add configurable receive timeout for POST requests

README.md

@@ -28,3 +28,8 @@ If you have an SSE MCP server available at `http://localhost:4000/tidewave/mcp`,
 ## Configuration
 
 `mcp-proxy` either accepts the SSE URL as argument or using the environment variable `SSE_URL`. For debugging purposes, you can also pass `--debug`, which will log debug messages on stderr.
+
+Other supported flags:
+
+* `--max-disconnected-time` the maximum amount of time for trying to reconnect while disconnected. When not set, defaults to infinity.
+* `--receive-timeout` the maximum amount of time to wait for an individual reply from the MCP server in milliseconds. Defaults to 60000 (60 seconds).

lib/mcp_proxy.ex

@@ -7,7 +7,13 @@ defmodule McpProxy do
   @doc false
   def main(args) do
     {opts, args} =
-      OptionParser.parse!(args, strict: [debug: :boolean, max_disconnected_time: :integer])
+      OptionParser.parse!(args,
+        strict: [
+          debug: :boolean,
+          max_disconnected_time: :integer,
+          receive_timeout: :integer
+        ]
+      )
 
     base_url =
       case args do
@@ -23,8 +29,9 @@ defmodule McpProxy do
       end
 
     Application.ensure_all_started(:req)
+    Application.put_all_env(mcp_proxy: opts)
 
-    {:ok, handler} = SSE.start_link({base_url, opts})
+    {:ok, handler} = SSE.start_link(base_url)
     ref = Process.monitor(handler)
 
     receive do

lib/mcp_proxy/sse.ex

@@ -34,13 +34,13 @@ defmodule McpProxy.SSE do
   end
 
   @impl true
-  def init({sse_url, opts}) do
+  def init(sse_url) do
     {:ok,
      %{
        url: sse_url,
        endpoint: nil,
-       debug: Keyword.get(opts, :debug, false),
-       max_disconnected_time: Keyword.get(opts, :max_disconnected_time),
+       debug: Application.get_env(:mcp_proxy, :debug, false),
+       max_disconnected_time: Application.get_env(:mcp_proxy, :max_disconnected_time),
        disconnected_since: nil,
        state: :connecting,
        connect_tries: 0,
@@ -263,11 +263,17 @@ defmodule McpProxy.SSE do
   end
 
   # regular events
-  def handle_info({:sse_event, {:message, event}}, state),
-    do: handle_event(event, state, &IO.puts(Jason.encode!(&1)))
+  def handle_info({:sse_event, {:message, event}}, state) do
+    handle_event(event, state, {&IO.puts(Jason.encode!(&1)), fn _ -> :ok end})
+  end
 
-  def handle_info({:io_event, event}, state),
-    do: handle_event(event, state, &forward_message(&1, state.endpoint, state.debug))
+  def handle_info({:io_event, event}, state) do
+    handle_event(
+      event,
+      state,
+      {&forward_message(&1, state.endpoint, state.debug), &IO.puts(Jason.encode!(&1))}
+    )
+  end
 
   # whenever the HTTP process dies, we try to reconnect
   def handle_info({:DOWN, _ref, :process, http_pid, _reason}, %{http_pid: http_pid} = state) do
@@ -327,52 +333,92 @@ defmodule McpProxy.SSE do
     Enum.reduce(messages, state, fn message, state -> handle_event(message, state, handler) end)
   end
 
-  defp handle_event(%{"jsonrpc" => "2.0", "id" => request_id} = event, state, handler)
+  defp handle_event(%{"jsonrpc" => "2.0", "id" => request_id} = event, state, {req, resp})
        when is_request(event) do
     # whenever we get a request from the client (OR the server!)
     # we generate a random ID to prevent duplicate IDs, for example when
     # a reconnected server decides to send a ping and always starts with ID 0
     new_id = random_id!()
     event = Map.put(event, "id", new_id)
 
-    handler.(event)
+    state =
+      case req.(event) do
+        :ok ->
+          %{state | id_map: Map.put(state.id_map, new_id, request_id)}
+
+        {:reply_error, reply} ->
+          resp.(Map.put(reply, "id", request_id))
+          # we don't store the new_id when we already replied to prevent duplicates;
+          # instead, we'll log an error if a server reply is received later and we already
+          # replied
+          state
+      end
 
-    {:noreply, %{state | id_map: Map.put(state.id_map, new_id, request_id)}}
+    {:noreply, state}
   end
 
-  defp handle_event(%{"jsonrpc" => "2.0", "id" => response_id} = event, state, handler)
+  defp handle_event(%{"jsonrpc" => "2.0", "id" => response_id} = event, state, {handler, _})
        when is_response(event) do
     # whenever we receive a response (from the client or server)
     # we fetch the original ID from the id map to present the expected
     # ID in the reply
-    original_id = Map.fetch!(state.id_map, response_id)
-    event = Map.put(event, "id", original_id)
+    case state.id_map do
+      %{^response_id => original_id} ->
+        event = Map.put(event, "id", original_id)
+
+        handler.(event)
+
+        {:noreply, %{state | id_map: Map.delete(state.id_map, response_id)}}
 
-    handler.(event)
+      _ ->
+        Logger.error(
+          "Did not find original ID for response: #{response_id}. Discarding response!"
+        )
 
-    {:noreply, %{state | id_map: Map.delete(state.id_map, response_id)}}
+        {:noreply, state}
+    end
   end
 
   # no id, so must be a notification that we can just forward as is
-  defp handle_event(%{"jsonrpc" => "2.0"} = event, state, handler) do
-    handler.(event)
+  defp handle_event(%{"jsonrpc" => "2.0"} = event, state, {req, _}) do
+    req.(event)
 
