7 **done** · **W4 code shipped** (live: Master `.env`에 FIREBASE_*/VAPID 후 `web` 재빌드)
+- **웹 고도화:** [`web-upgrade.md`](./web-upgrade.md) — W0~W3·W5~W7 **done** · **W4 live injected** (SW populated · accept: Web `:web:` APP_ID + push-test)
 - 후순위: **N4 Android FCM/UI** — Master 시크릿·실기기
 - UI: **iMessage-inspired light default** + soft charcoal dark 프로덕션 반영
 - **Track C 콘텐츠 갭 A~F**: 프로덕션 반영 완료 (`95422bb`대)
@@ -217,7 +217,7 @@ N2-A 전체 확정. 다음 구현 트랙은 **N1 스모크 → N2-B (Dockerfile/
 | **N4-W1** | 웹 로컬 지속성 (prefs/idb) | **done** (2026-08-04) | `app_database_web.dart` SharedPreferences · Drift WASM 비채택 · 라이브 하드 리프레시 스모크는 배포 후 |
 | **N4-W2** | 데모 메신저 UX | **done** (2026-08-04) | ≥900px 분할·WS 상태·ID 복사·웹 안내 배너 |
 | **N4-W3** | 인앱·Notification API | **done** (2026-08-04) | 웹 Timer+Notification · 포커스 past-due · 설정 권한 |
-| **N4-W4** | Web Push (FCM + VAPID) | **code** (2026-08-04) | dart-define+SW shipped · **live**는 Master `.env` FIREBASE_*/VAPID + `web` rebuild |
+| **N4-W4** | Web Push (FCM + VAPID) | **live-injected** (2026-08-04) | 서버 `.env` 동기·SW populated · **accept:** APP_ID를 `:web:`로 교체 후 push-test |
 | **N4-W5** | PWA | **done** (2026-08-04) | manifest 테마 · nginx no-cache · OfflineBanner |
 | **N4-W6** | 테스터 가이드 | **done** (2026-08-04) | `tester-guide.md` 웹 절 |
 | **N4-W7** | 품질 | **done** (2026-08-04) | `scripts/e2e_web_smoke.sh` + 부록 QA (Playwright는 후속) |
@@ -290,8 +290,8 @@ N1~N4(배포·품질에 필요한 최소분) 이후에만 착수. `roadmap.md` P
    확인 필요, 그때까지는 인앱 배지·배너가 실질적 대체 경로. Track C A~F 프로덕션
    재배포는 **완료** (`95422bb` / docs `f363e8e`, 2026-08-03).
 
-**바로 다음:** 웹 **W4 live** — Master가 Firebase Web config + VAPID를 서버 `.env`에 넣고 `docker compose up -d --build web` · 또는 Android N4-1/3.
-웹 W0~W7 코드 경로 **done** (W4 live accept만 시크릿 대기).
+**바로 다음:** 웹 **W4 accept** — Firebase Console에서 **Web** 앱(`:web:` APP_ID) 확인·`.env` 교체·`web` 재빌드·하드 리프레시·`/admin/push-test` · 또는 Android N4-1/3.
+웹 W0~W7 코드·시크릿 주입 **done** (W4 browser token accept만 남음).
 
 
 완료 시 본 표의 Status를 `done`으로 바꾸고, [`roadmap.md`](./roadmap.md) §4/§5의 대응 `[~]`/`[ ]`도 같이 갱신한다.
diff --git a/docs/fcm-setup.md b/docs/fcm-setup.md
index d8e2d01..22471c6 100644
--- a/docs/fcm-setup.md
+++ b/docs/fcm-setup.md
@@ -85,9 +85,11 @@ Android N4(실기기)와 **병행 가능**. 서버 서비스 계정(HTTP v1)은
 
 ### Master 준비물 (live accept)
 
-1. Firebase Console → **Web** 앱 추가 (authorized domain: `msn.iykyka.com`) — SA 프로젝트와 동일
+1. Firebase Console → **Web** 앱 추가 (authorized domain: `msn.iykyka.com`) — SA 프로젝트와 동일  
+   - `FIREBASE_APP_ID`는 **`1:…:web:…` 형태**여야 함. Android(`:android:`) / `google-services.json`의 mobilesdk_app_id를 넣으면 웹 토큰 발급이 실패할 수 있음.
 2. Cloud Messaging → Web Push certificates / **VAPID public key**
-3. 배포 호스트 `~/project/ykavu/.env`에 기입 (`.env.example` 키 이름 — **값 git 금지**):
+3. 배포 호스트 **`~/project/ykavu/.env`**(클라우드 에이전트 `/workspace/.env`가 아님)에 기입  
+   (`.env.example` 키 이름 — **값 git 금지**):
 
 ```bash
 FIREBASE_API_KEY=
@@ -95,7 +97,7 @@ FIREBASE_AUTH_DOMAIN=          # 비우면 {projectId}.firebaseapp.com
 FIREBASE_PROJECT_ID=
 FIREBASE_STORAGE_BUCKET=       # 비우면 {projectId}.appspot.com
 FIREBASE_MESSAGING_SENDER_ID=
-FIREBASE_APP_ID=
+FIREBASE_APP_ID=               # must be :web: app id
 FIREBASE_VAPID_KEY=
 ```
 
