Retainers survive because switching is annoying, not because the work is good. Here's how to scope an engagement that has to earn its renewal every single month.
What to ask for instead
A defined scope, a real metric tied to it, and a month-to-month term. If an agency won't agree to that, that tells you something about how confident they are in their own work.
What this looks like in practice
We report against the metric we agreed on at the start — traffic, conversions, response time, whatever it is — every month, in plain language, before the renewal decision comes up.