Should a marketing agency guarantee a lead count or refuse to promise numbers at all

A common decision point when hiring a marketing agency for owner lead generation is choosing between a proposal promising a fixed lead count, say 25 qualified leads a month or the next month is free, and one that refuses to promise a number at all and instead reports cost per lead weekly, citing lack of control over auction prices and competitor bidding. The guarantee looks like the safer buy on the surface. There is a known target and a stated penalty for missing it. The weakness is that anyone promising a lead count has to define qualified, and that definition is where the guarantee tends to go soft, since a contract that counts any form fill as qualified can hit its number with leads that never convert to a real client conversation. The no-guarantee version shifts more risk to the buyer, since real money can be spent for several months with no recourse beyond ending the relationship if results disappoint. Its advantage is that the agency is not shaping the campaign around hitting an arbitrary count, which is often what makes cheap, low-quality broad-match traffic attractive to an agency chasing a number. The more reliable diligence question either way is asking for the exact definition of a qualified lead in writing, and asking to see cost-per-lead and lead-to-appointment conversion data from an existing client in a comparable market before signing either version.