The phrase 'flakes out' can have a variety of antonyms depending on the context. For example, the opposite of flaking out when it comes to attending an event could be showing up or being present. If we refer to someone who is reliable and does not cancel plans, then the antonyms for flakes out could be dependable, consistent, or committed. Another antonym could be staying committed, as opposed to backing out of a commitment. Alternatively, if we take 'flakes out' to mean giving up on something, then the antonyms might include persevering, persisting, or seeing a task through to completion.