diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6631579..48c6c22 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -51,6 +51,7 @@ jobs:
           - 3.2
           - 3.3
           - 3.4
+          - "4.0"
 
       fail-fast: false
 
@@ -94,6 +95,7 @@ jobs:
           - 3.2
           - 3.3
           - 3.4
+          - "4.0"
 
       fail-fast: false
 
diff --git a/ffi/lib/llhttp.rb b/ffi/lib/llhttp.rb
index 236c81e..329b06c 100644
--- a/ffi/lib/llhttp.rb
+++ b/ffi/lib/llhttp.rb
@@ -10,6 +10,7 @@ module LLHttp
   require_relative "llhttp/version"
 
   extend FFI::Library
+
   ffi_lib(FFI::Compiler::Loader.find("llhttp-ext"))
 
   callback :llhttp_data_cb, [:pointer, :size_t], :void
diff --git a/spec/acceptance/server_spec.rb b/spec/acceptance/server_spec.rb
index 1efedb2..fba55be 100644
--- a/spec/acceptance/server_spec.rb
+++ b/spec/acceptance/server_spec.rb
@@ -29,9 +29,22 @@
     @bound_endpoint&.close
   end
 
-  it "parses" do
-    Timeout.timeout(1) do
-      expect(system("curl -v http://localhost:9000")).to be(true)
+  # The async-based test server requires Ruby >= 3.2: newer async drops 3.1, so
+  # on older Rubies bundle resolves to a stack that can't serve the request.
+  # llhttp core is still exercised by the rest of the suite on those versions.
+  it "parses", skip: RUBY_VERSION < "3.2" && "async test server requires Ruby >= 3.2" do
+    # Poll until the forked server is accepting and responding rather than
+    # asserting on a single curl under a tight timeout, so a slow fork + Async
+    # reactor cold start on a loaded CI runner costs a few retries, not a
+    # spurious failure.
+    result = false
+
+    Timeout.timeout(10) do
+      until (result = system("curl -sf http://localhost:9000", out: File::NULL, err: File::NULL))
+        sleep(0.05)
+      end
     end
+
+    expect(result).to be(true)
   end
 end
diff --git a/spec/initialize.rb b/spec/initialize.rb
index 2e21c11..b3e73df 100644
--- a/spec/initialize.rb
+++ b/spec/initialize.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require "pathname"
+require "pathname" # standard:disable Lint/RedundantRequireStatement -- needed on Ruby < 3.x where Pathname isn't autoloaded
 
 initializers = Pathname.new(File.expand_path("../initializers", __FILE__))