A diet of bread, water and peanut butter. A laptop with no internet connection. And intermittent access to millions of pages of digital evidence.

Sam Bankman-Fried, the 31-year-old cryptocurrency mogul, has spent nearly a month at the Metropolitan Detention Center in Brooklyn since a federal judge revoked his bail last month. As Mr. Bankman-Fried prepares for a fraud trial on Oct. 2 over the collapse of his crypto exchange, FTX, his lawyers have offered a picture of the conditions he has faced at the jail — a far cry from the Bahamas penthouse he once shared with other billionaire executives.

On Tuesday, Mr. Bankman-Fried’s lawyers and the prosecutors bringing the case against him are scheduled to submit a joint court filing discussing measures to help him prepare for his trial from the jail. The defense lawyers have argued in court filings that the jail conditions, especially a lack of internet service, are preventing Mr. Bankman-Fried from working on his case, while prosecutors have contended that they have made accommodations to help him.

A representative for Mr. Bankman-Fried declined to comment. A spokesman for the Federal Bureau of Prisons, which oversees the Brooklyn facility, declined to comment on Mr. Bankman-Fried’s conditions but said adults in custody at M.D.C. have “access to health care, telephones, a law library for legal research, hot meals, and they reside in certified environmental conditions.”

FTX collapsed in November, a symbol of crypto hubris gone awry. Since then, Mr. Bankman-Fried has spent months in relative comfort. After his arrest on fraud charges in December, he pleaded not guilty and was granted bail, allowing him to live in his parents’ home in Palo Alto, Calif. He was able to play video games and watch sports, while meeting with reporters and working with lawyers to construct a defense.

Last month, the judge overseeing his case, Lewis A. Kaplan, revoked Mr. Bankman-Fried’s bail after ruling that the FTX founder had twice tried to interfere with witnesses in the case. Mr. Bankman-Fried was taken from the courthouse to M.D.C., a jail that has struggled with staffing shortages, freezing conditions and other problems.

Mr. Bankman-Fried’s lawyers have filed an appeal seeking to have his bail reinstated. At a hearing last month, Mark Cohen, one of his lawyers, said the jail was not accommodating Mr. Bankman-Fried’s vegan diet, forcing him to subsist on bread, water and peanut butter.

“Notwithstanding multiple requests for a vegan diet, he continues to be served a flesh diet,” Mr. Cohen said in court.

Mr. Cohen also asked Judge Kaplan in a filing to give Mr. Bankman-Fried access to two medications that have been prescribed to him — Emsam, a transdermal patch that treats depression, and the A.D.H.D. medication Adderall. Mr. Bankman-Fried had brought only a few days’ worth of the medications with him to the jail, Mr. Cohen said.

Recently, most of the back and forth in court has focused on Mr. Bankman-Fried’s access to the millions of pages of digital evidence in the case.

In filings last month, his lawyers argued that M.D.C.’s conditions have made it virtually impossible for him to prepare for the trial. Mr. Bankman-Fried has access to an offline laptop that he can use during visiting hours, with his lawyers present, the filings said. But he cannot reach a key database of materials that is available only via the internet.

Twice a week, Mr. Bankman-Fried is allowed to leave the jail and meet with his lawyers at the federal courthouse in Manhattan, according to the filings. But an internet-enabled laptop he was given for those meetings has limited battery life, and Mr. Bankman-Fried has not been permitted an extension cord to charge it, the lawyers said. The internet connection has also been shaky.

“Time and again the government’s promises around providing him bona fide access to discovery have proven empty,” Mr. Bankman-Fried’s lawyers wrote in a filing.

In their own filing, the prosecutors said the Bureau of Prisons’ “nationwide security policies prohibit the use of any internet-enabled electronic device within the prison.”

They also argued that they had taken steps to help Mr. Bankman-Fried access evidence. During meetings at the courthouse, they said, his defense lawyers can communicate with him “through a glass panel, which permits counsel to review the defendant’s laptop in real time with the defendant’s consultation.”

The overall conditions at the M.D.C., which houses more than 1,500 male and female inmates in traditional cells and dormitory-style rooms, have long been a source of concern. When he revoked Mr. Bankman-Fried’s bail, Judge Kaplan acknowledged that the M.D.C. was “not on anyone’s list of five-star facilities.”

In February 2019, M.D.C. inmates endured subfreezing temperatures when a nearly weeklong power outage left the jail with limited heat and electricity. Last year, the National Association of Criminal Defense Lawyers issued a statement that said M.D.C.’s conditions were “inhumane” and noted that it had “the highest number of Covid-19 positive detainees in the nation” during the pandemic.

Ghislaine Maxwell, a former associate of the disgraced financier Jeffrey Epstein, frequently complained about the jail’s harsh living conditions when she was detained there before her trial on sex trafficking charges. She described it as “a living hell.”

The Bureau of Prisons spokesman said the M.D.C. provides “essential medical, dental and mental health services” and follows protocols designed to ensure appropriate temperatures for winter and summer.

Before they started working for Mr. Bankman-Fried, Mr. Cohen and his partner, Christian Everdell, helped represent Ms. Maxwell. While she was in the M.D.C., they argued that Ms. Maxwell needed access to a laptop to review discovery materials on weekends and holidays.

A judge granted that request over an objection from the Bureau of Prisons.