Via [url=https://list.valvesoftware.com/]HLDS[/url] and [url=https://steamcommunity.com/games/TF2/announcements/detail/3622613286455113145]the Steam Community[/url]:

[quote=Valve]- Updated the Festive Rack to fix missing paintable attribute
- Updated Musical Maelstrom, Verdant Virtuoso, and Silver Serenade effects to fix max particle count
- Updated pl_rumford_event to fix missing materials/models
- Updated cp_frostwatch (received via the workshop)
- Fixed being able to build on one of the roofs in stage 2
- Fixed a door going through the floor in stage 3

- Updated cp_gravelpit_snowy via the workshop
- Improved clipping
- Fixed various perch spots
- Decreased fog intensity
- Removed unused particle system entities
- Fixed a few floating trees
- Adjusted vertical positions of snowman bombs
- Adjusted lighting on metal sheets in blue spawn
- Fireplaces will now light arrows
- Added more func_dustmotes
- Fixed some visible z-fighting
- Minor detailing additions and adjustments

- VScript support changes
- Fixed lag compensation when friendly fire is enabled
- Added better support for meshing player upgrades
- Added GrantOrRemoveAllUpgrades method
- Exposed currency UI when upgrades are enabled
- Exposed upgrade history on the player when playing without a population manager[/quote]

Rumor has it:

- [b]Avoid playing on a server running Windows Server:[/b] a bug with the Spy's Sapper will cause a server crash when a building is sapped
- You can check what platform a server is running on with the "status" command in console
- Valve's servers run on Linux and are unaffected by this issue

- Size is ~80 MB