  The latest generation of radio surveys are now producing sky survey images
containing many millions of radio sources. In this context it is highly
desirable to understand the performance of radio image source finder (SF)
software and to identify an approach that optimises source detection
capabilities. We have created Hydra to be an extensible multi-SF and
cataloguing tool that can be used to compare and evaluate different SFs. Hydra,
which currently includes the SFs Aegean, Caesar, ProFound, PyBDSF, and Selavy,
provides for the addition of new SFs through containerisation and configuration
files. The SF input RMS noise and island parameters are optimised to a 90\%
''percentage real detections'' threshold (calculated from the difference
between detections in the real and inverted images), to enable comparison
between SFs. Hydra provides completeness and reliability diagnostics through
observed-deep ($\mathcal{D}$) and generated-shallow ($\mathcal{S}$) images, as
well as other statistics. In addition, it has a visual inspection tool for
comparing residual images through various selection filters, such as S/N bins
in completeness or reliability. The tool allows the user to easily compare and
evaluate different SFs in order to choose their desired SF, or a combination
thereof. This paper is part one of a two part series. In this paper we
introduce the Hydra software suite and validate its $\mathcal{D/S}$ metrics
using simulated data. The companion paper demonstrates the utility of Hydra by
comparing the performance of SFs using both simulated and real images.
