call_tool · Mcp Proxy MCP: Risk & Policy

WHAT IT DOES

What call_tool does on Mcp Proxy

AI agents invoke call_tool to trigger actions in Mcp Proxy. 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 call_tool needs a policy

This tool executes arbitrary operations on backend MCP servers. Since it can invoke any tool available on those servers without apparent restrictions, it has the blast radius of an Execute category tool — it can trigger code execution, shell commands, data modifications, or destructive operations depending on the backend tools available.

From the tool's definition Tool description states 'Execute a tool on a proxied MCP server.' The word 'Execute' directly indicates runtime code execution capabilities.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

call_tool Rate-limited

call_tool 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": {
    "call_tool": {
      "limits": [
        {
          "counter": "call_tool_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 Mcp Proxy, so you can run agents against it safely.

FAQ

Questions about call_tool

What does the call_tool tool do? +

Execute a tool on a proxied MCP server. It is categorised as a Execute tool in the Mcp Proxy MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on call_tool? +

Register the Mcp Proxy MCP server in PolicyLayer and add a rule for call_tool: 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 Mcp Proxy. Nothing to install.

What risk level is call_tool? +

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

Can I rate-limit call_tool? +

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

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

call_tool is provided by the Mcp Proxy MCP server (shengxiao20/mcp-proxy-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.