register_extra_tool

THE RISK

Medium Risk

Why register_extra_tool needs a policy

This tool dynamically modifies the server's tool registry at runtime by adding a new tool, which is a reversible write/configuration change. It also triggers a notification event. It does not execute arbitrary code, delete data, or involve finances.

From the tool's definition Adds the greet tool at runtime and notifies tools/list_changed

Documented attack patterns abuse exactly the kind of access register_extra_tool gives an agent:

POLICY

How to control register_extra_tool

PolicyLayer is an MCP gateway — it sits between your AI agents and MCP from Scratch Server, and nothing reaches the server without passing your rules. This is the rule we recommend for register_extra_tool:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "register_extra_tool": {
      "limits": [
        {
          "counter": "register_extra_tool_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

register_extra_tool 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.

Create a free account and register MCP from Scratch Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

LIMIT THIS TOOL →

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EXPLORE

Related tools and policies

More MCP from Scratch Server tools

Execute route_support_ticket Classifies a support message via sampling/createMessage, then routes it using server-side Execute unstable Throws on purpose to demonstrate handler exceptions → isError. Write add_note Registers a new demo://note/N resource and notifies list_changed. Write publish_status Updates demo://status and notifies subscribers. Read divide Divides a by b. Returns a tool error when b is zero. Read echo Returns its input Read get_time Returns the server clock as an ISO-8601 timestamp. Read greet Returns a short greeting (registered dynamically).

All 12 MCP from Scratch Server tools →

Write tools on other servers

M-Team MCP Server download_torrent YouTube MCP Server generate_video_title Android Forensics ADB MCP Server combine_reports 0xarchive web3_signup

Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about register_extra_tool

What does the register_extra_tool tool do? +

Adds the greet tool at runtime and notifies tools/list_changed. It is categorised as a Write tool in the MCP from Scratch 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 register_extra_tool? +

Register the MCP from Scratch Server MCP server in PolicyLayer and add a rule for register_extra_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 from Scratch Server. Nothing to install.

What risk level is register_extra_tool? +

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

Can I rate-limit register_extra_tool? +

Yes. Add a rate_limit block to the register_extra_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 register_extra_tool completely? +

Set action: deny in the PolicyLayer policy for register_extra_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 register_extra_tool? +

register_extra_tool is provided by the MCP from Scratch Server MCP server (pguso/mcp-from-scratch). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every MCP from Scratch Server tool call.

Start from MCP from Scratch Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →