Fastmail MCP Server

create_mailbox

Create a new mailbox folder in Fastmail. Provide a name and optionally a parent folder to create a nested subfolder.

Instructions

Create a new mailbox (folder) in the Fastmail account

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": {
      "name": "<name>"
    },
    "name": "create_mailbox"
  }
}

Input Schema

Name	Required	Description	Default
`name`	Yes	Name for the new mailbox/folder
`parentId`	No	Parent mailbox ID for creating a nested folder. Omit for top-level.

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.3.0Aug 2, 2026

TDQS

A3.5/5.0

Behavior2/5

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

With no annotations provided, the description must carry the full burden of behavioral disclosure. It indicates a mutating action but omits details about permission requirements, duplicate name handling, whether the operation is idempotent, or what the return value looks like.

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 a single concise sentence that is front-loaded with the action and resource. Every word contributes, with no repetition or filler.

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?

This is a simple two-parameter tool with full schema documentation, but no output schema or annotations. The description does not mention return values, error conditions, or behavioral details beyond creating a mailbox, leaving some gaps for a mutating operation.

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

Parameters3/5

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

The input schema covers 100% of parameters with clear descriptions (name and parentId). The tool description adds no additional meaning beyond what the schema already provides, so the baseline score of 3 is appropriate.

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 clearly states 'Create a new mailbox (folder) in the Fastmail account' with a specific verb and resource. It distinguishes itself from sibling tools like rename_mailbox, delete_mailbox, and list_mailboxes by focusing on the creation action.

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?

The usage is implied from the description: use this tool when you need to create a mailbox. However, it does not explicitly mention when not to use it or provide alternatives, and relies on the schema to explain parentId for nested folders.

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