create_transaction · YNAB MCP Server MCP: Risk & Policy

WHAT IT DOES

What create_transaction does on YNAB MCP Server

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

WHY IT NEEDS A POLICY

Why create_transaction needs a policy

This tool creates financial transaction records in YNAB. While it records transactions rather than directly moving money, it modifies budget/financial data in a way that tracks real spending or income.

From the tool's definition 'Create a new transaction. Amount should be negative for expenses and positive for income.'

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

create_transaction Rate-limited

create_transaction 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_transaction": {
      "limits": [
        {
          "counter": "create_transaction_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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Instant setup, no code required.

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

FAQ

Questions about create_transaction

What does the create_transaction tool do? +

Create a new transaction. Amount should be negative for expenses and positive for income. It is categorised as a Write tool in the YNAB MCP Server 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_transaction? +

Register the YNAB MCP Server MCP server in PolicyLayer and add a rule for create_transaction: 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 MCP Server. Nothing to install.

What risk level is create_transaction? +

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

Can I rate-limit create_transaction? +

Yes. Add a rate_limit block to the create_transaction 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_transaction completely? +

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

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