fix: force UTF-8 encoding for stdio streams

[iberianpig]

This change forces the stdin and stdout encodings to UTF-8, resolving platform-dependent issues—specifically, errors arising from handling multibyte characters (such as Japanese) under US-ASCII encoding.

Motivation and Context

On certain platforms the default encoding may be US-ASCII, which causes errors when processing multibyte characters like Japanese. By ensuring that the I/O streams always use UTF-8, it fix encoding-related issues that arise from varying default settings.

Related issues

• transport: force utf-8 for transports modelcontextprotocol#162 explicitly requires that MCP's JSON-RPC messages use UTF-8 encoding.
• Force stdin/stdout encoding to UTF-8 python-sdk#198 ensures that the Python SDK's stdin/stdout streams are always UTF-8 encoded, regardless of platform.

How Has This Been Tested?

The changes were tested in an environment where the MCP server (using the Ruby SDK) was executed via llm-functions. The testing steps and results were as follows:
ref: https://github.com/sigoden/llm-functions/tree/main/mcp/bridge

1. A server was configured using the following mcp.json settings:

{
  "mcpServers": {
    "example": {
      "command": "ruby",
      "args": [
        "/home/iberianpig/.ghq/github.com/modelcontextprotocol/ruby-sdk/mcp_demo.rb"
      ],
      "env": {
        "PATH": "/home/iberianpig/.rbenv/shims:/usr/bin"
      }
    }
  }
}

server:

#!/usr/bin/env ruby

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  gem "model_context_protocol", path: "/home/iberianpig/.ghq/github.com/modelcontextprotocol/ruby-sdk/"
  gem "debug"
end

require "model_context_protocol"
require "model_context_protocol/transports/stdio"

# Create a simple tool
class ExampleTool < ModelContextProtocol::Tool
  description "A simple example tool that echoes back its arguments"
  input_schema(
    properties: {
      message: { type: "string" },
    },
    required: ["message"]
  )

  class << self
    def call(message:, server_context:)
      ModelContextProtocol::Tool::Response.new([{
        type: "text",
        text: "Hello from example tool! Message: #{message}!!!!",
      }])
    end
  end
end

# Set up the server
server = ModelContextProtocol::Server.new(
  name: "example_server",
  tools: [ExampleTool],
)

# Create and start the transport
transport = ModelContextProtocol::Transports::StdioTransport.new(server)
transport.open

2. The server was started with the following command:

$ DEBUG=mcp:* argc mcp start  
Start MCP Bridge server...  
Merge MCP tools into functions.json  
Build bin/example_example_tool

3. The tool list was retrieved successfully:

$ curl http://localhost:8808/tools  
[{"name":"example_example_tool","description":"A simple example tool that echoes back its arguments","parameters":{"type":"object","properties":{"message":{"type":"string"}},"required":["message"]},"mcp":"example"}]

4. Before the fix, sending a POST request with a Japanese message resulted in an error:

$ curl -X POST http://localhost:8808/tools/example_example_tool -H 'content-type: application/json' -d '{"message": "こんにちは！"}'
{"code":-32000,"name":"McpError"}$

The server log indicated the following error:

$ argc mcp logs -f
/home/iberianpig/.ghq/github.com/modelcontextprotocol/ruby-sdk/lib/model_context_protocol/transports/stdio.rb:20:in `strip': invalid byte sequence in US-ASCII (Encoding::CompatibilityError)
      from .../ruby-sdk/lib/model_context_protocol/transports/stdio.rb:20:in `open'
      from .../ruby-sdk/mcp_demo.rb:42:in `<main>'

5. After the change, the same POST request returned a successful response:

$ curl -X POST http://localhost:8808/tools/example_example_tool -H 'content-type: application/json' -d '{"message": "こんにちは！"}'
{"code":0,"result":"Hello from example tool! Message: こんにちは！!!!!"}

Breaking Changes

None. This change does not introduce any breaking changes to the existing functionality of the MCP server or its tools.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This change is similar to the modifications made in modelcontextprotocol/python-sdk#198.