Borrowing it
Nothing to install: this file belongs to maniaclab/af-filesystem-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/maniaclab/af-filesystem-mcp/main/CLAUDE.mdgit clone --depth 1 https://github.com/maniaclab/af-filesystem-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/instructions/maniaclab/af-filesystem-mcp/claude-md)<a href="https://agentmods.dev/instructions/maniaclab/af-filesystem-mcp/claude-md"><img src="https://agentmods.dev/badge/instructions/maniaclab/af-filesystem-mcp/claude-md.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 | $0.03564 | $0.03564 |
| Opus 5 | $0.01782 | $0.01782 |
| Sonnet 5 | $0.00713 | $0.00713 |
| Haiku 4.5 | $0.00356 | $0.00356 |
Grade A, and why
af-filesystem-mcp CLAUDE.md scanned grade A with 1 finding 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 6d 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.
Runs shell commandslowCapability
Expected in a hook, worth knowing in a rule or an instructions file.
`minting.py` impersonation pattern, adapted from `subprocess.run` to How it starts
The opening of the file, as written. The whole thing — 280 lines — stays where its author put it; the contents beside it link to each section on GitHub.
af-filesystem-mcp — Contributor Guide
MCP server that gives an AF (Analysis Facility) user browse/read access to their
own files on the AF's shared NFS home (/home/<unixname>) and Ceph data area
(/data/<unixname>) — nothing more. Read-only in v1: there is no write, delete,
rename, or command-execution tool anywhere in this package.
Companion backends in the same af-mcp-platform: ami-mcp (AMI metadata) and rucio-mcp (Rucio data management). This repo follows their conventions (tool naming, broker-mode auth, Helm chart shape) wherever they apply, and departs from them explicitly where this backend's job is different — see "How this differs from ami-mcp/rucio-mcp" below.
Origin
Designed in maniaclab/af-mcp-platform#188, which also has the full design
rationale and the (rejected) alternative — an off-the-shelf
rust-mcp-filesystem server — and why it was rejected: stdio-only (no per-user
identity), and a dangling-symlink write-escape via a validate-then-open TOCTOU.
This package exists specifically to not have either problem.
Security model
The security boundary is impersonation, not path-string validation. Every
filesystem operation for user alice runs in a short-lived helper subprocess
that is started running AS alice's real uid/gid
(asyncio.create_subprocess_exec(..., user=uid, group=gid)), never inside the
long-lived async server process itself (which would mean a process-wide
seteuid() racing across concurrently in-flight requests for different users).
This means the kernel — and, for the NFS-mounted homes, the NFS server —
enforces every permission check against the real identity: even a bug in this
server's own path-pinning logic can only let alice reach what alice's real uid
could already reach. This mirrors [voms-token-service][voms-token-service]'s
minting.py impersonation pattern, adapted from subprocess.run to
asyncio.create_subprocess_exec.
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.
- 6d ago First seen · 280 lines · 3,564 tokens per session scan A 8699c529ff76
af-filesystem-mcp CLAUDE.md is an instructions file published in the GitHub repository maniaclab/af-filesystem-mcp (0 stars, last pushed 10d ago), licensed Apache-2.0. It adds 3,564 tokens to every session, about $0.0178 per session on Opus 5. A static security scan graded it A with 1 finding (runs shell commands). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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