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 Threat-Vector-Security/guardian-agent --skill mcp-buildergit clone --depth 1 https://github.com/Threat-Vector-Security/guardian-agentWrote 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/threat-vector-security/guardian-agent/mcp-builder)<a href="https://agentmods.dev/skills/threat-vector-security/guardian-agent/mcp-builder"><img src="https://agentmods.dev/badge/skills/threat-vector-security/guardian-agent/mcp-builder.svg" alt="Measured on agentmods" 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 | $0.00063 | $0.00452 |
| Opus 5 | $0.00032 | $0.00226 |
| Sonnet 5 | $0.00013 | $0.00090 |
| Haiku 4.5 | $0.00006 | $0.00045 |
Grade A, and why
mcp-builder 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 7d 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
MCP Builder
Build MCP servers so they are easy for agents to discover, reason about, and use safely.
Workflow
- Understand the target API or system first.
- Choose the transport intentionally.
- Design the tool surface for agent discoverability, not just endpoint parity.
- Implement shared auth, error handling, formatting, and pagination helpers before adding many tools.
- Test the server with realistic multi-step prompts, not only unit tests.
Design Principles
- Prefer clear verb-first or provider-prefixed tool names.
- Make tool descriptions concrete and concise.
- Use structured schemas with field constraints and examples.
- Return structured data whenever the SDK supports it.
- Keep read-only and mutating tools clearly separated.
- Write actionable errors that tell the agent what to do next.
Implementation Guidance
- Prefer TypeScript unless the repo already has a strong Python MCP pattern.
- Start with the most common read-only tools, then add mutating operations.
- Add pagination, filtering, and scoping so tools do not dump oversized payloads.
- Model auth once in a shared client layer.
- Use annotations and metadata that help approval and policy systems classify risk.
Review Checklist
- Can an agent find the right tool from the name alone?
- Are input and output schemas specific enough to prevent ambiguous calls?
- Do error messages explain the likely fix?
- Are mutating tools obviously distinct from inspection tools?
- Can common user tasks be solved without custom scripting around the server?
Read references/mcp-server-checklist.md when you need a compact implementation checklist.
Gotchas
- Do not mirror raw endpoint naming when it makes tool discovery worse for agents.
- Do not expose large unfiltered list endpoints without pagination, filters, or scoping.
- Do not mix read-only and mutating behavior behind one ambiguous tool.
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.
- 7d ago First seen · 50 lines · 63 tokens per session scan A c01ad746b102
mcp-builder is a skill published in the GitHub repository Threat-Vector-Security/guardian-agent (11 stars, last pushed today), licensed Apache-2.0. It adds 63 tokens to every session and 452 once invoked, about $0.0003 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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