Artifacts with mermaid blocks cannot be shared ("This version can't be shared publicly")

Summary

An artifact version that contains mermaid blocks (<pre class="mermaid"> in HTML, or `mermaid fences in Markdown) cannot be shared from the claude.ai Share dialog. The dialog shows:

This version can't be shared publicly. Publish a new version or change the shared version, then try again.

Republishing a new version does not help as long as the mermaid blocks remain — the message is misleading about the actual cause.

Environment

• Artifact published from a Claude Code session via the Artifact tool (claude.ai/code artifact)
• Self-contained HTML report (~74 KB), no capabilities declared, no external resources
• Account where public sharing of other artifacts works normally

Reproduction / bisection

Starting from a report artifact that could not be shared, we published controlled variants and tested the Share dialog on each:

| Variant | Result |
|---|---|
| Minimal control page (no mermaid) | ✅ shareable |
| Full report minus the 4 mermaid blocks | ✅ shareable |
| Full report minus inline scripts (mermaid kept) | ❌ blocked |
| Full report minus IPs/domains (mermaid kept) | ❌ blocked |
| Same content republished to a fresh artifact record | ❌ blocked |
| Same content overwriting a previously-shareable artifact | ❌ blocked (that record shared fine before and after with non-mermaid content) |

Only the presence of mermaid blocks predicts the failure. Replacing the diagrams with static HTML/CSS made the original artifact shareable at its existing URL.

Expected

Either mermaid-rendered artifacts should be shareable (the rendering has no external dependencies), or the Share dialog should say why the version is blocked (e.g. "this version uses diagram rendering and can't be shared") instead of suggesting a republish that cannot fix it.

Impact

The docs/tool guidance actively recommends mermaid for artifact diagrams ("Artifacts render mermaid diagrams natively"), so reports built the recommended way are silently unshareable, and the error message sends users down a dead-end republish loop.