diff --git a/mlx-rs/src/fast.rs b/mlx-rs/src/fast.rs
index 6d9d0935..669fc599 100644
--- a/mlx-rs/src/fast.rs
+++ b/mlx-rs/src/fast.rs
@@ -182,22 +182,28 @@ pub fn scaled_dot_product_attention_device<'a>(
 /// # Params
 ///
 /// - x: input array
-/// - weight: A multiplicative weight to scale the result by. The `weight` should be one-dimensional with the same size as the last axis of `x`.
+/// - weight: A multiplicative weight to scale the result by. The `weight` should be one-dimensional
+///   with the same size as the last axis of `x`. If not given, no scaling will occur (matches
+///   Python's `mx.fast.rms_norm(x, None, eps)` semantics — the underlying Metal kernel skips
+///   the per-element multiply, saving a kernel pass).
 /// - eps: A small additive constant for numerical stability
 /// - stream: stream or device to evaluate on
 #[generate_macro(customize(root = "$crate::fast"))]
 #[default_device]
-pub fn rms_norm_device(
-    x: impl AsRef<Array>,
-    weight: impl AsRef<Array>,
-    eps: f32,
+pub fn rms_norm_device<'a>(
+    #[named] x: impl AsRef<Array>,
+    #[optional] weight: impl Into<Option<&'a Array>>,
+    #[named] eps: f32,
     #[optional] stream: impl AsRef<Stream>,
 ) -> Result<Array> {
     Array::try_from_op(|res| unsafe {
         mlx_sys::mlx_fast_rms_norm(
             res,
             x.as_ref().as_ptr(),
-            weight.as_ref().as_ptr(),
+            weight
+                .into()
+                .map(|w| w.as_ptr())
+                .unwrap_or_else(|| mlx_sys::mlx_array_new()),
             eps,
             stream.as_ref().as_ptr(),
         )
@@ -308,7 +314,7 @@ mod tests {
         assert_eq!(a.dtype(), crate::Dtype::Float32);
 
         let weight = Array::ones::<f32>(&[16]).unwrap();
-        let result = rms_norm(a, weight, 1e-5).unwrap();
+        let result = rms_norm(a, &weight, 1e-5).unwrap();
         assert_eq!(result.shape(), [2, 8, 16]);
         assert_eq!(result.dtype(), crate::Dtype::Float32);
         assert_float_eq!(