Write your first permission

We’ll build a minimal permission that authorizes ERC-20 transfer calls to a fixed recipient, and only to that recipient. It demonstrates every rule from the implementation checklist: length-check, selector routing, target allowlist, value check, decode, fail-closed.

The contract

// SPDX-License-Identifier: MIT
pragma solidity 0.8.26;

import {IPermission, Context} from "@sail/interfaces/IPermission.sol";

/// Authorizes ERC-20 transfer(to, amount) calls, but only to `allowedRecipient`,
/// only on `allowedToken`, and only up to `maxAmount`.
contract TransferToRecipientPermission is IPermission {
    bytes4 private constant TRANSFER_SELECTOR = 0xa9059cbb; // transfer(address,uint256)

    address public immutable allowedToken;
    address public immutable allowedRecipient;
    uint256 public immutable maxAmount;

    constructor(address token, address recipient, uint256 max) {
        allowedToken = token;
        allowedRecipient = recipient;
        maxAmount = max;
    }

    function evaluate(bytes calldata txData, Context calldata ctx)
        external view returns (bool)
    {
        // 1. Token calls carry no ETH.
        if (ctx.value != 0) return false;

        // 2. Only the allowlisted token.
        if (ctx.target != allowedToken) return false;

        // 3. Only the transfer selector.
        if (ctx.selector != TRANSFER_SELECTOR) return false;

        // 4. Length check before decoding: 4 + 32 + 32 = 68 bytes.
        if (txData.length < 68) return false;

        // 5. Decode and check the arguments.
        (address to, uint256 amount) = abi.decode(txData[4:], (address, uint256));
        if (to != allowedRecipient) return false;
        if (amount > maxAmount) return false;

        return true;
    }

    function discriminator() external pure returns (bytes32) {
        return keccak256("TransferToRecipientPermission");
    }
}

Why each line is there

• ctx.value != 0 — an ERC-20 transfer should never carry ETH; a non-zero value is a red flag.
• ctx.target — the call must go to the token you intend; otherwise a manager could call transfer on any contract.
• ctx.selector — anything that isn’t transfer is denied (deny unknown selectors, don’t revert).
• length check before abi.decode — decoding short calldata reverts; the kernel treats a revert as false, but an explicit check is clearer and cheaper.
• to / amount checks — the actual bounds. Everything else was a guard so these checks are meaningful.
• view + no state writes — required: the kernel calls this under staticcall.

Gas

This permission does a handful of comparisons and one abi.decode — comfortably inside the 150,000-gas PERMISSION_GAS_CAP. If you add oracle reads or storage lookups, keep the total well under the cap (a cold SLOAD is 2,100 gas; an external oracle call can be 5–20k+).

Test it off-chain first

Before registering, you can confirm the permission would approve a given call without spending gas. With the Sailor harness this is sailor mandate simulate; at the protocol level you can eth_call the permission’s evaluate directly with a constructed Context. Only register a permission once you’ve verified it accepts the calls you intend and rejects the ones you don’t.

Next

Now that you have a permission contract, you need an account to attach it to → Deploy an SMA. For the multi-account pattern (one deployment serving many accounts with per-account config), see shared templates.