My experience is that the dress code depends a lot on the country the embassy is in as well. When in Rome sort of... Anything EU/US will be dark suit and tie (and stuffy) but in a lot of other countries embassy receptions tend to be much more informal, especially when a large part takes place outside as well.

Unless specifically stated otherwise I tend to go for slacks, nice shirt (sometimes even short sleeves...) and nice shoes. Last time I met a minister was in East Timor and there I was wearing cargo pants, polo shirt and dress sneakers (if there is such a thing). It was completely fine.