In 2015, Microsoft acquired the Havok middleware software suite, which includes a physics component that's been used in hundreds of games going back over 20 years. This includes major releases from Valve including Team Fortress 2, Half-Life 2, and even Half-Life: Alyx.
This has proved a barrier in that officially-supported Valve mods for these games then had to pay a hefty $50,000 license fee. In a good turn of news, it appears Valve has worked out a deal to remove these fees for modders going
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