Create a custom template from diagram code for reuse in current session.
AI agents use createCustomTemplate to create or update resources in Mcp Mermiad — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Mcp Mermiad environment.
This tool creates and stores a custom template within the current session. It is a reversible write operation with no destructive effect (templates can be cleared when the session ends or overwritten). It does not execute arbitrary code or trigger external side effects beyond diagram generation. The blast radius is minimal — a malicious template could only affect diagram rendering within that session.
From the tool's definition Tool description states 'Create a custom template from diagram code for reuse' — this is a create operation that modifies state (adds a template to the current session).
Documented attack patterns abuse exactly the kind of access createCustomTemplate gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Mcp Mermiad, and nothing reaches the server without passing your rules. This is the rule we recommend for createCustomTemplate:
{
"version": "1",
"default": "deny",
"tools": {
"createCustomTemplate": {
"limits": [
{
"counter": "createcustomtemplate_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}createCustomTemplate stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
Free to start. No card required.
Create a custom template from diagram code for reuse in current session. It is categorised as a Write tool in the Mcp Mermiad MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Mcp Mermiad MCP server in PolicyLayer and add a rule for createCustomTemplate: 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 Mcp Mermiad. Nothing to install.
createCustomTemplate 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 createCustomTemplate 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 createCustomTemplate. 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.
createCustomTemplate is provided by the Mcp Mermiad MCP server (narasimhaponnada/mermaid-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Mcp Mermiad, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
9 Mcp Mermiad tools catalogued and risk-classified — across an index of 43,000+ MCP servers.