COA red flag compilation — what worked for you?

Adding the numbers, since they settle part of this. The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as well as purity. A report failing any of those is weak evidence regardless of the percentage on it.

One thing that is still open after SarahChen_PharmD’s answer:

How people are distinguishing a supplier having a bad batch from a supplier on the way out?

TrialTracker_MD said:

The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as…

Adding the part of the answer the thread has not reached. They are not contradictory, they measure different things, and this is the single most common misreading of a certificate. Purity is a relative area measurement: the main peak as a percentage of total peak area in the chromatogram. Content is an absolute mass measurement against a reference standard. A vial can be 99% pure and contain 6.1mg — pure material, overfilled — or 99% pure and contain 3mg, which is pure material and a short fill. Purity tells you what fraction of what is there is the right molecule; content tells you how much of it there is. You need both, and a certificate with only purity is telling you half the story.

OP back with an update, since a thread like this is useless without one.

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality changed. I was reading the product and not the behaviour.

julia.endo said:

They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.

Right, and one flattering data point from a group buy is not consistency. Consistency means separate batches, separately commissioned, over months.

Ask again with the specifics and you will get a better answer than this one.