merge_category · Ynab MCP: Risk & Policy

WHAT IT DOES

What merge_category does on Ynab

AI agents use merge_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 merge_category needs a policy

This tool creates or modifies data reversibly (Write category) rather than being Destructive, as the source category merge is a reorganization operation, not an irreversible deletion. It is not Financial because it does not move money between accounts or commit financial obligations—it only reorganizes categorical metadata and historical budget allocations.

From the tool's definition The tool 'merge_category' modifies existing data by re-categorizing transactions and moving budgeted amounts.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

merge_category Rate-limited

merge_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": {
    "merge_category": {
      "limits": [
        {
          "counter": "merge_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 merge_category

What does the merge_category tool do? +

[Variable API calls] [Workflow] Merges a source category into a target category: re-categorizes all transactions and moves all historical budgeted amounts. 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 merge_category? +

Register the Ynab MCP server in PolicyLayer and add a rule for merge_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 merge_category? +

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

Can I rate-limit merge_category? +

Yes. Add a rate_limit block to the merge_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 merge_category completely? +

Set action: deny in the PolicyLayer policy for merge_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 merge_category? +

merge_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.