DesignJS

An open-source MCP design canvas that gives AI coding agents eyes.

DesignJS is a local-first, HTML/CSS-native visual canvas that AI coding agents (Claude Code, Cursor, Codex) read and write through the Model Context Protocol. Design and code converge into a single artifact — no translation gap between visual intent and production output.

Status: v0.1 foundations + v0.3 Chrome-extension capture working end-to-end. The agent ↔ canvas loop is stable, and a separate side channel lets you drop any live web page or DOM subtree onto the canvas as editable HTML via the extension. v0.2 polish (paste-import, transform handles, component instances) is in flight per the roadmap.

Why

AI coding agents currently generate frontend UI blind. They produce React components from text prompts alone, with no ability to see a visual design, iterate spatially, or maintain layout relationships. Design engineers spend 2–4 hours a day in prompt → preview → re-prompt cycles getting agents to match their visual intent.

Two proprietary tools are closing this gap from different angles. Paper.design ships an HTML/CSS-native canvas — the design is the production code — but as a hosted SaaS product tied to its own backend. Pencil.dev ships a local-first, git-native canvas that any agent can drive over MCP — but the design file is a vector format in a closed renderer, not the HTML/CSS that ships to production. DesignJS combines both bets: HTML/CSS-native like Paper, local-first and git-native like Pencil, MIT-licensed, and built on open-source foundations (GrapesJS, the MCP TypeScript SDK, html-to-image).

How it works

┌───────────────────┐     stdio      ┌─────────────────┐
│  Claude Code /    │──────────────▶│   MCP Server    │
│  Cursor / Codex   │     (JSON-RPC) │   (Node.js)     │
└───────────────────┘                └───────┬─────────┘
                                             │ WebSocket (127.0.0.1:29170)
                                             ▼
┌─────────────────────────────────────────────┐
│      Browser (Vite + React SPA)             │
│  ┌────────────┐  ┌───────────────────────┐  │
│  │ Editor UI  │  │  GrapesJS Canvas      │  │
│  │ (panels,   │  │  (iframe with         │  │
│  │  blocks,   │  │   real HTML/CSS       │  │
│  │  styles)   │  │   + Tailwind v4 CDN)  │  │
│  └────────────┘  └───────────────────────┘  │
└─────────────────────────────────────────────┘
                      │
              ┌───────┴───────┐
              │ .designjs   │
              │   .json       │
              └───────────────┘

Agent ↔ Canvas communication flows stdio → MCP server → WebSocket bridge → browser → GrapesJS API, with responses travelling back the same path. Both human edits and agent edits converge on the same GrapesJS component model — one source of truth.

Quickstart

Requires Node.js 20+.

1. Start the canvas

The canvas app runs locally (it's not hosted). Two ways:

From source (recommended while v0.2 is in development — get all the latest):

git clone https://github.com/rubychilds/DesignJS.git
cd DesignJS
pnpm install
pnpm dev

Opens at http://localhost:3000. The WebSocket bridge listens on 127.0.0.1:29170. Connection status is visible top-right in the editor shell.

2. Scaffold a project (or use one you already have)

npm create designjs@latest my-app
cd my-app

Drops a .mcp.json, CLAUDE.md, and README.md pointing at the published MCP server. If you already have a project directory and just want the MCP config, skip this step and use npx @designjs/cli init instead (writes .mcp.json / .cursor/mcp.json / .vscode/mcp.json based on which IDE config dirs it detects).

3. Connect your agent

With the canvas running and your project scaffolded:

claude          # Claude Code — reads .mcp.json automatically
# or
cursor .
# or
code .

On the first tool call, the agent runs npx -y @designjs/mcp-server and connects. The bridge dot in the canvas Topbar flips to green. Prompt:

"Create a Desktop artboard, then add a pricing section with three tier cards."

Manual MCP config

If you'd rather write the MCP config yourself, add this to your project's .mcp.json:

{
  "mcpServers": {
    "designjs": {
      "command": "npx",
      "args": ["-y", "@designjs/mcp-server"]
    }
  }
}

Capture web pages with the Chrome extension

The DesignJS Chrome extension lets you grab any element or full web page and drop it onto the canvas as editable HTML. The capture pipeline walks the DOM, serializes computed styles + author CSS, and routes everything into a fresh artboard on your running canvas — no copy-paste, no screenshot tracing.

