"Mistakes were made by Jeff and later Cindy" uses the passive and is quite clear about agency.

"A bomb exploded near midtown" uses the active but tells us nothing about who was responsible.

If we care about clarity of agency, we should look for it, not poor proxies.

You're responding to a much broader claim than anyone here is actually making. The point is not that the passive voice always conceals agency. The point is that people often use the passive voice in order to conceal agency.

I don't think I am responding to the claim you think I am.

If we want agency to be clearly attributed, we will do better to ask whether agency is clearly attributed than to ask if the passive voice is used. It's what we care about, and it is a determination people are often better at making in the first place.

Fair enough! Though I also don't think anyone was claiming that listening for the passive voice is the best or only way to identify when someone is dodging responsibility.

I don't think anyone was claiming that listening for the passive voice is the only way to identify when someone is dodging responsibility.

I do think it is being identified (at least) as an important or effective way of noticing when you are being evasive in your own writing - otherwise why is "the passive is often used to conceal agency" even relevant to the question of improving your writing? And I think to that end, asking the more direct question is better.