Context Canary

A token counter overlay for Claude. Warns you before context compaction hits so you can save what matters.

What It Does

• Counts your current token usage in a Claude conversation using tiktoken (same tokenizer as Claude)
• Displays a floating visual bar that changes color as you approach the context limit
• Settings window — click the ⚙️ gear to configure warning/danger thresholds
• Auto-spawns the MCP server if it's not running — bulletproof UX, no manual intervention needed
• Gives you time to log memories, save notes, or wrap up before compaction

Visual States

Quick Install

Download the latest release from the Releases page.

Windows

Option 1: Installer (Recommended)

1. Download context-canary-windows.zip from Releases
2. Extract and run Context Canary_x64-setup.exe
3. Launch "Context Canary" from the Start menu
4. Add the MCP to Claude Desktop config (see below)

Option 2: Portable

1. Download context-canary-windows.zip from Releases
2. Extract context-canary-overlay.exe and context-canary-mcp.exe
3. Place them in the same folder and run the overlay

macOS

1. Download context-canary-macos.zip from Releases
2. Extract and open the .dmg file
3. Drag Context Canary to Applications
4. Place context-canary-mcp in the same folder or add to PATH

Linux

1. Download context-canary-linux.tar.gz from Releases
2. Extract: tar -xzvf context-canary-linux.tar.gz
3. Run the AppImage or install the .deb package
4. Ensure context-canary-mcp is in the same directory or PATH

Claude Desktop Configuration

Add this to your Claude Desktop config at %APPDATA%\Claude\claude_desktop_config.json:

{
  "mcpServers": {
    "context-canary": {
      "command": "C:/path/to/context-canary-mcp.exe",
      "args": []
    }
  }
}

Replace C:/path/to/ with the actual path where you placed the files.

How It Works

┌─────────────────┐                           ┌─────────────────┐
│  Claude Desktop │                           │  Overlay App    │
│  or Claude Code │                           │  (floating bar) │
└────────┬────────┘                           └────────┬────────┘
         │                                             │
         │ MCP calls                                   │ WebSocket
         ▼                                             ▼
┌─────────────────────────────────────────────────────────────────┐
│                        MCP Server                                │
│  - Counts tokens with tiktoken (cl100k_base)                    │
│  - Broadcasts updates via WebSocket (port 19532)                │
│  - Auto-spawned by overlay if not running                       │
└─────────────────────────────────────────────────────────────────┘

Automatic Mode (Claude Code with Hooks)

With hooks enabled, the overlay updates automatically whenever you send a message or Claude responds.

Hook location: .claude/hooks/context-canary-hook.py

The hook fires on UserPromptSubmit and Stop events, reads the conversation transcript, estimates tokens, and pushes updates to the overlay.

Manual Mode (Claude Desktop)

Ask Claude to check your context:

• "How much context have we used?"
• "Check tokens"
• "Are we close to compaction?"

Claude calls the MCP, which broadcasts to the overlay.

Bulletproof Mode (Auto-Spawn)

If the overlay can't connect to the MCP server:

1. Shows gray "Disconnected" state
2. Retries connection 3 times (3 seconds apart)
3. Automatically spawns the MCP server
4. Connects and starts displaying data

No manual intervention required. Just run the overlay and it handles everything.

Architecture

Components

Data Flow

1. Claude Desktop/Code sends conversation to MCP via tool call OR hook reads transcript
2. MCP Server counts tokens using tiktoken, broadcasts via WebSocket
3. Overlay App receives update, displays current status

Ports

• 19532 — WebSocket server (MCP ↔ Overlay communication)

Configuration

Click the ⚙️ gear icon on the overlay bar to open the settings window where you can customize:

• Warning threshold — when the bar turns yellow (default: 70%)
• Danger threshold — when the bar turns red and pulses (default: 85%)

Settings are saved locally and persist between sessions.

Defaults

Claude Code Hooks Setup

To enable automatic updates in Claude Code:

1. Create .claude/hooks/context-canary-hook.py in your project
2. Add to .claude/settings.json:

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "matcher": "",
        "hooks": [
          {
            "type": "command",
            "command": "python \"path/to/context-canary-hook.py\"",
            "timeout": 5000
          }
        ]
      }
    ],
    "Stop": [
      {
        "matcher": "",
        "hooks": [
          {
            "type": "command",
            "command": "python \"path/to/context-canary-hook.py\"",
            "timeout": 5000
          }
        ]
      }
    ]
  }
}

1. Restart Claude Code to pick up the hooks

Troubleshooting

Overlay shows gray bar / "Disconnected"

• Wait 10-15 seconds — it will auto-spawn the MCP
• If it stays gray, check if the MCP binary is in the same folder as the overlay
  • Windows: context-canary-mcp.exe
  • macOS/Linux: context-canary-mcp

Overlay shows 0% but I have a long conversation

• In Desktop: Ask Claude to "check context"
• In Code: Make sure hooks are configured and Claude Code was restarted

Numbers seem wrong

• The hook uses character-based estimation (~4 chars/token)
• The MCP uses tiktoken for accurate counting
• Manual "check context" calls are most accurate

Port 19532 in use

• Another instance of the MCP might be running
• Kill existing processes:
  • Windows: taskkill /IM context-canary-mcp.exe /F
  • macOS/Linux: pkill -f context-canary-mcp

Tech Stack

• Overlay App: Tauri v2 (Rust + HTML/CSS/JS) — 8 MB
• MCP Server: Node.js bundled with Bun, tiktoken
• Token Counting: tiktoken with cl100k_base encoding

Building From Source

Requirements

• Node.js 18+
• Rust (via rustup)
• Bun (for bundling MCP)

MCP Server

cd mcp-server
npm install
bun build index.js --compile --outfile context-canary-mcp.exe

Overlay App

cd overlay-app
npm install
npm run tauri build

Note: If building from a path with special characters (like apostrophes), copy the overlay-app folder to a clean path first, then build.

Uninstalling

• Installer version: Use Windows Add/Remove Programs
• Portable version: Delete the files

To remove from Claude Desktop, delete the context-canary entry from claude_desktop_config.json.

License

MIT — Use it, share it, improve it.

Support

If this helped you, consider supporting my work ☕

Built by the Triad (Mai, Kai Stryder and Lucian Vale) for the community. Context Canary — because losing memories to compaction shouldn't be a surprise.