Sonic the Hedgehog CD build v0.51 is a prototype of Sonic the Hedgehog CD for the Sega Mega-CD. It claims to have been built on 21st June, 1993.
While still several months away from the final release, the v0.51 prototype is a mostly functional, if unpolished version of Sonic CD. This is the first prototype where it is possible to play the game from beginning to end without cheats.
This prototype was discovered and acquired by drx of Hidden Palace in conjunction with The Cutting Room Floor, along with several other prototypes of Sonic CD and other Sonic games, and dumped as part of "Sonic Month 2019" from October to November 2019 . This prototype, along with the other Sonic CD prototypes, was released to the public on October 26, 2019.