Bragg spectroscopy is a consolidated experimental method for high energy resolution X-ray measurements. However, this technique is limited to the detection of photons produced from point-like or well collimated sources and becomes quite inefficient for photons generated in extended and diffused ones; also, the possibility to perform simultaneous measurements of several X-ray lines is of great benefit when low rate signals are expected and individual angular scans require long exposure times. We present a prototype of a high resolution Highly Annealed Pyrolytic Graphite (HAPG) mosaic crystals based X-ray spectrometer, developed by the VOXES (high resolution VOn hamos X-ray spectrometer using HAPG for extended sources) collaboration at INFN Laboratories of Frascati, able to work with extended (millimetric) and isotropic sources. The spectrometer has been tested in the energy range 6–19 keV, with the aim to deliver a cost effective system having an energy resolution (FWHM) at the level of few eV, able to perform sub-eV precision measurements of single and multielement targets in a broad energy range. In this paper, the working principle of the VOXES spectrometer is presented, including a detailed description of the geometry, of the calibration methods and of the spectral fitting functions. Results of X-ray tracing simulations are compared to the experiment. The main results achieved by our spectrometer regard the peak resolutions. In particular, we report values as low as 4.02 ± 0.08 eV and 3.60 ± 0.05 eV for the Fe(Kα_1,2) and Cu(Kα_1,2) lines; for larger dynamic ranges, we report single shot spectra with Cu(Kα_1,2) Ni(Kβ) and Zn(Kα_1,2) lines with best resolution values of 5.15 ± 0.13 eV, 6.02 ± 0.24 eV and 6.20 ± 0.34 eV, respectively. Finally, we explored the capabilities of our spectrometer for energies above 15 keV, obtaining for Mo(Kα_1,2) and Nb(Kβ) lines resolution values of 19.44 ± 0.75 eV and 39.90 ± 1.48 eV, respectively. The proposed spectrometer has possible applications in several fields, going from fundamental physics, synchrotron radiation and X-FEL applications, medicine and industry to hadronic physics experiments, where its performances make it a fundamental tool for a series of measurements like the energies of kaonic atoms transitions, allowing to extract fundamental parameters in the low energy QCD in the strangeness sector.