trello-mcp

Add a URL attachment to a card

add_attachment

Attach a URL to a Trello card. Optionally set the attachment as the card cover if the image is public.

Instructions

Attach a URL to a card. Set setCover=true to make a publicly-fetchable image the card's cover (Trello must be able to preview the image).

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": {
      "id": "<id>",
      "url": "<url>"
    },
    "name": "add_attachment"
  }
}

Input Schema

Name	Required	Description
`id`	Yes	Card id
`url`	Yes	URL to attach
`name`	No	Display name for the attachment
`setCover`	No	Make this image attachment the card's cover

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.1.1Jul 14, 2026

TDQS

A3.8/5.0

Behavior3/5

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

Adds behavioral context: 'setCover=true' requires a publicly-fetchable image and Trello preview capability. However, no annotations exist, so the description carries full burden. It omits potential side effects, auth requirements, or rate limits, which is acceptable for a simple create operation but not exhaustive.

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 efficent sentences: first states core action, second adds critical nuance. No filler. Front-loaded with the primary purpose.

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 attachment tool with full schema coverage and no output schema, the description adequately covers the key behavioral constraint (setCover previewability). Missing return value info but that is not required given no output schema. Overall sufficiently complete for its complexity.

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

Parameters4/5

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

Schema coverage is 100% (all 4 params described). The description adds value beyond schema by clarifying the setCover condition ('publicly-fetchable image', Trello preview requirement). This extra context helps the agent use setCover correctly.

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?

Clearly states 'Attach a URL to a card' – specific verb and resource. Distinguishes from siblings like 'add_card_member' by focusing on URL attachment. Also mentions optional cover setting, clarifying scope.

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 on when to use this tool versus alternatives (e.g., other add_* tools). The description does not state prerequisites, when not to use, or mention sibling tools.

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