diff --git a/requirements/constraints.txt b/requirements/constraints.txt
index aaea43d..1b874c0 100644
--- a/requirements/constraints.txt
+++ b/requirements/constraints.txt
@@ -9,3 +9,7 @@
 # linking to it here is good.
 
 -c common_constraints.txt
+
+# tox>4.0.0 isn't yet compatible with many tox plugins, causing CI failures in almost all repos.
+# Details can be found in this discussion: https://github.com/tox-dev/tox/discussions/1810
+tox<4.0.0