diff --git a/models/cuboid/src/lib.rs b/models/cuboid/src/lib.rs
index 39800822e..6505e3612 100644
--- a/models/cuboid/src/lib.rs
+++ b/models/cuboid/src/lib.rs
@@ -1,44 +1,18 @@
-use fj::{
-    ArgumentMetadata, ContextExt, HostExt, Metadata, Model, ModelMetadata,
-};
-
-fj::register_model!(|host| {
-    host.register_model(Cuboid);
-
-    Ok(Metadata::new(
-        env!("CARGO_PKG_NAME"),
-        env!("CARGO_PKG_VERSION"),
-    ))
-});
-
-pub struct Cuboid;
-
-impl Model for Cuboid {
-    fn shape(
-        &self,
-        ctx: &dyn fj::Context,
-    ) -> Result<fj::Shape, Box<dyn std::error::Error + Send + Sync>> {
-        let x: f64 = ctx.parse_optional_argument("x")?.unwrap_or(3.0);
-        let y: f64 = ctx.parse_optional_argument("y")?.unwrap_or(2.0);
-        let z: f64 = ctx.parse_optional_argument("z")?.unwrap_or(1.0);
-
-        #[rustfmt::skip]
-        let rectangle = fj::Sketch::from_points(vec![
-            [-x / 2., -y / 2.],
-            [ x / 2., -y / 2.],
-            [ x / 2.,  y / 2.],
-            [-x / 2.,  y / 2.],
-        ]).with_color([100,255,0,200]);
-
-        let cuboid = fj::Sweep::from_path(rectangle.into(), [0., 0., z]);
-
-        Ok(cuboid.into())
-    }
-
-    fn metadata(&self) -> ModelMetadata {
-        ModelMetadata::new("Cuboid")
-            .with_argument(ArgumentMetadata::new("x").with_default_value("3.0"))
-            .with_argument(ArgumentMetadata::new("y").with_default_value("2.0"))
-            .with_argument(ArgumentMetadata::new("z").with_default_value("1.0"))
-    }
+#[fj::model]
+pub fn model(
+    #[param(default = 3.0)] x: f64,
+    #[param(default = 2.0)] y: f64,
+    #[param(default = 1.0)] z: f64,
+) -> fj::Shape {
+    #[rustfmt::skip]
+    let rectangle = fj::Sketch::from_points(vec![
+        [-x / 2., -y / 2.],
+        [ x / 2., -y / 2.],
+        [ x / 2.,  y / 2.],
+        [-x / 2.,  y / 2.],
+    ]).with_color([100,255,0,200]);
+
+    let cuboid = fj::Sweep::from_path(rectangle.into(), [0., 0., z]);
+
+    cuboid.into()
 }
diff --git a/models/spacer/src/lib.rs b/models/spacer/src/lib.rs
index 7e9c691bf..98724904d 100644
--- a/models/spacer/src/lib.rs
+++ b/models/spacer/src/lib.rs
@@ -1,56 +1,16 @@
-use fj::{
-    syntax::*, ArgumentMetadata, ContextExt, HostExt, Metadata, Model,
-    ModelMetadata,
-};
+use fj::syntax::*;
 
-fj::register_model!(|host| {
-    host.register_model(Spacer);
+#[fj::model]
+pub fn model(
+    #[param(default = 1.0, min = inner * 1.01)] outer: f64,
+    #[param(default = 0.5, max = outer * 0.99)] inner: f64,
+    #[param(default = 1.0)] height: f64,
+) -> fj::Shape {
+    let outer_edge = fj::Sketch::from_circle(fj::Circle::from_radius(outer));
+    let inner_edge = fj::Sketch::from_circle(fj::Circle::from_radius(inner));
 
-    Ok(Metadata::new(
-        env!("CARGO_PKG_NAME"),
-        env!("CARGO_PKG_VERSION"),
-    ))
-});
+    let footprint = outer_edge.difference(&inner_edge);
+    let spacer = footprint.sweep([0., 0., height]);
 
