بسم الله الرحمن الرحيم
As you may know already, Ahmad al-Hassan claims, that he is '13th Imam' & proof of Allah from Ahl al-Bayt (as), let's refer his claims to Allah (swt) i.e. al-hadith al-Qudsi, and see what Allah (swt) says: 12th Imam (as) wrote in his Tawqee [i.e. letter]:
الإحتجاج: وعن محمد بن عبد الله بن جعفر الحميري أنه قال: خرج توقيع من الناحية المقدسة - حرسها الله تعالى، بعد المسائل: [...] فقولوا كما قال الله تعالى " سلام على آل يس، [...] أشهدك يا مولاي أني أشهد أن لا إله إلا الله وحده لا شريك له وأن محمدا عبده ورسوله، لا حبيب إلا هو وأهله، وأشهدك أن أمير المؤمنين حجته، والحسن حجته، والحسين حجته، وعلي بن الحسين حجته، ومحمد بن علي حجته، وجعفر بن محمد حجته، وموسى بن جعفر حجته، وعلي بن موسى حجته ومحمد بن علي حجته، وعلي بن محمد حجته، والحسن بن علي حجته.´وأشهد أنك حجة الله، أنتم الأول والآخر، وأن رجعتكم حق لا ريب فيها، [...] فنفسي مؤمنة بالله وحده لا شريك له وبرسوله وبأمير المؤمنين وبكم يا مولاي أولكم وآخركم
[...] ''Say as Allah (Exalted be He) says: ''Peace to you, Oh Alee Yasin [i.e. Family of Muhammad]!'' [...]
''I take you as witness, Oh my master, that I bear witness, there is no god but Allah, One, without associates, and that Muammad is His servant and messenger. There is no one more beloved by Allah than him and his household. I take you as witness, O my master, that I bear witness, that Amīr al-Mu'minīn Ali, is His Hujjaty [i.e. proof of Allah], al-Hassan is His Hujjaty [i.e. proof of Allah], al-Hussain is His Hujjaty [i.e. proof of Allah], `Ali the son of al-Hussain is His Hujjaty [i.e. proof of Allah], Muammad the son of `Ali is His Hujjaty [i.e. proof of Allah], Ja`far the son of Muammad is His Hujjaty [i.e. proof of Allah], Musa the son of Ja`far is His Hujjaty [i.e. proof of Allah], `Ali the son of Musa is His Hujjaty [i.e. proof of Allah], Muammad the son of `Ali is His Hujjaty [i.e. proof of Allah], `Ali the son of Muammad is His Hujjaty [i.e. proof of Allah], and al-Hassan the son of `Ali is His Hujjaty [i.e. proof of Allah]. I also bear witness that you [i.e. 12th Imam] is verily the Hujjaty [i.e. proof] of Allah. You all are the first and the last. And [I bear witness] that your [promised] Return is undoubtedly true.'' [...]
Oh, My master! Died, who denied you [i.e. Ahl al-Bayt], and has found happiness one who obeyed you [i.e. Ahl al-Bayt]! So, bear witness to everything, that I have witnessed in front of you! I am, indeed, in friendship with you [i.e. Ahl al-Bayt] [...]
My soul is thus believing in Allah; One and Only Lord and having no associate, and also in His Messenger, in the Amīr al-Mu'minīn, and in you all [i.e. Imams], O my master, from the beginning to the end. [...]
Source: Bihār al-Anwār by Allamah al-Majlisī, Volume 53, page 171-172, Hadith# 5
 Note, that only the important parts of the letter has been translated.
I say: As we can see, Allah (swt) wants us to believe and testify only for 12 Imam (as). As he himself does, because, there are only 12 divinely appointed Mahdis / Imams. So, the claims of Ahmad al-Hassan, that he is divinely appointed Mahdi / Imam are false and should be rejected.
(Click on the image below - in order to enlarge ;)