“Bias and discrimination have no place on Airbnb, and we have zero tolerance for them,” Mr. Chesky wrote that accompanied the report. “Unfortunately, we have been slow to address these problems, and for this I am sorry.”

That reputation was stained in December, when Harvard University researchers that concluded it was harder for guests with African-American-sounding names to rent rooms through the site. Several Airbnb users have since shared stories on social media saying they were denied a rental because of their race. In May, an African-American Airbnb user against the company, seeking class-action status, saying he had been denied a place to stay because of his race.

On Thursday, Airbnb took its most forceful actions yet to combat discrimination. It told its rental hosts that they needed to agree to a “community commitment” starting on Nov. 1 and that they must hew to a new nondiscrimination policy. The company also said that it would try to reduce the prominence of user photographs, which indicate race and gender, and that it would accelerate the use of instant bookings, which lets renters book places immediately without host approval.

Airbnb, the short-term rental website, has moved quickly to tamp down the controversy. It embarked on a top-to-bottom review of how discrimination might creep into the site. It hired prominent advisers, including Eric H. Holder Jr., to help formulate anti-bias policies. And Brian Chesky, Airbnb’s chief executive, has repeatedly said that the company needed to do better in dealing with the issue.

Airbnb’s response is designed to quiet the questions over discrimination that have threatened to cloud growth of the company, which is based in San Francisco. Founded in 2008, Airbnb has spread to more than 34,000 cities and 191 countries, where people increasingly use the service as a replacement for hotels. The continued expansion of the privately held start-up, which is valued at $25 billion, depends partly on the idea that Airbnb can be a global company, providing a broad range of people with places to stay when they travel.