Conda channel_priority strict lacks documented tie-breaking for packages present in both defaults and conda-forge

Build-String Selection under Strict Priority

Under channel_priority: strict, the governing constraint is the channel order. Conda will select the package from the highest-priority channel that provides the requested version, regardless of the build string. Build strings are only used as tie-breakers when multiple builds of the same version exist within the same channel.

If defaults is listed above conda-forge in your configuration, Conda will install the build from defaults even if a newer or higher-numbered build string exists in conda-forge.

Solver Behavior in Conda 23.x+

In Conda 23.x and later, both the classic solver and the libmamba solver (now the default) adhere to this strict priority rule. While earlier iterations of libmamba occasionally exhibited different heuristics regarding build strings across channels, recent releases have aligned libmamba with the classic solver's behavior to ensure consistency.

Recommended Configuration for Reproducibility

To eliminate ambiguity and ensure the same build string is selected across different environments or solvers, use the following methods:

• Explicit Channel Pinning: Specify the channel directly in the environment.yml file for critical packages:

  dependencies:
    - conda-forge::openssl=3.0.8

• Override Channels: Use the --override-channels flag during environment creation to ignore global .condarc settings and rely solely on the provided list:

  conda env create -f environment.yml --override-channels -c defaults -c conda-forge

• Explicit Specification Files: For absolute reproducibility, export the environment to an explicit lockfile:

  conda list --explicit > spec-file.txt

  This records the exact URL and build string of every package.

Verification

You can verify the source of an installed package by checking the conda-meta directory of the environment:

# Check the channel and build of a specific package
cat /path/to/env/conda-meta/package_name.json | grep -E "channel|build"

Diagnostic Note: Please confirm your exact Conda version (conda --version). If you are using a version prior to 23.x, the libmamba solver may not yet be fully aligned with the classic solver's strict priority implementation.