fix: MCP screenshot returns invalid base64 with puppeteer >=22

[Fieldnote-Echo]

Description

The coursecode_screenshot MCP tool fails on every call with MCP error -32602: Invalid tools/call result: ... "Invalid Base64 string", so MCP clients (Claude Code, Cursor, etc.) can never receive screenshots.

Root cause: page.screenshot() / elementHandle.screenshot() return a Uint8Array since Puppeteer v22 (this project pins puppeteer-core@^24.43.1). lib/headless-browser.js calls screenshotBuffer.toString('base64'), but Uint8Array.prototype.toString() ignores the encoding argument and returns comma-separated byte values ("255,216,255,..."), which then fails the MCP SDK's image-content validation.

Fix: wrap the result in Buffer.from(...) before encoding, which is correct whether Puppeteer returns a Buffer or a Uint8Array.

Repro

1. coursecode create test-course && cd test-course && coursecode preview
2. Start coursecode mcp and send over stdio:

   {"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"coursecode_screenshot","arguments":{}}}

3. Before: MCP error -32602 ... "Invalid Base64 string". After: valid base64 JPEG (/9j/4AAQSkZJRg...).

Related Issue

None filed — happy to open one first if you prefer issues for tracking.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• Other (describe):

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's code style
• I have run npm run lint and it passes
• I have tested my changes with npm run preview
• I have updated documentation if needed (N/A — no user-facing behavior change beyond the fix)
• My commits follow the conventional commit format

Testing

• Full unit suite: 1340/1340 passing (npm test).
• SCORM 1.2 and SCORM 2004 e2e driver suites: 14/14 each.
• Manual verification against a live preview server via a raw MCP stdio client: coursecode_screenshot previously returned the -32602 validation error on every call; with this change it returns a valid image/jpeg content block (base64 starts with the JFIF magic /9j/). Also exercised coursecode_state, coursecode_navigate, and coursecode_lint in the same session to confirm no regressions in the MCP surface.

Screenshots (if applicable)

N/A (the fix is screenshots 🙂)

🤖 Generated with Claude Code

[svincent240]

Thank you for the contribution! The fix looks good and is much appreciated!