diff --git a/changelog.d/7461-layer1-migrate-urlnew.md b/changelog.d/7461-layer1-migrate-urlnew.md
new file mode 100644
index 0000000000..5b737521cb
--- /dev/null
+++ b/changelog.d/7461-layer1-migrate-urlnew.md
@@ -0,0 +1 @@
+- **`new URL(input, base)` is the first lowering migrated onto the Layer 1 rooting API, and the move closed a window the hand-fix left.** `FnCtx` has no interior mutability, so #7459's borrow-carrying handle cannot be built on it — the shape that works against a `&mut`-only emitter is the combinator the runtime already settled on for layer 3: `call_rooted` emits the collecting call and roots its result in one step, so no unrooted register ever exists to be held across a later collection point. Migrating the path revealed that #7453's hand-written fix left `base_ptr` live and unrooted across `js_gc_temp_root_get` — a small window, but one nobody spotted in review, which is the argument for an API over a checklist. 11/11 URL gap tests byte-identical to the Node oracle; clean under forced evacuation. One lowering: this starts the migration, it does not finish Layer 1. (#7461)
diff --git a/crates/perry-codegen/src/expr/url_main.rs b/crates/perry-codegen/src/expr/url_main.rs
index de38fef4c4..d20d528054 100644
--- a/crates/perry-codegen/src/expr/url_main.rs
+++ b/crates/perry-codegen/src/expr/url_main.rs
@@ -54,15 +54,25 @@ pub(crate) fn lower(ctx: &mut FnCtx<'_>, expr: &Expr) -> Result<String> {
                 // would root an already-stale pointer.
                 let url_slot = super::temp_root::temp_root_push_i64(ctx, &url_ptr);
                 let base_v = lower_expr(ctx, base)?;
-                let base_ptr = ctx
-                    .block()
-                    .call(I64, "js_url_coerce_string", &[(DOUBLE, &base_v)]);
+                // Layer 1 migration (#7459): `call_rooted` emits the collecting
+                // call and roots its result in one step, so no unrooted
+                // register for `base_ptr` ever exists to be held across a later
+                // collection point. The window that made #7453 a bug is not
+                // expressible here.
+                let base_slot = crate::rooting::call_rooted(
+                    ctx,
+                    I64,
+                    "js_url_coerce_string",
+                    &[(DOUBLE, &base_v)],
+                );
                 let url_ptr = super::temp_root::temp_root_get_i64(ctx, &url_slot);
+                let base_ptr = base_slot.read(ctx);
                 let obj = ctx.block().call(
                     I64,
                     "js_url_new_with_base",
                     &[(I64, &url_ptr), (I64, &base_ptr)],
                 );
+                base_slot.release(ctx);
                 super::temp_root::temp_root_truncate(ctx, &url_slot);
                 obj
             } else {
diff --git a/crates/perry-codegen/src/rooting.rs b/crates/perry-codegen/src/rooting.rs
index a35cedc68d..d77575fd5d 100644
--- a/crates/perry-codegen/src/rooting.rs
+++ b/crates/perry-codegen/src/rooting.rs
@@ -238,3 +238,67 @@ mod tests {
         assert_eq!(len.0, "%r0");
     }
 }
+
+// ---------------------------------------------------------------------------
+// Applying the design to the REAL emitter.
+//
+// `FnCtx` has no interior mutability -- `ctx.block()` needs `&mut` -- so the
+// borrow-carrying `Raw` above cannot be built on it directly: `root(self)`
+// would need a second borrow while the handle still holds the first (the same
+// E0499 the RFC's own API hits, see `Raw`'s doc).
+//
+// The shape that DOES work against a `&mut`-only emitter is the combinator, and
+// it is the same one the runtime settled on for layer 3 (`RuntimeHandle::
+// across_*`): never hand out an unrooted handle at all. `call_rooted` emits the
+// collecting call and roots its result in one step, so there is no window in
+// which an unrooted register exists to be misused, and `read` re-reads through
+// the slot every time.
+//
+// This is weaker than the borrow formulation -- it prevents the bug rather than
+// detecting attempts to write it -- but it needs no emitter rewrite, which is
+// what makes it migratable one call site at a time.
+// ---------------------------------------------------------------------------
+
+use crate::expr::FnCtx;
+use crate::types::{I32, I64};
+
+/// A slot holding a GC-managed pointer for the duration of a lowering.
+#[derive(Debug, Clone)]
+pub struct RootedSlot {
+    idx: String,
+}
+
+impl RootedSlot {
+    /// Re-read the slot. Called afresh at every use: the returned register is
+    /// only valid until the next emission that can collect, and re-reading is
+    /// cheaper than reasoning about whether one has happened.
+    pub fn read(&self, ctx: &mut FnCtx<'_>) -> String {
+        ctx.block()
+            .call(I64, "js_gc_temp_root_get", &[(I32, &self.idx)])
+    }
+
+    /// Release the slot. Call after the last [`RootedSlot::read`].
+    pub fn release(self, ctx: &mut FnCtx<'_>) {
+        ctx.block()
+            .call_void("js_gc_temp_root_truncate", &[(I32, &self.idx)]);
+    }
+}
+
+/// Emit a call that can collect and root its result in one step.
+///
+/// The point is what this function does NOT return: an unrooted register. A
+/// caller cannot hold the result across a later collection point because it
+/// never has the result -- only a slot -- which is what makes the #7453 shape
+/// unwritable here rather than merely reviewable.
+pub fn call_rooted(
+    ctx: &mut FnCtx<'_>,
+    ret_ty: crate::types::LlvmType,
+    callee: &str,
+    args: &[(crate::types::LlvmType, &str)],
+) -> RootedSlot {
+    let reg = ctx.block().call(ret_ty, callee, args);
+    let idx = ctx
+        .block()
+        .call(I32, "js_gc_temp_root_push", &[(I64, &reg)]);
+    RootedSlot { idx }
+}