Valve has officially released Team Fortress 2's source code to all players

Valve has officially added Team Fortress 2's game and server code to the Source SDK, enabling modders to create entirely new games and mods.

The details:

As announced in a new blog post, the latest update to the Source SDK now includes all code used to create and power Team Fortress 2. This allows players to create more complex mods and even entire games, which Valve has encouraged, as long as they are not sold commercially. “Unlike the Steam Workshop or local content mods, this SDK gives mod makers the ability to change, extend or rewrite TF2, making anything from small tweaks to complete conversions possible,” the post reads. “The SDK is licensed to users on a non-commercial basis, meaning that any mod created using the SDK must be free, and any content in those mods must be free.”
Valve also confirmed that mods created with the SDK can be published on Steam and appear as new games in the user's library. With modders now given the legal go-ahead, some are planning to port their projects to the SDK, including TF2 Classic, which was put on hold for eight months in 2021 at Valve’s request.
The update also included several new features and improvements to both the SDK and TF2 itself. Valve has updated its previous multiplayer source titles with 64-bit binary support, scalable HUD/UI, prediction fixes, and other improvements, while TF2 now has borderless windowed support and some new in-game visual effects. It's worth noting that some existing mods for the game are now broken as a result of the update, but as more modders switch to the new SDK, they will likely be updated soon.

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