Fault attacks present unique safety and security challenges that require dedicated countermeasures, even for bug-free programs. Models of these complex attacks are made workable by approximating their effects to a suitable level of abstraction. The common practice of targeting the Instruction Set Architecture (ISA) level isn't ideal because it discards important micro-architectural information, leading to weaker security guarantees. Conversely, including microarchitectural details makes countermeasures harder to model and reason about, creating a new challenge in validating and trusting protections. We show that a semantic approach to modeling faults makes micro-architectural models workable, and enables precise cooperation between software and hardware in the design of countermeasures. We demonstrate the approach by designing and implementing a compiler/hardware countermeasure, which protects against a state-of-the-art pipeline fetch attack that generalizes multi-fault instruction skips. Crucially, we provide a formal security proof that guarantees faults are detected by the end of every basic block. This result shows that carefully embracing the complexity of lowlevel systems enables finer, more secure countermeasures.