mcp-builder

A guide for building MCP servers. MCP, or Model Context Protocol, is a way for AI agents to use tools that connect them to external services and APIs.

In plain words
What is it for?
Use it to plan and build MCP integrations in Python or Node/TypeScript, especially when an agent must perform multi-step work through an external service.
Why use it?
Simply exposing individual API endpoints may not support a complete real-world task. The guide focuses on designing tool workflows that are useful and concise for agents with limited context.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/hamr0/agentic-toolkit/mcp-builder
Any agent
npx skills add hamr0/agentic-toolkit --skill mcp-builder
Clone the repo
git clone --depth 1 https://github.com/hamr0/agentic-toolkit

Made for: Claude Code, Codex.

Per session 61 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 2,865 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin 100% copy Near-identical to another mod in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00061 $0.02865
Opus 5 $0.00030 $0.01432
Sonnet 5 $0.00012 $0.00573
Haiku 4.5 $0.00006 $0.00286

Measured 2d ago against content hash b1010e90adcb, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

mcp-builder scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

The scan reads SKILL.md. This mod also ships 2 executable files (scripts/connections.py, scripts/evaluation.py), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

This is a copy

100% identical to mcp-builder — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

ai/customize/skill-to-command/skills/mcp-builder/SKILL.md · 329 lines

How it starts

The opening of the file, as written. The whole thing — 329 lines — stays where its author put it; the contents beside it link to each section on GitHub.

MCP Server Development Guide

Overview

To create high-quality MCP (Model Context Protocol) servers that enable LLMs to effectively interact with external services, use this skill. An MCP server provides tools that allow LLMs to access external services and APIs. The quality of an MCP server is measured by how well it enables LLMs to accomplish real-world tasks using the tools provided.


Process

🚀 High-Level Workflow

Creating a high-quality MCP server involves four main phases:

Phase 1: Deep Research and Planning

1.1 Understand Agent-Centric Design Principles

Before diving into implementation, understand how to design tools for AI agents by reviewing these principles:

Build for Workflows, Not Just API Endpoints:

  • Don't simply wrap existing API endpoints - build thoughtful, high-impact workflow tools
  • Consolidate related operations (e.g., schedule_event that both checks availability and creates event)
  • Focus on tools that enable complete tasks, not just individual API calls
  • Consider what workflows agents actually need to accomplish

Optimize for Limited Context:

  • Agents have constrained context windows - make every token count
  • Return high-signal information, not exhaustive data dumps
  • Provide "concise" vs "detailed" response format options
  • Default to human-readable identifiers over technical codes (names over IDs)
  • Consider the agent's context budget as a scarce resource

Design Actionable Error Messages:

  • Error messages should guide agents toward correct usage patterns
  • Suggest specific next steps: "Try using filter='active_only' to reduce results"
  • Make errors educational, not just diagnostic
  • Help agents learn proper tool usage through clear feedback

Follow Natural Task Subdivisions:

  • Tool names should reflect how humans think about tasks
  • Group related tools with consistent prefixes for discoverability
  • Design tools around natural workflows, not just API structure

Use Evaluation-Driven Development:

  • Create realistic evaluation scenarios early
  • Let agent feedback drive tool improvements
  • Prototype quickly and iterate based on actual agent performance

Read the full file on GitHub · 329 lines

Files

What ships with it

9 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 329 lines · 61 tokens per session scan A b1010e90adcb

Subscribe to this mod's changes

mcp-builder is a skill published in the GitHub repository hamr0/agentic-toolkit (22 stars, last pushed 3d ago), licensed Apache-2.0. It adds 61 tokens to every session and 2,865 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to mcp-builder, differing in 0 lines, and is treated as a copy.