Ronald Reagan famously said of a nuclear agreement with the then Soviet Union that it was based on an attitude of 'trust, but verify'. Perhaps slightly contradictory but very realpolitik nonetheless. Thirty years later, Secretary of State John Kerry's admission that the latest Syrian cessation of hostilities agreement is 'not built on trust' tells you perhaps all you need to know about its likelihood of long-term success.
There are holes in the agreement that you could drive a truck through. Among other things, it requires the practical assent of the Assad regime, the Iranian government and the pro-Assad militias that are largely supported by Iran. There is no enforcement mechanism in place for those who breach the conditions of the agreement. It requires seven days of ceasefire (excluding actions against Islamic State) before a US-Russian coordination centre is established to target agreed locations of Jabhat Fatah al-Sham. And it requires non-proscribed armed groups to dissociate themselves from Jabhat Fatah al-Sham (there are already reports that the islamist group Ahrar al-Sham has come out against the plan). Meeting any one of these conditions is difficult. All of them together is much more so but if there was any easy way out of this impasse it would have been tried long ago. Something needs to be done but in an environment as complex an environment as Syria, expectations must remain low.
The parties have taken advantage of the holy Muslim holiday of Eid al-Adha and a 'natural' lessening of conflict to use this as the starting point for the ceasefire. If it is to have any chance of success, it will be essential that the humanitarian aid that is supposed to flow during this seven days is both able to be delivered and allowed to be delivered. In the absence of any enforcement mechanism, the cost of restarting hostilities needs to be raised in order to dissuade parties from doing so. The loss of humanitarian assistance to the civilian population would be a tangible, albeit limited cost.
Of course, as with most ceasefire agreements, groups will likely seek military gains right up to the hour of implementation, knowing that their opponents will use the ceasefire to regroup, replenish and re-equip in anticipation of renewed fighting once the ceasefire breaks down. Such is the nature of these things. And the reaction from the concerned groups is what we would have expected. Outright rejection from Jabhat Fatah al-Sham, caution from the Higher Negotiating Committee who wanted to see details of the enforcement mechanism (which doesn't appear to exist), and scepticism from elements of the Free Syrian Army.