diff --git a/frontend/dcViewer.css b/frontend/dcViewer.css
index 3320b54..4e9f48c 100644
--- a/frontend/dcViewer.css
+++ b/frontend/dcViewer.css
@@ -3220,11 +3220,17 @@ header {
 }
 
 body[data-viewer-mode="guest"] .tb-btn:not([data-action="measure"]):not([data-action="section"]):not([data-action="projection"]),
+html[data-share-startup="guest"] body:not([data-viewer-mode="standard"]) .tb-btn:not([data-action="measure"]):not([data-action="section"]):not([data-action="projection"]),
 body[data-viewer-mode="guest"] #settingsPanel,
+html[data-share-startup="guest"] body:not([data-viewer-mode="standard"]) #settingsPanel,
 body[data-viewer-mode="guest"] #templatesPanel,
+html[data-share-startup="guest"] body:not([data-viewer-mode="standard"]) #templatesPanel,
 body[data-viewer-mode="guest"] #projectNameSection,
+html[data-share-startup="guest"] body:not([data-viewer-mode="standard"]) #projectNameSection,
 body[data-viewer-mode="guest"] #saveProjectSection,
-body[data-viewer-mode="guest"] #modelVersionUploadButton {
+html[data-share-startup="guest"] body:not([data-viewer-mode="standard"]) #saveProjectSection,
+body[data-viewer-mode="guest"] #modelVersionUploadButton,
+html[data-share-startup="guest"] body:not([data-viewer-mode="standard"]) #modelVersionUploadButton {
   display: none !important;
 }
 
diff --git a/frontend/dcViewer.js b/frontend/dcViewer.js
index 172c51d..6f0cdbb 100644
--- a/frontend/dcViewer.js
+++ b/frontend/dcViewer.js
@@ -184,6 +184,7 @@ const AUTH_SESSION_API = "/api/auth/session";
 const SHARES_API = "/api/shares";
 const COMMENTS_API = "/api/comments";
 const UPLOAD_VERSIONS_API = "/api/uploads/versions";
+const SHARE_AUTH_GUEST_PARAM = "ndc_bim_guest";
 const SHARE_PROMOTION_TIMEOUT_MS = 4500;
 
 const acceptByType = {
@@ -624,13 +625,39 @@ const getShareTokenFromPath = () => {
 const getCurrentReturnPath = () => `${window.location.pathname}${window.location.search || ""}${window.location.hash || ""}`;
 
 const getShareTopLevelPromotionUrl = () => {
-  const promoteUrl = startupSharePayload?.promoteUrl;
+  const promoteUrl = startupSharePayload?.optionalPromoteUrl || startupSharePayload?.promoteUrl;
   return typeof promoteUrl === "string" && promoteUrl.trim() ? promoteUrl : "";
 };
 
+const hasShareGuestAuthMarker = () => {
+  try {
+    return new URLSearchParams(window.location.search || "").get(SHARE_AUTH_GUEST_PARAM) === "1";
+  } catch (_) {
+    return false;
+  }
+};
+
+const clearShareGuestAuthMarker = () => {
+  if (!hasShareGuestAuthMarker()) return;
+  try {
+    const url = new URL(window.location.href);
+    url.searchParams.delete(SHARE_AUTH_GUEST_PARAM);
+    window.history.replaceState(window.history.state, "", `${url.pathname}${url.search}${url.hash}`);
+  } catch (_) {
+    // keep current URL if the History API is unavailable
+  }
+};
+
 const scheduleShareSessionPromotion = (shareToken) => {
   const silentPromoteUrl = startupSharePayload?.silentPromoteUrl;
-  if (!shareToken || viewerMode !== "guest" || runtimeSessionInfo.authenticated || typeof silentPromoteUrl !== "string" || !silentPromoteUrl.trim()) {
+  if (
+    !shareToken ||
+    hasShareGuestAuthMarker() ||
+    viewerMode !== "guest" ||
+    runtimeSessionInfo.authenticated ||
+    typeof silentPromoteUrl !== "string" ||
+    !silentPromoteUrl.trim()
+  ) {
     return;
   }
   let storageKey = "";
@@ -1810,6 +1837,7 @@ const applyRuntimeMode = () => {
 
 const initializeRuntimeMode = async () => {
   const shareToken = getShareTokenFromPath();
+  const shareGuestAuthMarker = hasShareGuestAuthMarker();
   if (shareToken) {
     viewerMode = "guest";
     inspectorCollapsed = true;
@@ -1849,7 +1877,8 @@ const initializeRuntimeMode = async () => {
   }
 
   applyRuntimeMode();
-  if (shareToken && viewerMode === "guest") {
+  clearShareGuestAuthMarker();
+  if (shareToken && viewerMode === "guest" && !shareGuestAuthMarker) {
     scheduleShareSessionPromotion(shareToken);
   }
 };
@@ -5182,13 +5211,13 @@ const applyViewerState = (state) => {
 };
 
 const initViewer = async () => {
-  // remove preload hidden style ASAP
+  await initializeRuntimeMode();
+
   const preloadHidden = document.getElementById("preload-hidden");
   if (preloadHidden) {
     preloadHidden.remove();
   }
   document.body.style.opacity = "1";
-  await initializeRuntimeMode();
 
   const sdk = await import(SDK_URL);
   sdkRef = sdk;
diff --git a/frontend/index.html b/frontend/index.html
index eb38494..5bc6142 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -13,6 +13,13 @@
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
   <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Manrope:wght@500;600;700&display=swap">
   <script src="/theme.js?v=3"></script>
+  <script>
+    (function () {
+      if (/^\/share\/[^/]+\/?$/.test(window.location.pathname || "")) {
+        document.documentElement.dataset.shareStartup = "guest";
+      }
+    }());
+  </script>
   <script>
     // Apply persisted theme ASAP to avoid initial flash of defaults
     (function () {
diff --git a/server/index.js b/server/index.js
index fed3d88..813a044 100644
--- a/server/index.js
+++ b/server/index.js
@@ -56,6 +56,7 @@ const BIM_SESSION_COOKIE = process.env.NODEDC_BIM_SESSION_COOKIE || "nodedc_bim_
 const BIM_SESSION_TTL_MS = Number(process.env.NODEDC_BIM_SESSION_TTL_MS || 12 * 60 * 60 * 1000);
 const BIM_COOKIE_SECURE = parseBooleanEnv(process.env.NODEDC_BIM_COOKIE_SECURE, false);
 const BIM_COOKIE_SAMESITE = process.env.NODEDC_BIM_COOKIE_SAMESITE || (BIM_COOKIE_SECURE ? "None" : "Lax");
+const SHARE_AUTH_GUEST_PARAM = "ndc_bim_guest";
 const CORS_ALLOWED_ORIGINS = String(process.env.NODEDC_BIM_ALLOWED_ORIGINS || "")
     .split(",")
     .map((origin) => origin.trim())
@@ -812,6 +813,14 @@ const buildLauncherLaunchUrl = (nextPath = "/") => {
     return launchUrl.toString();
 };
 
+const buildLauncherOptionalLaunchUrl = (nextPath = "/", fallbackPath = nextPath) => {
+    const launchPath = `/api/services/${encodeURIComponent(BIM_SERVICE_SLUG)}/optional-launch`;
+    const launchUrl = new URL(launchPath, LAUNCHER_BASE_URL);
+    launchUrl.searchParams.set("returnTo", sanitizeReturnTo(nextPath));
+    launchUrl.searchParams.set("fallbackTo", sanitizeReturnTo(fallbackPath, nextPath));
+    return launchUrl.toString();
+};
+
 const buildLauncherLoginUrl = (req, nextPath = "/") => {
     const loginUrl = new URL("/auth/login", LAUNCHER_BASE_URL);
     const launchUrl = new URL(buildLauncherLaunchUrl(nextPath));
@@ -822,16 +831,47 @@ const buildLauncherLoginUrl = (req, nextPath = "/") => {
 const buildSharePromotionUrls = (token) => {
     const safeToken = safeShareToken(token);
     if (!safeToken) {
-        return {promoteUrl: null, silentPromoteUrl: null};
+        return {promoteUrl: null, silentPromoteUrl: null, optionalPromoteUrl: null};
     }
     const sharePath = `/share/${encodeURIComponent(safeToken)}`;
+    const guestPath = `${sharePath}?${SHARE_AUTH_GUEST_PARAM}=1`;
     const bridgePath = `/auth/share-promoted?next=${encodeURIComponent(sharePath)}`;
     return {
         promoteUrl: buildLauncherLaunchUrl(sharePath),
-        silentPromoteUrl: buildLauncherLaunchUrl(bridgePath)
+        silentPromoteUrl: buildLauncherLaunchUrl(bridgePath),
+        optionalPromoteUrl: buildLauncherOptionalLaunchUrl(sharePath, guestPath)
     };
 };
 
+const shouldPromoteShareNavigation = (req, url) => {
+    if (!BIM_AUTH_REQUIRED || !INTERNAL_ACCESS_TOKEN || !isPublicSharePath(url)) {
+        return false;
+    }
+    if (req.method !== "GET" && req.method !== "HEAD") {
+        return false;
+    }
+    if (url.searchParams.get(SHARE_AUTH_GUEST_PARAM) === "1") {
+        return false;
+    }
+    if (getCurrentBimSession(req)) {
+        return false;
+    }
+    const accept = typeof req.headers.accept === "string" ? req.headers.accept : "";
+    return accept.includes("text/html") || accept.includes("*/*") || !accept;
+};
+
+const redirectShareNavigationToPromotion = (res, token) => {
+    const urls = buildSharePromotionUrls(token);
+    if (!urls.optionalPromoteUrl) {
+        return false;
+    }
+    res.statusCode = 302;
+    res.setHeader("Location", urls.optionalPromoteUrl);
+    res.setHeader("Cache-Control", "no-store");
+    res.end();
+    return true;
+};
+
 const sendAuthRequired = (req, res, url) => {
     const nextPath = sanitizeReturnTo(`${url.pathname}${url.search || ""}`);
     if (!INTERNAL_ACCESS_TOKEN) {
@@ -1777,6 +1817,7 @@ const handleGetShare = async (req, res, token) => {
         canShare: !BIM_AUTH_REQUIRED || authenticated,
         promoteUrl: promotionUrls.promoteUrl,
         silentPromoteUrl: promotionUrls.silentPromoteUrl,
+        optionalPromoteUrl: promotionUrls.optionalPromoteUrl,
         user: session?.user || null,
         viewer: {
             src: publicUrlForUploadSrc(req, share.src),
@@ -3267,6 +3308,10 @@ const requestHandler = async (req, res) => {
         return handleGetShare(req, res, shareTokenFromPath(url.pathname));
     }
 
+    if (shouldPromoteShareNavigation(req, url) && redirectShareNavigationToPromotion(res, shareTokenFromPath(url.pathname))) {
+        return;
+    }
+
     if (ensureBimSessionForRequest(req, res, url)) {
         return;
     }