[STDIO] BadMapError when processing incoming messages

[b-serra]

Description

The STDIO transport crashes with a BadMapError when processing any incoming JSON-RPC message.

Steps to Reproduce

1. Start an MCP server with stdio transport
2. Send any valid JSON-RPC message via stdin (e.g., an initialize request)
3. Server crashes with BadMapError

Example:

echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}}' | mix mcp.server

Expected Behavior

The server should process the message and return a valid JSON-RPC response.

Actual Behavior

** (BadMapError) expected a map, got:
    [
      %{
        "id" => 1,
        "jsonrpc" => "2.0",
        "method" => "initialize",
        ...
      }
    ]

Root Cause

In lib/hermes/server/transport/stdio.ex, the handle_incoming_data/2 function calls Message.decode/1 which returns {:ok, messages} where messages is a list of decoded messages. However, this list is passed directly to process_message/2 which expects a single message map.

# Current code (line 241-243):
case Message.decode(data) do
  {:ok, messages} ->
    process_message(messages, state)  # Bug: messages is a list, not a map

Proposed Fix

Iterate over the messages list before calling process_message/2:

case Message.decode(data) do
  {:ok, messages} ->
    Enum.each(messages, &process_message(&1, state))

Environment

• hermes_mcp version: 0.14.1
• Elixir version: 1.15+
• OS: macOS/Linux