If you have previously been running Boost.Build on toolset=msvc-14.0 (Visual Studio 2015 Preview), installing CTP 6 causes some bjam/b2 builds to fail because of missing include files. What's happening is that Boost.Build caches several files in C:\Users\<name>\AppData\Local\Temp. For my installation these files are: b2_msvc_14.0_vcvarsall_x86.cmd b2_msvc_14.0_vcvarsall_x86_amd64.cmd b2_msvc_14.0_vcvarsall_x86_arm.cmd but the introduction of the new universal runtime stuff in CTP 6 added some additional paths to vcvarsall, so the cached files are no longer correct. The workaround is to simply delete the b2_msvc_14.0_vcvarsall* temporary files. The next time bjam/b2 is run correct versions of the files will be regenerated. --Beman