Danny Ray Ferguson seeks judicial review of a denial of Social Security benefits. The district court affirmed the denial of benefits, and Ferguson timely filed this appeal. We have jurisdiction under 28 U.S.C. § 1291 . We review de novo the district court’s order. Smartt v. Kijakazi, 53 F.4th 489 , 494 (9th Cir. 2022) (citation omitted). The only issue presented is whether the Administrative Law Judge (“ALJ”) erred in rejecting Ferguson’s testimony regarding the severity of his headaches. “When an ALJ determines that a claimant for Social Security benefits is not malingering and has provided objective medical evidence of an underlying impairment which might reasonably produce the pain or other symptoms [he] alleges, the ALJ may reject the claimant’s testimony about the severity of those symptoms only by providing specific, clear, and convincing reasons for doing so.” Brown-Hunter v. Colvin, 806. F.3d 487, 488–89 (9th Cir. 2015). Here, the ALJ failed to provide clear and convincing reasons for rejecting Ferguson’s symptom testimony regarding his headaches. Further, the district court erred by affirming for reasons other than those asserted by the ALJ. Accordingly, we reverse and remand. I. Background To determine whether a claimant is disabled, the ALJ follows a five-step process. 20 C.F.R. § 416.920 (a)(4). At step two of the disability analysis, the ALJ must determine whether the claimant has any “severe medically determinable” impairments. 20 C.F.R. § 416.920 (a)(4)(ii). A “severe impairment” is one that significantly limits a claimant’s ability to perform basic work activities. 20 C.F.R.  FERGUSON V. O’MALLEY 5 § 416.920(c). After reviewing Ferguson’s testimony and the medical record, the ALJ determined that Ferguson has multiple severe medically determinable impairments, including a history of epilepsy, Arnold-Chiari malformation type I, headaches, a depressive disorder, borderline intellectual functioning, a specified learning disorder with impairment in reading and math, and a mild neurocognitive disorder. The Arnold-Chiari malformation diagnosis means that part of Ferguson’s brain bulges into his vertebral canal. In 2015, Ferguson underwent surgery to treat the malformation. At his hearing before the ALJ, Ferguson presented medical records that demonstrate he has experienced headaches since he was a child, including prolonged weekly headaches that could last up to a day or two. Ferguson also testified about the symptoms he experienced before and after his 2015 surgery. Before the surgery, he experienced seizures, passing out, and “incredible” pressure in both the front and back of his head. After the surgery, Ferguson enjoyed relief from those symptoms for about two to three months. But then, in his words, “it all rushed back.” Ferguson testified that he currently experiences headaches that last up to two days, two or three times a week, and that the headaches cause pressure so intense that he cannot leave his room. In the past, his headaches were triggered by physical labor, but now, there is no identifiable trigger. A claimant’s subjective symptoms, if credited, are relevant to the determination of a claimant’s residual function capacity (“RFC”), which is “the most [one] can still do despite [one’s] limitations.” 20 C.F.R. § 416.945 (a)(1). The RFC is used at step four to determine if a claimant can do past relevant work and at step five to determine if a claimant can adjust to other work. 20 C.F.R. § 416.920 (e). 6 FERGUSON V. O’MALLEY In this case, the ALJ discounted Ferguson’s testimony regarding the severity and frequency of his headaches when assessing his RFC. 1 In turn, based on that RFC, the ALJ determined at step five that there were sufficient jobs in the national economy that Ferguson could perform and, therefore, that Ferguson was not disabled under the Social Security Act. The district court affi