diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7afbf20..1ca2d73 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## [Unreleased]
+
+- `Scheduler#io_close` and `Async::Selector#io_close` now take a file descriptor (Integer), matching Ruby's Fiber::Scheduler protocol. (Thanks to [Samuel Williams](https://github.com/samuel-williams-shopify)).
+
 ## [0.1.3] - 2026-05-08
 
 - Update to Zig 0.16.0; update libxev and zig.rb to the latest versions.
diff --git a/lib/carbon_fiber/async.rb b/lib/carbon_fiber/async.rb
index d6c0ab8..d389e9c 100644
--- a/lib/carbon_fiber/async.rb
+++ b/lib/carbon_fiber/async.rb
@@ -143,11 +143,10 @@ def io_write(fiber, io, buffer, length, offset = 0)
         ruby_io_write(fiber, io, buffer, length, offset)
       end
 
-      # Cancel pending waiters and close the descriptor.
-      # @param io [IO]
-      def io_close(io)
-        fd = io.respond_to?(:fileno) ? io.fileno : io.to_i
-        super(fd, IOError.new("stream closed while waiting"))
+      # Cancel pending waiters on the descriptor.
+      # @param descriptor [Integer]
+      def io_close(descriptor)
+        super(descriptor, IOError.new("stream closed while waiting"))
       end
 
       # Wait for a child process on a background thread.
diff --git a/lib/carbon_fiber/scheduler.rb b/lib/carbon_fiber/scheduler.rb
index 4f8e088..29d1e36 100644
--- a/lib/carbon_fiber/scheduler.rb
+++ b/lib/carbon_fiber/scheduler.rb
@@ -238,15 +238,14 @@ def io_select(...)
       end
     end
 
-    # Cancel pending waiters on an IO and close the descriptor.
-    # @param io [IO]
-    def io_close(io)
-      descriptor = io.respond_to?(:to_i) ? io.to_i : io
+    # Cancel pending waiters on a descriptor and close it.
+    # @param descriptor [Integer]
+    def io_close(descriptor)
       @selector.io_close(descriptor, IOError.new("stream closed while waiting"))
 
       Fiber.blocking do
-        target = io.is_a?(IO) ? io : IO.for_fd(descriptor.to_i)
-        target.close unless target.closed?
+        io = IO.for_fd(descriptor)
+        io.close unless io.closed?
       end
 
       true