diff --git a/test/finch/alpn_integration_test.exs b/test/finch/alpn_integration_test.exs
new file mode 100644
index 0000000..2318ba0
--- /dev/null
+++ b/test/finch/alpn_integration_test.exs
@@ -0,0 +1,121 @@
+defmodule Finch.ALPNIntegrationTest do
+  use ExUnit.Case, async: false
+
+  @moduletag :capture_log
+
+  setup_all do
+    {:ok, listen_socket} = :ssl.listen(0, mode: :binary)
+    {:ok, {_address, port}} = :ssl.sockname(listen_socket)
+    :ssl.close(listen_socket)
+
+    {:ok, _} = Finch.ALPNServer.start(port)
+
+    {:ok, url: "https://localhost:#{port}"}
+  end
+
+  # This test reproduces issue #265 where sending POST requests with bodies larger than 64KB
+  # fails when using protocols: [:http1, :http2] due to HTTP/2 window size constraints.
+  # The HTTP/1 pool doesn't implement HTTP/2 flow control, causing the request to exceed
+  # the HTTP/2 window size (65535 bytes).
+  @tag :skip
+  test "POST request with body larger than 64KB using ALPN negotiation", %{url: url} do
+    # Start Finch with ALPN negotiation (both HTTP/1 and HTTP/2)
+    start_supervised!(
+      {Finch,
+       name: ALPNFinch,
+       pools: %{
+         default: [
+           protocols: [:http1, :http2],
+           conn_opts: [
+             transport_opts: [
+               verify: :verify_none
+             ]
+           ]
+         ]
+       }}
+    )
+
+    # Create a body larger than 64KB (65538 bytes as per issue)
+    large_body = :crypto.strong_rand_bytes(65_538)
+
+    # This should fail with {:exceeds_window_size, :request, 65535}
+    # when the connection upgrades to HTTP/2 via ALPN
+    result =
+      Finch.build(:post, "#{url}/echo", [], large_body)
+      |> Finch.request(ALPNFinch)
+
+    # Currently this fails with the window size error
+    # Once fixed, this should succeed
+    case result do
+      {:ok, response} ->
+        assert response.status == 200
+        body = Jason.decode!(response.body)
+        assert body["received_bytes"] == 65_538
+
+      {:error, %Mint.HTTPError{reason: {:exceeds_window_size, :request, 65_535}}} ->
+        flunk("""
+        Request failed with window size error - this is the bug we're trying to fix.
+        The HTTP/1 pool doesn't implement HTTP/2 flow control when connections upgrade via ALPN.
+        """)
+
+      {:error, error} ->
+        flunk("Unexpected error: #{inspect(error)}")
+    end
+  end
+
+  # Test that confirms HTTP/2-only works correctly with large bodies
+  test "POST request with body larger than 64KB using HTTP/2 only (should work)", %{url: url} do
+    start_supervised!(
+      {Finch,
+       name: HTTP2Finch,
+       pools: %{
+         default: [
+           protocols: [:http2],
+           conn_opts: [
+             transport_opts: [
+               verify: :verify_none
+             ]
+           ]
+         ]
+       }}
+    )
+
+    large_body = :crypto.strong_rand_bytes(65_538)
+
+    {:ok, response} =
+      Finch.build(:post, "#{url}/echo", [], large_body)
+      |> Finch.request(HTTP2Finch)
+
+    assert response.status == 200
+    body = Jason.decode!(response.body)
+    assert body["received_bytes"] == 65_538
+  end
+
+  # Test that confirms HTTP/1-only works correctly with large bodies
+  test "POST request with body larger than 64KB using HTTP/1 only (should work)", %{url: url} do
+    start_supervised!(
+      {Finch,
+       name: HTTP1Finch,
+       pools: %{
+         default: [
+           protocols: [:http1],
+           conn_opts: [
+             transport_opts: [
+               verify: :verify_none
+             ]
+           ]
+         ]
+       }}
+    )
+
+    large_body = :crypto.strong_rand_bytes(65_538)
+
+    {:ok, response} =
+      Finch.build(:post, "#{url}/echo", [], large_body)
+      |> Finch.request(HTTP1Finch)
+
+    assert response.status == 200
+    body = Jason.decode!(response.body)
+    assert body["received_bytes"] == 65_538
+  end
+end
diff --git a/test/support/alpn_server.ex b/test/support/alpn_server.ex
new file mode 100644
index 0000000..673bc12
--- /dev/null
+++ b/test/support/alpn_server.ex
@@ -0,0 +1,60 @@
+defmodule Finch.ALPNServer do
+  @moduledoc false
+  # A test server that supports ALPN negotiation between HTTP/1 and HTTP/2
+
+  @fixtures_dir Path.expand("../fixtures", __DIR__)
+
+  def child_spec(opts) do
+    Plug.Cowboy.child_spec(
+      scheme: :https,
+      plug: Finch.ALPNServer.PlugRouter,
+      options: [
+        port: Keyword.fetch!(opts, :port),
+        cipher_suite: :strong,
+        certfile: Path.join([@fixtures_dir, "selfsigned.pem"]),
+        keyfile: Path.join([@fixtures_dir, "selfsigned_key.pem"]),
+        # Enable ALPN negotiation between HTTP/2 and HTTP/1.1
+        alpn_preferred_protocols: ["h2", "http/1.1"],
+        otp_app: :finch,
+        protocol_options: [
+          idle_timeout: 3_000,
+          inactivity_timeout: 5_000,
+          max_keepalive: 1_000,
+          request_timeout: 10_000,
+          shutdown_timeout: 10_000
+        ]
+      ]
+    )
+  end
+
+  def start(port) do
+    Supervisor.start_link([child_spec(port: port)], strategy: :one_for_one)
+  end
+end
+
+defmodule Finch.ALPNServer.PlugRouter do
+  @moduledoc false
+
+  use Plug.Router
+
+  plug(:match)
+  plug(:dispatch)
+
+  get "/" do
+    conn
+    |> send_resp(200, "Hello world!")
+    |> halt()
+  end
+
+  post "/echo" do
+    {:ok, body, conn} = read_body(conn)
+    body_size = byte_size(body)
+
+    response = Jason.encode!(%{received_bytes: body_size})
+
+    conn
+    |> put_resp_content_type("application/json")
+    |> send_resp(200, response)
+    |> halt()
+  end
+end