diff --git a/flake.lock b/flake.lock
index bd4e5bfa0..b90b72d81 100644
--- a/flake.lock
+++ b/flake.lock
@@ -35,17 +35,17 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1776927958,
-        "narHash": "sha256-XOzEtft7E0P6TgQViLUOQeGHlEYiQ0+FY24BPEksj6s=",
+        "lastModified": 1783284758,
+        "narHash": "sha256-tiQ8/qi8I45OOaBBYlVbXoAVkeQzvvTQOv5I45rMw5o=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "fec2c46cca5bf9767486a290abae51200b656d69",
+        "rev": "ec1a11210589d294f0ac99d3290a27e6c73dfa1d",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "nixos-unstable-small",
         "repo": "nixpkgs",
+        "rev": "ec1a11210589d294f0ac99d3290a27e6c73dfa1d",
         "type": "github"
       }
     },
@@ -64,11 +64,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1776914043,
-        "narHash": "sha256-qug5r56yW1qOsjSI99l3Jm15JNT9CvS2otkXNRNtrPI=",
+        "lastModified": 1783320166,
+        "narHash": "sha256-l7C/OsjcnWDOk2K3ssj+SBduwL67LashjBqis9+t468=",
         "owner": "oxalica",
         "repo": "rust-overlay",
-        "rev": "2d35c4358d7de3a0e606a6e8b27925d981c01cc3",
+        "rev": "20ee15370c9256669d66968b89ee20a4b0a4e673",
         "type": "github"
       },
       "original": {
diff --git a/flake.nix b/flake.nix
index cc3993a75..1d7430a16 100644
--- a/flake.nix
+++ b/flake.nix
@@ -10,7 +10,10 @@
 
   inputs = {
     flake-utils.url = "github:numtide/flake-utils";
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable-small";
+    # Temporarily pin this commit to fix ucx issues:
+    # https://github.com/NixOS/nixpkgs/pull/538738
+    nixpkgs.url = "github:NixOS/nixpkgs/ec1a11210589d294f0ac99d3290a27e6c73dfa1d";
+    #nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable-small";
     flake-compat.url = "github:edolstra/flake-compat";
     rust-overlay = {
       url = "github:oxalica/rust-overlay";
diff --git a/nix-builder/lib/extension/torch/arch.nix b/nix-builder/lib/extension/torch/arch.nix
index 0fa51eaf2..004cc55a5 100644
--- a/nix-builder/lib/extension/torch/arch.nix
+++ b/nix-builder/lib/extension/torch/arch.nix
@@ -204,7 +204,7 @@ stdenv.mkDerivation (prevAttrs: {
 
       # Make dependent on build configuration dependencies once
       # the Torch dependency is gone.
-      cuda_cccl
+      cccl
       libcublas
       libcusolver
       libcusparse
diff --git a/nix-builder/lib/extension/tvm-ffi/arch.nix b/nix-builder/lib/extension/tvm-ffi/arch.nix
index ef1e1d2d9..5290994d4 100644
--- a/nix-builder/lib/extension/tvm-ffi/arch.nix
+++ b/nix-builder/lib/extension/tvm-ffi/arch.nix
@@ -200,7 +200,7 @@ stdenv.mkDerivation (prevAttrs: {
 
       # Make dependent on build configuration dependencies once
       # the Torch dependency is gone.
-      cuda_cccl
+      cccl
       libcublas
       libcusolver
       libcusparse
diff --git a/nix-builder/lib/mk-build-set.nix b/nix-builder/lib/mk-build-set.nix
index b12308250..920d51cc1 100644
--- a/nix-builder/lib/mk-build-set.nix
+++ b/nix-builder/lib/mk-build-set.nix
@@ -52,7 +52,6 @@ let
             });
           }
         );
-
   };
 
   overlayForRocmVersion = rocmVersion: self: super: {
diff --git a/nix-builder/overlay.nix b/nix-builder/overlay.nix
index b7328dbe6..f5bcb994f 100644
--- a/nix-builder/overlay.nix
+++ b/nix-builder/overlay.nix
@@ -7,6 +7,7 @@
 
 final: prev:
 {
+  # Local packages/hooks.
   kernel-builder = final.callPackage ./pkgs/kernel-builder { inherit builderProvenance; };
 
   cmakeNvccThreadsHook = final.callPackage ./pkgs/cmake-nvcc-threads-hook { };
@@ -44,6 +45,17 @@ final: prev:
     }
   );
 
+  # nixpkgs overrides
+  ucc = prev.ucc.overrideAttrs (
+    _: prevAttrs: {
+      # Concurrent build flags do not seem to get passed through? So instead, let's
+      # add parallelism to building CUDA files.
+      preConfigure = prevAttrs.preConfigure + ''
+        export NVCC_CFLAGS="--threads $NIX_BUILD_CORES"
+      '';
+    }
+  );
+
   ucx = prev.ucx.overrideAttrs (
     _: prevAttrs: {
       buildInputs = prevAttrs.buildInputs ++ [ final.cudaPackages.cuda_nvcc ];