AI agents use update_month_category to commit financial operations through Ynab — usually the final step of a payment, billing, or trading workflow. A call moves real money.
This tool directly modifies financial budget allocations — moving/assigning money to categories within a budget. Misuse could result in incorrect fund allocation, misrepresentation of financial plans, or unintended redistribution of funds across budget categories, qualifying it as a Financial action.
From the tool's definition Update the budgeted/assigned amount for a category in a specific month. This is how you allocate money to categories.
Attacks that exploit this kind of access
[1 API call] Update the budgeted/assigned amount for a category in a specific month. This is how you allocate money to categories. It is categorised as a Financial tool in the Ynab MCP Server, which means it involves financial transactions. Block by default and require explicit approval.
Register the Ynab MCP server in PolicyLayer and add a rule for update_month_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.
update_month_category is a Financial tool with critical risk. Critical-risk tools should be blocked by default and only enabled with explicit human approval.
Yes. Add a rate_limit block to the update_month_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.
Set action: deny in the PolicyLayer policy for update_month_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.
update_month_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.
Every MCP server has a record like this.
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