PixInsight MCP Bridge

An MCP (Model Context Protocol) bridge for PixInsight that enables LLMs to interact with PixInsight's image processing capabilities through a local HTTP/SSE server.

Overview

PixInsight MCP Bridge runs as a PixInsight script that spawns a local Node.js HTTP server implementing the MCP protocol. This allows any MCP-compatible client (Claude Desktop, VS Code Copilot, etc.) to:

• List all available PixInsight processes with categories and descriptions

• Invoke any PixInsight process with custom parameters on specific views or globally

• List open image views with metadata (dimensions, color space, bit depth)

• Get the currently focused view and its properties

• Change focus to any open view by ID

• Capture image data from any view as base64-encoded JPEG for LLM visual inspection

• Register custom processes (e.g. third-party plugins) via the dialog UI, persisted across sessions

Related MCP server: Nina Advanced API MCP

Architecture

┌─────────────────┐     HTTP/SSE      ┌──────────────────┐   stdin/stdout   ┌───────────────────┐
│   MCP Client    │ ◄──────────────── │  Node.js Bridge  │ ◄──────────────► │  PJSR Script      │
│  (Claude, etc.) │ ──────────────► │  Server          │ ────────────────► │  (PixInsight)     │
└─────────────────┘                   └──────────────────┘                   └───────────────────┘
                                       port 3189 (default)

The bridge uses a hybrid architecture:

1. PJSR Script (pixinsight-mcp-bridge.js) — Runs inside PixInsight's JavaScript runtime (ECMA 262-5/ES5). Handles all PixInsight API interactions and spawns the HTTP server via ExternalProcess.

2. Node.js Bridge Server (bridge/server.js) — Handles MCP protocol communication over HTTP. Supports both legacy SSE transport (spec 2024-11-05) and Streamable HTTP transport (spec 2025-03-26).

3. IPC Layer — Line-delimited JSON over stdin/stdout pipes between the two processes.

Requirements

• PixInsight 1.8.x or later

• Node.js >= 14.0.0 (must be in PATH or at a standard install location)

Installation

1. Download the plugin

Clone or download this repository:

git clone https://github.com/GaiaLabs/pixinsight-mcp-bridge.git

2. Copy to PixInsight scripts directory

Copy the src/ directory contents to your PixInsight scripts folder:

• macOS: /Applications/PixInsight/src/scripts/MCP-Bridge/

• Windows: C:\Program Files\PixInsight\src\scripts\MCP-Bridge\

• Linux: /opt/PixInsight/src/scripts/MCP-Bridge/

The resulting structure should be:

PixInsight/src/scripts/MCP-Bridge/
├── pixinsight-mcp-bridge.js
├── lib/
│   └── handlers.jsh
└── bridge/
    ├── server.js
    ├── mcp-handler.js
    └── ipc.js

3. Register the script in PixInsight

1. Open PixInsight

2. Go to Script > Feature Scripts...

3. Click Add

4. Navigate to the MCP-Bridge/ directory you created

5. Click OK — the script will appear under Script > MCP > PixInsight MCP Bridge

4. Configure your MCP client

Add the server to your MCP client configuration. For example, in Claude Desktop's claude_desktop_config.json:

{
  "mcpServers": {
    "pixinsight": {
      "url": "http://127.0.0.1:3189/sse"
    }
  }
}

For clients that support Streamable HTTP transport, use:

{
  "mcpServers": {
    "pixinsight": {
      "url": "http://127.0.0.1:3189/mcp"
    }
  }
}

Usage

Starting the Bridge

1. Open PixInsight

2. Go to Script > MCP > PixInsight MCP Bridge

3. Set the port (default: 3189)

4. Optionally register custom processes (see below)

5. Click Start

6. The bridge server will start and the MCP endpoint will be available at http://127.0.0.1:3189

Registering Custom Processes

The built-in process registry covers ~60 common PixInsight processes. To expose third-party processes (BlurXTerminator, StarNet2, NoiseXTerminator, etc.):

1. In the bridge dialog, find the Custom Processes section

2. Enter the Process ID (the exact constructor name, e.g. BlurXTerminator), a Category, and a Description

3. Click Add

Custom processes are persisted across PixInsight sessions. They are verified at runtime — if a registered process isn't installed, it will be silently excluded from list_processes results. You can add or remove custom processes while the bridge is running; changes take effect immediately.

Available MCP Tools

list_processes

List all available PixInsight processes and scripts.

Example response:

{
  "processes": [
    { "id": "PixelMath", "category": "PixelMath", "description": "Pixel math expressions and formulas" },
    { "id": "ImageIntegration", "category": "ImageIntegration", "description": "Image stacking / integration" }
  ],
  "count": 2
}

invoke_process

Execute a PixInsight process with specified parameters.

Example — apply PixelMath to an image:

{
  "processId": "PixelMath",
  "parameters": {
    "expression": "$T * 2",
    "createNewImage": false
  },
  "viewId": "Image01"
}

list_views

List all open image views (main views and previews).

