MCP Server: validate_and_render_mermaid_diagram returns unsupported image media type (API Error 400)

[fabn]

Describe the bug

When using the Mermaid Chart MCP server (https://mcp.mermaidchart.com/mcp) with Claude Code, the validate_and_render_mermaid_diagram tool returns an image with an unsupported media type, causing the Anthropic API to reject the response with a 400 error.

Error message:

API Error: 400 messages.124.content.1.tool_result.content.2.image.source.base64.media_type: Input
should be 'image/jpeg', 'image/png', 'image/gif' or 'image/webp'

Expected behavior

The MCP server should return images in a format supported by the Anthropic API:

• image/png
• image/jpeg
• image/gif
• image/webp

Actual behavior

The server returns images with an unsupported media type. Based on the error, we hypothesize it might be image/svg+xml, but we don't have visibility into the actual media type being returned.

Configuration:

{
  "servers": {
    "mermaid-mcp": {
      "url": "https://mcp.mermaidchart.com/mcp",
      "type": "http"
    }
  }
}

Environment:

• Client: Claude Code
• MCP endpoint: https://mcp.mermaidchart.com/mcp
• Tool used: validate_and_render_mermaid_diagram

Please let us know if you need any additional details to investigate this issue.

[fabn]

Additional finding: Same issue occurs on Claude.ai web interface

I can confirm that the same problem also affects the Claude.ai web chat interface (not just Claude Code).

When using the MCP server from Claude.ai, the tool output explicitly shows:

"Image format 'image/svg+xml' is not currently supported. Supported formats are: GIF, JPG, PNG, WEBP."

Behavior comparison:

• Claude Code: The Anthropic API returns an explicit 400 error rejecting the unsupported media type
• Claude.ai: The image simply doesn't render in the chat (the frontend silently ignores the unsupported format)

The root cause is the same: the MCP server returns image/svg+xml instead of a supported raster format (PNG/JPEG/GIF/WEBP).

This confirms that the issue affects all Anthropic clients consuming the MCP server output, not just Claude Code.

[rajat-ht]

Hi @fabn, thanks for the detailed report.

Just a quick update, we’ve released a fix for this issue. The MCP server now returns images in a supported format instead of image/svg+xml.

Could you please try again and let us know if everything works as expected on your side?

[fabn]

Confirmed! The fix is working correctly.

I just tested validate_and_render_mermaid_diagram and the image was returned successfully without any API errors. The diagram rendered properly.

Thank you for the quick fix! 🎉