What is the test for a prison officer to be liable for failing to prevent harm to an inmate?

First, objectively, the alleged deprivation must be "sufficiently serious" and where a failure to prevent harm is alleged, "the inmate must show that he is incarcerated under conditions posing a substantial risk of serious harm." Id. at 834 (quoting Rhodes v. Chapman, 452 U.S. 337, 349 (1981)).

Second, subjectively, the prison official must "know of and disregard an excessive risk to inmate health or safety." Id. at 837; Anderson v. Cty. of Kern, 45 F.3d 1310, 1313 (9th Cir. 1995). A prison official must "be aware of facts from which the inference could be drawn that a substantial risk of serious harm exists, and . . . must also draw the inference." Farmer, 511 U.S. at 837. Liability may follow only if a prison official "knows that inmates face a substantial risk of serious harm and disregards that risk by failing to take reasonable measures to abate it." Id. at 847.