Fastmail MCP Server

get_email_headers

Fetch specific email headers such as List-Unsubscribe, Reply-To, Return-Path, and Authentication-Results without retrieving the full message body. Useful for inspecting routing and authentication details.

Instructions

Get selected raw email headers for a message without returning the full body. Defaults to unsubscribe, reply routing, return path, and authentication headers.

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>"
    },
    "name": "get_email_headers"
  }
}

Input Schema

Name	Required	Description	Default
`emailId`	Yes	The email ID whose headers should be retrieved
`headers`	No	Header names to retrieve. Defaults to List-Unsubscribe, List-Unsubscribe-Post, Reply-To, Return-Path, and Authentication-Results.

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, the description carries the transparency burden. It explains that the tool retrieves raw headers and that it can use defaults, which is helpful. However, it does not disclose potential limitations such as whether missing headers are omitted, whether header names are case-sensitive, or what the response format looks like. This is adequate but has gaps.

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?

Two sentences, front-loaded with the action and resource, then a valuable default detail. No wasted words, and the structure makes it easy to scan.

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?

The tool is simple (2 params, no output schema, no annotations). The description covers purpose, key behavior (no body), and defaults. It lacks a note about the response shape or error conditions, but for a lightweight getter this is sufficient. Slightly below a 5 due to these omissions.

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?

Schema description coverage is 100%, so the schema already explains both parameters. The description adds contextual meaning by stating the default header list and reinforcing that headers are selected/raw, which supplements the schema but does not significantly extend it. Baseline 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 the verb 'Get', the resource 'selected raw email headers', and the scope 'for a message'. It also distinguishes itself from siblings by explicitly stating 'without returning the full body', which differentiates it from get_email and similar tools.

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 phrase 'without returning the full body' indicates when to use this tool over alternatives like get_email. It also lists default headers, giving concrete context for typical use. However, it does not explicitly name alternative tools or mention scenarios where one might want to avoid this tool, so it falls short of a 5.

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