Add ERC: Contract Feature Detection

ERCS/eip-draft_features.md

@@ -0,0 +1,76 @@
+---
+title: Contract Feature Detection
+description: Standard method to publish and detect contract features that lack an ERC-165 interface
+author: raffy.eth (@adraffy)
+discussions-to: https://ethereum-magicians.org/t/eip-discussion-contract-feature-detection/24975
+status: Draft
+type: Informational
+created: 2025-07-07
+requires: 165
+---
+
+## Abstract
+
+Creates a standard method to publish and detect what features a smart contract implements that lack an [ERC-165](https://eips.ethereum.org/EIPS/eip-165) interface.
+
+## Motivation
+
+Ethereum Name Service (ENS) has maintained backwards compatibility with contracts created in 2016 through extensive use of ERC-165.  Unfortunately, not all contract capabilities can be expressed through an unique interface.
+
+Features allow expression of contract capabilities that preserve existing interfaces.  This proposal standardizes the concept of features and standardizes the identification (naming) of features.
+
+## Specification
+
+### How Interfaces are Identified
+
+For this standard, a *feature* is any property of a contract that cannot be expressed via ERC-165.
+
+A feature name should be a reverse domain name that uniquely defines its implication, eg. `eth.ens.resolver.extended.multicall` is the multicall feature for an extended ENS resolver contract.
+
+A feature identifier is defined as the first four-bytes of the keccak256-hash of its name, eg. `bytes4(keccak256("eth.ens.resolver.extended.multicall")) = 0x96b62db8`.
+
+### How a Contract will Publish the Interfaces it Implements 
+
+A contract that is compliant with this specification shall implement the following interface:
+
+```solidity
+interface IFeatureSupporter {
+    /// @notice Check if a feature is supported.
+    /// @param featureId The feature identifier.
+    /// @return `true` if the feature is supported by the contract.
+    function supportsFeature(bytes4 featureId) external view returns (bool);
+}
+```
+
+The ERC-165 interface identifier for this interface is `0x582de3e7`.
+
+### How to Detect if a Contract Implements Features
+
+1. Check if the contract supports the interface above according to [ERC-165](https://eips.ethereum.org/EIPS/eip-165#how-to-detect-if-a-contract-implements-erc-165).
+
+### How to Detect if a Contract Implements any Given Feature
+
+1. If you are not sure if the contract implements features, use the above procedure to confirm.
+1. If it implements features, then call `supportsFeature(feature)` to determine if it implements the desired feature.
+
+Note: a contract that implements features may implement no features.
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119](https://www.rfc-editor.org/rfc/rfc2119) and [RFC 8174](https://www.rfc-editor.org/rfc/rfc8174).
+
+## Rationale
+
+Defining a new standard avoids unnecessary pollution of the ERC-165 selector namespace with synthetic interfaces representing features.
+
+## Backwards Compatibility
+
+Callers unaware of features or any specific feature experience no change in behavior.
+
+ENS already implements this EIP.
+
+## Security Considerations
+
+As with ERC-165, declaring support for a feature does not guarantee that the contract implements it.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](../LICENSE.md).