AI agents invoke mod to trigger actions in Calculator MCP. 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.
The tool executes a mathematical computation (modulo operation) on two numbers. While it has no side effects and no destructive potential, it is an active computation rather than a passive read. The blast radius is minimal — misuse could only result in incorrect arithmetic results, hence low severity.
From the tool's definition 'Mod two numbers' — performs a modulo arithmetic operation, which is a computation/execution of a mathematical function rather than a simple data retrieval or storage operation.
Documented attack patterns abuse exactly the kind of access mod gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Calculator MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for mod:
{
"version": "1",
"default": "deny",
"tools": {
"mod": {
"limits": [
{
"counter": "mod_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}mod 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.
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Mod two numbers. It is categorised as a Execute tool in the Calculator MCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Calculator MCP server in PolicyLayer and add a rule for mod: 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 Calculator MCP. Nothing to install.
mod is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the mod 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.
Set action: deny in the PolicyLayer policy for mod. 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.
mod is provided by the Calculator MCP server (wrtnlabs/calculator-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Calculator MCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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6 Calculator MCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.