Borrowing it
Nothing to install: this file belongs to cervonwong/ILSpy-MCP. 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/cervonwong/ILSpy-MCP/main/.claude/skills/mcp-tool-design/SKILL.mdgit clone --depth 1 https://github.com/cervonwong/ILSpy-MCPWrote 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/cervonwong/ilspy-mcp/mcp-tool-design)<a href="https://agentmods.dev/skills/cervonwong/ilspy-mcp/mcp-tool-design"><img src="https://agentmods.dev/badge/skills/cervonwong/ilspy-mcp/mcp-tool-design/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.
<a href="https://agentmods.dev/skills/cervonwong/ilspy-mcp/mcp-tool-design"><img src="https://agentmods.dev/badge/skills/cervonwong/ilspy-mcp/mcp-tool-design.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00030 | $0.01987 |
| Opus 5 | $0.00015 | $0.00993 |
| Sonnet 5 | $0.00006 | $0.00397 |
| Haiku 4.5 | $0.00003 | $0.00199 |
Grade A, and why
mcp-tool-design 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 11d 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 — 123 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MCP Tool Design Principles
When to use this skill
Apply this skill whenever you are: (a) adding a new MCP tool under Transport/Mcp/Tools/, (b) modifying an existing tool's signature, parameters, or [Description] attribute, (c) reviewing the output schema returned by a use case wired to an MCP tool, or (d) writing the v1.1 cleanup tasks derived from the 260410 audit. The audience for these rules is the developer; the audience for the tools themselves is an AI agent that has never seen this codebase before.
Core mental model
AI agents are lazy. They will not chase round-trips. They will not infer hidden affordances. They will not read the tool source. They will read the tool name, the description, and the parameter list — once — and then either pick the tool or ignore it. Every design choice below flows from that fact.
Principles
1. Scenario-oriented descriptions
Rule: Every [Description] starts by answering "when would a user reach for this tool?", not "what does it do mechanically?". Prefer "Use this when…" or "Use this to…" framings.
Why: An agent reading a catalogue of 28+ tools needs to filter to "is this relevant to my current intent?" in one pass. Mechanical descriptions force the agent to translate verbs into intents on the fly, which it does badly.
// BAD — describes mechanics
[Description("Find all call sites, field accesses, and property usages of a member.")]
// GOOD — describes the user's situation
[Description("Use this when you're about to change a method/field and need to know what will break. Returns every call site so you can assess impact before refactoring.")]
2. Nested references by default
Rule: When a tool's output references something the agent will obviously want next (a metadata token, a type name, a member signature), resolve it inline. Do not return bare identifiers that force a follow-up call.
Why: IL disassembly that says call 0x0A000023 is useless without a second tool call. IL disassembly that says call instance string [System.Runtime]System.Object::ToString() is self-sufficient. The cost of resolving the reference once on the server is paid back many times in saved round-trips and reduced agent confusion.
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.
- 11d ago First seen · 123 lines · 30 tokens per session scan A 918ebe087179
mcp-tool-design is a skill published in the GitHub repository cervonwong/ILSpy-MCP (25 stars, last pushed 1mo ago), licensed MIT. It adds 30 tokens to every session and 1,987 once invoked, about $0.0002 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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