Known limitations and differences

conda-rattler-solver aims to be a drop-in replacement for the classic and libmamba solvers, but it is still in beta and there are some areas where it behaves differently. This page lists these known limitations and differences.

Tip

You can always use --solver=classic or --solver=libmamba to run a single operation with another solver, even after setting rattler as the default.

Features and track_features

The legacy features metadata, as used by old mkl and nomkl variant packages, is not supported. This limitation is shared with libmamba. If you depend on packages that select variants through features, use the classic solver for those environments.

Channel priority

With channel_priority: disabled, classic prefers the higher-priority channel when the same version is available from several channels. conda-rattler-solver may pick a build from a lower-priority channel instead. See conda/rattler#2823. The default, flexible, and strict are not affected.

With channel_priority: strict, if an installed package came from a channel that is no longer in your configuration and one of your active channels also provides it, conda-rattler-solver replaces it with the active channel’s build, even when that means a downgrade. classic keeps the installed package. If you need to keep it, add the original channel back or pin the package.

Error messages

For unsatisfiable dependency conflicts, conda-rattler-solver reports RattlerUnsatisfiableError. Its diagnostic walks through the packages that were tried and explains which requirements could not be satisfied.

A request for an unavailable package can raise PackagesNotFoundError. A direct conflict between requested specs and configured pins can raise SpecsConfigurationConflictError. See pinned_packages for pinning behavior.

Solutions that differ from classic

These are not bugs, but they can be surprising if you are used to the classic solver:

• When an existing environment is re-solved with relaxed constraints, conda-rattler-solver may keep installed packages that classic would remove, as long as they are still compatible with the solution.

• conda-rattler-solver tries to minimise changes to your environment, but it does not implement classic’s multi-step retry logic with --freeze-installed. If a solve fails, it retries with progressively looser constraints on the conflicting packages. During an update this can downgrade a conflicting package if that is the only way to find a solution, which is what libmamba does too.

Reporting a new difference

If you find a case where conda-rattler-solver and classic or libmamba disagree and it is not listed here, please open an issue with the command you ran, your channel configuration, and the output of conda info.