Text Mirror

A tiny MCP (Model Context Protocol) service written in Go. It mirrors (reverses) UTF-8 text while preserving grapheme clusters.

This repository implements a minimal MCP server and a single mirror tool to help me/us learn MCP basics and to build something that at minimum works with VS Code's Copilot (via stdio transport).

[!TIP]

Article about this project in Japanese: 「【Golang】最もシンプルな MCP Server を Go で堅牢に実装してみる」@ Qiita

Features

• MCP tool that reverses UTF‑8 text
• Unicode grapheme cluster–safe (handles emoji, combining marks, ZWJ sequences)
• stdio transport only (HTTP/SSE transports not implemented)

Prerequisites

• Requires Go 1.25 or newer (see go.mod).
  • Go 1.25 is already released as of 2025-08.
• golangci-lint (optional; make test runs lint)
• make (optional; for build and test automation)
• MCP client that supports stdio transport
  • e.g., VS Code v1.102 or newer + Copilot enabled (tested with VS Code v1.107.1)

Quick Start

This app aims to work with VS Code's Copilot extension via MCP. Any MCP-compatible client that supports stdio transport, Claude Desktop for example, should also work.

Build & Test

• Build:

  make build
  # or
  go build -o text-mirror .
  

• Tests & lint:

  make test
  # or
  golangci-lint run --fix
  go test -cover -race ./...
  

Using with VS Code (Copilot)

MCP support in VS Code is generally available as of version 1.102 and later.

1. Build the text-mirror as executable binary (as above) and copy its absolute path.

   • e.g., /home/user/go/bin/text-mirror

2. Register as an MCP server in VS Code's MCP config file.

   • Open VS Code's command palette (F1 or Ctrl+Shift+P) and search for:

     • MCP: Open User Configuration

   • Edit the generated mcp.json as follows (bare minimal example):

     {
       "servers": {
         "text-mirror": {
           "command": "/full/path/to/text-mirror",
           "env": {
             "MCP_TEXT_MIRROR_DEBUG_LOG": "/full/path/to/text-mirror.log"
           }
         }
       }
     }
     

     • If mcp.json already has other servers, just add the text-mirror entry under servers.
     • command is required.
       • Replace /full/path/to/text-mirror with the actual path to the built binary.
     • If type is omitted, VS Code assumes "stdio" by default for local servers. So no need to specify it here.
     • env is optional.
       • If MCP_TEXT_MIRROR_DEBUG_LOG is present, it enables debug logging to the specified log file.
       • Replace /full/path/to/text-mirror.log with the desired log file path.
     • For more details about the configuration format, see the VS Code MCP documentation.

3. Restart VS Code

   • The text-mirror MCP server should appear in the VS Code extension list (as an Installed MCP server pane).
   • Click the configuration icon next to text-mirror and check what can be done.
     • You may but don't have to start the server manually here; VS Code will start it automatically when needed.
   • For more details, see the VS Code MCP documentation.

4. Call the mirror tool via Copilot Chat.

   • In Copilot Chat, select an "Agent" model that is MCP-capable.

     • If a model supports MCP tools, the “tools” icon appears.

   • The mirror tool should now be available. You can ask Copilot to use it:

     Reverse this text using text-mirror: 🙂🙃🙂👩‍💻👨‍💻123

     or

     Use the mirror tool to reverse "こんにちは"

How It Works

If the MCP server is locally running, MCP clients like VS Code MCP/Claude Desktop communicate with it in a very Unix-like way.

1. The MCP clients start the text-mirror binary as a subprocess (spawns the MCP server).
2. The MCP server listens for tool calls over stdio.
3. MCP clients send instruction data (JSON-RPC) to the server’s stdin.
4. The server processes the request, reverses the text, and sends the result back via stdout.
5. The MCP client receives the response and displays it to the user.

Development notes

• Tests with edge cases and 100% test coverage
• Linting via golangci-lint
• Tests include table-driven cases for combining marks, ZWJ sequences, flags, and long strings to exercise tricky Unicode behavior.
• .editorconfig is included to keep consistent formatting (tabs for Go files, spaces for other files, LF endings, UTF‑8 charset).

Contributing

This project is primarily a learning resource, but contributions are welcome (fixes, tests, documentation).

License

• MIT License. See LICENSE for details.