-struct Spacer;
-
-impl Model for Spacer {
-    fn shape(
-        &self,
-        ctx: &dyn fj::Context,
-    ) -> Result<fj::Shape, Box<dyn std::error::Error + Send + Sync>> {
-        let outer: f64 = ctx.parse_optional_argument("outer")?.unwrap_or(1.0);
-        let inner: f64 = ctx.parse_optional_argument("inner")?.unwrap_or(0.5);
-        let height: f64 = ctx.parse_optional_argument("height")?.unwrap_or(1.0);
-
-        if outer < inner * 1.01 {
-            todo!("Return a suitable error");
-        }
-        if inner > outer * 0.99 {
-            todo!("Return a suitable error");
-        }
-
-        let outer_edge =
-            fj::Sketch::from_circle(fj::Circle::from_radius(outer));
-        let inner_edge =
-            fj::Sketch::from_circle(fj::Circle::from_radius(inner));
-
-        let footprint = outer_edge.difference(&inner_edge);
-        let spacer = footprint.sweep([0., 0., height]);
-
-        Ok(spacer.into())
-    }
-
-    fn metadata(&self) -> ModelMetadata {
-        ModelMetadata::new("Spacer")
-            .with_argument(
-                ArgumentMetadata::new("outer").with_default_value("1.0"),
-            )
-            .with_argument(
-                ArgumentMetadata::new("inner").with_default_value("0.5"),
-            )
-            .with_argument(
-                ArgumentMetadata::new("height").with_default_value("1.0"),
-            )
-    }
+    spacer.into()
 }
diff --git a/models/star/src/lib.rs b/models/star/src/lib.rs
index 34fa9bab4..3d1775680 100644
--- a/models/star/src/lib.rs
+++ b/models/star/src/lib.rs
@@ -1,85 +1,40 @@
 use std::f64::consts::PI;
 
-use fj::{
-    ArgumentMetadata, ContextExt, HostExt, Metadata, Model, ModelMetadata,
-};
-
-fj::register_model!(|host| {
-    host.register_model(Star);
-
-    Ok(Metadata::new(
-        env!("CARGO_PKG_NAME"),
-        env!("CARGO_PKG_VERSION"),
-    ))
-});
-
-struct Star;
-
-impl Model for Star {
-    fn shape(
-        &self,
-        ctx: &dyn fj::Context,
-    ) -> Result<fj::Shape, Box<dyn std::error::Error + Send + Sync>> {
-        let num_points: u64 =
-            ctx.parse_optional_argument("num_points")?.unwrap_or(5);
-        let r1: f64 = ctx.parse_optional_argument("r1")?.unwrap_or(1.0);
-        let r2: f64 = ctx.parse_optional_argument("r2")?.unwrap_or(2.0);
-        let h: f64 = ctx.parse_optional_argument("h")?.unwrap_or(1.0);
-
-        if num_points < 3 {
-            todo!();
-        }
-        if r1 < 1.0 {
-            todo!();
-        }
-        if r2 < 2.0 {
-            todo!();
-        }
-
-        let num_vertices = num_points * 2;
-        let vertex_iter = (0..num_vertices).map(|i| {
-            let angle =
-                fj::Angle::from_rad(2. * PI / num_vertices as f64 * i as f64);
-            let radius = if i % 2 == 0 { r1 } else { r2 };
-            (angle, radius)
-        });
-
-        // Now that we got that iterator prepared, generating the vertices is just a
-        // bit of trigonometry.
-        let mut outer = Vec::new();
-        let mut inner = Vec::new();
-        for (angle, radius) in vertex_iter {
-            let (sin, cos) = angle.rad().sin_cos();
-
-            let x = cos * radius;
-            let y = sin * radius;
-
-            outer.push([x, y]);
-            inner.push([x / 2., y / 2.]);
-        }
-
-        let outer = fj::Sketch::from_points(outer);
-        let inner = fj::Sketch::from_points(inner);
+#[fj::model]
+pub fn model(
+    #[param(default = 5, min = 3)] num_points: u64,
+    #[param(default = 1.0, min = 1.0)] r1: f64,
+    #[param(default = 2.0, min = 2.0)] r2: f64,
+    #[param(default = 1.0)] h: f64,
+) -> fj::Shape {
+    let num_vertices = num_points * 2;
+    let vertex_iter = (0..num_vertices).map(|i| {
+        let angle =
+            fj::Angle::from_rad(2. * PI / num_vertices as f64 * i as f64);
+        let radius = if i % 2 == 0 { r1 } else { r2 };
+        (angle, radius)
+    });
+
+    // Now that we got that iterator prepared, generating the vertices is just a
+    // bit of trigonometry.
+    let mut outer = Vec::new();
+    let mut inner = Vec::new();
+    for (angle, radius) in vertex_iter {
+        let (sin, cos) = angle.rad().sin_cos();
+
+        let x = cos * radius;
+        let y = sin * radius;
+
+        outer.push([x, y]);
+        inner.push([x / 2., y / 2.]);
+    }
 
