Conventional false discovery rate (FDR) control limits the expected proportion of selected features that are unrelated to the response. Directional FDR extends this criterion to also account for incorrect sign estimates. In some applications, however, discoveries are sought only in a direction specified in advance. We therefore introduce the prespecified directional FDR, which counts as errors both null effects and effects opposite to the target direction in context of the fixed-design Gaussian linear model with unknown noise variance. A straightforward adaptation of the knockoff filter provides prespecified directional FDR control. To improve power, we develop the response-guided knockoff filter, which uses a noise-perturbed response to guide knockoff construction in favour of features likely to have the target sign, while retaining finite-sample prespecified directional FDR control. Notably, our improved filter maintains this guarantee with a sample size $n>p+2$ in contrast to standard constructions that require $n\geq2p$. Simulations across a range of settings demonstrate power gains of the response-guided knockoff filter. An application to HIV drug-resistance data illustrates improved recovery of externally annotated resistance-associated mutations.

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