MCP Server: Playground URL encoding breaks with <br/> tags in participant names

[comanche]

Description:

When using the Mermaid Chart MCP server (mcp.mermaidchart.com/mcp) via the validate_and_render_mermaid_diagram tool,
the generated playground URL (mermaidchart.com/play#pako:...) doesn't properly encode/decode diagrams containing

HTML tags in participant labels.

Steps to Reproduce:

1. Call validate_and_render_mermaid_diagram with a sequence diagram containing
   in participant names
2. Example:
   sequenceDiagram
   participant User as User
   (Client)
   participant API as API Server
   /api/endpoint
   User->>API: Send request
   API-->>User: Return response
3. Copy the generated playground URL from the tool response
4. Paste URL into browser

Expected Behavior:
The playground URL should open and display the diagram correctly with line breaks in participant names.

Actual Behavior:
The URL fails to properly decode the
tags, resulting in a broken/corrupted diagram display or parsing error.

Environment:

• MCP Server: mcp.mermaidchart.com/mcp
• Tool: validate_and_render_mermaid_diagram
• Client: Claude Code

Additional Context:
The diagram itself renders correctly when the MCP returns the image - only the shareable playground URL is affected.
This appears to be an issue with the pako compression/encoding step when generating the URL hash.

[rajat-ht]

Hi @comanche Thanks for reporting this and for the detailed steps, that really helps.

We’re looking into the encoding issue affecting
tags in participant names when generating the playground URL. We’ll update the issue once we have more information.

Thanks for your patience!

[mmphego]

Any update wrt this bug?