Named for the Red Fort, hedged for the Taj
Ustad Ahmad Lahori came from Lahore — his family were Timurid architects, originally from Herat, who kept the epithet "Lahori" even after the family moved to Delhi (Wikipedia, 'Life'). Shah Jahan later gave him the title Nadir-ul-Asar, "wonder of the age" (Wikipedia, 'Life'). What the record — built mainly from his son Lutfullah Muhandis's writing — says about him is precise for one building and hedged for the other: "He is credited with designing the Red Fort in Delhi and may have also been responsible for the design of the Taj Mahal in Agra, along with Mir Abd-ul Karim and Makramat Khan" (Wikipedia, lede). For the fort alone the hedge drops: "Ustad Ahmad Lahori laid the foundations of the Red Fort at Delhi, which was built between 1638 and 1648" (Wikipedia, 'Career'), and "the building of the city, including the Red Fort, was complete by 1648" (Wikipedia, 'Career'). He died in the city he had just finished building: "He died in 1649, before the foundations of mosque were laid" (Wikipedia, 'Career').
A credit the sources split three ways
This archive records the Taj Mahal attribution as a claim, not a fact — the same page naming Ahmad Lahori also names two other men for the same building: "Mir Abd-ul Karim counted as the favourite architect of the previous emperor, Jahangir, and is mentioned as a supervisor, together with Makramat Khan, for the construction of the Taj Mahal" (Wikipedia, 'Career'). Its own cross-reference points to a fourth name again, unadjudicated: "Ustad Isa, another architect of the Taj Mahal" (Wikipedia, 'See also'). What survives a century on is a name attached to the empire's most famous building by inference from his son's memoir, never by an inscription or a contract in his own hand — the fort is undisputedly his; the tomb is a name among several.