Metal Sonic is the boss of Stardust Speedway and the sixth overall boss of Sonic the Hedgehog CD. While in the original version he is only faced by Sonic, the 2011 re-release and Sonic Origins makes it possible to face him with Tails as well.
Instead of a direct fight, the player will face Metal Sonic in a race to the finish across Stardust Speedway. Before the race, Metal Sonic will wait for the playable character in front of a closed gate. Meanwhile, Dr. Robotnik will arrive above them in the Egg Mobile, which has a laser attached to its lower part. Once the race begins, Robotnik will activate his laser and the gate will open.
The race takes place across a long road with two kinds of obstacles. The first obstacle type is the topography itself. There are elevations and depressions in the road that can slow down the playable character. The second type of obstacles are the spikes located along the entire length of the route. Metal Sonic, however, can destroy them on contact, which can be helpful in building up speed when the playable character is falling behind. In addition, the racers are followed by Dr. Robotnik and his laser-armed Egg Mobil. Any contact with Robotnik's laser will result in an immediate loss of a life.
During the race, Metal Sonic will use two forms of attack. When he overtakes the playable character, he uses the Ring Spark Field, creating a energy shield around himself. This shield will deal damage when touched, but it also slows Metal Sonic down, which will allow the player to catch up to Metal Sonic and overtake him. The second attack is used when the player is in the lead, and is called the V. Maximum Overdrive Attack. This technique creates a golden aura around Metal Sonic and allows him to move forward quickly in a horizontal direction. This attack can be avoided by jumping over Metal Sonic, but it often comes at the cost of the player's lead.
Several shortcuts appear along the route of the race in the form of platforms above the road. Metal Sonic cannot reach them, so the player is not vulnerable to his attacks on them. Getting up to them, however, requires fast reactions and the opportunities to get up to them are easily overlooked.
The race itself ends just like it started, namely with a gate. This gates closes when the first racer crosses it. If it is Metal Sonic, the player will be defeated due to Robotnik's laser catching up to them. However, if the player wins, Metal Sonic will crash into the gate and fall off-screen in pieces. Robotnik will then turn off his laser and run away. The player, on the other hand, will be able to go further, eventually reaching Amy, who is kept suspended on one of the beams. The player has to use the Spin Jump to free Amy and let her hug the playable character in order to complete this boss fight.
- In the Japanese and European Sega Mega-CD versions of Sonic the Hedgehog CD, the bad future music plays during the race even if the player is in the good future. This does not happen in the American version. However, in the 2011 remaster, the bad future music plays no matter which region's soundtrack the player is using.