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mcp-builder

MCP Builder

MCP Builder is Anthropic's open-source agent skill for building MCP servers in TypeScript or Python, from API research and tool design to testing and a 10-question evaluation.

Agent skill by Anthropic Official · Updated

Install MCP Builder

In Claude Code, add Anthropic's skills marketplace

Terminal
/plugin marketplace add anthropics/skills

Then install the example-skills plugin, which holds this skill and Anthropic's other example skills

Terminal
/plugin install example-skills@anthropic-agent-skills

Or install only this skill, for any agent, with the skills CLI

Terminal
npx skills add anthropics/skills --skill mcp-builder

In Claude.ai, Anthropic says its example skills are already available on paid plans: switch them on under Customize > Skills.

Or paste this into your coding agent: Install the agent skill mcp-builder from github.com/anthropics/skills. A skill can include scripts that run on your computer, so read its source first.

What MCP Builder does

MCP Builder is Anthropic's guide to building MCP (Model Context Protocol) servers, which let AI models use an external API or service through tools. It loads when you build an MCP server in Python with FastMCP or in TypeScript with the official MCP SDK. It recommends TypeScript, with streamable HTTP for remote servers and stdio for local ones.

With MCP Builder, the agent works in four phases. It studies the MCP specification and the service's API, then plans which endpoints to cover, starting with the most common operations. It implements tools with Zod or Pydantic input schemas, consistent name prefixes, pagination, actionable error messages and hints such as read-only or destructive. Then it builds the server and tests it with the MCP Inspector. Reference files cover best practices and each language.

The last MCP Builder phase is an evaluation: 10 realistic, read-only questions, each with one verifiable answer, saved as an XML file. A bundled Python script runs those questions against your server through Claude, over stdio, SSE or HTTP, and reports the results along with Claude's feedback on the tools. It needs the anthropic and mcp Python packages and an ANTHROPIC_API_KEY.

When to use MCP Builder

  • You want Claude or another agent to use your product's API and need an MCP server for it.
  • You are designing MCP tools and want naming, pagination and error handling conventions to follow.
  • You built an MCP server and want to test whether a model can answer real questions with it.

When to pick something else

  • You want to package know-how or a workflow rather than tools: write a skill with Skill Creator or Writing Skills.

Which agents MCP Builder works in

Anthropic documents MCP Builder for Claude Code, Claude.ai, Claude API. The open skills CLI also installs it into 78 agents, including Claude Code, Codex, Cursor, Gemini CLI, GitHub Copilot, OpenCode (we listed it with the CLI on October 1, 2026). See where each agent looks for skills.

Building the server works in any agent, but the evaluation script calls Claude through the Anthropic API, so that step needs an ANTHROPIC_API_KEY whichever agent you use.

MCP Builder license

MCP Builder is published under Apache-2.0. Apache-2.0 comes from the LICENSE.txt in the skill's own folder (read 2026-10-01); the anthropics/skills repository has no root license file. In practice you may use, change and share the skill in any agent, commercially too, as long as you keep the license text and notices and mark your changes.

Questions people ask

Do I need an Anthropic API key for mcp-builder?

Only for the evaluation: its Python script runs your 10 questions through Claude and needs the anthropic and mcp packages plus an ANTHROPIC_API_KEY. Researching, designing and building the server works without one.

Should I build my MCP server in Python or TypeScript?

The skill supports both, FastMCP for Python and the official MCP SDK for TypeScript, and recommends TypeScript, with streamable HTTP for remote servers and stdio for local ones.

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