diff --git a/docs/web-upgrade.md b/docs/web-upgrade.md
index 79d7bbd..29f01b8 100644
--- a/docs/web-upgrade.md
+++ b/docs/web-upgrade.md
@@ -255,7 +255,7 @@ flowchart LR
 
 | | |
 |--|--|
-| **Status** | **code shipped** (2026-08-04) — **live accept**는 Master Firebase Web config + VAPID를 서버 `.env`에 넣고 `web` 재빌드 후 |
+| **Status** | **live injected** (2026-08-04) — 서버 `.env`+SW populated · **accept 대기:** `FIREBASE_APP_ID`가 `:android:`로 보여 Web 앱(`:web:`) 교체 후 브라우저 토큰·push-test 확인 |
 | **목표** | 웹 탭이 백그라운드여도 에스컬레이션 등 서버 `notifyUser`가 **실 FCM 웹 토큰**으로 전달 |
 | **비범위** | 레거시 `FCM_SERVER_KEY` 신규 의존, 커스텀 푸시 프로토콜 |
 
diff --git a/mobile/Dockerfile b/mobile/Dockerfile
index ef6fbf9..a42c533 100644
--- a/mobile/Dockerfile
+++ b/mobile/Dockerfile
@@ -2,6 +2,9 @@
 #   docker build -f mobile/Dockerfile --build-arg CORE_API_BASE=https://msn.iykyka.com .
 # Optional Web Push (N4-W4) — pass FIREBASE_* / FIREBASE_VAPID_KEY as build-args
 # (never commit real values; see .env.example and docs/fcm-setup.md).
+#
+# Secrets are copied into ENV for the build stage only (not the final nginx image).
+# The RUN line calls a script so values are not expanded into Docker build logs.
 ARG FLUTTER_IMAGE=ghcr.io/cirruslabs/flutter:3.32.7
 FROM ${FLUTTER_IMAGE} AS build
 
@@ -15,39 +18,20 @@ ARG FIREBASE_MESSAGING_SENDER_ID=
 ARG FIREBASE_APP_ID=
 ARG FIREBASE_VAPID_KEY=
 
+ENV CORE_API_BASE=${CORE_API_BASE} \
+    FIREBASE_API_KEY=${FIREBASE_API_KEY} \
+    FIREBASE_AUTH_DOMAIN=${FIREBASE_AUTH_DOMAIN} \
+    FIREBASE_PROJECT_ID=${FIREBASE_PROJECT_ID} \
+    FIREBASE_STORAGE_BUCKET=${FIREBASE_STORAGE_BUCKET} \
+    FIREBASE_MESSAGING_SENDER_ID=${FIREBASE_MESSAGING_SENDER_ID} \
+    FIREBASE_APP_ID=${FIREBASE_APP_ID} \
+    FIREBASE_VAPID_KEY=${FIREBASE_VAPID_KEY}
+
 COPY mobile/pubspec.yaml mobile/pubspec.lock ./
 RUN flutter pub get
 
 COPY mobile/ .
-
-# Populate firebase-messaging-sw.js when Web config is present; keep stub otherwise.
-RUN set -eu; \
-    if [ -n "${FIREBASE_API_KEY}" ] && [ -n "${FIREBASE_APP_ID}" ] && \
-       [ -n "${FIREBASE_MESSAGING_SENDER_ID}" ] && [ -n "${FIREBASE_PROJECT_ID}" ]; then \
-      AUTH_DOMAIN="${FIREBASE_AUTH_DOMAIN:-${FIREBASE_PROJECT_ID}.firebaseapp.com}"; \
-      STORAGE_BUCKET="${FIREBASE_STORAGE_BUCKET:-${FIREBASE_PROJECT_ID}.appspot.com}"; \
-      sed \
-        -e "s|__FIREBASE_API_KEY__|${FIREBASE_API_KEY}|g" \
-        -e "s|__FIREBASE_AUTH_DOMAIN__|${AUTH_DOMAIN}|g" \
-        -e "s|__FIREBASE_PROJECT_ID__|${FIREBASE_PROJECT_ID}|g" \
-        -e "s|__FIREBASE_STORAGE_BUCKET__|${STORAGE_BUCKET}|g" \
-        -e "s|__FIREBASE_MESSAGING_SENDER_ID__|${FIREBASE_MESSAGING_SENDER_ID}|g" \
-        -e "s|__FIREBASE_APP_ID__|${FIREBASE_APP_ID}|g" \
-        web/firebase-messaging-sw.js.template > web/firebase-messaging-sw.js; \
-      echo "firebase-messaging-sw.js: populated for project ${FIREBASE_PROJECT_ID}"; \
-    else \
-      echo "firebase-messaging-sw.js: stub (FIREBASE_* build-args empty)"; \
-    fi
-
-RUN flutter build web --release \
-      --dart-define=CORE_API_BASE=${CORE_API_BASE} \
-      --dart-define=FIREBASE_API_KEY=${FIREBASE_API_KEY} \
-      --dart-define=FIREBASE_AUTH_DOMAIN=${FIREBASE_AUTH_DOMAIN} \
-      --dart-define=FIREBASE_PROJECT_ID=${FIREBASE_PROJECT_ID} \
-      --dart-define=FIREBASE_STORAGE_BUCKET=${FIREBASE_STORAGE_BUCKET} \
-      --dart-define=FIREBASE_MESSAGING_SENDER_ID=${FIREBASE_MESSAGING_SENDER_ID} \
-      --dart-define=FIREBASE_APP_ID=${FIREBASE_APP_ID} \
-      --dart-define=FIREBASE_VAPID_KEY=${FIREBASE_VAPID_KEY}
+RUN chmod +x scripts/docker_build_web.sh && ./scripts/docker_build_web.sh
 
