Now that Iran has dropped its demand to keep 20 centrifuges while freezing the rest of its uranium-enrichment programme, the way looks clear for a six-month freeze negotiated by European diplomats to begin. This at least buys time for a more permanent deal to be struck.
But for a permanent agreement to hold, flaws in the temporary pact must be fixed. Sanctions for breaking pledges have to be spelled out, including referral to the United Nations Security Council, and United States endorsement of the agreement will have to be more than just passive.
Iran's record on these matters is not a pretty one and there is no reason to give it the benefit of the doubt, or to leave potential loopholes. An earlier deal, similar to the current one, made with the European Union fell apart after Tehran continued with uranium-enrichment programmes. Expediency won the day in securing the latest deal, as the new agreement offers political and economic benefits but includes no negative consequences for breaking any of the covenants. There is only the unspoken threat of having the matter sent to the security council to keep the Iranians in line.
The wording of the International Atomic Energy Agency's expected resolution should also allow Iran to save face by acknowledging the positive steps the country has taken to disclose its nuclear activities and by emphasising that the freeze is voluntary. If the freeze holds - long enough for a meaningful and permanent treaty - the Europeans' calculated risks and sensitivity will have paid off.
However, the measures needed to buy time for further negotiations should not be considered good enough for the longer-term agreement. Verifiable compliance should bring benefits such as greater diplomatic contact with other countries and access to bodies such as the World Trade Organisation, but any failure to keep promises should also carry penalties. Iran has for years flouted deals with limited consequences and may well carry on with the same pattern unless given incentives to behave otherwise.
Possibly more important than having both 'carrots' and 'sticks' will be US involvement in framing and enforcing the next deal. American indifference, bordering on scepticism, about the negotiations between Britain, France, Germany and Iran, has seriously undermined the whole process. The US, meanwhile, has carried on with veiled threats of strong action from the sidelines.
