Add an attachment to a card. Use this tool to attach a URL to a card.
AI agents use add_attachment to create or update resources in Trello — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Trello environment.
This tool creates/adds a new attachment resource to an existing card, modifying the card's state reversibly. The attachment can be removed, making it a Write operation rather than Destructive. Severity is medium because a malicious agent could attach malicious files or phishing URLs to cards, potentially affecting team collaboration and security, but the action is reversible and doesn't delete data or move money.
From the tool's definition Tool description states 'Add an attachment to a card' and 'attach a URL to a card', which modifies card data by appending an attachment resource.
Documented attack patterns abuse exactly the kind of access add_attachment gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Trello, and nothing reaches the server without passing your rules. This is the rule we recommend for add_attachment:
{
"version": "1",
"default": "deny",
"tools": {
"add_attachment": {
"limits": [
{
"counter": "add_attachment_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}add_attachment stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
Free to start. No card required.
Add an attachment to a card. Use this tool to attach a URL to a card. It is categorised as a Write tool in the Trello MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Trello MCP server in PolicyLayer and add a rule for add_attachment: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Trello. Nothing to install.
add_attachment is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the add_attachment rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for add_attachment. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
add_attachment is provided by the Trello MCP server (v4lheru/trello-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Trello, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
76 Trello tools catalogued and risk-classified — across an index of 43,000+ MCP servers.