Install

The extension isn't on the Chrome Web Store yet (planned for v0.3 public). For now, load it locally from this repo:

pnpm install                                       # if you haven't already
pnpm --filter @designjs/chrome-extension build     # builds packages/chrome-extension/dist/

Then in Chrome:

1. Open chrome://extensions

2. Toggle Developer mode on (top-right)

3. Click Load unpacked and select packages/chrome-extension/dist/

4. Pin the DesignJS icon to the toolbar (puzzle-piece menu → pin)

When you next change extension source, rebuild and click the circular reload arrow on the DesignJS card in chrome://extensions.

Use

With pnpm dev running (canvas reachable at localhost:3000):

1. Navigate to any web page in Chrome.

2. Click the DesignJS icon in the toolbar. An overlay drops into the page; the green dot top-left confirms it can reach the canvas.

3. Two capture modes:

   • Start capture — hover any element to highlight it. Use ↑ ↓ ← → to traverse up/down the DOM tree, Enter to commit, Esc to cancel. The selection plus its descendants and computed styles land in the canvas as a new component.

   • Capture page — grabs the full <html> tree, scrolls + settles lazy-mounted sections, takes a full-page screenshot for an opacity-0.15 backplate, creates a fresh artboard sized to the source page, and ships everything to the canvas. Progress phases (Capturing → Sending → Rendering) display in the overlay during long captures.

Captured content lands as real, editable HTML/CSS — every element shows up in the layer tree, every property is inspectable + editable, classes survive intact for Tailwind round-trips, and the result saves to your .designjs.json like any other canvas state.

What's captured today (v0.3)

• ✅ Element selection + whole-page capture

• ✅ Computed-style serialization (~150 CSS properties: layout, typography, color, flex/grid, multi-column, tables, lists, transforms, filters)

• ✅ Same-origin <iframe> content (inlined as srcdoc)

• ✅ Cross-origin image / media URL rewriting (absolute paths so canvas iframe loads them)

• ✅ Source page's author CSS (via editor.Css.addRules so dedup + author rules land in the canvas's CSS Manager rather than being stripped by GrapesJS' HTML parser — see ADR-0011 2026-05-24 addendum)

• ✅ Allowlisted font-CDN <link> tags hoisted into the capture (Google Fonts / Bunny / TypeKit / fonts loaded via <link>)

• ✅ Style dedup hoist — repeated computed-style blocks share auto-generated _djhN classes to keep payload bounded

• ✅ Progress UX — granular phase events shown in-overlay during capture

Known limitations (planned for v0.4)

• ❌ Shadow DOM — silently skipped today; needs CDP DOM.getDocument traversal

• ❌ Cross-origin iframes — pass through as <iframe> with absolute src, content not inlined

• ❌ Hotlink-protected images — render as broken-image placeholders

• ❌ Authed-only content — captured from the live session but the canvas can't re-auth

• ❌ Non-CDN-served fonts — font-family value preserved, but the font file doesn't follow (Phase 1 of font preservation is in docs/font-preservation-plan.md)

• ⚠️ Wikipedia-class long pages (~7k components) take 60-180s to land — add_components is the bottleneck; chunked dispatch is tracked as Q1 in epic-8-followups.md §9

Diagnose a stuck or off-looking capture

The overlay shows a progress phase + an inline error if anything fails. For more detail:

• Inspect the Wikipedia tab's DevTools console — the content script logs [designjs] page captured: N nodes, MKB, serialized in Tms plus a [designjs] extracted N <style> block(s) line confirming the CSS extraction step.

• Inspect the extension's service worker (chrome://extensions → DesignJS card → blue "service worker" link) — the background logs [designjs] dispatching add_css_rules: NKB then the canvas response.

• Inspect the canvas tab (localhost:3000) — the bridge logs [designjs:bridge] add_css_rules: NKB → N chunks → M rules parsed confirming rules landed in the CSS Manager.

If [designjs:bridge] add_css_rules doesn't appear after a capture, the canvas isn't running an add_css_rules-aware build — restart pnpm dev from scratch (not just HMR).

