render_pdf_page fails in Node.js: "Cannot read properties of undefined (reading 'createElement')"

[augusto-afontes]

Bug Report

Description

The render_pdf_page tool fails on Windows with the following error:
Cannot read properties of undefined (reading 'createElement')

This happens because the rendering code calls document.createElement('canvas'), which is a browser-only DOM API that does not exist in any version of Node.js. As a result, document is undefined at runtime and the call throws.

Environment

• OS: Windows 11
• Node.js: v24.16.0
• Plugin: pdf-filler-simple (Claude Desktop Extension)

Steps to Reproduce

1. Install the extension in Claude Desktop
2. Ask Claude to use render_pdf_page on any PDF file
3. The tool returns the error above

Root Cause

The code is using document.createElement('canvas') as if it were running in a browser. Since the plugin runs in a Node.js process, document is undefined.

The @napi-rs/canvas package (already listed as a dependency) provides createCanvas() specifically for Node.js environments and does not require the DOM. The fix is to replace:

// ❌ Browser-only — breaks in Node.js
const canvas = document.createElement('canvas');

with:

// ✅ Node.js compatible
const { createCanvas } = require('@napi-rs/canvas');
const canvas = createCanvas(width, height);

Additional Notes

All other tools in the plugin (merge_pdfs, read_pdf_content, split_pdf, etc.) work correctly. Only render_pdf_page is affected.

After running npm install from scratch (removing node_modules and package-lock.json), the original module-not-found error for @napi-rs/canvas-win32-x64-msvc was resolved. The createElement error only appeared after that fix, confirming the native binding now loads correctly — the issue is purely in the canvas initialization logic.

[silverstein]

Fixed on master in e76b372.

The server now passes explicit Node-capable CanvasFactory and FilterFactory implementations to pdf.js raster loads. This prevents Claude Desktop’s Electron utility-process shape from making pdf.js select its DOM factories and call document.createElement.

Regression coverage now starts the MCP server with Electron-like process.type/process.versions.electron and verifies:

• render_pdf_page
• render_pdf_region
• scanned/image-only read_pdf_content fallback

Verification: focused 12/12 tests; full suite 185/185; UI production build passed. I’m leaving this issue open until the fix is packed and validated in a clean Windows Claude Desktop install.