diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a974cbc08..1083ffde9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -52,6 +52,7 @@ jobs:
       #
       # caching id for this ghc's build artifacts:
       # XXX ideally should match the default ghc in stack.yaml, though it's not critical
+      # XXX supposed to be interpolated by $ghc below, but this is not working
       ghc: 910
       # stack config for this ghc:
       stack: stack --system-ghc
diff --git a/.github/workflows/oldest.yml b/.github/workflows/oldest.yml
index 057795824..b2fa85a6e 100644
--- a/.github/workflows/oldest.yml
+++ b/.github/workflows/oldest.yml
@@ -18,6 +18,7 @@ jobs:
       # Keep these synced with the latest ghc version at https://github.com/actions/runner-images/blob/ubuntu22/20240514.2/images/ubuntu/Ubuntu2404-Readme.md#haskell-tools
       #
       # caching id for this ghc's build artifacts:
+      # XXX supposed to be interpolated by $ghc below, but this is not working
       ghc: 8107
       # stack config for this ghc:
       stack: stack --stack-yaml=stack8.10.yaml