MetaMask's Added Protection checks whether a transaction executes under the conditions shown in its simulation. If execution differs, it reverts. The feature is supported by smart accounts on MetaMask Extension. MetaMask documentation
A normal preview estimates balance changes before signing. A malicious contract can recognize that test environment and behave differently when the transaction reaches the blockchain. Enforcing conditions onchain addresses that mismatch.
Why the fee increases
The extra verification requires computation, increasing gas consumption. A reverted transaction still incurs the network fee. MetaMask says it charges no separate fee for Added Protection.
The distinction matters: a failed operation can prevent the unwanted transfer while still costing gas. Token Primer's gas sponsorship explainer describes a separate question, how another party can fund transaction fees.
What to check before signing
Support depends on the account, network and contract interaction. The documentation covers token and NFT balance changes; it does not describe universal protection for every transfer. Matching a preview also does not establish that an investment or counterparty is trustworthy.
There is a privacy tradeoff. Estimated balance changes use a MetaMask server for simulation data. Disabling that feature also disables Added Protection. Check the current confirmation screen for both the protection setting and the network fee.