Cassandra Clare took to Tumblr to address some important topics that have been brought up to her, including her involvement in The Mortal Instruments: City of Bones.
To answer the original question: I did not have control over the film. I’ve said that a thousand times, and I know it’s confusing, because I have thanked the moviemakers for letting me have input. For listening to me at all. I have talked about having influence and being asked questions and being allowed to contribute and collaborate. But those things are not control. Control in Hollywood over a film is actually a legal issue. If you have a certain amount of control over decision-making, or if you wrote a certain amount of the script, you must legally be credited. There is a pretty easy way to tell if an author has control over a movie: look for a producer credit or a co-writing credit attached to their name. If there isn’t one*, if all it says is “based on the books by”, then you can assume no, they didn’t have control over the movie. In the end, it isn’t a matter of he-said-she-said, or sound bytes in interviews, or hastily dashed off tweets, but what’s in black and white in a contract and on the screen. When I said I did not have control, that is a contractual fact, and that is what I mean when I say it.
Cassie addressed the confusion surrounding her supposedly “changing her story” about having a lot of influence to having none. Like she’s always said from the beginning, she’s had some involvement, including casting, props, submitting script notes (which the studio doesn’t have to use), etc., but she’s never had control. Movies and books are two different things.
Nothing I have said since the movie’s release has been any different from exactly what I have said for the past year and a half. (And indeed, it is kind of weird even to imagine why I would about-face, since it would only be completely harmful to me and my career — it wouldn’t be so much a move of selfishness as total stupidity. The City of Ashes movie is still on the cards: why would I want to alienate the people making it? Why would I want to annoy them considering they also own Magisterium and I’m writing a screenplay for them right now? That would be dumb and bad for me on a million levels. There’s just no advantage to it. None.)