A small, brown bird of rainforest canopy. Note upturned mandible, white whisker mark, and pale streaking on underparts and back. Follows mixed-species flocks in the canopy but is rarely encountered. Bores into dead twigs and vines for insects, and may climb briefly on larger branches to probe bark crevices. Song is 4–6 high notes, dropping slightly in pitch at end; delivered forcefully but with a thin quality and thus easily overlooked. In central and western Amazonia, compare with very similar Streaked Xenops, from which it is best separated by Slender-billed’s slower song. Readily distinguished from Rufous-tailed Xenops by prominent white whisker streak.