Published artifacts render blank again — possible regression of #42064 (postMessage origin mismatch)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Artifacts published via the Artifact tool in a Claude Code session render as a completely blank page when opened via the direct claude.ai/code/artifact/<id> link, and also via the claude.ai/code/artifacts gallery — while signed in to the correct account.

This reproduces even with a trivial, static HTML test page (no JavaScript, minimal CSS), which rules out anything specific to page complexity or script content in the artifact that triggered the report. The same HTML file, when instead served from a local file server and viewed in the Claude Code Browser pane, renders correctly with no issues.

Console output on the blank artifact page shows no fatal/red script errors — only a benign "Unrecognized Content-Security-Policy directive 'webrtc'." warning from the artifact's own sandboxed frame (frame.claudeusercontent.com), plus unrelated browser-extension logs.

What Should Happen?

Published artifacts should render their HTML content when opened via the artifact URL or the gallery, the same way they render when the identical file is served locally.

Error Messages/Logs

Unrecognized Content-Security-Policy directive 'webrtc'.

Steps to Reproduce

1. Publish any HTML artifact from a Claude Code session (reproduced with both a content-heavy page and a trivial static page with no JavaScript).
2. Open the published claude.ai/code/artifact/<id> URL directly, or open it via the claude.ai/code/artifacts gallery, while signed in.
3. Observe: the page is completely blank — no content renders.
4. Open DevTools console: no fatal/red script errors, only a benign "Unrecognized Content-Security-Policy directive 'webrtc'." warning from the sandboxed frame.
5. Compare with the identical HTML file served from a plain local file server (e.g. python3 -m http.server) — it renders correctly with no issues, confirming the file itself is not the problem.

Note: this looks like it may be a regression of #42064 ("Published artifacts show blank screen - postMessage origin mismatch"), which was closed on 2026-04-02 as fixed by a frontend deploy. That issue is now locked, so filing a new report per its own bot's instructions ("If you're experiencing a similar issue, please file a new issue and reference this one"). Our console did not surface the same postMessage SyntaxError text #42064 documented — only the CSP warning above — so it may be the same root cause resurfacing or a related-but-distinct regression in the same artifact-loading path.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

N/A — worked as of 2026-04-02 per #42064's resolution

Claude Code Version

N/A — this is a claude.ai Artifact-hosting/web issue, not the Claude Code CLI (same as #42064)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_