update_category · Ynab MCP: Risk & Policy

WHAT IT DOES

What update_category does on Ynab

AI agents use update_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 update_category needs a policy

The tool creates or modifies category data in YNAB (You Need A Budget), which is a financial management system. While it affects financial tracking, the action itself is a reversible update to metadata (a category definition), not a direct movement of money or irreversible deletion.

From the tool's definition Tool name is 'update_category' and description states 'Update an existing category' – this modifies data reversibly.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

update_category Rate-limited

update_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": {
    "update_category": {
      "limits": [
        {
          "counter": "update_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 update_category

What does the update_category tool do? +

[1 API call] Update an existing category. 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? +

Register the Ynab MCP server in PolicyLayer and add a rule for update_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 update_category? +

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

Can I rate-limit update_category? +

Yes. Add a rate_limit block to the update_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 update_category completely? +

Set action: deny in the PolicyLayer policy for update_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 update_category? +

update_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.