How to enable the libmamba Conda solver

Slow Conda solves can block environment creation, package installs, and updates long before any files are changed. Enabling the libmamba solver tells Conda to use the newer dependency resolver for package decisions instead of the older classic solver.

Recent Conda releases already use libmamba by default, but older installations or explicit .condarc settings can still point to classic. The active value is visible through conda config and conda info, so check the current solver before changing a shared workstation or build runner.

The conda-libmamba-solver package must be present in the base environment because Conda loads solver plugins from there. conda config --set solver libmamba writes the setting to the user .condarc by default, and new Conda commands read the setting without restarting the shell.

1. Check the current solver setting.

   $ conda config --show solver
   solver: classic

   If the output already shows solver: libmamba, skip the change step and run the smoke test to confirm Conda can still solve a package request.

2. Check whether the libmamba solver plugin is installed in base.

   $ conda list --name base conda-libmamba-solver
   # packages in environment at /opt/conda:
   #
   # Name                     Version          Build            Channel
   conda-libmamba-solver      26.4.0           pyh3785b3c_0

3. Install the solver plugin if the package row is missing.

   $ conda install --name base conda-libmamba-solver

   If Conda stops with CondaToSNonInteractiveError on Anaconda channels, review the listed channel URLs and accept the required Terms of Service or install the package from an approved channel before continuing.

   Related: How to install a package with Conda

4. Set libmamba as the default solver.

   $ conda config --set solver libmamba

5. Confirm the saved solver setting.

   $ conda config --show solver
   solver: libmamba

6. Inspect the configuration source if the active value is not the expected one.

   $ conda config --show-sources
   ==> /home/user/.condarc <==
   channels:
     - defaults
   solver: libmamba

   Multiple files can appear in this output. A later file or environment-specific setting can override an earlier solver value.

7. Confirm the runtime solver reported by Conda.

   $ conda info
   
        active environment : None
             conda version : 26.3.2
                    solver : libmamba (default)
   ##### snipped #####

8. Run a dry-run solve with the enabled solver.

   $ conda create --dry-run --name solver-smoke --override-channels --channel conda-forge zlib
   Channels:
    - conda-forge
   Platform: linux-64
   Collecting package metadata (repodata.json): done
   Solving environment: done
   
   DryRunExit: Dry run. Exiting.
   
   ## Package Plan ##
   
     environment location: /opt/conda/envs/solver-smoke
   
     added / updated specs:
       - zlib
   
   The following NEW packages will be INSTALLED:
   
     libzlib            conda-forge/linux-64::libzlib-1.3.2-hdc9db2a_2
     zlib               conda-forge/linux-64::zlib-1.3.2-hdc9db2a_2

   The dry run does not create the solver-smoke environment. The explicit conda-forge channel keeps the smoke test independent of Anaconda channel Terms of Service prompts.
   Related: How to enable conda-forge in Conda