diff --git a/repeater/handler_helpers/acl.py b/repeater/handler_helpers/acl.py
index e2ad745..d863a9d 100644
--- a/repeater/handler_helpers/acl.py
+++ b/repeater/handler_helpers/acl.py
@@ -106,13 +106,27 @@ class ACL:
         if not password:
             client = self.clients.get(pub_key)
             if client is None:
-                if self.allow_read_only:
-                    logger.info("Blank password, allowing read-only guest access")
-                    return True, PERM_ACL_GUEST
-                else:
+                if not self.allow_read_only:
                     logger.info("Blank password, sender not in ACL and read-only disabled")
                     return False, 0
-            logger.info(f"ACL-based login for {pub_key[:6].hex()}...")
+                if len(self.clients) >= self.max_clients:
+                    logger.warning("ACL full, cannot add read-only guest")
+                    return False, 0
+                # Read-only guests must still land in the ACL: the room server's
+                # text handler and sync loop only see ACL members, so an absent
+                # entry means the client's posts are silently dropped (never
+                # decrypted or ACKed) and posts are never pushed to it.
+                client = ClientInfo(client_identity, PERM_ACL_GUEST)
+                self.clients[pub_key] = client
+                logger.info(f"Blank password, added read-only guest {pub_key[:6].hex()}...")
+            else:
+                logger.info(f"ACL-based login for {pub_key[:6].hex()}...")
+            client.last_activity = int(time.time())
+            client.last_login_success = int(time.time())
+            client.shared_secret = shared_secret
+            if sync_since is not None:
+                client.sync_since = sync_since
+                logger.debug(f"Stored sync_since={sync_since} for client")
             return True, client.permissions
 
         permissions = 0
diff --git a/tests/test_handler_helpers_acl_advert.py b/tests/test_handler_helpers_acl_advert.py
index fecc80f..5e732f9 100644
--- a/tests/test_handler_helpers_acl_advert.py
+++ b/tests/test_handler_helpers_acl_advert.py
@@ -37,10 +37,42 @@ def test_acl_blank_password_guest_rules_and_room_server_password_requirements():
         shared_secret=b"secret",
         password="",
         timestamp=10,
+        sync_since=42,
     )
     assert ok is True
     assert perms == PERM_ACL_GUEST
 
+    # The read-only guest must land in the ACL with its shared secret: the
+    # room server's text handler and sync loop only see ACL members, so an
+    # absent entry means posts are dropped un-ACKed and pushes never happen.
+    guest = acl.get_client(b"A" * 32)
+    assert guest is not None
+    assert guest.is_guest() is True
+    assert guest.shared_secret == b"secret"
+    assert guest.sync_since == 42
+
+    # A repeat blank-password login reuses the existing entry.
+    ok_again, perms_again = acl.authenticate_client(
+        client_identity=identity,
+        shared_secret=b"secret",
+        password="",
+        timestamp=11,
+    )
+    assert ok_again is True
+    assert perms_again == PERM_ACL_GUEST
+    assert acl.get_num_clients() == 1
+
+    # A full ACL rejects new blank-password guests.
+    acl_full = ACL(max_clients=0, allow_read_only=True)
+    full_ok, full_perms = acl_full.authenticate_client(
+        client_identity=identity,
+        shared_secret=b"secret",
+        password="",
+        timestamp=10,
+    )
+    assert full_ok is False
+    assert full_perms == 0
+
     acl_ro_disabled = ACL(allow_read_only=False)
     ok2, perms2 = acl_ro_disabled.authenticate_client(
         client_identity=identity,