The boy, who hid a hammer and knife in his school bag and wrote a “martyrdom letter” after being radicalised online, was convicted of preparing for terrorist acts following a nine-day trial at Birmingham Crown Court.
In total the teenager was convicted of five terror offences, which he had denied.
At the start of the boy’s trial it emerged that he had written a note apparently aimed for distribution after his death reading: “I am a soldier of the Islamic State and I have attacked Cardiff today because your government keep on bombing targets in Syria and Iraq. There will be more attacks in the future.”
The note – found with the gutting knife and claw hammer – also featured bullet points including “run down the non-believers with a car” and “strike the infidels, who oppose Allah, in the neck”.
A court heard the boy, who is from a white British background, may have also been looking to target Cardiff Castle.
Prosecutors said the boy uploaded an image of the castle to social media with a caption, “Cardiff are you prepared?”, alongside a picture of a jeep, knife and a bomb.
Another image was captioned: “Oh my Islamic state brothers you are the role models of these worlds. ‘The attack on Cardiff will be deadly.”