🧠 Simple MCP Server with Node.js

🧠 Simple MCP Server with Node.js

This is a simple prototype of a Model Context Protocol (MCP) server built with Node.js and Express.
It interacts with OpenAI's API to simulate a context-aware assistant—for example, one that helps write stories.

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🚀 What You Can Do

• Structure your context with system, user, steps, and resources
• Call OpenAI API to generate story plots using the context
• Add characters dynamically via API
• See how changing context affects LLM responses

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📦 Requirements

• Node.js v18+
• npm
• OpenAI API key (Get one here)

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🛠 Setup

1. Clone this repository

git clone https://github.com/takehisa10098/plot-mcp-server.git
cd plot-mcp-server

2. Install dependencies

npm install

3. Create .env file

OPENAI_API_KEY=sk-xxxxxxxxxxxxxxxxxxxxxxxx

4. Create context.json file

{
  "system": "あなたは小説執筆のサポートAIです。",
  "user": {
    "goal": "和風ファンタジーの短編小説を書きたい",
    "constraints": ["文字数は3000字以内", "テーマは『喪失と再生』"]
  },
  "steps": [
    { "name": "キャラクター作成", "status": "done" },
    { "name": "プロット生成", "status": "in_progress" }
  ],
  "resources": {
    "characters": [
      { "name": "アカネ", "role": "主人公", "trait": "寡黙で芯が強い" }
    ],
    "world": "戦国時代風の異世界。陰陽術が存在する。"
  }
}

5. Create server.js

const express = require('express');
const fs = require('fs');
const dotenv = require('dotenv');
const { getPlotSuggestion } = require('./openai');

dotenv.config();
const app = express();
app.use(express.json());

const PORT = 3000;
const CONTEXT_PATH = './context.json';

const readContext = () => JSON.parse(fs.readFileSync(CONTEXT_PATH, 'utf-8'));

app.get('/context', (req, res) => {
  const context = readContext();
  res.json(context);
});

app.post('/step/plot', async (req, res) => {
  const context = readContext();
  const plot = await getPlotSuggestion(context);
  res.json({ plot });
});

app.post('/update/characters', (req, res) => {
  const context = readContext();
  const newCharacter = req.body;

  if (!newCharacter.name || !newCharacter.role || !newCharacter.trait) {
    return res.status(400).json({ error: 'name, role, and trait are required' });
  }

  context.resources.characters.push(newCharacter);
  fs.writeFileSync(CONTEXT_PATH, JSON.stringify(context, null, 2), 'utf-8');
  res.json({ message: 'Character added', character: newCharacter });
});

app.listen(PORT, () => {
  console.log(`MCP server running at http://localhost:${PORT}`);
});

6. Create openai.js

const { OpenAI } = require('openai');
require('dotenv').config();

const openai = new OpenAI({
  apiKey: process.env.OPENAI_API_KEY,
});

async function getPlotSuggestion(context) {
  const { system, user, resources } = context;

  const messages = [
    { role: 'system', content: system },
    {
      role: 'user',
      content: `
目標: ${user.goal}
制約: ${user.constraints.join(', ')}

キャラクター: ${JSON.stringify(resources.characters)}
世界観: ${resources.world}

これを踏まえて、小説のプロット案を出してください。
      `
    }
  ];

  const res = await openai.chat.completions.create({
    model: 'gpt-4-turbo',
    messages,
  });

  return res.choices[0].message.content;
}

module.exports = { getPlotSuggestion };

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🧪 How to Use

Start the server

node server.js

Get current context

curl http://localhost:3000/context

Generate a plot

curl -X POST http://localhost:3000/step/plot

Add a new character

curl -X POST http://localhost:3000/update/characters \
  -H "Content-Type: application/json" \
  -d '{
    "name": "ユキ",
    "role": "謎の旅人",
    "trait": "静かで何かを知っているような雰囲気"
  }'

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💡 What is MCP?

Model Context Protocol (MCP) is a proposed format for structuring contextual information for LLMs.
It allows applications to define system, user, steps, and resources that represent the ongoing state of a conversation or project.

This project shows how you can design an LLM interaction by modifying structured context, instead of rewriting your prompt each time.

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🔗 Related

• Anthropic MCP Docs
• OpenAI Function Calling

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