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 vikasudasi/skill-vault --skill fastmcp-mcp-servergit clone --depth 1 https://github.com/vikasudasi/skill-vaultWrote 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/vikasudasi/skill-vault/fastmcp-mcp-server)<a href="https://agentmods.dev/skills/vikasudasi/skill-vault/fastmcp-mcp-server"><img src="https://agentmods.dev/badge/skills/vikasudasi/skill-vault/fastmcp-mcp-server/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/vikasudasi/skill-vault/fastmcp-mcp-server"><img src="https://agentmods.dev/badge/skills/vikasudasi/skill-vault/fastmcp-mcp-server.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 | $0.00032 | $0.00474 |
| Opus 5 | $0.00016 | $0.00237 |
| Sonnet 5 | $0.00006 | $0.00095 |
| Haiku 4.5 | $0.00003 | $0.00047 |
Grade A, and why
fastmcp-mcp-server 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 10d 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
Building MCP Servers with FastMCP
Use when exposing capabilities or data to AI agents via the Model Context Protocol.
Minimal tool server
from fastmcp import FastMCP
mcp = FastMCP("my-server")
@mcp.tool()
def add(a: int, b: int) -> int:
"""Add two integers."""
return a + b
if __name__ == "__main__":
mcp.run()
Transports matter
- stdio — local, one process. Must NOT print to stdout (pure JSON-RPC); send
logs to stderr. Do NOT pass host/port kwargs to
run()in stdio mode — it silently breaks the handshake. - streamable-http — remote; pass
host/port. Mount the app viamcp.http_app()behind your web framework when serving remotely.
Keep tool surfaces thin
Expose a small number of discovery tools that return lightweight metadata
first, then a fetch tool for full payloads on demand — progressive disclosure.
This is exactly Skill Vault's search_skills → get_skill pattern and keeps
agent context small.
Testing tools directly
Invoke through the runtime so the error/translation layer is exercised:
from fastmcp.exceptions import ToolError
with pytest.raises(ToolError):
await server.call_tool("add", {"a": 1, "b": "x"})
Pitfalls
- Keep tool schemas narrow and typed; agents rely on them.
- Never log request payloads/secrets to stdout.
- For errors, raise typed exceptions the framework translates to MCP error codes.
What ships with it
2 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.
- 10d ago First seen · 66 lines · 32 tokens per session scan A 1274de56f895
fastmcp-mcp-server is a skill published in the GitHub repository vikasudasi/skill-vault (0 stars, last pushed 25d ago), licensed Apache-2.0. It adds 32 tokens to every session and 474 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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