prepareERC20Approval

THE RISK

Critical Risk

Why prepareERC20Approval needs a policy

ERC20 approvals grant a spender unlimited (or specified) access to a user's tokens. While this tool only 'prepares' the transaction data rather than executing it, the output is ready to be signed and broadcast, making it a direct precursor to a financial operation that could result in token loss if misused. Token approvals are a well-known attack vector in DeFi.

From the tool's definition Prepare an ERC20 token approval transaction for signing. Returns transaction data that can be signed and broadcast.

Documented attack patterns abuse exactly the kind of access prepareERC20Approval gives an agent:

POLICY

How to control prepareERC20Approval

PolicyLayer is an MCP gateway — it sits between your AI agents and MCP Ethers Wallet, and nothing reaches the server without passing your rules. This is the rule we recommend for prepareERC20Approval:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "prepareERC20Approval": {
      "deny_if": [
        {
          "conditions": [],
          "on_deny": "Requires human approval."
        }
      ]
    }
  }
}

Any call to prepareERC20Approval is blocked until a human approves it. The rest of the server keeps working.

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

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EXPLORE

Related tools and policies

More MCP Ethers Wallet tools

Financial prepareERC1155BatchTransfer Prepare an ERC1155 batch transfer transaction for signing. Returns transaction data that c Financial safeBatchTransferERC1155 Safely transfer multiple tokens from the connected wallet to another address in a single t Financial safeTransferERC1155 Safely transfer tokens from the connected wallet to another address Financial safeTransferERC721 Safely transfer an NFT from the connected wallet to another address, with additional data Financial transferERC20 Transfer ERC20 tokens from the connected wallet to another address Financial transferERC721 Transfer an NFT from the connected wallet to another address Financial transferFromERC20 Transfer ERC20 tokens from one address to another (requires approval) Financial transferNFT transferNFT

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Go deeper

Govern MCP payment tools → Approval gates, spend caps, and per-identity scopes on every refund, payout, and charge your agents make.
Critical-risk tools in MCP Ethers Wallet → Every critical-risk tool this server exposes, in one view.
All critical-risk MCP tools → Every critical-risk tool across the catalogue, and the policy pattern that contains them.
The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Human-in-the-loop → When a tool call should wait for a person — and when policy can decide alone.
Tool call approval → Approval gates on individual tool calls: auto-allow, prompt, or deny.
How to Add Spending Controls to Any MCP Agent →
Rate Limiting MCP Tool Calls: A Practical Guide →
PolicyLayer vs alternatives → How deterministic, per-call MCP policy compares with gateways, identity layers, and authorisation platforms.

FAQ

Questions about prepareERC20Approval

What does the prepareERC20Approval tool do? +

Prepare an ERC20 token approval transaction for signing. Returns transaction data that can be signed and broadcast. It is categorised as a Financial tool in the MCP Ethers Wallet MCP Server, which means it involves financial transactions. Block by default and require explicit approval.

How do I enforce a policy on prepareERC20Approval? +

Register the MCP Ethers Wallet MCP server in PolicyLayer and add a rule for prepareERC20Approval: 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 Ethers Wallet. Nothing to install.

What risk level is prepareERC20Approval? +

prepareERC20Approval is a Financial tool with critical risk. Critical-risk tools should be blocked by default and only enabled with explicit human approval.

Can I rate-limit prepareERC20Approval? +

Yes. Add a rate_limit block to the prepareERC20Approval 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 prepareERC20Approval completely? +

Set action: deny in the PolicyLayer policy for prepareERC20Approval. 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 prepareERC20Approval? +

prepareERC20Approval is provided by the MCP Ethers Wallet MCP server (crazyrabbitltc/mcp-ethers-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every MCP Ethers Wallet tool call.

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

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