Render a Mermaid diagram definition and return the image with metadata. The definition should be valid Mermaid syntax (e.g. flowchart, sequence, class, ER, state, or Gantt diagram). Returns a list of content blocks: the rendered image plus a JSON text block with metadata including a mermaid.live ...
Risk signalsAccepts file system path (filename)
Part of the Diagrams MCP server.
Free to start. No card required.
AI agents use render_mermaid to create or modify resources in Diagrams MCP. Write operations carry medium risk because an autonomous agent could trigger bulk unintended modifications. Rate limits prevent a single agent session from making hundreds of changes in rapid succession. Argument validation ensures the agent passes expected values.
Without a policy, an AI agent could call render_mermaid repeatedly, creating or modifying resources faster than any human could review. PolicyLayer's rate limiting ensures write operations happen at a controlled pace, and argument validation catches malformed or unexpected inputs before they reach Diagrams MCP.
Write tools can modify data. A rate limit prevents runaway bulk operations from AI agents.
{
"version": "1",
"default": "deny",
"tools": {
"render_mermaid": {
"limits": [
{
"counter": "render_mermaid_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}See the full Diagrams MCP policy for all 9 tools.
These attack patterns abuse exactly the kind of access render_mermaid gives an agent. Each links to the full case and the policy that stops it:
Other write tools across the catalogue. The same approach applies to each: rate-limit and validate the arguments.
Render a Mermaid diagram definition and return the image with metadata. The definition should be valid Mermaid syntax (e.g. flowchart, sequence, class, ER, state, or Gantt diagram). Returns a list of content blocks: the rendered image plus a JSON text block with metadata including a mermaid.live edit link for opening the diagram in a browser editor. Args: definition: Mermaid diagram definition text. filename: Output filename without extension. format: Output format — "png" (default), "svg", or "pdf". download_link: If True, return a temporary download URL path (/images/{token}) that expires after 15 minutes; if False, return inline image bytes. Defaults to True (URL) — set DIAGRAMS_INLINE_DEFAULT=true on the server to flip the default. SVG/PDF and PNGs larger than the inline limit always use a download link.. It is categorised as a Write tool in the Diagrams MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Diagrams MCP server in PolicyLayer and add a rule for render_mermaid: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Diagrams MCP. Nothing to install.
render_mermaid is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the render_mermaid rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for render_mermaid. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
render_mermaid is provided by the Diagrams MCP server (pypi:diagrams-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 9 Diagrams MCP tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
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