proton-mcp

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Apply a label

add_label

Idempotent

Applies an existing label to a Proton Mail message while keeping it in its current folder and retaining its other labels.

Instructions

Applies an existing label to a message. In Proton a message lives in exactly one folder but can carry any number of labels, so this adds to the message rather than moving it: it stays where it is and keeps the labels it already has. The label has to exist already; this server does not create labels.

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": {
      "messageId": "<messageId>",
      "label": "<label>"
    },
    "name": "add_label"
  }
}

Input Schema

Name	Required	Description
`label`	Yes	The label name, for example "Work". The "Labels/" prefix may be included and is optional. Use list_folders to see which labels exist. Labels are created in Proton itself, not here.
`mailbox`	No	The mailbox the message is in, if known. Only a shortcut for the lookup.
`messageId`	Yes	The message id from a listing or a search, with or without angle brackets.

Schema Changelog

Changes observed during successful MCP inspections.

Addedv0.4.11Nov 1, 2026

TDQS

A4.2/5.0

Behavior4/5

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

Annotations already declare idempotentHint=true, destructiveHint=false, and readOnlyHint=false, so the safety profile is covered. The description adds meaningful context beyond those annotations: the label must pre-exist, the server does not create labels, and the operation is additive rather than a folder move. It stops short of describing error behavior when a label is missing, but the added behavioral context is substantial.

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 three tight sentences with zero waste. The core action is front-loaded, the crucial non-moving semantics follow immediately, and the pre-existence constraint closes it out. Every sentence contributes to correct tool selection and invocation.

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 small, three-parameter mutation tool with full schema coverage and safety annotations, the description covers the essential behavior: additive labeling, no folder move, and the pre-existence requirement. It does not describe the return value or failure mode for a nonexistent label, but with no output schema and strong annotations, the definition is largely complete.

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?

With 100% schema description coverage, the schema already documents all three parameters, including the optional 'Labels/' prefix and the list_folders pointer. The description reinforces the 'label must exist' constraint but adds no new syntax or format details beyond what the schema provides. Baseline 3 is appropriate when the schema carries parameter 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 and resource: 'Applies an existing label to a message.' It immediately distinguishes this from moving a message by explaining that the message stays in its folder and retains existing labels. This lets an agent differentiate it from siblings like move_messages and remove_label without additional inference.

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 clearly establishes the context for use: the label must already exist and is additive rather than a move. It does not explicitly name remove_label or move_messages as alternatives, but the 'adds to the message rather than moving it' framing implicitly guides the agent away from those tools when the goal is to apply a label without relocation.

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