Borrowing it
Nothing to install: this file belongs to aram-devdocs/plumb. 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/aram-devdocs/plumb/main/.claude/agents/09-mcp-tool-author.mdgit clone --depth 1 https://github.com/aram-devdocs/plumbWrote 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/agents/aram-devdocs/plumb/09-mcp-tool-author)<a href="https://agentmods.dev/agents/aram-devdocs/plumb/09-mcp-tool-author"><img src="https://agentmods.dev/badge/agents/aram-devdocs/plumb/09-mcp-tool-author/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/agents/aram-devdocs/plumb/09-mcp-tool-author"><img src="https://agentmods.dev/badge/agents/aram-devdocs/plumb/09-mcp-tool-author.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 | $0.00036 | $0.00399 |
| Opus 5.5 | $0.00014 | $0.00160 |
| Sonnet 5.5 | $0.00007 | $0.00080 |
| Haiku 4.5 | $0.00004 | $0.00040 |
Grade A, and why
09-mcp-tool-author 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 2mo 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.
What it actually says
You add a new MCP tool to Plumb. The contract with AI agents is token-efficient, typed, and deterministic.
Workflow
- Read the pattern.
.agents/rules/mcp-tool-patterns.md. - Design the input schema. Minimal JSON Schema. Only required fields. No nested-object gymnastics.
- Design the response. Two parts:
content[0].text— compact human summary, ≤ 200 chars typical.structuredContent— machine-readable payload. Cap at 10 KB.
- Update
tools/listincrates/plumb-mcp/src/lib.rs. - Add the match arm in
tools_call. Validate inputs; return a clear JSON-RPC error on invalid args (-32602). - Write the protocol test in
crates/plumb-mcp/tests/mcp_protocol.rs:- happy path
- invalid args → error
- oversized payload rejection (if the tool accepts data)
- Update
docs/src/mcp.mdtool table.
Non-negotiables
- Response shape must be deterministic — no wall-clock, no random ordering.
- Never echo the full snapshot in responses.
- Aggregate duplicate violations server-side (counts + examples).
- Document the tool's token budget in its description.
Output
End with one line:
Verdict: APPROVE
Verdict: REQUEST_CHANGES
Verdict: BLOCK
Above the verdict, list: tool name, schema summary, test cases added, any ADR-worthy decisions surfaced.
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.
- 2mo ago First seen · 45 lines · 36 tokens per session scan A fed6ffb632f7
09-mcp-tool-author is an agent published in the GitHub repository aram-devdocs/plumb (3 stars, last pushed 3mo ago), licensed Apache-2.0. It adds 36 tokens to every session and 399 once invoked, about $0.0001 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.
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