diff --git a/build-constraints.yaml b/build-constraints.yaml
index aea9c196..17871cde 100644
--- a/build-constraints.yaml
+++ b/build-constraints.yaml
@@ -2693,7 +2693,7 @@ packages:
 
     "Peter Simons <simons@cryp.to> @peti":
         - cabal2nix
-        - cabal2spec
+        - cabal2spec < 2.6.3  # https://github.com/commercialhaskell/stackage/issues/6448
         - cgi
         - distribution-nixpkgs
         - distribution-opensuse