MCP server incorrectly returns errors for JSON-RPC notifications

[rickhull]

$ cat .mise.toml
[tools]
erlang = "28"
elixir = "1.19-otp-28"
elixir-ls = "0.30.0"

The ElixirLS MCP server returns error -32600 "Invalid request" for JSON-RPC notifications (messages without an id field). According to the JSON-RPC 2.0 spec and MCP spec, notifications should be silently ignored if not supported, not return error responses.

This breaks compatibility with MCP clients like Codex that send the required initialized notification after the initialize request.

Reproduction:

(echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}'; echo '{"jsonrpc":"2.0","method":"initialized"}'; sleep 0.5) | elixir /path/to/tcp_to_stdio_bridge.exs 3798

Expected: Only the initialize response
Actual: Initialize response + {"error":{"code":-32600,"message":"Invalid request"},"id":null,"jsonrpc":"2.0"}

Impact: MCP clients that send the initialized notification (per spec) fail to connect. This includes the codex CLI from OpenAI.

Full repro:

$ (echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}'; echo '{"jsonrpc":"2.0","method":"initialized"}'; sleep 0.5) | elixir /home/rwh/.local/share/mise/installs/elixir-ls/0.30.0/tcp_to_stdio_bridge.exs 3798
{"id":1,"jsonrpc":"2.0","result":{"capabilities":{"tools":{}},"protocolVersion":"2024-11-05","serverInfo":{"name":"ElixirLS MCP Server","version":"1.0.0"}}}
{"error":{"code":-32600,"message":"Invalid request"},"id":null,"jsonrpc":"2.0"}

[rickhull]

WORKAROUND: mcp_filter.sh

#!/bin/bash

# Filter error responses to notifications (messages with id:null)
elixir "$@" | awk '
{
  # Check if this is an error response with id:null
  if ($0 ~ /"error"/ && $0 ~ /"id":null/) {
    # Skip this line
    next
  }
  # Forward all other lines
  print $0
  fflush()
}
'

codex mcp add elixir-ls -- /path/to/mcp_filter.sh /path/to/tcp_to_stdio_bridge.exs $MCP_PORT