create_category · Ynab MCP: Risk & Policy

WHAT IT DOES

What create_category does on Ynab

AI agents use create_category to create or update resources in Ynab — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Ynab environment.

WHY IT NEEDS A POLICY

Why create_category needs a policy

This tool creates new budget categories, which is a reversible modification to budget metadata. It falls under Write rather than Financial (no money movement), Destructive (reversible), or Execute (no arbitrary code execution).

From the tool's definition Tool name 'create_category' and description 'Create a new category in a budget' indicate data creation. The tool adds a new category resource to a budget, which is a reversible write operation.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

create_category Rate-limited

create_category 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "create_category": {
      "limits": [
        {
          "counter": "create_category_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Ynab, so you can run agents against it safely.

FAQ

Questions about create_category

What does the create_category tool do? +

[1 API call] Create a new category in a budget. It is categorised as a Write tool in the Ynab MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on create_category? +

Register the Ynab MCP server in PolicyLayer and add a rule for create_category: 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 Ynab. Nothing to install.

What risk level is create_category? +

create_category is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit create_category? +

Yes. Add a rate_limit block to the create_category 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.

How do I block create_category completely? +

Set action: deny in the PolicyLayer policy for create_category. 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.

What MCP server provides create_category? +

create_category is provided by the Ynab MCP server (justmytwospence/ynab-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.