--- name: mcp-builder-6aa80d83cf96 description: Build, register, and use custom MCP servers within this environment. Use this when asked to create a new tool, capability, or integration as an MCP server. allowed-tools: Bash, Read, Write, Edit --- # Building Custom MCP Servers MCP (Model Context Protocol) servers extend what I can do by exposing new tools. I can write, register, and use them across sessions. ## File locations - Server code: `/workspace/group/mcp-servers//index.js` - Shared dependencies: `/workspace/group/mcp-servers/package.json` - Registration: `/workspace/group/.mcp.json` All paths under `/workspace/group/` persist across container restarts. --- ## Step-by-step: Create and register an MCP server ### 1. Set up shared dependencies (first time only) All MCP servers share a single `node_modules` at the `mcp-servers/` root. Only do this once: ```bash mkdir -p /workspace/group/mcp-servers cd /workspace/group/mcp-servers if [ ! -f package.json ]; then npm init -y # Add "type": "module" for ES module support node -e "const p=require('./package.json'); p.type='module'; require('fs').writeFileSync('package.json', JSON.stringify(p, null, 2)+'\n')" npm install @modelcontextprotocol/sdk zod fi ``` ### 2. Write the server Create `/workspace/group/mcp-servers//index.js`: ```js import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js"; import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js"; import { z } from "zod"; const server = new McpServer({ name: "", version: "1.0.0" }); server.tool( "tool-name", "Description of what this tool does", { param: z.string().describe("A parameter") }, async ({ param }) => ({ content: [{ type: "text", text: `Result: ${param}` }], }) ); const transport = new StdioServerTransport(); await server.connect(transport); ``` Node resolves imports from the shared `node_modules` up the directory tree — no per-server install needed. ### 3. Test it ```bash echo '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"capabilities":{},"clientInfo":{"name":"test"},"protocolVersion":"2024-11-05"}}' | node /workspace/group/mcp-servers//index.js ``` Should return a JSON response without errors. ### 4. Register in `.mcp.json` Add the server to `/workspace/group/.mcp.json`. If the file doesn't exist, create it: ```json { "mcpServers": { "": { "command": "node", "args": ["/workspace/group/mcp-servers//index.js"] } } } ``` If `.mcp.json` already exists, merge the new entry into the existing `mcpServers` object — don't overwrite other servers. ### 5. Tell the user After creating and registering the server, tell the user: > I've created the `` MCP server. It will be available as a tool next time you message me. The current session cannot pick up newly registered MCP servers. They take effect on the next container invocation. --- ## Common patterns ### Tool with no params ```js server.tool("get-time", "Get current time", {}, async () => ({ content: [{ type: "text", text: new Date().toISOString() }], })); ``` ### Tool that calls an external API ```js server.tool( "fetch-weather", "Get weather for a city", { city: z.string() }, async ({ city }) => { const res = await fetch(`https://wttr.in/${city}?format=3`); return { content: [{ type: "text", text: await res.text() }] }; } ); ``` ### Tool that reads/writes files ```js import { readFileSync, writeFileSync } from "fs"; server.tool("read-notes", "Read saved notes", {}, async () => ({ content: [{ type: "text", text: readFileSync("/workspace/group/notes.md", "utf8") }], })); server.tool( "save-note", "Save a note", { text: z.string() }, async ({ text }) => { writeFileSync("/workspace/group/notes.md", text); return { content: [{ type: "text", text: "Saved." }] }; } ); ``` ### Tool that runs a shell command ```js import { execSync } from "child_process"; server.tool( "run-command", "Run a shell command", { command: z.string() }, async ({ command }) => { try { const output = execSync(command, { timeout: 30000 }).toString(); return { content: [{ type: "text", text: output || "(no output)" }] }; } catch (err) { return { content: [{ type: "text", text: `Error: ${err.message}` }] }; } } ); ``` ### Multiple tools in one server Group related tools into a single server rather than creating one server per tool: ```js const server = new McpServer({ name: "project-tools", version: "1.0.0" }); server.tool("lint", "Run linter", {}, async () => { /* ... */ }); server.tool("test", "Run tests", {}, async () => { /* ... */ }); server.tool("deploy", "Deploy to staging", {}, async () => { /* ... */ }); ``` --- ## Debugging - **Server crashes silently**: Wrap all tool handlers in try/catch. Unhandled errors kill the process and Claude Code just sees "MCP server disconnected." - **Tools don't appear**: Check that `/workspace/group/.mcp.json` is valid JSON. A trailing comma or syntax error silently breaks registration. - **Import errors**: Make sure the shared `package.json` has `"type": "module"` and that `node_modules` exists at `/workspace/group/mcp-servers/`. - **Logging**: Use `console.error()` for debug output. Stdout is reserved for the JSON-RPC protocol — anything else on stdout breaks communication. --- ## Python alternative ```python # /workspace/group/mcp-servers//server.py from mcp.server.fastmcp import FastMCP mcp = FastMCP("") @mcp.tool() def my_tool(param: str) -> str: """Description of the tool""" return f"Result: {param}" if __name__ == "__main__": mcp.run() ``` Install: `pip install mcp` Register in `/workspace/group/.mcp.json`: ```json { "mcpServers": { "": { "command": "python3", "args": ["/workspace/group/mcp-servers//server.py"] } } } ``` --- Source attribution: sskarz Original project: https://github.com/sskarz/nanoclawbster/blob/798fee5371feaafe2929bdaed7efb9b88a2ea45d/container/skills/mcp-builder/SKILL.md License: MIT License information: https://opensource.org/license/mit Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice MIT License Copyright (c) 2026 Gavriel Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. # Mega AI Hub resource metadata Creator / publisher: sskarz Original project: https://github.com/sskarz/nanoclawbster/blob/798fee5371feaafe2929bdaed7efb9b88a2ea45d/container/skills/mcp-builder/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Backend Developer, DevOps / SRE, ML / AI Engineer Specialties: Backend Tools: Bash Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 778ed5fad503c8e448cc35c0f4de3ee213ea8e61f1f763bcdda5f7eca469d8e3