Look at it from a business, rather than engineering perspective. Forget the achievability of the 100% target for a moment -- what target can you realistically a

Look at it from a business, rather than engineering perspective. Forget the achievability of the 100% target for a moment -- what target can you realistically achieve? Then, what does the contract say the remedy is for breach? As long as the remedy is not huge and -- this is very important -- is clearly quantifiable, it may be just fine to enter into such an agreement. You need the remedy to be clearly quantifiable (X dollars per Y minutes, for example) because otherwise you create an opportunity for dispute when the inevitable occurs and you breach. Resolving such a dispute could very well cost more than the remedy itself, even in the worst case. From an ethical standpoint, I would only enter into such an agreement with an understanding that "while we agree that it makes sense for you to request that target, we think realistically that we'll be closer to 99.9% (or whatever you truly believe)". Entering into an agreement with a 100% uptime clause is different from setting an expectation that uptime will actually be 100%.