diff --git a/.github/workflows/CI.yaml b/.github/workflows/CI.yaml
index fe97733ce..c0368f5af 100644
--- a/.github/workflows/CI.yaml
+++ b/.github/workflows/CI.yaml
@@ -51,6 +51,8 @@ jobs:
           environment-name: test
           channels: jaimergp/label/unsupported-cudatoolkit-shim,conda-forge/label/openmm_dev/linux-64,conda-forge,openeye
           channel-priority: flexible
+          cache-env: true
+          cache-downloads: true
           extra-specs: |
             python==${{ matrix.python-version }}
             openmm==8.0.0dev3
@@ -61,6 +63,8 @@ jobs:
         with:
           environment-file: devtools/conda-envs/test_env.yaml
           environment-name: test
+          cache-env: true
+          cache-downloads: true
           extra-specs: |
             python==${{ matrix.python-version }}
             openmm==${{ matrix.openmm }}
diff --git a/devtools/conda-envs/test_env.yaml b/devtools/conda-envs/test_env.yaml
index 6ced88068..b0b3d9019 100644
--- a/devtools/conda-envs/test_env.yaml
+++ b/devtools/conda-envs/test_env.yaml
@@ -28,7 +28,7 @@ dependencies:
   - openff-units >=0.1.8
   - openmm >=7.7
   - openmmforcefields >=0.9.0
-  - openmmtools >=0.21.2 # may need to sort out ambermini/ambertools/parmed dependencies
+  - openmmtools >=0.21.5 # may need to sort out ambermini/ambertools/parmed dependencies
   - openmoltools # may need to sort out ambermini/ambertools/parmed dependencies (we don't want ambertools)
   - parmed # may need to sort out ambermini/ambertools/parmed dependencies
   - pdbfixer