3dstreet-mcp

A Model Context Protocol relay that lets Claude Desktop or Claude Code drive a 3DStreet scene running in your browser.

Is this the right tool? This relay is the fallback for MCP clients that cannot use WebMCP. If your agent runs inside a browser (ChatGPT's desktop browser, Gemini in Chrome, Chrome 149+ with the WebMCP origin trial or chrome://flags/#enable-webmcp-testing), you do not need this package: open https://3dstreet.app and the page registers the same tools with the browser via document.modelContext — nothing to install, no pairing. See the 3DStreet WebMCP docs. Use this relay only for clients with no browser of their own or no WebMCP support (Claude Desktop, Claude Code, Cursor, headless scripts). It will be retired once those clients read WebMCP natively.

Listed on the MCP Registry as app.3dstreet/3dstreet.

Status: alpha — protocol may change. Tracks 3DStreet#1582 (design) and 3DStreet#1600 (browser side). Scene creation works in any 3dstreet.app tab; saving and geospatial features require a free 3DStreet account.

How it works

Claude Desktop  ── stdio MCP ──▶  3dstreet-mcp  ── ws://127.0.0.1:51735 ──▶  3DStreet tab
or Claude Code                    (this package)                              (your browser)

The relay speaks MCP over stdio and bridges every tools/list and tools/call to a 3DStreet tab over a localhost WebSocket. The tab does the actual work — it's already signed in, has the catalog loaded, owns the cloud-save flow. The relay is a dumb pipe.

No auth tokens cross the WebSocket. The relay's port binds to 127.0.0.1 only and rejects WebSocket upgrades from origins outside the 3DStreet allowlist (https://3dstreet.app, https://dev-3dstreet.web.app, http://localhost:3333, http://127.0.0.1:3333).

Setup

Claude Desktop

Edit ~/Library/Application Support/Claude/claude_desktop_config.json (macOS) or %APPDATA%/Claude/claude_desktop_config.json (Windows):

{
  "mcpServers": {
    "3dstreet": {
      "command": "npx",
      "args": ["-y", "3dstreet-mcp"]
    }
  }
}

Restart Claude Desktop.

Claude Code

claude mcp add 3dstreet -- npx -y 3dstreet-mcp

Pair the browser tab

On startup the relay prints an auto-pair URL to its log, like:

open this URL to pair a 3DStreet tab: https://3dstreet.app/#mcp

Open that URL in a signed-in browser and the editor will detect the #mcp fragment, open the AI Assistant pane, and pair with the relay automatically — no console commands needed. (If you ran the relay on a non-default port the URL will look like https://3dstreet.app/#mcp=PORT.)

If you'd rather pair manually: open https://3dstreet.app, click AI Assistant in the right panel, type /mcp, and click Reconnect.

You can now ask Claude things like:

• "What's in my current scene?"
• "Add a bike lane on the left side of the street."
• "Change the environment preset to night."

Toggle Read-only in the status bar to block scene mutations (useful when you just want Claude to explain a scene).

CLI

3dstreet-mcp [options]

Options:
  -p, --port <number>    WebSocket port (default 51735)
  -o, --origin <url>     Origin used in the printed auto-pair URL and
                         the MCP `instructions` hint (default
                         https://3dstreet.app — set to
                         http://localhost:3333 when running 3DStreet
                         from a local dev server)
  -h, --help             Show this help
  -v, --version          Print version and exit

If you need a different port (e.g. running two Claude clients against two different tabs), pass --port and open the auto-pair URL the relay prints — https://3dstreet.app/#mcp=PORT — which both selects the port and triggers auto-pair.

For local 3DStreet development, point --origin at your dev server so the printed URL and the LLM's instructions hint both resolve to the right tab:

node src/cli.js --origin http://localhost:3333
# → open this URL to pair a 3DStreet tab: http://localhost:3333/#mcp

Telling the LLM how to pair

The relay returns an instructions string on every initialize (an optional MCP field clients may fold into the LLM's system prompt). It explains that scene tools are forwarded to a paired tab, names the auto-pair URL, and tells the model to surface that URL to the user when no tab is connected — so the LLM proactively guides users through pairing instead of waiting for an opaque tool-call timeout.

When no tab has ever paired this relay session, tools/call also fast-fails with the auto-pair URL in the error message, rather than queuing for 30 seconds. Once a tab has paired, brief disconnects revert to the queued retry behaviour so reconnecting peers can drain in-flight calls.

Tools exposed

The tool list is fetched from the connected browser tab on pair, so it always matches whatever the editor's command registry exposes. As of #1600 that includes:

• Reads: getScene, getEntity, getSelectedEntity, getManagedStreet, listMixins, getSessionInfo
• Selection / camera: selectEntity, focusCamera
• Mutations: entityCreate, entityUpdate, entityRemove, entityClone, entityReparent, componentAdd, componentRemove, segmentAdd, segmentUpdate, segmentRemove, replaceManagedStreet, …and the rest of the registry
• History: undo, redo

Run tools/list from your MCP client to see the current set.

Wire protocol (for contributors)

Stdio side: line-delimited JSON-RPC 2.0, MCP 2024-11-05.

WebSocket side: same JSON-RPC envelope, with the relay handling initialize / ping / notifications locally and forwarding only tools/list and tools/call to the peer. The peer assigns its own ids on incoming frames; replies match by id.

The single-peer policy: first browser tab to connect holds the slot. Second connection gets WebSocket close code 4001 with reason paired-elsewhere.

Development

git clone https://github.com/3DStreet/3dstreet-mcp.git
cd 3dstreet-mcp
npm install
npm test

Smoke-test against a real 3DStreet build:

# Terminal 1 — the relay, REPL-free, just stdio
node src/cli.js

# Terminal 2 — exercise it with the MCP inspector
npx @modelcontextprotocol/inspector node src/cli.js

License

AGPL-3.0-or-later, matching 3DStreet itself.