diff --git a/pkgs/development/compilers/cmdstan/default.nix b/pkgs/development/compilers/cmdstan/default.nix
index ad84e13d369e..817234f79991 100644
--- a/pkgs/development/compilers/cmdstan/default.nix
+++ b/pkgs/development/compilers/cmdstan/default.nix
@@ -11,14 +11,14 @@
 
 stdenv.mkDerivation rec {
   pname = "cmdstan";
-  version = "2.33.1";
+  version = "2.34.1";
 
   src = fetchFromGitHub {
     owner = "stan-dev";
     repo = pname;
     rev = "v${version}";
     fetchSubmodules = true;
-    hash = "sha256-c+L/6PjW7YgmXHuKhKjiRofBRAhKYCzFCZ6BOX5AmC4=";
+    hash = "sha256-gze8kd5zSs9nUlSY7AJwpx+jnc9Y21ahzDJmynlqm1Y=";
   };
 
   postPatch = ''
diff --git a/pkgs/development/python-modules/cmdstanpy/default.nix b/pkgs/development/python-modules/cmdstanpy/default.nix
index c3c46546695d..717f85dc8e54 100644
--- a/pkgs/development/python-modules/cmdstanpy/default.nix
+++ b/pkgs/development/python-modules/cmdstanpy/default.nix
@@ -2,6 +2,7 @@
 , buildPythonPackage
 , fetchFromGitHub
 , substituteAll
+, fetchpatch
 , cmdstan
 , pythonRelaxDepsHook
 , setuptools
@@ -31,6 +32,11 @@ buildPythonPackage rec {
       src = ./use-nix-cmdstan-path.patch;
       cmdstan = "${cmdstan}/opt/cmdstan";
     })
+    (fetchpatch {
+      name = "stan-2.34-fix-parsing-of-unit_e-output-files.patch";
+      url = "https://github.com/stan-dev/cmdstanpy/commit/144d641739ccd1109055d13b5b96e4e76607305d.patch";
+      hash = "sha256-21hcbK3Xs7vGBNRs4hMfY5g7jIwEG49WYnsOxYJ6ccs=";
+    })
   ];
 
   postPatch = ''