calculate_mirr · Excel Finance MCP: Risk & Policy

WHAT IT DOES

What calculate_mirr does on Excel Finance MCP

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

WHY IT NEEDS A POLICY

Why calculate_mirr needs a policy

An AI agent can call calculate_mirr faster than any human can review — one bad instruction and it creates or modifies resources in Excel Finance MCP by the hundred, each call as confident as the last.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

calculate_mirr Rate-limited

calculate_mirr 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": {
    "calculate_mirr": {
      "limits": [
        {
          "counter": "calculate_mirr_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 Excel Finance MCP, so you can run agents against it safely.

FAQ

Questions about calculate_mirr

What does the calculate_mirr tool do? +

Calculate Modified Internal Rate of Return. It is categorised as a Write tool in the Excel Finance MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on calculate_mirr? +

Register the Excel Finance MCP server in PolicyLayer and add a rule for calculate_mirr: 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 Excel Finance MCP. Nothing to install.

What risk level is calculate_mirr? +

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

Can I rate-limit calculate_mirr? +

Yes. Add a rate_limit block to the calculate_mirr 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 calculate_mirr completely? +

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

calculate_mirr is provided by the Excel Finance MCP server (jeremycharlesgillespie/excel-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.