An AI document reviewer — open a progress report, RFP, or compliance filing and Claude reviews it alongside you in real time. Highlights, comments, suggestions, and questions appear as first-class annotations you accept, dismiss, or discuss. The original file is never modified unless you save.

Quick Start

Prerequisites

• Node.js 22+ (download)
• Claude Code (irm https://claude.ai/install.ps1 | iex)

Install and Run

npm install -g tandem-editor
tandem setup     # registers MCP tools + installs Claude Code skill
tandem           # starts server + opens browser

tandem setup auto-detects Claude Code and Claude Desktop, writes MCP configuration, and installs a skill (~/.claude/skills/tandem/SKILL.md) that teaches Claude how to use Tandem's tools effectively. Re-run after upgrading (npm update -g tandem-editor && tandem setup).

Connect Claude Code

Start Claude Code with channel push for real-time notifications:

claude --dangerously-load-development-channels server:tandem-channel

Then try:

"Review the welcome document with me"

Claude calls tandem_open, the document appears in the browser, and annotations start flowing. Chat messages, annotation actions, and text selections push to Claude instantly.

Without channels: Use the /loop skill in Claude Code to poll:

/loop 30s check tandem inbox and respond to any new messages

Verify

npm run doctor    # checks Node.js, MCP config, server health, ports

Or check the raw health endpoint:

curl http://localhost:3479/health
# → {"status":"ok","version":"0.1.2","transport":"http","hasSession":false}

hasSession becomes true once Claude Code connects.

Development Setup (contributing / building from source)

git clone https://github.com/bloknayrb/tandem.git
cd tandem
npm install
npm run dev:standalone   # starts server (:3478/:3479) + browser client (:5173)

Open http://localhost:5173 — you'll see sample/welcome.md loaded automatically on first run. The .mcp.json in the repo configures Claude Code automatically when run from this directory.

Features

Annotations

Claude adds highlights, comments, suggestions, and flags directly in the document. The side panel lists all annotations with filtering by type, author, and status. Accept, dismiss, or edit each one individually — or use bulk actions to process them in batches.

Chat

Send freeform messages to Claude alongside annotation review. Select text before sending to attach it as a clickable anchor — clicking it later scrolls back to that passage.

Review Mode

Press Ctrl+Shift+R to enter keyboard review mode. Navigate with Tab, accept with Y, dismiss with N, examine with E. A 10-second undo window lets you reverse accidental accepts. The side panel tracks your position.

More

• Multi-document tabs — open .md, .txt, .docx files side by side; drag to reorder
• .docx review-only mode — open Word documents for annotation; imported Word comments appear alongside Claude's
• Session persistence — documents and annotations survive server restarts
• Real-time channel push — annotation actions, chat, and selections push to Claude instantly
• Keyboard shortcuts — press ? for the full reference
• Unsaved-changes indicator — dot on tab title when a document has pending edits
• Configurable display name — set your name so Claude knows who's reviewing
• Atomic file saves — write to temp, then rename, preventing partial writes
• E2E tested — Playwright tests cover the annotation lifecycle end-to-end

Documentation

• User Guide — How to use Tandem: browser UI, annotations, chat, review mode, keyboard shortcuts
• MCP Tool Reference — 30 MCP tools + channel API endpoints
• Architecture — System design, data flows, coordinate systems, channel push
• Workflows — Claude Code usage patterns: document review, cross-referencing, multi-model
• Roadmap — Phase 2+ roadmap, known issues, future extensions
• Design Decisions — ADR-001 through ADR-021
• Lessons Learned — 31 implementation lessons

CLI Commands

Command	What it does
tandem	Start server and open browser (global install)
tandem setup	Register MCP tools with Claude Code / Claude Desktop
tandem setup --force	Register to default paths regardless of auto-detection
tandem --version	Show installed version
tandem --help	Show usage

MCP Configuration

Tandem uses two MCP connections: HTTP for document tools (28 tools including annotation editing), and a channel shim for real-time push notifications.

Global install (tandem setup): Automatically writes both entries to ~/.claude/mcp_settings.json (Claude Code) and/or claude_desktop_config.json (Claude Desktop) with absolute paths. No manual configuration needed.

Development setup (.mcp.json): The repo includes a .mcp.json that configures both entries automatically when Claude Code runs from the repo directory:

{
  "mcpServers": {
    "tandem": {
      "type": "http",
      "url": "http://localhost:3479/mcp"
    },
    "tandem-channel": {
      "command": "npx",
      "args": ["tsx", "src/channel/index.ts"],
      "env": { "TANDEM_URL": "http://localhost:3479" }
    }
  }
}

Both entries are cross-platform — no platform-specific configuration needed.

Environment Variables

All optional — defaults work out of the box.

Variable	Default	Description
TANDEM_PORT	3478	Hocuspocus WebSocket port
TANDEM_MCP_PORT	3479	MCP HTTP + REST API port
TANDEM_URL	http://localhost:3479	Channel shim server URL
TANDEM_TRANSPORT	http	Transport mode (http or stdio)
TANDEM_NO_SAMPLE	unset	Set to 1 to skip auto-opening sample/welcome.md
TANDEM_CLAUDE_CMD	claude	Claude Code executable name (for tandem setup auto-detection)

See .env.example for a copy-paste template.

Troubleshooting

Run npm run doctor for a quick diagnostic of your setup. It checks Node.js version, .mcp.json config, server health, and port status.

Claude Code says "MCP failed to connect" Start the server first (tandem for global install, or npm run dev:standalone for dev setup), then open Claude Code. The server must be running before Claude Code probes the MCP URL. If you restart the server, run /mcp in Claude Code to reconnect.

Port already in use Tandem kills stale processes on :3478/:3479 at startup. If another app uses those ports, set TANDEM_PORT / TANDEM_MCP_PORT to different values and update TANDEM_URL to match.

Channel shim fails to start The tandem-channel entry spawns a subprocess. For global installs, tandem setup writes absolute paths to the bundled dist/channel/index.js — re-run tandem setup after upgrading. For dev setup, if you see MODULE_NOT_FOUND with a production config (node dist/channel/index.js), run npm run build. The default dev config uses npx tsx and doesn't require a build step.

Browser shows "Cannot reach the Tandem server" The browser connects to the server via WebSocket. For global installs, run tandem to start the server. For dev setup, use npm run dev:standalone (or npm run dev:server). The message appears after 3 seconds of failed connection.

Empty browser with no document On first run, sample/welcome.md auto-opens. If you've cleared sessions or deleted the sample file, click the + button in the tab bar or drop a file onto the editor.

Development

Command	What it does
npm run dev:standalone	Recommended — both frontend + backend (via concurrently)
npm run dev:server	Backend only: Hocuspocus (:3478) + MCP HTTP (:3479)
npm run dev:client	Frontend only: Vite dev server (:5173)
npm run build	Production build (dist/server/ + dist/channel/ + dist/cli/ + dist/client/)
npm test	Run vitest (unit tests)
npm run test:e2e	Run Playwright E2E tests
npm run test:e2e:ui	Playwright UI mode

Tech Stack: React 19, Tiptap, Vite, TypeScript | Node.js, Hocuspocus (Yjs WebSocket), MCP SDK, Express | Yjs (CRDT), y-prosemirror | mammoth.js (.docx), unified/remark (.md)