In order to predict r i+1 given r 1..i and s, for each candidate r i+1 , in principle one can marginalize over all possible completions of r 1..i+1 , and rank candidate r i+1 by that.
let P(n) be the distribution of response length, let r be a possible completion of r 1..i+1 (i.e., a response whose first i + 1 tokens match r 1..i+1 ).