The four evaluation guarantees

Permissions are arbitrary, user-deployed Solidity. The kernel does not trust them — it contains them. Every permission evaluation, on every dispatch, is bounded by four structural guarantees. Together they mean the worst a buggy or malicious permission can do is deny calls, never widen authority or harm the kernel.

1. Static evaluation

Permissions are invoked via staticcall:

(bool success, bytes memory ret) =
    permission.staticcall{gas: PERMISSION_GAS_CAP}(
        abi.encodeCall(IPermission.evaluate, (data, ctx))
    );

staticcall prohibits state mutation. A permission cannot modify any contract's storage during evaluation, and reentrancy through the permission surface is structurally impossible — there is no write path to re-enter.

Protects against: reentrancy, permissions that try to mutate state mid-evaluation, hidden side effects.

2. Gas isolation

Each permission is called with a fixed gas cap, PERMISSION_GAS_CAP = 150_000. A permission that exceeds its cap reverts and is treated as returning false. (Batch permissions get a larger BATCH_EVAL_GAS_CAP = 1_000_000 because they must inspect every subcall.)

Protects against: a pathological permission denying service to the kernel or burning the manager's entire gas budget. The cap bounds the cost of evaluation no matter what the permission does.

3. Selective authorization

The manager's signature names one registered permission. The kernel evaluates that permission alone; no other registered permission is consulted. The named permission must be registered on the account — registration is the Permission Signer's trust anchor.

Protects against: unrelated permissions falsely denying each other, and lets one account hold many independent mandates. See the mandate.

4. Fail-closed

Any permission that returns false, reverts, runs out of gas, or returns malformed data causes the entire dispatch to revert. There is no partial-allow path, and an account with no registered permissions cannot dispatch at all (deny-by-default). The kernel decodes the return as a uint256 and requires it to equal 1, so a non-canonical boolean word is treated as denial, not acceptance.

Protects against: a buggy permission accidentally allowing a call. The default behavior of anything that goes wrong is to deny.

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These four properties protect the kernel from the permission. Everything a permission expresses inside that envelope — which tokens, which venues, what amounts — is the permission author's responsibility. That division is what makes the kernel auditable in isolation and the permission set permissionlessly extensible. See full expressiveness.