calculate_expression · FastMCP Multi-Tool Server MCP

WHAT IT DOES

What calculate_expression does on FastMCP Multi-Tool Server

AI agents invoke calculate_expression to trigger actions in FastMCP Multi-Tool Server. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why calculate_expression needs a policy

Evaluating expressions involves executing code or an expression parser at runtime. While described as 'safe', expression evaluators can have unexpected behaviors depending on implementation (e.g., if it uses eval() or similar). The blast radius is medium since a misconfigured or insufficiently sandboxed evaluator could be exploited for code execution, though the stated intent is math-only evaluation.

From the tool's definition "Safely evaluate a mathematical expression" — the tool executes/evaluates an expression at runtime

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

calculate_expression Rate-limited

calculate_expression stays usable, but rate-capped — a runaway agent can't fire it dozens of times 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_expression": {
      "limits": [
        {
          "counter": "calculate_expression_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about calculate_expression

What does the calculate_expression tool do? +

Safely evaluate a mathematical expression. It is categorised as a Execute tool in the FastMCP Multi-Tool Server MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on calculate_expression? +

Register the FastMCP Multi-Tool Server MCP server in PolicyLayer and add a rule for calculate_expression: 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 FastMCP Multi-Tool Server. Nothing to install.

What risk level is calculate_expression? +

calculate_expression is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit calculate_expression? +

Yes. Add a rate_limit block to the calculate_expression 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_expression completely? +

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

calculate_expression is provided by the FastMCP Multi-Tool Server MCP server (rt0120-ramco/mcp-py). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.