diff --git a/src/twocore/frontend/js/lower.gleam b/src/twocore/frontend/js/lower.gleam
index be1f19d..921952c 100644
--- a/src/twocore/frontend/js/lower.gleam
+++ b/src/twocore/frontend/js/lower.gleam
@@ -4358,6 +4358,24 @@ fn lower_call_fixed(
     )
       if sym == "match" || sym == "replace" || sym == "search" || sym == "split"
     -> lower_regex_symbol_method(sym, object, arguments, env, ctx, ctr)
+    // `Number.prototype.toString(radix)` — `Number.prototype` is itself a Number
+    // object whose [[NumberData]] is +0 (ES2023 §21.1.3), so per §21.1.3.6 it
+    // renders as "0" in every radix (2–36) and for an absent/undefined/10 radix.
+    // Route it through `num_to_string_radix` with a literal 0 receiver; without
+    // this the receiver is the opaque `builtin_prototype("Number")` marker, whose
+    // ToNumber is NaN, so toString falls through to the generic object form
+    // "[object Object]".
+    ast.MemberExpression(
+      object: ast.MemberExpression(
+        object: ast.Identifier(name: "Number", ..),
+        property: ast.Identifier(name: "prototype", ..),
+        computed: False,
+        ..,
+      ),
+      property: ast.Identifier(name: "toString", ..),
+      computed: False,
+      ..,
+    ) -> lower_number_proto_tostring(arguments, env, ctx, ctr)
     // recv.method(args) — method dispatch (array/string methods).
     ast.MemberExpression(
       object:,
@@ -5539,6 +5557,37 @@ fn lower_method(
   }
 }
 
+/// `Number.prototype.toString(radix)` — render the [[NumberData]] of
+/// `Number.prototype`, which is +0 (ES2023 §21.1.3), in `radix`.
+///
+/// Parameters: `arguments` is the call's argument list; the first argument (if
+/// present) is the radix and any excess is ignored. An absent or `undefined`
+/// radix means base 10 (§21.1.3.6 step 3).
+///
+/// Returns the lowered `#(binds, value, ctr)` for a `num_to_string_radix` call
+/// whose receiver is a literal 0, so the result is always the string "0"
+/// regardless of the radix (0 is the same digit in every base). Never returns
+/// `Error` for this shape (the radix expression itself may still error while
+/// lowering, propagated via `result_try`).
+fn lower_number_proto_tostring(
+  arguments: List(ast.Expression),
+  env: Env,
+  ctx: Ctx,
+  ctr: Int,
+) -> Result(#(List(Bind), ir.Value, Int), Error) {
+  use #(ba, argvals, ctr) <- result_try(lower_args(arguments, env, ctx, ctr))
+  let radix = case argvals {
+    [r, ..] -> r
+    [] -> undefined()
+  }
+  let #(bz, zero, ctr) = number_literal(0.0, ctr)
+  Ok(bind_after(
+    list.append(ba, bz),
+    ir.CallHost("js", "num_to_string_radix", [zero, radix]),
+    ctr,
+  ))
+}
+
 /// The arity of `C.method` if `C` is a known class with static method `method`.
 fn static_method(
   object: ast.Expression,
diff --git a/test/js_compiler_test.gleam b/test/js_compiler_test.gleam
index f455b5a..0bad27e 100644
--- a/test/js_compiler_test.gleam
+++ b/test/js_compiler_test.gleam
@@ -7328,3 +7328,36 @@ pub fn wave14_arrow_callback_ignores_thisarg_test() {
   val("[1, 2, 3].map((x) => x * 2, { k: 9 }).join(',')")
   |> should.equal(dyn("2,4,6"))
 }
+
+// ==== Number (wave 15) ====
+
+// `Number.prototype.toString()` — `Number.prototype` is itself a Number object whose
+// [[NumberData]] is +0 (ES2023 §21.1.3), so §21.1.3.6 renders it as "0" for an absent
+// radix (base 10). test262 S15.7.4.2_A1_T01.
+pub fn number_proto_tostring_default_test() {
+  val("Number.prototype.toString()") |> should.equal(dyn("0"))
+}
+
+// The same value with an explicit radix of 10 or `undefined` is still base 10 → "0".
+// test262 S15.7.4.2_A1_T02 / _A1_T03.
+pub fn number_proto_tostring_radix10_and_undefined_test() {
+  val("Number.prototype.toString(10)") |> should.equal(dyn("0"))
+  val("Number.prototype.toString(undefined)") |> should.equal(dyn("0"))
+}
+
+// For every radix 2..36 the digit for 0 is "0", so `Number.prototype.toString(r)`
+// is "0" throughout the whole legal radix range. test262 S15.7.4.2_A2_T01..T34.
+pub fn number_proto_tostring_all_radixes_test() {
+  val("Number.prototype.toString(2)") |> should.equal(dyn("0"))
+  val("Number.prototype.toString(8)") |> should.equal(dyn("0"))
+  val("Number.prototype.toString(16)") |> should.equal(dyn("0"))
+  val("Number.prototype.toString(36)") |> should.equal(dyn("0"))
+}
+
+// A plain number renders its own value in the requested radix (§21.1.3.6): the radix
+// path is unaffected by the Number.prototype receiver special-case above.
+pub fn number_tostring_radix_plain_value_test() {
+  val("(255).toString(16)") |> should.equal(dyn("ff"))
+  val("(5).toString(2)") |> should.equal(dyn("101"))
+  val("(-1).toString(2)") |> should.equal(dyn("-1"))
+}