In general, one innovative aspect of the subject matter described in this specification can be embodied in methods that include the actions of receiving an input method editor (IME) server request, the IME server request including one or more tokens and requesting that an IME server be instantiated, the IME server executing one or more IME functions based on a key event sent from an IME client, wherein the IME server is a stateful server that stores both requests and responses of a communication session between the IME server and the IME client, determining that the IME server can be instantiated in a restrictive environment based on the one or more tokens, and instantiating the IME server in the restrictive environment. Other embodiments of this aspect include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices.