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FerrumMCP: Instructions file for Claude Code

CLAUDE.md

FerrumMCP CLAUDE.md is an instructions file for Claude Code from Eth3rnit3/FerrumMCP. It costs 3,987 tokens per session, scanned A, original, MIT.

Development instructions for FerrumMCP, a browser automation server that lets AI assistants control browsers through the Model Context Protocol. It uses Ferrum, a Ruby driver for headless Chrome, and organises more than 27 tools into browser-operation categories.

In plain words
What is it for?
Use them when working on browser sessions, navigation, page interaction, waiting, data extraction, or advanced browser automation.
Why use it?
They explain the server's session-based design and how browser tools are organised, helping contributors understand where automation behaviour belongs.

Instructions file for Claude Code

Written for Claude Code: the file is CLAUDE.md. Also seen: mentions CLAUDE.md; mentions Claude Code.

This is Eth3rnit3/FerrumMCP's own configuration. It tells Claude Code how to work on FerrumMCP itself, so it is not a mod to install elsewhere. Copy it as a starting point and replace the rules that are about this project. Everything FerrumMCP configures →

not rated 30repo changed 29d ago A scan 3,987 tokens original MIT
View source ↗ Eth3rnit3/FerrumMCP eth3rnit3.github.io
Reuse

Borrowing it

Nothing to install: this file belongs to Eth3rnit3/FerrumMCP. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file
curl -O https://raw.githubusercontent.com/Eth3rnit3/FerrumMCP/main/CLAUDE.md
Clone the repo
git clone --depth 1 https://github.com/Eth3rnit3/FerrumMCP

Made for: Claude Code.

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Per session 3,987 This file is loaded in full into every session.
When invoked 3,987 The same file — it is already loaded in full.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5.1 $0.03987 $0.03987
Opus 5.5 $0.01595 $0.01595
Sonnet 5.5 $0.00797 $0.00797
Haiku 4.5 $0.00399 $0.00399

Measured 29d ago against content hash 26691900bd6e, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

FerrumMCP CLAUDE.md 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 29d 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.

CLAUDE.md · 368 lines

How it starts

The opening of the file, as written. The whole thing — 368 lines — stays where its author put it; the contents beside it link to each section on GitHub.

CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Project Overview

FerrumMCP is a browser automation server implementing the Model Context Protocol (MCP). It provides AI assistants with browser automation capabilities through a standardized interface, using Ferrum (Ruby's headless Chrome driver) and optional BotBrowser integration for anti-detection.

Architecture

Core Components

Server Layer (lib/ferrum_mcp/bin/ferrum-mcp)

  • FerrumMCP::Server: Main MCP server implementation
  • Manages 40 browser automation tools organized into 7 categories (Session Management, Navigation, Interaction, Extraction, Waiting, Tabs & Viewport, Advanced)
  • Tools are defined in TOOL_CLASSES constant and registered with the MCP server at initialization
  • Session-based architecture: All browser operations require an explicit session

Session Management (lib/ferrum_mcp/session_manager.rb, lib/ferrum_mcp/session.rb)

  • SessionManager: Thread-safe session pool with automatic cleanup
  • Session: Encapsulates a browser instance with custom configuration
  • Supports multiple concurrent browser sessions with different configurations
  • Each session can use different browser types (Chrome, BotBrowser), options, and profiles
  • Session lifecycle: create → use → auto-cleanup (after 30min idle) or manual close
  • Important: All browser tools require a valid session_id parameter

Browser Management (lib/ferrum_mcp/browser_manager.rb)

  • BrowserManager: Handles Ferrum browser lifecycle
  • Supports both standard Chrome/Chromium and BotBrowser (anti-detection mode)
  • Browser options configured via browser_options method with anti-automation flags
  • Each session has its own BrowserManager with custom configuration

Transport Layer (lib/ferrum_mcp/transport/)

  • Two transport implementations:
    • HTTPServer: Uses MCP's StreamableHTTPTransport with Puma/Rack
    • StdioServer: Uses MCP's StdioTransport for standard I/O communication
  • Transport is selected at startup via --transport flag (http or stdio)

Read the full file on GitHub · 368 lines

Changes

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.

  1. 29d ago Changed · +9 lines · +247 tokens per session 26691900bd6e
  2. 1mo ago Changed · +17 lines · +573 tokens per session 9dc8ea663dcb
  3. 2mo ago First seen · 342 lines · 3,167 tokens per session scan A 54559ed65673

Subscribe to this mod's changes

FerrumMCP CLAUDE.md is an instructions file published in the GitHub repository Eth3rnit3/FerrumMCP (30 stars, last pushed 28d ago), licensed MIT. It adds 3,987 tokens to every session, about $0.0159 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 2026-08-30.

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