update_event · Fastmail Calendar MCP Server MCP

WHAT IT DOES

What update_event does on Fastmail Calendar MCP Server

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

WHY IT NEEDS A POLICY

Why update_event needs a policy

The update_event tool modifies existing calendar data but does not delete or irreversibly destroy information. Updates are reversible—events can be modified again or restored. While calendar manipulation carries medium severity due to potential scheduling disruptions or conflicts an AI agent could introduce, it does not rise to Destructive (irreversible deletion) or Execute (arbitrary code execution) levels.

From the tool's definition Tool description explicitly states 'Modify an existing event' and allows updates to calendar event fields. This is a reversible modification operation.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

update_event Rate-limited

update_event 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": {
    "update_event": {
      "limits": [
        {
          "counter": "update_event_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 Fastmail Calendar MCP Server, so you can run agents against it safely.

FAQ

Questions about update_event

What does the update_event tool do? +

Modify an existing event. PREREQUISITE: You must first call list_calendars, then list_events to get the eventUrl. Only include fields you want to change; omitted fields stay the same. It is categorised as a Write tool in the Fastmail Calendar MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on update_event? +

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

What risk level is update_event? +

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

Can I rate-limit update_event? +

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

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

update_event is provided by the Fastmail Calendar MCP Server MCP server (nova-3951/fastmail-calendar-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.