.zst File Extension

What kind of file is a .zst?

A .zst file contains data compressed with Zstandard, usually called zstd. It normally uses the application/zstd MIME type. Zstandard is lossless, so decompression restores the original bytes exactly.¹

Zstandard compresses a byte stream, not a folder tree. A backup.tar.zst file therefore has two layers: TAR supplies filenames, directories, permissions, and other archive metadata, while Zstandard compresses the complete TAR stream.

A regular Zstandard frame starts with the bytes 28 B5 2F FD. Checking that signature is more reliable than trusting the suffix, although a decoder must still validate the complete stream.¹

How do I open a .zst on my computer?

Install the reference zstd command on Windows, macOS, or Linux, then decompress while keeping the input:

zstd -dk report.csv.zst

You can first test the compressed stream with zstd --test report.csv.zst. The unzstd command is equivalent to zstd -d, while zstdcat writes decompressed data to standard output.²

For a TAR archive compressed with Zstandard, GNU tar can list or extract the members:

tar --zstd -tf backup.tar.zst
tar --zstd -xf backup.tar.zst

GNU tar recognizes both .zst and .tzst when automatic compression selection is enabled.³ The decompressed result may be a document, database, disk image, TAR archive, or any other file type, so use an appropriate app for that result.

Poly support for .zst

Poly recognizes .zst as Zstandard-compressed data. It stores, syncs, shares, versions, and finds the file by name, but treats the compressed payload as opaque. Poly does not decompress it, preview archive members, index the expanded content, or send that content to the Poly agent.

Download and decompress the file with a trusted utility. If you then add the expanded file to Poly, it can receive the preview, indexing, and agent features available for its own format.

History of .zst

Yann Collet developed Zstandard at Facebook for fast, general-purpose lossless compression. Facebook announced the stable 1.0 release on August 31, 2016.⁴ RFC 8478 documented the format in 2018, and RFC 8878 replaced it in 2021. The newer RFC registers .zst, application/zstd, the zstd HTTP content coding, and the +zstd structured syntax suffix.¹

The name does not imply that RFC 8878 is an Internet Standards Track specification. The RFC is Informational, while the media type and filename extension are registered for common use.

Working with .zst and alternatives

Choose the surrounding format based on the job:

Zstandard's ordinary CLI compression levels run from 1 through 19; --ultra enables levels 20 through 22 at greater memory cost.² Test representative data rather than assuming one level or codec will always be smallest.

Compressed data may expand dramatically. Apply size, time, and memory limits to untrusted inputs, and inspect TAR member paths before extraction. A valid optional Zstandard checksum detects corruption, but it does not establish that the expanded content is safe.¹