update_transaction · YNAB MCP Server MCP: Risk & Policy

WHAT IT DOES

What update_transaction does on YNAB MCP Server

AI agents use update_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 update_transaction needs a policy

This tool modifies existing financial transaction records in YNAB. While it is reversible (not Destructive like delete_transaction), and involves financial data (which could suggest Financial category), the core action is data modification rather than movement of money or commitment of financial obligations.

From the tool's definition Tool name 'update_transaction' and description 'Update an existing transaction' indicate modification of financial data.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

update_transaction Rate-limited

update_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": {
    "update_transaction": {
      "limits": [
        {
          "counter": "update_transaction_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 MCP Server, so you can run agents against it safely.

FAQ

Questions about update_transaction

What does the update_transaction tool do? +

Update an existing transaction. Only provide the fields you want to change. 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 update_transaction? +

Register the YNAB MCP Server MCP server in PolicyLayer and add a rule for update_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 update_transaction? +

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

Can I rate-limit update_transaction? +

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

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

update_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.