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.
npx skills add sigistry/marketplace --skill mcp-tool-designgit clone --depth 1 https://github.com/sigistry/marketplaceWrote 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/sigistry/marketplace/mcp-tool-design)<a href="https://agentmods.dev/skills/sigistry/marketplace/mcp-tool-design"><img src="https://agentmods.dev/badge/skills/sigistry/marketplace/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/sigistry/marketplace/mcp-tool-design"><img src="https://agentmods.dev/badge/skills/sigistry/marketplace/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 | $0.00097 | $0.01097 |
| Opus 5.5 | $0.00039 | $0.00439 |
| Sonnet 5.5 | $0.00019 | $0.00219 |
| Haiku 4.5 | $0.00010 | $0.00110 |
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 1mo 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 — 61 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MCP Tool Design
Purpose
An MCP server's tools are the API the model reasons over, its quality decides whether the agent uses the server well or thrashes. The failure mode is not usually a bug; it is too many thin, ambiguous tools with loose schemas, which burn context, invite wrong calls, and hide destructive actions behind innocent names. This skill is the design discipline: few powerful tools, complete schemas, honest annotations, safe errors, bounded output.
Tools vs resources vs prompts
| Primitive | For | Example |
|---|---|---|
| Tool | an action the model invokes (may have side effects) | create_issue, run_query, search_docs |
| Resource | readable content addressed by URI, app-controlled | file://…, db://schema, a document the host attaches |
| Prompt | a user-initiated templated workflow | a /summarize-pr slash command |
| Do not model a read as a destructive tool, or a user workflow as an always-on tool. Picking the right primitive removes whole classes of misuse. |
Few, powerful tools
- Prefer one
search_orders(status?, customer?, since?)overlist_orders+filter_by_status+filter_by_customer. Each extra tool is permanent context cost and another way to be called wrong. - A powerful tool has a clear verb-noun name, does one coherent job, and exposes its variations through parameters, not through sibling tools.
- If two tools are almost always called in sequence, consider merging them; if one tool has a boolean that flips it into a different operation, consider splitting, judgment, but bias toward fewer.
Complete input schemas
Every tool needs a full JSON Schema inputSchema:
- Type every field; use
enumfor closed sets; markrequiredaccurately; set sanedefaults. - Give every parameter a
descriptionwritten for the model ("ISO-8601 date; results on or after this instant"), the description is how the model knows what to pass. - Constrain:
minimum/maximum,maxLength,format,patternwhere they prevent bad calls. - Reject unknown fields (
additionalProperties: false) so a hallucinated parameter fails loudly. A tool whose description invites an action the schema does not constrain is a design bug (and a security one): the model may attempt the unconstrained thing.
What ships with it
1 file 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.
- 1mo ago First seen · 61 lines · 97 tokens per session scan A 9c5f44e50d6d
mcp-tool-design is a skill published in the GitHub repository sigistry/marketplace (3 stars, last pushed 4d ago), licensed MIT. It adds 97 tokens to every session and 1,097 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 2026-09-03.
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