create_custom_field · Campaign Monitor MCP: Risk & Policy

WHAT IT DOES

What create_custom_field does on Campaign Monitor MCP

AI agents use create_custom_field to create or update resources in Campaign Monitor MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Campaign Monitor MCP environment.

WHY IT NEEDS A POLICY

Why create_custom_field needs a policy

This tool creates (writes) a new custom field in a subscriber list. Creation is a reversible operation—the field can be deleted or modified later—so it falls under Write rather than Destructive.

From the tool's definition Tool name 'create_custom_field' and description 'Create a new custom field for a subscriber list' indicate data creation. The verb 'create' and the action of adding a new field to a list are reversible modifications to the data model.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

create_custom_field Rate-limited

create_custom_field 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "create_custom_field": {
      "limits": [
        {
          "counter": "create_custom_field_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Campaign Monitor MCP, so you can run agents against it safely.

FAQ

Questions about create_custom_field

What does the create_custom_field tool do? +

Create a new custom field for a subscriber list. It is categorised as a Write tool in the Campaign Monitor MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on create_custom_field? +

Register the Campaign Monitor MCP server in PolicyLayer and add a rule for create_custom_field: 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 Campaign Monitor MCP. Nothing to install.

What risk level is create_custom_field? +

create_custom_field is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit create_custom_field? +

Yes. Add a rate_limit block to the create_custom_field 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.

How do I block create_custom_field completely? +

Set action: deny in the PolicyLayer policy for create_custom_field. 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.

What MCP server provides create_custom_field? +

create_custom_field is provided by the Campaign Monitor MCP server (pauliowest/cmon-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.