Known quirks in v0.1

One .designjs.json per canvas session, not per project

The Vite dev server writes all canvas state to a single .designjs.json at the root of the cloned DesignJS repo, regardless of which user project an agent is operating in. A user who scaffolds /tmp/my-app via create-designjs, points Claude Code there, and starts designing will find the resulting .designjs.json inside the DesignJS clone — not inside /tmp/my-app.

For v0.1 this is a known limitation. The practical workaround if you're designing for multiple projects:

• Keep one pnpm dev session per project. Before switching projects, save (Cmd+S), copy .designjs.json out of the DesignJS clone into your project's directory, then git restore .designjs.json to reset the canvas.

• Or commit the per-project design files somewhere inside your own project structure and pull them back into the DesignJS clone when you want to resume.

A proper fix — per-project .designjs.json discovery via a get_project_context MCP handshake plus a Paper-style file switcher in the Topbar — is tracked as a v0.2 feature. It's the single biggest UX gap in v0.1.

Agent picks a competing design MCP instead of DesignJS

If you have other design MCPs configured globally in ~/.claude.json (pencil, paper, figma, etc.), Claude Code can pick any of them for a "design this" prompt. The CLAUDE.md that create-designjs drops tells agents to prefer DesignJS, but isn't foolproof — reinforce in-prompt ("use designjs, not pencil") or remove competing entries from your user config to isolate the test.

Stale opencanvas entry in Claude Code config

Pre-rebrand users may have an opencanvas MCP server sitting in ~/.claude.json's mcpServers block pointing at @opencanvas/mcp-server (which doesn't exist on npm anymore). Claude Code retries the connection on every launch and reports opencanvas · ✘ failed. Open the file and delete that key — restart Claude Code and the error stops.

Packages

MCP tools

Twenty-one bidirectional tools, grouped by area. Full input/output schemas in packages/bridge/src/tools.ts.

Inspect

Mutate

Artboards (multi-frame canvas)

Comparison

Roadmap

v0.1 — canvas + MCP foundation (largely shipped; a few gaps remain)

Foundations

• Three-pane editor shell — resizable panels, layers tree, semantic inspector, all keyboard shortcuts

• GrapesJS canvas with Tailwind v4 (CDN in iframe), light + dark themes

• Save/load to .designjs.json — Cmd+S, 30s autosave, reload-restore, git-diffable

• MCP server (stdio) + WebSocket bridge on 127.0.0.1:29170, multi-peer routing

• designjs init CLI — auto-detects Claude Code / Cursor / VS Code and writes the right MCP config

• CI: typecheck, build, smoke tests (bridge round-trip, MCP stdio, init), Playwright E2E (160+ tests across 28 specs)

• Repo: MIT, CONTRIBUTING, RELEASING (Changesets), ADR-driven design log

Multi-frame spatial canvas (originally v0.2 — landed early)

• Infinite, pannable, zoomable canvas hosting multiple artboard frames at world coordinates

• Frames as top-level layer-tree roots (ADR-0004)

• Snap-to-edge frame movement, automatic placement of new artboards, resize via Breakpoint toolbar (Desktop / Tablet / Mobile / Custom)

• Minimap with viewport indicator

• Page-root model — first frame is "the page"; loose primitives land there (ADR-0006)

Primitive vocabulary (ADR-0005)

• Frame, Rectangle, Ellipse, Text, Image, Group as first-class concepts mapped to HTML/CSS storage

• InsertRail for click-to-insert primitives (drag-to-canvas still pending)

• Block palette — 25 ready-made HTML/Tailwind blocks across Layout / Typography / Form / Media

• Per-frame counter naming (Rectangle 1, Rectangle 2, …)

• Point-text behaviour for the Text primitive — sizes to content, edits cleanly under contenteditable

Semantic inspector (panel reshape from sectioned style manager)

• Position — alignment, X/Y, rotation

• Layout — W/H with Hug/Fill/Fixed sizing, auto-layout (flex + grid), padding, margin, clip-content checkbox

• Appearance — opacity, radius (uniform + per-corner), Figma-style blend picker, cursor, z-index

• Typography — family, weight, size, LH, LS, alignment, case, decoration; gated to text-bearing tags

• Fill — multi-layer stack with reorder, hide, opacity, color picker

• Stroke — color + width + style (solid/dashed/dotted/double)

