Fastmail MCP Server

update_calendar_event

Update an existing calendar event's start time, title, location, or status. Only specified fields are changed.

Instructions

Update an existing calendar event. Only specified fields will be changed.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "eventId": "<eventId>"
    },
    "name": "update_calendar_event"
  }
}

Input Schema

Name	Required	Description
`start`	No	New start date/time (YYYY-MM-DDTHH:MM:SS format)
`title`	No	New event title
`status`	No	New event status
`eventId`	Yes	The calendar event ID to update
`duration`	No	New duration in ISO 8601 format (e.g. PT1H, PT30M)
`location`	No	New event location name
`timeZone`	No	New IANA timezone (e.g. America/New_York)
`description`	No	New event description
`freeBusyStatus`	No	New free/busy status

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.3.0Aug 2, 2026

TDQS

A3.9/5.0

Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries the full burden. It does disclose a key behavioral trait: 'Only specified fields will be changed,' revealing partial-update semantics. However, it does not mention error handling, idempotency, permissions, or what the response contains, leaving significant gaps for a mutation tool.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is two short sentences, front-loaded with the primary action and a concise clarification. Every word earns its place, with no repetition of schema details.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness3/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a tool with 9 parameters, no annotations, and no output schema, the description provides only the fundamental update behavior and partial-update note. It lacks guidance on return values, error conditions, or prerequisites, making it adequate but not comprehensive.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the parameters are already well-documented. The description adds value by clarifying that unspecified fields remain unchanged, which is not explicitly stated in the schema. This helps an agent understand that optional parameters only override when provided.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource: 'Update an existing calendar event.' This clearly distinguishes it from sibling tools like create_calendar_event and delete_calendar_event. The additional clause 'Only specified fields will be changed' further clarifies the partial-update semantics.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines3/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage context is implied by the verb 'Update' and the required eventId, but the description does not explicitly state when to use this tool versus alternatives, nor does it mention exclusions or prerequisites. The guidance is minimal and relies on the tool's name and schema.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.