Channels and Branches

“Channel” is an overloaded term in the Nix ecosystem. It refers to two related but distinct things: the legacy nix-channel mechanism for subscribing to a nixpkgs feed, and the release branches that those feeds track. Understanding both is useful because the branch names appear everywhere — in flake inputs, in pinning tools, and in documentation — regardless of whether you use the legacy channel mechanism at all.

Release branches

Nixpkgs is developed on a rolling master branch and periodically stabilised into versioned release branches. The main branches you will encounter are:

NixOS stable releases follow a YY.MM scheme and are cut twice a year, in May and November. Once a release branch is cut, it receives only security and critical bug fixes — package versions are frozen.

nixpkgs-unstable vs nixos-unstable

Both branches track master but are bumped independently by Hydra, the NixOS build infrastructure. A branch is only advanced once all of its required builds succeed, so either branch may lag behind master by hours or days.

The practical difference is that nixos-unstable requires additional NixOS system-level tests to pass before advancing, which means it can lag behind nixpkgs-unstable but offers slightly higher confidence for NixOS configurations. For non-NixOS workflows — development shells, macOS, WSL — nixpkgs-unstable is preferred.

The legacy nix-channel mechanism

Before flakes and pinning tools, nix-channel was how users subscribed to a nixpkgs branch. It works by pointing at a URL that Hydra keeps updated to the latest successful build of a branch:

https://nixos.org/channels/nixos-unstable
https://nixos.org/channels/nixos-24.11

Managing channels looks like this:

# Subscribe to a channel
nix-channel --add https://nixos.org/channels/nixpkgs-unstable nixpkgs

# Download the latest version of all subscribed channels
nix-channel --update

# List current channels
nix-channel --list

After an update, <nixpkgs> in Nix expressions resolves to the downloaded channel. The major drawback is that nix-channel --update silently changes the version of nixpkgs used by every expression on the machine, making builds non-reproducible between updates. For this reason, channels have largely been superseded by explicit pinning and flakes, which are covered in the following sections.

Choosing a branch

Unstable (nixpkgs-unstable or nixos-unstable) is the right choice when you want the latest package versions and are comfortable with occasional breakage. It is the most popular choice among individual developers and is generally more stable in practice than its name suggests, since Hydra gates advancement on CI passing.

Stable (nixos-24.11, etc.) is the right choice when predictability matters more than freshness — production servers, shared developer environments, or anywhere that an unexpected package update would be disruptive.

Mixing both is also possible and common: pin most of your configuration to a stable branch, then selectively pull individual packages from unstable when you need a newer version.