diff --git a/.ci/scripts/setup-conda.sh b/.ci/scripts/setup-conda.sh
index a725c90dd82..1571ac66cf7 100755
--- a/.ci/scripts/setup-conda.sh
+++ b/.ci/scripts/setup-conda.sh
@@ -9,7 +9,22 @@ set -ex
 
 install_conda() {
   pushd .ci/docker || return
-  ${CONDA_INSTALL} -c conda-forge -y --file conda-env-ci.txt
+
+  # The env created by pytorch/test-infra's setup-miniconda is pre-populated
+  # with cmake=3.22 ninja=1.10 pkg-config=0.29 wheel=0.37 from the anaconda
+  # defaults channel. Mixing those with conda-forge's newer transitive deps
+  # required by our cmake=3.31.2 pin (libzlib>=1.3.1, rhash>=1.4.5) has been
+  # intermittently failing the libmamba solver, especially on ephemeral
+  # GitHub-hosted macOS runners where the env is fresh every job. Tear down
+  # the pre-populated env and recreate it from conda-forge only so the solve
+  # never has to reconcile two channels.
+  PY_VERSION=$(conda run -p "${CONDA_PREFIX}" python -c \
+    'import sys; print(f"{sys.version_info.major}.{sys.version_info.minor}")')
+
+  conda env remove --prefix "${CONDA_PREFIX}" -y
+  conda create --prefix "${CONDA_PREFIX}" -c conda-forge --override-channels -y \
+    "python=${PY_VERSION}" --file conda-env-ci.txt
+
   popd || return
 }