Micro-blogging outfit Twitter has commited itself not use any patents derived from employee inventions in offensive lawsuits without the inventor's permission.

The move is highly unusual in the technology industry, where everyone is using the US's somewhat rubbish patents system to shut down rivals. Twitter has written up a draft of what it calls the "Innovator's Patent Agreement," or IPA, which encourages its developers to invent without the fear that their inventions will be used for trolling.

Such promises have been made in the past. Yahoo promised that it would use patents for defensive purposes only, only to use its  patents to sue Facebook. Twitter will put its own promise in writing, theoretically making it a lot more binding than any spoken commitments.