diff --git a/shared/components/rstack-identity.ts b/shared/components/rstack-identity.ts
index b465d53b..c9fcc26a 100644
--- a/shared/components/rstack-identity.ts
+++ b/shared/components/rstack-identity.ts
@@ -283,6 +283,16 @@ function getAllKnownUsernames(): string[] {
 		for (const a of accounts) add(a.username);
 	} catch { /* ignore */ }
 
+	// 5. Cross-subdomain hint cookie on .rspace.online — fresh subdomain has no
+	//    localStorage yet; login-button writes username here on successful auth.
+	try {
+		const match = document.cookie.split(/;\s*/).find(c => c.startsWith("rspace_hint="));
+		if (match) {
+			const parsed = JSON.parse(decodeURIComponent(match.slice("rspace_hint=".length)));
+			if (parsed?.username) add(parsed.username);
+		}
+	} catch { /* ignore */ }
+
 	return result;
 }
 
diff --git a/src/encryptid/ui/login-button.ts b/src/encryptid/ui/login-button.ts
index 577d34ef..be07ae58 100644
--- a/src/encryptid/ui/login-button.ts
+++ b/src/encryptid/ui/login-button.ts
@@ -21,10 +21,17 @@ import { getVaultManager, resetVaultManager } from '../vault';
 import { syncWalletsOnLogin } from '../wallet-sync';
 
 // ============================================================================
-// KNOWN ACCOUNTS (localStorage)
+// KNOWN ACCOUNTS (localStorage + .rspace.online hint cookie)
 // ============================================================================
+//
+// Cross-subdomain UX hint ONLY. The cookie carries username + optional
+// displayName so the passkey button can say "Sign in as jeff" on a freshly
+// visited subdomain. No session/JWT/credential ever crosses origins — the
+// actual auth is still a WebAuthn ceremony bound to RP ID rspace.online.
 
 const KNOWN_ACCOUNTS_KEY = 'encryptid-known-accounts';
+const HINT_COOKIE = 'rspace_hint';
+const HINT_TTL_SECONDS = 60 * 60 * 24 * 30; // 30 days
 const ENCRYPTID_AUTH = ''; // same-origin — avoids cross-origin issues on Safari
 
 interface KnownAccount {
@@ -32,21 +39,74 @@ interface KnownAccount {
   displayName?: string;
 }
 
-function getKnownAccounts(): KnownAccount[] {
+function parentDomainForCookie(): string | null {
+  if (typeof location === 'undefined') return null;
+  const host = location.hostname;
+  // Share across *.rspace.online. Localhost / other hosts → per-origin only.
+  if (host === 'rspace.online' || host.endsWith('.rspace.online')) return 'rspace.online';
+  return null;
+}
+
+function readHintCookie(): KnownAccount | null {
+  if (typeof document === 'undefined') return null;
+  const match = document.cookie.split(/;\s*/).find(c => c.startsWith(`${HINT_COOKIE}=`));
+  if (!match) return null;
   try {
-    return JSON.parse(localStorage.getItem(KNOWN_ACCOUNTS_KEY) || '[]');
-  } catch { return []; }
+    const raw = decodeURIComponent(match.slice(HINT_COOKIE.length + 1));
+    const parsed = JSON.parse(raw);
+    if (parsed && typeof parsed.username === 'string') {
+      return { username: parsed.username, displayName: parsed.displayName };
+    }
+  } catch { /* malformed — ignore */ }
+  return null;
+}
+
+function writeHintCookie(account: KnownAccount): void {
+  const domain = parentDomainForCookie();
+  if (!domain) return; // dev / non-rspace host: skip cross-origin hint
+  const value = encodeURIComponent(JSON.stringify({
+    username: account.username,
+    ...(account.displayName ? { displayName: account.displayName } : {}),
+  }));
+  document.cookie = `${HINT_COOKIE}=${value}; Domain=.${domain}; Path=/; Max-Age=${HINT_TTL_SECONDS}; Secure; SameSite=Lax`;
+}
+
+function clearHintCookie(): void {
+  const domain = parentDomainForCookie();
+  if (!domain) return;
+  document.cookie = `${HINT_COOKIE}=; Domain=.${domain}; Path=/; Max-Age=0; Secure; SameSite=Lax`;
+}
+
+function getKnownAccounts(): KnownAccount[] {
+  let accounts: KnownAccount[] = [];
+  try {
+    accounts = JSON.parse(localStorage.getItem(KNOWN_ACCOUNTS_KEY) || '[]');
+  } catch { accounts = []; }
+
+  // Fresh subdomain → localStorage empty; fall back to cross-subdomain hint.
+  if (accounts.length === 0) {
+    const hint = readHintCookie();
+    if (hint) accounts = [hint];
+  }
+  return accounts;
 }
 
 function addKnownAccount(account: KnownAccount): void {
-  const accounts = getKnownAccounts().filter(a => a.username !== account.username);
+  const existing: KnownAccount[] = (() => {
+    try { return JSON.parse(localStorage.getItem(KNOWN_ACCOUNTS_KEY) || '[]'); }
+    catch { return []; }
+  })();
+  const accounts = existing.filter(a => a.username !== account.username);
   accounts.unshift(account);  // most recent first
   localStorage.setItem(KNOWN_ACCOUNTS_KEY, JSON.stringify(accounts));
+  writeHintCookie(account);
 }
 
 function removeKnownAccount(username: string): void {
   const accounts = getKnownAccounts().filter(a => a.username !== username);
   localStorage.setItem(KNOWN_ACCOUNTS_KEY, JSON.stringify(accounts));
+  const hint = readHintCookie();
+  if (hint && hint.username === username) clearHintCookie();
 }
 
 function escapeHtml(s: string): string {