     {:noreply, state}
   end
 
   ## other helpers
 
   defp forward_message(message, endpoint, debug) do
-    try do
-      if debug, do: Logger.debug("Forwarding request to server: #{inspect(message)}")
-      Req.post!(endpoint, json: message)
-    rescue
-      error ->
-        Logger.error(
-          "Failed to forward message: #{Exception.format(:error, error, __STACKTRACE__)}"
-        )
-
-        # TODO: store message and replay later?
+    if debug, do: Logger.debug("Forwarding request to server: #{inspect(message)}")
+
+    case Req.post(endpoint,
+           json: message,
+           receive_timeout: Application.get_env(:mcp_proxy, :receive_timeout, 60_000)
+         ) do
+      {:ok, %{status: status}} when status in 200..299 ->
+        :ok
+
+      {:ok, %{status: status}} ->
+        {:reply_error,
+         %{
+           jsonrpc: "2.0",
+           error: %{
+             code: -32011,
+             message: "Failed to forward request. Request failed with status code: #{status}"
+           }
+         }}
+
+      {:error, %Req.TransportError{reason: reason}} ->
+        Logger.error("Failed to forward message #{inspect(message)}:\n#{inspect(reason)}")
+
+        {:reply_error,
+         %{
+           jsonrpc: "2.0",
+           error: %{
+             code: -32011,
+             message: "Failed to forward request. Reason: #{inspect(reason)}"
+           }
+         }}
     end
   end
 

test/mcp_proxy_test.exs

@@ -6,7 +6,7 @@ defmodule McpProxyTest do
 
   alias McpProxy.SSEServer
 
-  setup do
+  setup context do
     {result, _output} =
       with_io(:stderr, fn ->
         _server_pid = start_supervised!(SSEServer, restart: :temporary)
@@ -17,7 +17,10 @@ defmodule McpProxyTest do
         _main_pid =
           spawn_link(fn ->
             Process.group_leader(self(), parent)
-            McpProxy.main(["http://localhost:#{port}/sse", "--debug"])
+
+            McpProxy.main(
+              ["http://localhost:#{port}/sse", "--debug"] ++ (context[:extra_args] || [])
+            )
           end)
 
         assert_receive {:io_request, io_pid, reply_as, {:get_line, :unicode, []}}
@@ -116,6 +119,32 @@ defmodule McpProxyTest do
     assert io =~ "Flushing buffer"
   end
 
+  @tag extra_args: ["--receive-timeout", "50"]
+  test "handles receive timeout", %{io_pid: io_pid, reply_as: reply_as} do
+    capture_io(:stderr, fn ->
+      send_message(
+        io_pid,
+        reply_as,
+        %{
+          jsonrpc: "2.0",
+          id: "call-1",
+          method: "tools/call",
+          params: %{"name" => "sleep", "arguments" => %{"time" => 100}}
+        }
+      )
+
+      assert_receive {:io_request, _io_pid, _reply_as, {:get_line, :unicode, []}}
+      assert_receive {:io_request, put_pid, put_reply_as, {:put_chars, :unicode, json}}, 100
+      send(put_pid, {:io_reply, put_reply_as, :ok})
+
+      assert %{"id" => "call-1", "error" => %{"message" => message}} = Jason.decode!(json)
+      assert message =~ "Failed to forward request"
+
+      # wait an extra 100 milliseconds for the log about discarding a duplicate response
+      Process.sleep(100)
+    end) =~ "Discarding!"
+  end
+
   defp send_message(io_pid, reply_as, json) do
     send(io_pid, {:io_reply, reply_as, Jason.encode_to_iodata!(json)})
   end

test/support/sse_server.ex

@@ -75,7 +75,7 @@ defmodule McpProxy.SSEServer do
     post "/message" do
       with %{query_params: %{"session_id" => session_id}} <- conn,
            [{pid, _}] <- Registry.lookup(McpProxy.SSEServer.Registry, session_id) do
-        GenServer.cast(pid, {:request, conn.body_params})
+        GenServer.call(pid, {:request, conn.body_params})
 
         conn
         |> put_resp_content_type("application/json")
@@ -93,16 +93,16 @@ defmodule McpProxy.SSEServer do
     end
 
     @impl GenServer
-    def handle_cast({:request, message}, state) do
+    def handle_call({:request, message}, _from, state) do
       result = handle_message(message)
 
       if result do
         case Plug.Conn.chunk(state.conn, ["event: message\ndata: ", result]) do
-          {:ok, conn} -> {:noreply, %{state | conn: conn}}
+          {:ok, conn} -> {:reply, :ok, %{state | conn: conn}}
           {:error, :closed} -> {:stop, :shutdown, state}
         end
       else
-        {:noreply, state}
+        {:reply, :ok, state}
       end
     end
 
@@ -164,6 +164,20 @@ defmodule McpProxy.SSEServer do
       })
     end
 
+    defp handle_message(%{
+           "id" => request_id,
+           "method" => "tools/call",
+           "params" => %{"name" => "sleep", "arguments" => %{"time" => time}}
+         }) do
+      Process.sleep(time)
+
+      Jason.encode_to_iodata!(%{
+        jsonrpc: "2.0",
+        id: request_id,
+        result: %{content: [%{text: "Ok"}]}
+      })
+    end
+
     defp handle_message(%{"id" => id, "method" => other} = _message) do
       Jason.encode_to_iodata!(%{
         jsonrpc: "2.0",