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You must own Chivalry to use the SDK. To download it, open up your Steam Library, right click on Chivalry: Medieval Warfare Beta and go to Betas. Choose the SDK beta. You'll then be subject to a ~3-5 GB download. If you ever want to switch back to the normal Chivalry Beta, you'll again be subjected to a large download. This will be true every time you try to switch between the two.
The SDK branch of the dedicated server has the server side of the SDK build.
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