diff --git a/pkgs/development/python-modules/msprime/default.nix b/pkgs/development/python-modules/msprime/default.nix
index 83e1314a5501..c0cd877ccaaa 100644
--- a/pkgs/development/python-modules/msprime/default.nix
+++ b/pkgs/development/python-modules/msprime/default.nix
@@ -28,6 +28,12 @@ buildPythonPackage rec {
     hash = "sha256-2K55gHYWf2Mrj9fszVCJ+qqEyQNMppQi+IZCX5SlsBs=";
   };
 
+  postPatch = ''
+    # build-time constriant, used to ensure forward and backward compat
+    substituteInPlace pyproject.toml \
+      --replace-fail "numpy>=2" "numpy"
+  '';
+
   nativeBuildInputs = [
     gsl
     oldest-supported-numpy
diff --git a/pkgs/development/python-modules/tskit/default.nix b/pkgs/development/python-modules/tskit/default.nix
index 775284b8990b..e91fba90dc77 100644
--- a/pkgs/development/python-modules/tskit/default.nix
+++ b/pkgs/development/python-modules/tskit/default.nix
@@ -21,6 +21,12 @@ buildPythonPackage rec {
     hash = "sha256-Wq7ar96w5hdAyKMMvK4zjYln74RmX1l/VhvS++CN+Xk=";
   };
 
+  postPatch = ''
+    # build-time constriant, used to ensure forward and backward compat
+    substituteInPlace pyproject.toml \
+      --replace-fail "numpy>=2.0" "numpy"
+  '';
+
   build-system = [ setuptools ];
 
   dependencies = [