hermes-pen-bridge

Unofficial. Community MCP bridge for the Pencil design tool, built primarily for Hermes but compatible with any MCP client. Not affiliated with or endorsed by High Agency, Inc. "Pencil" is their trademark; this project uses the name descriptively only.

A hybrid MCP server that spawns Pencil's bundled MCP binary and layers extra developer tools on top — file validation, design-token extraction, layer trees, structural diffs, and batch export.

Pencil already ships an MCP server out of the box. This project doesn't replace it — it proxies all 13 native tools through unchanged and adds 7 utility tools that are convenient when scripting against .pen files from outside the app.

Architecture

[MCP client] ─stdio MCP─► [hermes-pen-bridge] ─stdio MCP─► [Pencil's bundled binary] ─WS─► [Pencil app]
                              │
                              └── 7 custom local tools (file utils)

1. Spawns Pencil's own MCP binary (mcp-server-darwin-arm64) as a subprocess
2. Forwards all standard MCP tool calls (batch_get, batch_design, get_editor_state, …)
3. Handles 7 custom tools locally
4. Relies on Pencil's stored credentials — no extra auth setup

Why "for Hermes"?

Hermes (Nous Research's local agent) is the primary audience — it's where this was first used. The server itself is provider-agnostic standard MCP and works fine with Claude Desktop, Cursor, Claude Code, and any other MCP client. The "Hermes-first" framing is about who we're solving for, not a technical lock-in.

Native tools (proxied)

batch_design, batch_get, export_nodes, find_empty_space_on_canvas, get_editor_state, get_guidelines, get_screenshot, get_variables, open_document, replace_all_matching_properties, search_all_unique_properties, set_variables, snapshot_layout

Custom tools

Tool	Purpose
pencil_version	Show binary path and server status
pencil_list_recent_files	List recent .pen files from Pencil's config
pencil_validate_file	Validate a .pen file's structure
pencil_extract_design_tokens	Extract colors, fonts, spacing, variables
pencil_list_layers	Hierarchical tree of all elements
pencil_diff_files	Structural diff between two .pen files
pencil_batch_export	Batch export to PNG/SVG

(Tool names retain the pencil_ prefix because they operate on .pen files — descriptive, not a brand claim.)

Installation

Prerequisites

• macOS (Pencil ships an arm64 binary)
• Pencil installed at /Applications/Pencil.app with a valid account/license
• Python 3.11+

Install

From PyPI:

pip install hermes-pen-bridge

Or zero-install with uv:

uvx hermes-pen-bridge --pencil-binary /Applications/Pencil.app/Contents/Resources/app.asar.unpacked/out/mcp-server-darwin-arm64

Or from source:

git clone https://github.com/devvgwardo/hermes-pen-bridge
cd hermes-pen-bridge
python3 -m venv .venv && source .venv/bin/activate
pip install -e .

MCP client configs

Hermes (~/.hermes/config.yaml)

mcp_servers:
  pen-bridge:
    command: hermes-pen-bridge
    args:
      - --pencil-binary
      - /Applications/Pencil.app/Contents/Resources/app.asar.unpacked/out/mcp-server-darwin-arm64
    tools: all

Claude Desktop / Cursor / Claude Code

{
  "mcpServers": {
    "pen-bridge": {
      "command": "hermes-pen-bridge",
      "args": ["--pencil-binary", "/Applications/Pencil.app/Contents/Resources/app.asar.unpacked/out/mcp-server-darwin-arm64"]
    }
  }
}

Restart the client. All 20 tools (13 native + 7 custom) appear automatically.

Usage

pencil_version
pencil_list_recent_files
pencil_validate_file --path "/path/to/file.pen"
pencil_extract_design_tokens --path "/path/to/file.pen"
pencil_list_layers --path "/path/to/file.pen" --max_depth 10
pencil_diff_files --path_a "v1.pen" --path_b "v2.pen"
pencil_batch_export --paths '["a.pen","b.pen"]' --format png --scale 2

Open-source alternative

If you want a fully open-source design tool with built-in MCP (no closed-source binary in the loop), see OpenPencil or ZSeven-W/openpencil.

License & disclaimers

• Wrapper code: MIT (see LICENSE)
• Pencil's binary and .pen file format: proprietary to High Agency, Inc. This project does not redistribute Pencil's binary, source, or assets. Users must install Pencil themselves and comply with pencil.dev's Terms of Use.
• This is an independent community project. No affiliation with or endorsement by High Agency, Inc.

Related: the design pattern

The proxy approach used here is documented as a generic skill at desktop-app-mcp-proxy-pattern — applies to any Electron app that bundles its own MCP binary.