List all available custom fields for an entity type
AI agents call list_custom_fields to retrieve information from CiviCRM MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool retrieves metadata about custom fields without modifying, deleting, or executing operations. It is a straightforward read operation that returns configuration/schema information, posing minimal risk if misused by an AI agent.
From the tool's definition Tool name is 'list_custom_fields' and description states it 'List[s] all available custom fields for an entity type' — a pure query operation with no side effects.
Attacks that exploit this kind of access
List all available custom fields for an entity type. It is categorised as a Read tool in the CiviCRM MCP Server MCP Server, which means it retrieves data without modifying state.
Register the CiviCRM MCP Server MCP server in PolicyLayer and add a rule for list_custom_fields: 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 CiviCRM MCP Server. Nothing to install.
list_custom_fields is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the list_custom_fields 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 list_custom_fields. 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.
list_custom_fields is provided by the CiviCRM MCP Server MCP server (johncallhub/civicrm-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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