diff --git a/.github/workflows/_test.yml b/.github/workflows/_test.yml
index 2465aa1ace..ebb0791737 100644
--- a/.github/workflows/_test.yml
+++ b/.github/workflows/_test.yml
@@ -46,7 +46,11 @@ jobs:
               elif [ "${{ matrix.test_type }}" == "core-pydantic-2" ]; then
                 echo "Running core-pydantic-v2 tests, installing dependencies with poetry..."
                 poetry install
-                poetry add pydantic@2.1
+
+                # Install via `pip` instead of `poetry add` to avoid changing lockfile,
+                # which would prevent caching from working: the cache would get saved
+                # to a different key than where it gets loaded from.
+                poetry run pip install 'pydantic>=2.1,<3'
               else
                 echo "Running extended tests, installing dependencies with poetry..."
                 poetry install -E extended_testing
@@ -59,10 +63,10 @@ jobs:
             EXPECTED_VERSION=1
           fi
           echo "Checking pydantic version... Expecting ${EXPECTED_VERSION}"
-          
+
           # Determine the major part of pydantic version
           VERSION=$(poetry run python -c "import pydantic; print(pydantic.__version__)" | cut -d. -f1)
-          
+
           # Check that the major part of pydantic version is as expected, if not
           # raise an error
           if [[ "$VERSION" -ne $EXPECTED_VERSION ]]; then