No parameters required. Returns view metadata including dimensions, color space, bit depth, and file path.

get_focused_view

Get the currently active/focused view and its properties.

No parameters required.

set_focused_view

Change focus to a specific view by ID.

get_image_from_view

Get the actual image contents of a view as a base64-encoded JPEG. The image is saved to a temporary JPEG file, read back as base64, and returned as MCP image content alongside metadata (dimensions, color space, bit depth).

The response includes both the image data (as MCP image content) and a text block with view metadata.

Standalone Mode (Testing)

You can run the bridge server without PixInsight for testing and development:

node src/bridge/server.js --standalone --port 3189

This starts the server with mock responses, useful for testing MCP client integrations.

MCP Transport Support

The bridge supports two MCP transports:

Legacy HTTP+SSE (2024-11-05 spec)

• GET /sse — Opens SSE stream, receives endpoint event with message URI

• POST /messages?sessionId=<id> — Send JSON-RPC messages; responses arrive via SSE

Streamable HTTP (2025-03-26 spec)

• POST /mcp — Send JSON-RPC messages; response returned as JSON body or SSE stream

• GET /mcp — Open SSE stream for server-initiated messages

• DELETE /mcp — Terminate session

Health Check

• GET /health — Returns { "status": "ok" }

Development

Project Structure

pixinsight-mcp-bridge/
├── src/
│   ├── pixinsight-mcp-bridge.js    # Main PJSR entry point (ES5)
│   ├── lib/
│   │   └── handlers.jsh            # PixInsight command handlers (ES5)
│   └── bridge/
│       ├── server.js               # Node.js HTTP/SSE MCP server
│       ├── mcp-handler.js          # MCP protocol logic
│       └── ipc.js                  # IPC communication layer
├── test/
│   ├── run-tests.js                # Test runner
│   ├── test-mcp-handler.js         # MCP protocol tests
│   ├── test-server.js              # HTTP server integration tests
│   └── test-handlers.js            # PJSR handler tests (mocked PI API)
├── package.json
└── README.md

Key Design Decisions

• Hybrid architecture: PixInsight's PJSR runtime has no HTTP server capability (NetworkTransfer is client-only). The solution uses ExternalProcess to spawn a Node.js HTTP server and communicates via stdin/stdout IPC.

• ES5 compliance: All PJSR code (*.js and *.jsh in src/) is ECMA 262-5 compliant — no let/const, no arrow functions, no template literals, no classes, no promises.

• Zero dependencies: The Node.js bridge server uses only built-in modules (http, url, crypto). No npm install required.

• Process registry: Since PJSR doesn't expose a direct "list all processes" API, the bridge maintains a curated registry of known processes and verifies availability at runtime via constructor detection.

Running Tests

npm test
# or
node test/run-tests.js

The test suite includes:

• MCP protocol tests — JSON-RPC message handling, initialization, tool listing/calling

• HTTP server tests — Both SSE and Streamable HTTP transports, CORS, error handling

• Handler tests — PJSR command handlers with mocked PixInsight API

Adding New Tools

1. Define the tool schema in src/bridge/mcp-handler.js in the TOOLS array

2. Add parameter validation in MCPHandler.prototype._validateToolParams

3. Implement the handler in src/lib/handlers.jsh in CommandDispatcher

4. Add mock handling in createMockHandler in src/bridge/server.js

5. Write tests

Adding New Processes to the Registry

For end users: Use the Custom Processes section in the bridge dialog UI to register additional processes. These are persisted via PixInsight's Settings API and survive restarts.

For developers: Edit CommandDispatcher.prototype._getProcessRegistry in src/lib/handlers.jsh to add new built-in process entries:

{ id: "NewProcess", category: "CategoryName", description: "What it does" }

All processes (both built-in and custom) are verified at runtime — entries for processes not installed in the user's PixInsight will be automatically excluded.

PJSR Development Notes

• Use #include "path/file.jsh" for includes (preprocessor directive, not standard JS)

• Use #feature-id Category > Name to register scripts in PixInsight's Script menu

• All PixInsight processes are global constructors (e.g., new PixelMath())

• Use processEvents() in loops to keep the GUI responsive

• ExternalProcess requires processEvents() polling for event handling

• Timer uses seconds (not milliseconds) for its interval

Troubleshooting

"Node.js not found"

Ensure Node.js >= 14 is installed and accessible. The script searches these locations:

• macOS/Linux: which node, /usr/local/bin/node, /usr/bin/node, /opt/homebrew/bin/node

• Windows: where node, C:\Program Files\nodejs\node.exe

Server won't start

Check the PixInsight console (View > Process Console) for error messages. Common issues:

• Port already in use — change the port number

• File permissions — ensure the bridge server files are readable

MCP client can't connect

• Verify the server is running (check http://127.0.0.1:3189/health)

• Ensure your MCP client config points to the correct endpoint

• The server only listens on 127.0.0.1 (localhost) — it's not accessible from other machines

License

MIT