Negotiations can work in an atmosphere of mistrust, under three circumstances.
i) The stakes and level of ambition are so low future deviation by either side wouldn't be a big blow. Some developing country FTAs probably fall into this category.
ii) Both sides, anticipating chicanery, include maximally specific and legally perceptive language everywhere, leaving nothing to interpretation.
This is agonising to negotiate and agree.
iii) The agreement includes incredibly robust dispute settlement and enforcement provisions. Both sides sign knowing that if there's funny business, they'll have strong options.
This very rarely actually works, but in theory it's possible.
The stakes are too high for i, there's no time to negotiate every detail as envisaged by ii, and neither side will accept a supranational court to enforce the deal as per iii.