Borrowing it
Nothing to install: this file belongs to Neschadin/askell-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/Neschadin/askell-mcp/main/.agents/skills/mcp-tool-design/SKILL.mdgit clone --depth 1 https://github.com/Neschadin/askell-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/neschadin/askell-mcp/mcp-tool-design)<a href="https://agentmods.dev/skills/neschadin/askell-mcp/mcp-tool-design"><img src="https://agentmods.dev/badge/skills/neschadin/askell-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/neschadin/askell-mcp/mcp-tool-design"><img src="https://agentmods.dev/badge/skills/neschadin/askell-mcp/mcp-tool-design.svg" alt="Reviewed on agentmods" width="80" 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 | $0.00047 | $0.00696 |
| Opus 5 | $0.00023 | $0.00348 |
| Sonnet 5 | $0.00009 | $0.00139 |
| Haiku 4.5 | $0.00005 | $0.00070 |
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 9d 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 — 94 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MCP Tool Design & Evaluation
Design MCP tools that LLMs can discover, compose, and use reliably. For project scaffolding, SDK v2 patterns, and transport setup, use the typescript-mcp-server-generator skill.
When to Use
- Choosing which API endpoints become tools vs higher-level workflows
- Naming tools, shaping inputs/outputs, and writing descriptions agents can act on
- Adding tool annotations (
readOnlyHint,destructiveHint, etc.) - Building evaluation suites to verify agents can solve real tasks with your tools
Tool Design
API Coverage vs Workflow Tools
Balance comprehensive API endpoint coverage with specialized workflow tools. Workflow tools are convenient for specific tasks; broad coverage lets agents compose operations. When uncertain, prioritize comprehensive API coverage.
Naming and Discoverability
Use consistent, action-oriented prefixes (github_create_issue, github_list_repos). Names should make the tool's purpose obvious without reading the full description.
Context Management
Return focused, relevant data. Support pagination and filtering where the backing API allows. Concise descriptions beat exhaustive parameter dumps.
Error Messages
Errors should guide agents toward a fix — include what failed, why, and a concrete next step (retry with different params, call another tool first, etc.).
Tool Annotations
Set hints so clients can surface appropriate UX:
readOnlyHint— no side effectsdestructiveHint— irreversible or high-impact changesidempotentHint— safe to retryopenWorldHint— interacts with external/unbounded data
Planning Checklist
Before implementing, review the service API docs for endpoints, auth, and data models. List tools to implement starting with the most common read operations, then writes.
Evaluation Suite
After tools are implemented, verify agents can use them effectively.
Load Evaluation Guide for the full process and runner scripts.
What ships with it
5 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.
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.
- 9d ago First seen · 94 lines · 47 tokens per session scan A 6004b0d70b11
mcp-tool-design is a skill published in the GitHub repository Neschadin/askell-mcp (0 stars, last pushed 4d ago), licensed MIT. It adds 47 tokens to every session and 696 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 2026-08-31.
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