diff --git a/constraints.txt b/constraints.txt
index 579c9ad8..0a29d56a 100644
--- a/constraints.txt
+++ b/constraints.txt
@@ -1 +1,2 @@
-black<24
+# NB: Keep in sync with run_tests.vpython3.
+black<26
diff --git a/run_tests.vpython3 b/run_tests.vpython3
index e4c62c11..e6dfe7c6 100644
--- a/run_tests.vpython3
+++ b/run_tests.vpython3
@@ -54,6 +54,7 @@ wheel: <
   version: "version:21.4.0"
 >
 
+# NB: Keep in sync with constraints.txt.
 wheel: <
   name: "infra/python/wheels/black-py3"
   version: "version:25.1.0"