• Effects — shadow stack + filter functions (blur, brightness, contrast, saturate, grayscale, hue-rotate) + backdrop-blur

• Export — JSX (Tailwind or inline mode), HTML, CSS — copy-to-clipboard

• "Other CSS" Raw-CSS escape hatch — only appears when orphan properties are set

• Applicability gating — controls grey out (radius on inline text) or hide entirely (auto-layout on text) when not meaningful for the selection

• Lucide-only iconography, drag-to-scrub number inputs, color field with hex + alpha

MCP tools (21 tools, full list above — all verified via Playwright specs + unit tests)

• Inspect: ping, get_tree, get_html, get_css, get_jsx, get_screenshot, get_selection, get_variables

• Mutate: add_components, add_css_rules, update_styles, add_classes, remove_classes, set_text, set_variables, delete_nodes, select, deselect

• Artboards: create_artboard, list_artboards, find_placement, fit_artboard

Remaining for v0.1

• HTML/Tailwind clipboard paste import (Epic 3 — partial: paste-import.ts exists for image paste)

• npm create designjs@latest scaffolder (shipped — packages/create-designjs)

• Drag-to-canvas from the InsertRail (click-to-insert works today)

• User-extensible block config + custom Tailwind config loading

• Demo GIF, per-tool examples, troubleshooting docs (Epic 4)

• GitHub issue templates + Projects board

v0.2 — collaboration & export polish (weeks 5–8)

• Figma copy-paste import (clipboard payload → primitive vocabulary)

• Selection-overlay polish + transform handles (resize/rotate by drag)

• Component instances and props (variants beyond ADR-0005's terminal primitives)

• Style sources panel — surface where each computed value comes from (component, class, instance)

• Export presets — single-file React component, multi-file with extracted CSS, plain HTML

v0.3 — extension, multi-agent, IDE (weeks 9–12)

Chrome extension for site capture (substantially shipped — load-unpacked today; Web Store submission pending)

• Content-script overlay (not a browser-action popup — see ADR-0011)

• Keyboard-driven DOM walker — hover highlights, ↑↓←→ traverses, Enter commits, Esc exits

• Computed-style serializer (hybrid inline / inherited-diff, ~150 CSS properties: layout, type, color, flex/grid, multi-column, tables, lists, transforms, filters)

• Whole-page capture — auto-scroll-and-settle for lazy-mounted sections, <html> capture with <body> → <div> swap, fresh artboard sized to source

• Hybrid screenshot backplate (ADR-0012 §1) — full-page stitched screenshot composited under the HTML tree at opacity 0.15

• Same-origin iframe inlining (via srcdoc)

• Author-CSS supplement — extracts source page's @font-face, @keyframes, ::before / ::after rules

• Style-dedup hoist — repeated computed-style blocks share _djhN classes; capped at 100 to keep GrapesJS' CSS Manager surface bounded

• add_css_rules bridge tool routes captured CSS (~2,400 rules on Wikipedia-class pages) directly into the canvas's CSS Manager, bypassing parseHtml stripping

• Granular capture progress (Serializing → Screenshotting → Sending → Rendering) with per-phase events

• Font-CDN allowlist hoist — preserves Google Fonts / Bunny / TypeKit <link> tags

• Capture-fidelity diff tool — scripts/capture-diff.mjs scores source vs captured per-element drift with ε-tolerance and walk-alignment invariants

• Chrome Web Store submission (gating public v0.3 availability)

• Shadow DOM / cross-origin iframes / authed content — deferred to v0.4 CDP per ADR-0012 §2

• Non-CDN font preservation (Phase 1 Google Fonts name fallback + Phase 2 binary inlining — see docs/font-preservation-plan.md)

• Chunked add_components — eliminates 180s timeout race on Wikipedia-class captures (Q1 in epic-8-followups.md §9)

Other v0.3 items

• Concurrent MCP sessions with workspace isolation (foundation already in the bridge)

• VS Code custom editor for .designjs.json

• shadcn/ui block library

Detailed stories and acceptance criteria live in docs/epic-8-followups.md and the relevant ADRs.

Development

See CONTRIBUTING.md.

Releasing

Maintainer workflow for publishing releases: see RELEASING.md.

License

MIT — see LICENSE.