Preview features

uv includes opt-in preview features to provide an opportunity for community feedback and increase confidence that changes are a net-benefit before enabling them for everyone.

Enabling preview features

To enable all preview features, use the --preview flag:

$ uv run --preview ...

Or, set the UV_PREVIEW environment variable:

$ UV_PREVIEW=1 uv run ...

To enable specific preview features, use the --preview-features flag:

$ uv run --preview-features foo ...

The --preview-features flag can be repeated to enable multiple features:

$ uv run --preview-features foo --preview-features bar ...

Or, features can be provided in a comma separated list:

$ uv run --preview-features foo,bar ...

The UV_PREVIEW_FEATURES environment variable can be used similarly, e.g.:

$ UV_PREVIEW_FEATURES=foo,bar uv run ...

For backwards compatibility, enabling preview features that do not exist will warn, but not error.

Using preview features

Often, preview features can be used without changing any preview settings if the behavior change is gated by some sort of user interaction, For example, while pylock.toml support is in preview, you can use uv pip install with a pylock.toml file without additional configuration because specifying the pylock.toml file indicates you want to use the feature. However, a warning will be displayed that the feature is in preview. The preview feature can be enabled to silence the warning.

Other preview features change behavior without changes to your use of uv. For example, when the python-upgrade feature is enabled, the default behavior of uv python install changes to allow uv to upgrade Python versions transparently. This feature requires enabling the preview flag for proper usage.

Available preview features

The following preview features are available:

• add-bounds: Allows configuring the default bounds for uv add invocations.
• json-output: Allows --output-format json for various uv commands.
• package-conflicts: Allows defining workspace conflicts at the package level.
• pylock: Allows installing from pylock.toml files.
• python-install-default: Allows installing python and python3 executables.
• python-upgrade: Allows transparent Python version upgrades.
• format: Allows using uv format.

Disabling preview features

The --no-preview option can be used to disable preview features.