Configuration

Build-time configuration

Package specs, channels, and exclusions are defined in the [tool.cx] section of pixi.toml. These are read both by build.rs (at compile time) and embedded into the binary.

[tool.cx]
channels = ["conda-forge"]
packages = [
    "python >=3.12",
    "conda >=25.1",
    "conda-rattler-solver",
    "conda-spawn",
    "conda-pypi",
    "conda-self",
]
exclude = ["conda-libmamba-solver"]

channels

List of conda channels to solve against. Defaults to conda-forge.

packages

List of MatchSpec strings defining the packages to install in the base prefix.

exclude

List of package names to exclude from the installation. cx will also remove any dependencies that are exclusively required by the excluded packages.

Runtime configuration

.condarc

cx writes a .condarc into the prefix with these settings:

solver: rattler
auto_activate_base: false
notify_outdated_conda: false
show_channel_urls: true
default_channels:
  - conda-forge

.cx.json

cx writes metadata about the installation into .cx.json at the prefix root:

{
  "version": "0.1.0",
  "channels": ["conda-forge"],
  "packages": ["python >=3.12", "conda >=25.1", "conda-rattler-solver"],
  "excludes": ["conda-libmamba-solver"]
}

This is used by cx status and will be used by the future conda-self update backend to detect cx-managed prefixes.

conda-meta/frozen

A CEP 22 frozen marker file that prevents accidental modification of the base prefix:

{
  "message": "This base environment is managed by cx (conda-express).\nCreate a new environment instead: conda create -n myenv\nTo re-bootstrap: cx bootstrap --force\nTo override: pass --override-frozen-env"
}

Customizing the build

To change what cx installs, edit the [tool.cx] section in pixi.toml and rebuild:

pixi run build

The first build after a config change triggers a compile-time re-solve. A content hash ensures the solve is skipped when the config hasn’t changed.

Environment variable overrides

For custom builds without editing pixi.toml (e.g. via the GitHub Action or CI), build.rs supports environment variable overrides:

Variable	Overrides	Format
CX_PACKAGES	packages	Comma-separated MatchSpec strings
CX_CHANNELS	channels	Comma-separated channel names
CX_EXCLUDE	exclude	Comma-separated package names

Empty values are ignored (the pixi.toml defaults are used).

# Build with extra packages baked in
CX_PACKAGES="python >=3.12, conda >=25.1, conda-rattler-solver, conda-spawn, numpy" pixi run build

# Build with a different channel
CX_CHANNELS="conda-forge, bioconda" pixi run build

When overrides are active:

• The checked-in cx.lock is skipped (a fresh solve is performed)

• The lockfile cache still works based on a hash of the config + overrides

• The repo-root cx.lock is not overwritten (the solve is one-off)

Default prefix

The default installation prefix is ~/.cx. Override it per-command with the --prefix flag:

cx bootstrap --prefix /opt/cx
cx status --prefix /opt/cx