Detainee Mohamedou Ould Slahi (of the film's title) has stated that his treatment in Guantanamo was much more brutal and inhumane than portrayed in this movie.
Civil rights lawyer Nancy Hollander (played here by Jodie Foster) has stated in an interview that she gasped when she first walked onto the 'Guantanamo' set for this movie, it was so eerily like the very place she'd conducted so many of her interviews with actual detainee Mohamedou Ould Slahi. Hollander also fielded numerous phone calls from the production team in their pursuit of accuracy for the interview line of questioning.
In all of the prison scenes, none of the guards are wearing name tapes standard elements of all Army uniforms. At first glance, this may seem like a common Hollywood military uniform error, but it is actually realistic. Before new uniforms came out, guards and other personnel that had direct contact with the detainees were to cover their name tapes with duct tape. After newer uniforms were issued, the name tapes were Velcro and easier to remove to hide the identity of the guards and to prevent the detainees from learning their identities.
Top-billed French actor Tahar Rahim - playing detained terrorist suspect Mohamedou Ould Slahi - ironically portrayed real-life FBI agent Ali Soufan charged with investigating the 9/11 terrorist threats in The Looming Tower (2018).