The Slender-billed Greenbul needs to be described with its source limits in view. A Liberian subspecies record from 1899 notes a narrow, flattened bill, while a 2026 survey in the DRC caught it in primary forest sites.
The 1899 account describes a narrow, flattened bill with serrations near its tip. Its Liberian subspecies specimen had a black bill and feet, with the lower mandible slightly paler underneath.
In the eastern DRC survey, mist-net captures were associated with primary forest sites. The same paper notes records of the species from secondary forest, farmland and gardens elsewhere.
An extinction-risk assessment has not been added yet.