openpencil/packages/pen-mcp

@zseven-w/pen-mcp

MCP server for OpenPencil — enables Claude, GPT, Gemini, and other LLMs to read, create, and modify designs through a standard tool protocol.

Note: pen-mcp is shipped as part of the OpenPencil app (desktop + web) and is not a standalone CLI. The published package ships TypeScript source against workspace-only dependencies and has no bin entry, so npx @zseven-w/pen-mcp does not work. Run the server from the OpenPencil monorepo or connect external clients to the HTTP endpoint exposed by a running OpenPencil instance.

Overview

pen-mcp exposes OpenPencil's full editing API as MCP tools. External AI agents can open documents, inspect the canvas, insert/update/delete nodes, and generate complete designs — all through structured tool calls.

Three workflows are supported:

Workflow	Tools	Best for
Single-shot	insert_node, batch_design	Quick edits, single components
Layered	design_skeleton → design_content × N → design_refine	Full-page designs with high fidelity
CRUD	batch_get → update_node / delete_node	Reading & modifying existing content

Running the MCP Server

The server supports both stdio and streamable HTTP transports. The default HTTP endpoint is http://localhost:3100/mcp.

From the monorepo (development)

git clone https://github.com/ZSeven-W/openpencil.git
cd openpencil && bun install
bun run mcp:dev              # starts stdio + HTTP on port 3100
# flags: --http (HTTP only), --stdio (stdio only), --port <n>

Built-in to the OpenPencil app

Launching the desktop or web app automatically starts the MCP server in the background. External MCP clients should connect over HTTP to the running instance — no separate install required.

Connecting an MCP client

Most MCP-aware clients (Claude Desktop, Cursor, Continue, etc.) accept an HTTP URL pointing at a running server. Point them at http://localhost:3100/mcp while the OpenPencil app or bun run mcp:dev is running.

Tools

Document & Read Tools

Tool	Description
open_document	Open an .op file or connect to the live Electron canvas. Always call first.
batch_get	Search and read nodes by type, name regex, or specific IDs. Controls read depth for nested content.
get_selection	Get the currently selected nodes on the live canvas.
snapshot_layout	Get a compact bounding-box layout tree — useful for spatial understanding.
find_empty_space	Find available canvas space in a given direction for placing new content.
get_design_prompt	Retrieve segmented design knowledge (schema, layout, roles, text, style, icons, examples).

Node CRUD Tools

Tool	Description
insert_node	Insert a new node with full PenNode data. Supports postProcess for auto-defaults.
update_node	Shallow-merge properties into an existing node.
delete_node	Delete a node and all its children.
move_node	Reparent a node to a new container.
copy_node	Deep-clone a node with new IDs under a target parent.
replace_node	Replace a node entirely with new data at the same position.
import_svg	Import a local SVG file as editable PenNodes.

Batch Design DSL

batch_design accepts a compact DSL — one operation per line:

root=I(null, { "type": "frame", "name": "Page", "width": 1200, ... })
header=I(root, { "type": "frame", "name": "Header", ... })
U(header, { "fill": [{ "type": "solid", "color": "#1A1A2E" }] })
logo=C("existing-logo", header, { "x": 24 })
M("floating-btn", header)
D("old-section")

Op	Syntax	Description
I	binding=I(parent, { data })	Insert node
U	U(path, { updates })	Update properties
C	binding=C(source, parent, { overrides })	Copy node
R	binding=R(path, { newData })	Replace node
M	M(nodeId, parent, index?)	Move node
D	D(nodeId)	Delete node

Layered Generation Workflow

For high-fidelity multi-section designs:

1. design_skeleton  → Create root frame + section placeholders
2. design_content   → Fill each section with content nodes (call per section)
3. design_refine    → Run full-tree validation and auto-fixes

Page Management

Tool	Description
add_page	Add a new page to the document
remove_page	Remove a page (cannot remove the last one)
rename_page	Rename a page
reorder_page	Move a page to a new index
duplicate_page	Deep-clone a page with new IDs

Post-Processing

All creation tools support postProcess=true for automatic:

• Semantic role defaults (button padding, input height, card radius, etc.)
• Icon name → SVG path resolution (Lucide icon set)
• Card row equalization in horizontal layouts
• Text height estimation
• Frame height expansion when content overflows
• clipContent auto-addition for frames with cornerRadius + images

Design Prompt Sections

get_design_prompt(section) returns focused subsets of design knowledge:

Section	Content
schema	PenNode type definitions and property reference
layout	Flexbox layout engine rules (gap, padding, justify, align)
roles	Semantic roles and their auto-defaults (button, input, card, navbar, ...)
text	Typography rules, CJK support, copywriting guidelines
style	Visual style policy (colors, fonts, aesthetic)
icons	Feather/Lucide icon naming conventions
examples	Complete design examples with DSL
guidelines	Design tips (cards, inputs, phone mockups, hero sections)
planning	Layered workflow guide with section decomposition rules

Live Canvas Sync

When connected to a running OpenPencil desktop app, changes made via MCP tools appear on the canvas in real-time. The sync is bidirectional — user edits on the canvas are reflected in subsequent batch_get / snapshot_layout calls.

Programmatic Usage

import { configureMcpHooks, MCP_DEFAULT_PORT } from '@zseven-w/pen-mcp';

// Configure custom hooks (optional)
configureMcpHooks({
  onDocumentOpen: (path) => console.log(`Opened: ${path}`),
  onNodeInsert: (node) => console.log(`Inserted: ${node.id}`),
});

License

MIT