diff --git a/mlx/backend/cuda/custom_kernel.cpp b/mlx/backend/cuda/custom_kernel.cpp
index 560429b4a..d85e02e51 100644
--- a/mlx/backend/cuda/custom_kernel.cpp
+++ b/mlx/backend/cuda/custom_kernel.cpp
@@ -370,13 +370,13 @@ void CustomKernel::eval_gpu(
   for (const auto& t : copies) {
     encoder.add_temporary(t);
   }
-  auto kernel = mod.get_kernel(kernel_name);
-  if (shared_memory_ > 0 && shared_memory_ > 48000) {
-    cuFuncSetAttribute(
-        kernel,
-        CU_FUNC_ATTRIBUTE_MAX_DYNAMIC_SHARED_SIZE_BYTES,
-        shared_memory_);
-  }
+  auto kernel =
+      mod.get_kernel(kernel_name, [smem = shared_memory_](CUfunction kernel) {
+        if (smem > 0 && smem > 48000) {
+          cuFuncSetAttribute(
+              kernel, CU_FUNC_ATTRIBUTE_MAX_DYNAMIC_SHARED_SIZE_BYTES, smem);
+        }
+      });
   encoder.add_kernel_node(kernel, grid, block, shared_memory_, args.args());
 }
 
diff --git a/mlx/backend/cuda/jit_module.cpp b/mlx/backend/cuda/jit_module.cpp
index 9805b7cf6..367c94392 100644
--- a/mlx/backend/cuda/jit_module.cpp
+++ b/mlx/backend/cuda/jit_module.cpp
@@ -312,7 +312,7 @@ JitModule::JitModule(
   for (const auto& [name, mangled] : ptx_kernels) {
     CUfunction kernel;
     CHECK_CUDA_ERROR(cuModuleGetFunction(&kernel, module_, mangled.c_str()));
-    kernels_[name] = kernel;
+    kernels_[name] = std::make_pair(kernel, false);
   }
 }
 
@@ -327,7 +327,7 @@ JitModule::JitModule(
       CU_JIT_ERROR_LOG_BUFFER, CU_JIT_ERROR_LOG_BUFFER_SIZE_BYTES};
   void* values[] = {jit_log, reinterpret_cast<void*>(std::size(jit_log) - 1)};
   CUresult jit_result = cuModuleLoadDataEx(
-      &module_, ptx.c_str(), std::size(options), options, values);
+      &module_, ptx.data(), std::size(options), options, values);
   if (jit_result != CUDA_SUCCESS) {
     throw std::runtime_error(fmt::format(
         "Failed to load compiled {} kernel: {}.", module_name, jit_log));
@@ -337,7 +337,7 @@ JitModule::JitModule(
   for (const auto& name : kernel_names) {
     CUfunction kernel;
     CHECK_CUDA_ERROR(cuModuleGetFunction(&kernel, module_, name.c_str()));
-    kernels_[name] = kernel;
+    kernels_[name] = std::make_pair(kernel, false);
   }
 }
 
@@ -345,13 +345,23 @@ JitModule::~JitModule() {
   CHECK_CUDA_ERROR(cuModuleUnload(module_));
 }
 
-CUfunction JitModule::get_kernel(const std::string& kernel_name) {
+CUfunction JitModule::get_kernel(
+    const std::string& kernel_name,
+    std::function<void(CUfunction)> configure_kernel) {
   auto it = kernels_.find(kernel_name);
   if (it == kernels_.end()) {
     throw std::runtime_error(
         fmt::format("There is no kernel named {}.", kernel_name));
   }
-  return it->second;
+
+  // If it is the first time we run this kernel then configure it. Do it only
+  // once!
+  if (!it->second.second) {
+    configure_kernel(it->second.first);
+    it->second.second = true;
+  }
+
+  return it->second.first;
 }
 
 std::unordered_map<std::string, JitModule>& get_jit_module_cache() {
diff --git a/mlx/backend/cuda/jit_module.h b/mlx/backend/cuda/jit_module.h
index 1de77bf26..3aa7ffa8a 100644
--- a/mlx/backend/cuda/jit_module.h
+++ b/mlx/backend/cuda/jit_module.h
@@ -94,11 +94,16 @@ class JitModule {
 
   JitModule(const JitModule&) = delete;
   JitModule& operator=(const JitModule&) = delete;
-  CUfunction get_kernel(const std::string& kernel_name);
+  CUfunction get_kernel(
+      const std::string& kernel_name,
+      std::function<void(CUfunction)> configure_kernel);
+  CUfunction get_kernel(const std::string& kernel_name) {
+    return get_kernel(kernel_name, [](auto k) {});
+  }
 
  private:
   CUmodule module_{nullptr};
-  std::unordered_map<std::string, CUfunction> kernels_;
+  std::unordered_map<std::string, std::pair<CUfunction, bool>> kernels_;
 };
 
 std::unordered_map<std::string, JitModule>& get_jit_module_cache();
diff --git a/python/src/fast.cpp b/python/src/fast.cpp
index b359f84bd..7105d12cb 100644
--- a/python/src/fast.cpp
+++ b/python/src/fast.cpp
@@ -388,7 +388,7 @@ void init_fast(nb::module_& parent_module) {
   m.def(
       "precompiled_custom_kernel",
       [](const std::string& name,
-         const std::string& compiled_source,
+         const nb::bytes compiled_source,
          const std::vector<ScalarOrArray>& inputs_,
          const std::vector<mx::Shape>& output_shapes,
          const std::vector<mx::Dtype>& output_dtypes,
@@ -429,7 +429,9 @@ void init_fast(nb::module_& parent_module) {
 
         return mx::fast::precompiled_custom_kernel(
             name,
-            compiled_source,
+            std::string(
+                static_cast<const char*>(compiled_source.data()),
+                compiled_source.size()),
             inputs,
             output_shapes,
             output_dtypes,