Fastmail MCP Server

add_labels

Add mailbox labels to an email while keeping existing assignments. Apply new labels without removing current ones.

Instructions

Add one or more mailbox labels to an email without removing its existing mailbox assignments.

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": {
      "emailId": "<emailId>",
      "mailboxIds": "<mailboxIds>"
    },
    "name": "add_labels"
  }
}

Input Schema

Name	Required	Description	Default
`emailId`	Yes	The email ID to label
`mailboxIds`	Yes	Mailbox IDs to add as labels

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.3.0Aug 2, 2026

TDQS

A4/5.0

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 discloses the key behavioral trait (not removing existing assignments) but does not mention other relevant behavior such as idempotency, failure modes, or permission requirements. This is adequate but not richly transparent.

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 sentence, front-loaded with the action, and contains no filler or redundant information. Every word adds value.

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

Completeness4/5

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

For a simple two-parameter tool with a clear schema and no output schema, the description adequately explains the core operation and its key constraint. It could explicitly mention that this applies to a single email (as opposed to bulk), but the schema and tool name make that evident.

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 already provides 100% coverage with clear descriptions for both parameters. The description adds no additional parameter-level detail beyond what the schema states, so it meets the baseline but does not enhance semantics.

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 uses a specific verb ('add') and resource ('mailbox labels to an email'), and clearly states a key differentiator: it does not remove existing mailbox assignments. This distinguishes it from sibling tools like remove_labels and move_email.

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

Usage Guidelines4/5

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

The description gives clear context by stating that existing mailbox assignments are preserved, implying when to use this tool (additive labeling). However, it does not explicitly mention alternatives like bulk_add_labels or when not to use it, so it falls short of explicit exclusions.

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