 FROM nginx:1.27-alpine
 COPY --from=build /app/build/web /usr/share/nginx/html
diff --git a/mobile/scripts/docker_build_web.sh b/mobile/scripts/docker_build_web.sh
new file mode 100755
index 0000000..a8f0a92
--- /dev/null
+++ b/mobile/scripts/docker_build_web.sh
@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# Runs inside the Flutter build stage. Reads FIREBASE_* from the environment
+# (set via Dockerfile ENV from build-args) so secret values are not expanded
+# into the Dockerfile RUN instruction / build logs.
+set -euo pipefail
+
+CORE_API_BASE="${CORE_API_BASE:-https://msn.iykyka.com}"
+FIREBASE_API_KEY="${FIREBASE_API_KEY:-}"
+FIREBASE_AUTH_DOMAIN="${FIREBASE_AUTH_DOMAIN:-}"
+FIREBASE_PROJECT_ID="${FIREBASE_PROJECT_ID:-}"
+FIREBASE_STORAGE_BUCKET="${FIREBASE_STORAGE_BUCKET:-}"
+FIREBASE_MESSAGING_SENDER_ID="${FIREBASE_MESSAGING_SENDER_ID:-}"
+FIREBASE_APP_ID="${FIREBASE_APP_ID:-}"
+FIREBASE_VAPID_KEY="${FIREBASE_VAPID_KEY:-}"
+
+if [[ -n "${FIREBASE_API_KEY}" && -n "${FIREBASE_APP_ID}" && \
+      -n "${FIREBASE_MESSAGING_SENDER_ID}" && -n "${FIREBASE_PROJECT_ID}" ]]; then
+  AUTH_DOMAIN="${FIREBASE_AUTH_DOMAIN:-${FIREBASE_PROJECT_ID}.firebaseapp.com}"
+  STORAGE_BUCKET="${FIREBASE_STORAGE_BUCKET:-${FIREBASE_PROJECT_ID}.appspot.com}"
+  # Detect common misconfiguration: Android app id used for web build.
+  if [[ "${FIREBASE_APP_ID}" == *":android:"* ]]; then
+    echo "WARN: FIREBASE_APP_ID looks like an Android app id (:android:)."
+    echo "WARN: Web Push needs a Firebase Web app id (:web:). Token registration may fail."
+  fi
+  sed \
+    -e "s|__FIREBASE_API_KEY__|${FIREBASE_API_KEY}|g" \
+    -e "s|__FIREBASE_AUTH_DOMAIN__|${AUTH_DOMAIN}|g" \
+    -e "s|__FIREBASE_PROJECT_ID__|${FIREBASE_PROJECT_ID}|g" \
+    -e "s|__FIREBASE_STORAGE_BUCKET__|${STORAGE_BUCKET}|g" \
+    -e "s|__FIREBASE_MESSAGING_SENDER_ID__|${FIREBASE_MESSAGING_SENDER_ID}|g" \
+    -e "s|__FIREBASE_APP_ID__|${FIREBASE_APP_ID}|g" \
+    web/firebase-messaging-sw.js.template > web/firebase-messaging-sw.js
+  echo "firebase-messaging-sw.js: populated for project ${FIREBASE_PROJECT_ID}"
+else
+  echo "firebase-messaging-sw.js: stub (FIREBASE_* build-args empty)"
+fi
+
+flutter build web --release \
+  --dart-define="CORE_API_BASE=${CORE_API_BASE}" \
+  --dart-define="FIREBASE_API_KEY=${FIREBASE_API_KEY}" \
+  --dart-define="FIREBASE_AUTH_DOMAIN=${FIREBASE_AUTH_DOMAIN}" \
+  --dart-define="FIREBASE_PROJECT_ID=${FIREBASE_PROJECT_ID}" \
+  --dart-define="FIREBASE_STORAGE_BUCKET=${FIREBASE_STORAGE_BUCKET}" \
+  --dart-define="FIREBASE_MESSAGING_SENDER_ID=${FIREBASE_MESSAGING_SENDER_ID}" \
+  --dart-define="FIREBASE_APP_ID=${FIREBASE_APP_ID}" \
+  --dart-define="FIREBASE_VAPID_KEY=${FIREBASE_VAPID_KEY}"