-        let footprint =
-            fj::Difference2d::from_shapes([outer.into(), inner.into()]);
+    let outer = fj::Sketch::from_points(outer);
+    let inner = fj::Sketch::from_points(inner);
 
-        let star = fj::Sweep::from_path(footprint.into(), [0., 0., h]);
+    let footprint = fj::Difference2d::from_shapes([outer.into(), inner.into()]);
 
-        Ok(star.into())
-    }
+    let star = fj::Sweep::from_path(footprint.into(), [0., 0., h]);
 
-    fn metadata(&self) -> ModelMetadata {
-        ModelMetadata::new("Star")
-            .with_argument(
-                ArgumentMetadata::new("num_points").with_default_value("5"),
-            )
-            .with_argument(
-                ArgumentMetadata::new("r1").with_default_value("1.0"),
-            )
-            .with_argument(
-                ArgumentMetadata::new("r2").with_default_value("2.0"),
-            )
-            .with_argument(ArgumentMetadata::new("h").with_default_value("1.0"))
-    }
+    star.into()
 }
diff --git a/models/test/src/lib.rs b/models/test/src/lib.rs
index 11abd4883..76f955cf6 100644
--- a/models/test/src/lib.rs
+++ b/models/test/src/lib.rs
@@ -1,41 +1,18 @@
 use std::f64::consts::PI;
 
-use fj::{syntax::*, Angle, HostExt, Metadata, ModelMetadata};
-
-fj::register_model!(|host| {
-    host.register_model(Test);
-
-    Ok(
-        Metadata::new(env!("CARGO_PKG_NAME"), env!("CARGO_PKG_VERSION"))
-            .with_short_description(env!("CARGO_PKG_DESCRIPTION"))
-            .with_description(include_str!("../README.md"))
-            .with_homepage(env!("CARGO_PKG_HOMEPAGE"))
-            .with_repository(env!("CARGO_PKG_REPOSITORY"))
-            .with_license(env!("CARGO_PKG_LICENSE")),
-    )
-});
-
-struct Test;
-
-impl fj::Model for Test {
-    fn shape(
-        &self,
-        _ctx: &dyn fj::Context,
-    ) -> Result<fj::Shape, Box<dyn std::error::Error + Send + Sync>> {
-        let a = star(4, 1., [0, 255, 0, 200]);
-        let b = star(5, -1., [255, 0, 0, 255])
-            .rotate([1., 1., 1.], Angle::from_deg(45.))
-            .translate([3., 3., 1.]);
-        let c = spacer().translate([6., 6., 1.]);
-
-        let group = a.group(&b).group(&c);
-
-        Ok(group.into())
-    }
+use fj::{syntax::*, Angle};
 
-    fn metadata(&self) -> ModelMetadata {
-        ModelMetadata::new("Test")
-    }
+#[fj::model]
+pub fn model() -> fj::Shape {
+    let a = star(4, 1., [0, 255, 0, 200]);
+    let b = star(5, -1., [255, 0, 0, 255])
+        .rotate([1., 1., 1.], Angle::from_deg(45.))
+        .translate([3., 3., 1.]);
+    let c = spacer().translate([6., 6., 1.]);
+
+    let group = a.group(&b).group(&c);
+
+    group.into()
 }
 
 fn star(num_points: u64, height: f64, color: [u8; 4]) -> fj::Shape {