What is the authenticity of the hadith: "There is no Mahdi except Jesus son of Mary," and what is the response to those who claim it is weak? And does it contradict the fact that the Mahdi does not live at the same time as Jesus, peace be upon them both?

The narrations about the Awaited Mahdi have reached the level of Tawatur (mass transmission), and this was mentioned by Hadith imams such as Ibn Hajar, Al-Sakhawi, and Al-Saffarini. Al-Shawkani stated that fifty hadiths and twenty-eight narrations about the Mahdi have been reported, all of which reached the level of Tawatur. As for the hadith, "There is no Mahdi except 'Isa ibn Maryam," its chain of narration is weak. Shaykh al-Islam deemed it weak, and Al-Shawkani considered it among the fabricated hadiths. Ibn al-Qayyim also explained that Muhammad ibn Khalid al-Jundi was the only one who narrated it, and he is unknown, and Aban ibn Abi Ayyash is matruk (abandoned), and the hadith is munqati' (broken). He further stated that the hadiths reported about the advent of the Mahdi have more authentic chains of narration.