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nacre: Skill for Claude Code

.claude/skills/mcp-tool/SKILL.md

mcp-tool is a skill for Claude Code from nacre-work/nacre. It costs 110 tokens per session (1,330 once invoked), scanned A, original, Apache-2.0.

A set of rules for building and changing MCP tools. MCP is a standard way for AI agents to call external tools and services.

In plain words
What is it for?
Use it when adding MCP tools, changing how they connect, listing available tools, or updating OAuth and authorization behavior.
Why use it?
It keeps tool requests, permissions, errors, and HTTP or command-line connections consistent. This reduces accidental data exposure and deployment problems.

Skill for Claude Code

Written for Claude Code: installed under .claude/.

This is nacre-work/nacre's own configuration. It tells Claude Code how to work on nacre itself, so it is not a mod to install elsewhere. Copy it as a starting point and replace the rules that are about this project. Everything nacre configures →

not rated 2repo +1 changed 17d ago A scan 110 tokens original Apache-2.0
View source ↗ nacre-work/nacre nacre.work
Reuse

Borrowing it

Nothing to install: this file belongs to nacre-work/nacre. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file
curl -O https://raw.githubusercontent.com/nacre-work/nacre/main/.claude/skills/mcp-tool/SKILL.md
Clone the repo
git clone --depth 1 https://github.com/nacre-work/nacre

Made for: Claude Code.

Wrote 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.

agentmods badge for mcp-tool

README.md
[![agentmods](https://agentmods.dev/badge/skills/nacre-work/nacre/mcp-tool/github.svg)](https://agentmods.dev/skills/nacre-work/nacre/mcp-tool)
Your own site
<a href="https://agentmods.dev/skills/nacre-work/nacre/mcp-tool"><img src="https://agentmods.dev/badge/skills/nacre-work/nacre/mcp-tool/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for mcp-tool

Your own site · 80×15
<a href="https://agentmods.dev/skills/nacre-work/nacre/mcp-tool"><img src="https://agentmods.dev/badge/skills/nacre-work/nacre/mcp-tool.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 110 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,330 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found 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 5.1 $0.00110 $0.01330
Opus 5.5 $0.00044 $0.00532
Sonnet 5.5 $0.00022 $0.00266
Haiku 4.5 $0.00011 $0.00133

Measured 17d ago against content hash ea4e0c368d47, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

mcp-tool 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 17d 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.

.claude/skills/mcp-tool/SKILL.md · 107 lines

How it starts

The opening of the file, as written. The whole thing — 107 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Adding or changing an MCP tool

Contract: docs/mcp.md, target revision 2026-07-28.

Transport rules that constrain every tool

  • Stateless, in both eras. The protocol is @modelcontextprotocol/server 2.x: factory.ts builds one McpServer per request (STDIO, per connection), and it answers a legacy client's initialize and a modern client's server/discover alike. No Mcp-Session-Id is ever issued and nothing is kept between requests. Any request is served by any replica.
  • A tool needing state between calls returns an explicit descriptor in its result and takes it back as an argument next time. Hidden state in the transport is not allowed — it is what makes the round-robin deployment work.
  • The mirrored headers — MCP-Protocol-Version, Mcp-Method, and Mcp-Name on tools/call, resources/read and prompts/get — are required of a modern client (one sending the _meta envelope) and compared against the body; a legacy client sends none and is not asked to. Present and disagreeing is -32020.
  • tools/list returns ttlMs: 0 and cacheScope: "private". The catalog depends on the caller's permissions, so it is never shared across callers, and a grant can change it with no list_changed, so it is never fresh.
  • A /mcp tool is registered in factory.ts and nowhere else; transport-parity.test.ts refuses a second site (admin.ts is its one written exemption).

Every tool declares

  1. Which permission it requires — read, write or admin (ToolPermission in tools.ts), from the model in docs/authz.md. Remember write does not imply read: an ingest-only service account must not be able to search. A delegation's ceiling also removes the tool from that connection's catalog — through the ceiling field, never by name.
  2. What it returns on no permission — the same isError result with the text Not found that a genuinely missing object gets. Never a 403, and never an HTTP 404 either: on Streamable HTTP that tells a client its session is gone.
  3. Its title and MCP annotations — the type requires them.

Read the full file on GitHub · 107 lines

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. 17d ago Changed · +30 lines ea4e0c368d47
  2. 1mo ago First seen · 77 lines · 110 tokens per session scan A 668e1b90e651

Subscribe to this mod's changes

mcp-tool is a skill published in the GitHub repository nacre-work/nacre (2 stars, last pushed 18d ago), licensed Apache-2.0. It adds 110 tokens to every session and 1,330 once invoked, about $0.0004 per session on Opus 5.5. 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 2026-08-31.

Backend and APIsBuilding agentsRAG and vectorsTypeScriptmcpself-hostedsemantic-searchvector-database
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