approve_underlying · Justlend MCP: Risk & Policy

WHAT IT DOES

What approve_underlying does on Justlend

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

WHY IT NEEDS A POLICY

Why approve_underlying needs a policy

This tool creates or modifies token allowances—a reversible Write action that enables future transfers of user assets. While not immediately destructive, it carries high severity because approving excessive token spending could lead to unauthorized fund transfers if the jToken contract is compromised or if an AI agent grants unchecked approvals.

From the tool's definition Tool name 'approve_underlying' and description 'Approve the jToken contract to spend your underlying TRC20 tokens' indicate this grants spending permissions on user tokens via smart contract approval, a reversible but critical authorization action.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

approve_underlying Rate-limited

approve_underlying 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": {
    "approve_underlying": {
      "limits": [
        {
          "counter": "approve_underlying_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 Justlend, so you can run agents against it safely.

FAQ

Questions about approve_underlying

What does the approve_underlying tool do? +

Approve the jToken contract to spend your underlying TRC20 tokens. It is categorised as a Write tool in the Justlend MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on approve_underlying? +

Register the Justlend MCP server in PolicyLayer and add a rule for approve_underlying: 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 Justlend. Nothing to install.

What risk level is approve_underlying? +

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

Can I rate-limit approve_underlying? +

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

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

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