diff --git a/.ci/build.sh b/.ci/build.sh
index e73e540b52..9b77a33469 100755
--- a/.ci/build.sh
+++ b/.ci/build.sh
@@ -23,10 +23,15 @@ if [[ "$GHC_HEAD" != "yes" ]]; then
 fi
 set -u
 
-cabal v2-build clash-prelude --write-ghc-environment-files=always
-# Undo mmap crash workaround for clash-cosim, see PR #2572.
-# It uses a custom setup, which can't be linked with -rtsopts
-GHCRTS="" cabal v2-build clash-cosim --write-ghc-environment-files=always
+# Any package with a custom setup or using happy can only be build with GHCRTS=""
+# see PR #2572.
+if [[ "$GHCRTS" != "" ]]; then
+CANT_BUILD_WITH_RTSOPTS="ghc-paths haskell-src-exts pretty-show singletons"
+cabal v2-build --only-dependencies $CANT_BUILD_WITH_RTSOPTS
+GHCRTS="" cabal v2-build $CANT_BUILD_WITH_RTSOPTS
+cabal v2-build clash-prelude
+GHCRTS="" cabal v2-build clash-cosim
+fi
 
 # Build with default constraints
 cabal v2-build all --write-ghc-environment-files=always