diff --git a/packages/core/src/impl.ts b/packages/core/src/impl.ts
index 99b02e3..7be8d11 100644
--- a/packages/core/src/impl.ts
+++ b/packages/core/src/impl.ts
@@ -532,7 +532,7 @@ export const fn = <const P extends readonly any[], const R>(
 };
 
 /** Construct an opaque function whose implementation runs `f`. */
-export const opaque = <const P extends readonly Reals[], const R extends Reals>(
+export const opaque = <const P extends readonly any[], const R>(
   params: P,
   ret: R,
   f: (...args: JsArgs<SymbolicParams<P>>) => ToJs<Value<R>>,
@@ -604,6 +604,8 @@ type ToJs<T> = [T] extends [Null]
   ? number
   : [T] extends [Nat]
   ? number
+  : [T] extends [Vec<infer V>]
+  ? ToJs<V>[]
   : { [K in keyof T]: ToJs<T[K]> };
 
 /** Map from an abstract value type array to a concrete argument type array. */
diff --git a/packages/core/src/index.test.ts b/packages/core/src/index.test.ts
index 4528f64..961e9e6 100644
--- a/packages/core/src/index.test.ts
+++ b/packages/core/src/index.test.ts
@@ -942,4 +942,46 @@ describe("valid", () => {
     const h = await compile(g);
     expect(h({ v: [2], i: 0 })).toEqual({ v: [1], i: 0 });
   });
+
+  test("sort", async () => {
+    const n = 3;
+
+    const sortWithIndices = opaque(
+      [Vec(n, Real)],
+      Vec(n, struct({ x: Real, i: n })),
+      (v) => v.map((x, i) => ({ x, i })).sort((a, b) => a.x - b.x),
+    );
+    sortWithIndices.jvp = fn([Vec(n, Dual)], Vec(n, { x: Dual, i: n }), (v) => {
+      const w = sortWithIndices(vec(n, Real, (i) => v[i].re));
+      return vec(n, { x: Dual, i: n }, (j) => {
+        const { i } = w[j];
+        return { x: v[i], i };
+      });
+    });
+
+    const sort = fn([Vec(n, Real)], Vec(n, Real), (v) => {
+      const w = sortWithIndices(v);
+      return vec(n, Real, (i) => w[i].x);
+    });
+
+    const sortGrad = fn([Vec(n, Real), Vec(n, Real)], Vec(n, Real), (v, g) =>
+      vjp(sort)(v).grad(g),
+    );
+
+    const interpreted = interp(sortGrad);
+    expect(interpreted([42, 121, 342], [-1, -2, -3])).toEqual([-1, -2, -3]);
+    expect(interpreted([42, 342, 121], [-1, -2, -3])).toEqual([-1, -3, -2]);
+    expect(interpreted([121, 42, 342], [-1, -2, -3])).toEqual([-2, -1, -3]);
+    expect(interpreted([121, 342, 42], [-1, -2, -3])).toEqual([-2, -3, -1]);
+    expect(interpreted([342, 42, 121], [-1, -2, -3])).toEqual([-3, -1, -2]);
+    expect(interpreted([342, 121, 42], [-1, -2, -3])).toEqual([-3, -2, -1]);
+
+    const compiled = await compile(sortGrad);
+    expect(compiled([42, 121, 342], [-1, -2, -3])).toEqual([-1, -2, -3]);
+    expect(compiled([42, 342, 121], [-1, -2, -3])).toEqual([-1, -3, -2]);
+    expect(compiled([121, 42, 342], [-1, -2, -3])).toEqual([-2, -1, -3]);
+    expect(compiled([121, 342, 42], [-1, -2, -3])).toEqual([-2, -3, -1]);
+    expect(compiled([342, 42, 121], [-1, -2, -3])).toEqual([-3, -1, -2]);
+    expect(compiled([342, 121, 42], [-1, -2, -3])).toEqual([-3, -2, -1]);
+  });
 });