mcp-stdio

Zero-dependency MCP server for stdio transport. Just tools, nothing else.

Why

The official MCP SDK has 17 dependencies (Express, Hono, OAuth, JWT...). Most MCP servers just need stdio + tools. This package is 222 lines, zero runtime dependencies.

| | @modelcontextprotocol/sdk | mcp-stdio | |---|---|---| | Dependencies | 17 | 0 | | Transport | stdio, HTTP, SSE | stdio | | Features | tools, resources, prompts, sampling, auth | tools | | Lines | ~5000+ | 222 |

Install

npm install mcp-stdio

Usage

import { createMcpServer } from 'mcp-stdio';

createMcpServer({
  name: 'my-server',
  version: '1.0.0',
  tools: {
    greet: {
      description: 'Say hello',
      parameters: {
        type: 'object',
        properties: { name: { type: 'string' } },
        required: ['name'],
      },
      handler: async ({ name }) => `Hello ${name}!`,
    },

    add: {
      description: 'Add two numbers',
      parameters: {
        type: 'object',
        properties: {
          a: { type: 'number' },
          b: { type: 'number' },
        },
        required: ['a', 'b'],
      },
      handler: async ({ a, b }) => `${Number(a) + Number(b)}`,
    },
  },
});

Configure in Claude Code

{
  "mcpServers": {
    "my-server": {
      "command": "node",
      "args": ["my-server.js"]
    }
  }
}

API

createMcpServer(options)

Starts an MCP server reading JSON-RPC from stdin, writing to stdout.

Options:

| Field | Type | Required | Description | |-------|------|----------|-------------| | name | string | yes | Server name (shown to clients) | | version | string | no | Server version (default: '0.1.0') | | tools | Record<string, ToolDefinition> | yes | Tool definitions |

ToolDefinition:

| Field | Type | Required | Description | |-------|------|----------|-------------| | description | string | yes | What the tool does | | parameters | JsonSchema | no | JSON Schema for input | | handler | (params) => Promise<string \| ToolContent[]> | yes | Implementation |

Handler return types:

• string — wrapped as [{ type: 'text', text: '...' }]
• ToolContent[] — returned as-is (supports text, image, resource)

Errors in handlers are caught and returned as { isError: true, content: [{ type: 'text', text: 'Error: ...' }] } — the server never crashes.

What it implements

• JSON-RPC 2.0 over stdio (newline-delimited)
• initialize with capability negotiation
• tools/list with JSON Schema input schemas
• tools/call with structured content responses
• ping
• Proper error codes (-32700, -32600, -32601, -32603)
• Notification handling (no response for messages without id)
• Logs to stderr (stdout is protocol-only)

What it doesn't implement

• HTTP/SSE/Streamable HTTP transport
• Resources, Prompts, Sampling
• OAuth, authentication
• Schema validation (trusts the caller — your handler validates)

These are intentional omissions. If you need them, use the official SDK.

License

MIT