update_category_group · Ynab MCP: Risk & Policy

WHAT IT DOES

What update_category_group does on Ynab

AI agents use update_category_group 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 update_category_group needs a policy

This tool modifies budget metadata (category group structure) reversibly. It does not delete data (ruling out Destructive), does not move money (ruling out Financial), and does not execute arbitrary code (ruling out Execute). The sibling context shows both Write operations (create_*) and Destructive operations (delete_*), confirming this falls in the Write category.

From the tool's definition Tool name 'update_category_group' indicates modification of existing data. Description states 'Update a category group'. In the YNAB financial management context, category groups organize budget categories.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

update_category_group Rate-limited

update_category_group 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": {
    "update_category_group": {
      "limits": [
        {
          "counter": "update_category_group_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 update_category_group

What does the update_category_group tool do? +

[1 API call] Update a category group. 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 update_category_group? +

Register the Ynab MCP server in PolicyLayer and add a rule for update_category_group: 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 update_category_group? +

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

Can I rate-limit update_category_group? +

Yes. Add a rate_limit block to the update_category_group 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 update_category_group completely? +

Set action: deny in the PolicyLayer policy for update_category_group. 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 update_category_group? +

update_category_group 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.