diff --git a/action.yml b/action.yml
index c1639266..999e6690 100644
--- a/action.yml
+++ b/action.yml
@@ -201,6 +201,7 @@ runs:
         GITHUB_HEAD_REF: ${{ github.head_ref }}
         GITHUB_ACTION_PATH: ${{ github.action_path }}
         GITHUB_WORKSPACE: ${{ github.workspace }}
+        GITHUB_PULL_REQUEST_BASE_SHA: ${{ github.event.pull_request.base.sha }}
         # INPUT_<VARIABLE_NAME> is not available in Composite run steps
         # https://github.community/t/input-variable-name-is-not-available-in-composite-run-steps/127611
         INPUT_SHA: ${{ steps.sha.outputs.sha }}