Borrowing it
Nothing to install: this file belongs to TheDecipherist/claude-code-mastery-project-starter-kit. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/TheDecipherist/claude-code-mastery-project-starter-kit/main/.claude/skills/mcp-builder/SKILL.mdgit clone --depth 1 https://github.com/TheDecipherist/claude-code-mastery-project-starter-kitWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/thedecipherist/claude-code-mastery-project-starter-kit/mcp-builder)<a href="https://agentmods.dev/skills/thedecipherist/claude-code-mastery-project-starter-kit/mcp-builder"><img src="https://agentmods.dev/badge/skills/thedecipherist/claude-code-mastery-project-starter-kit/mcp-builder.svg" alt="Measured on agentmods" height="20"></a>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.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00066 | $0.00928 |
| Opus 5 | $0.00033 | $0.00464 |
| Sonnet 5 | $0.00013 | $0.00186 |
| Haiku 4.5 | $0.00007 | $0.00093 |
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 7d ago.
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.
How it starts
The opening of the file, as written. The whole thing — 58 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Build an MCP Server
An MCP server is judged by one thing: whether an LLM can use its tools to finish real tasks. Tool count, endpoint coverage, and cleverness are means, not the goal. Build for the agent that will call it.
Default stack
- TypeScript, on the MCP SDK. Strong typing and good model familiarity. Drop to Python (FastMCP) only when the service it wraps is Python-native.
- Transport: streamable HTTP with stateless JSON for remote servers; stdio for local. Stateless scales and debugs far more easily than session streaming.
- Resolve the SDK version at build time, don't hardcode from memory. The current SDK README is the source of truth.
The four phases
1. Understand the target. Read the service's API or schema first. Identify the handful of tasks a user actually wants done through it, not every endpoint. A wrapper that mirrors 60 REST routes one-to-one is worse than 8 tools shaped around real workflows.
2. Design the tools. This is where quality is won or lost.
- Discoverable names:
verb_noun, scoped by service, for examplegithub_create_issue,orders_find_by_customer. The name should tell the model when to reach for it. - Descriptions written for the model, stating what the tool does and when to use it, not just what it returns.
- Explicit input and output schemas. Constrain inputs; return structured, parseable output, not prose the model has to scrape.
- Annotations: mark each tool
readOnlyHint,destructiveHint, oridempotentHintso the client can reason about safety. - Actionable errors. "Invalid date" is useless; "expected ISO 8601, got 03/04/2026, did you mean 2026-03-04" lets the model self-correct.
- Balance coverage against workflow tools. Comprehensive low-level tools give the agent room to compose; a few high-level workflow tools cut steps for the common path. Most good servers ship both.
3. Implement. Shared infrastructure first (client, auth, pagination, error handling), then the tools on top. If the server touches a database, it follows the same kit rules as everything else: one shared client, data access through the adapter, StrictDB if installed otherwise the native driver, never Mongoose.
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.
- 7d ago First seen · 58 lines · 66 tokens per session scan A ced321eec4e3
mcp-builder is a skill published in the GitHub repository TheDecipherist/claude-code-mastery-project-starter-kit (337 stars, last pushed 2mo ago), licensed MIT. It adds 66 tokens to every session and 928 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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