http://steamcommunity.com/app/440/discussions/0/135509472115112472/
Jill said:Testers wanted - New compilers, same everything else!
Hi everyone,
In an upcoming update we are modernizing the compilers and associated tools used to build Team Fortress 2. In order to make sure this change doesn't introduce any new bugs or otherwise ruin anyone's day, we've made a beta branch available using this new setup!
You can access this beta by selecting Team Fortress 2 in Steam and going to Properties -> Betas and selecting the branch titled "toolchainbeta"
The main differences you should see in the beta are... nothing! Ideally, users should not notice any difference in their experience. The changes within, however, enable us to make use of newer tools and future proof the game for updates to come.
So, if you are interested in helping us test our nothing, give the beta a shot, and let us know if you do find something amiss!