What is a .dem.zst file?

It is two things stacked: a .dem file, which is the Source 2 demo recording of a Counter-Strike 2 match, wrapped in Zstandard compression (.zst). Zstandard is a modern compression format that packs faster and smaller than gzip, which is why FACEIT switched to it for demo downloads.

Why you keep running into it

A CS2 demo of a full match is large. Compressed, a download is typically less than half the size, which matters when a platform serves thousands of them a day. The side effect: tools that only know .dem or .dem.gz reject the file, and the game cannot play it directly either.

Option 1: open it as it is

DemoLens reads .dem.zst directly — it detects the compression from the file header, decompresses it in a worker and parses it in your browser. No unpacking step, no upload, no account.

Open a .dem.zst in DemoLens →

Option 2: unpack it for the game

To watch the demo inside CS2 you need the plain .dem:

Command line: zstd -d match.dem.zst with the official zstd tool.
GUI: archivers with Zstandard support, such as NanaZip, PeaZip or the 7-Zip ZS fork. Plain 7-Zip builds may not list .zst.

Then move the unpacked file into the replays folder and start it with playdemo replays/match — see the demo commands.

Related formats

.dem.gz is the same idea with gzip, used by some league sites and by bo3.gg. DemoLens handles that one as well, so in practice you can drop whichever file you were given.

DemoLens is a community project, not affiliated with or endorsed by Valve Corporation or FACEIT. Counter-Strike is a trademark of Valve Corporation.