Trello MCP Server

Add Label to Card

add_label_to_card

Attach an existing label to a Trello card using its card ID and label ID. Organize cards by categorizing them with labels.

Instructions

Add an existing label to a card.

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": {
      "cardId": "<cardId>",
      "labelId": "<labelId>"
    },
    "name": "add_label_to_card"
  }
}

Input Schema

Name	Required	Description	Default
`cardId`	Yes	The ID of the card
`labelId`	Yes	The ID of the label to add

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.2.0Sep 6, 2026

TDQS

C2.9/5.0

Behavior2/5

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It clarifies that the label must already exist and that the operation is additive, but it does not state whether the operation is idempotent, what happens if the label is already on the card, whether board membership is required, or what response is returned.

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

Conciseness4/5

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

The description is a single, direct sentence with no filler, and the core action is front-loaded. However, it is so terse that it omits useful usage and behavioral context, so it is not a perfect conciseness score.

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 simple two-parameter operation, the description plus full schema coverage provide enough to identify the call. However, with no annotations and no output schema, the agent is left without information about side effects, idempotency, or expected response behavior, making it minimally sufficient rather than fully 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?

Schema description coverage is 100%, so the schema already documents cardId and labelId. The description adds slight nuance by specifying that the label must be an existing label, but it largely repeats what the schema already conveys.

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

Purpose4/5

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

The description uses a specific verb ('Add') with a clear object ('existing label') and target resource ('card'), and the qualifier 'existing' implicitly distinguishes it from create_label. However, it does not explicitly differentiate from sibling tools like remove_label_from_card or create_label.

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

Usage Guidelines2/5

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

No guidance is given about when to use this tool versus alternatives. It does not mention that the label must already exist, that the label likely belongs to the same board, or that removal is handled by remove_label_from_card. The agent must infer usage from the tool name and sibling list.

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

Deploy Server