list_calendars

THE RISK

Low Risk

Why list_calendars needs a policy

This tool performs a query operation that retrieves calendar data without creating, modifying, or deleting any information. The action is non-destructive and returns information only.

From the tool's definition Tool name 'list_calendars' and description 'List all calendars' indicate a retrieval operation with no modification or side effects.

Documented attack patterns abuse exactly the kind of access list_calendars gives an agent:

POLICY

How to control list_calendars

PolicyLayer is an MCP gateway — it sits between your AI agents and Fastmail MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for list_calendars:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "list_calendars": {}
  }
}

list_calendars is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.

Create a free account and register Fastmail MCP Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

CAP THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Data exfiltration → How read access becomes an exfiltration channel when chained with untrusted content.
MCP token cost → What connecting this server costs in context-window tokens on every request.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about list_calendars

What does the list_calendars tool do? +

List all calendars. It is categorised as a Read tool in the Fastmail MCP Server MCP Server, which means it retrieves data without modifying state.

How do I enforce a policy on list_calendars? +

Register the Fastmail MCP Server MCP server in PolicyLayer and add a rule for list_calendars: 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 Fastmail MCP Server. Nothing to install.

What risk level is list_calendars? +

list_calendars is a Read tool with low risk. Read-only tools are generally safe to allow by default.

Can I rate-limit list_calendars? +

Yes. Add a rate_limit block to the list_calendars 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 list_calendars completely? +

Set action: deny in the PolicyLayer policy for list_calendars. 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 list_calendars? +

list_calendars is provided by the Fastmail MCP Server MCP server (madllama25/fastmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Fastmail MCP Server tool call.

Start from Fastmail MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →