Librarian is an open-access, web-based application for building and sharing high-quality MS² spectral libraries for nontarget high-resolution mass spectrometry. It provides a streamlined, end-to-end workflow to retrieve and harmonize chemical metadata from PubChem, design compound mixtures for HRMS acquisition, and assemble standardized, metadata-rich MS² records ready for deposition in public repositories such as MassBank. Designed to lower technical barriers, Librarian is modular, interoperable with common HRMS preprocessing tools, and supports scalable, community-driven expansion of open spectral libraries for metabolomics, exposomics, and environmental research.