Legacy Contract Compatibility Shim Deployer

WHAT IT DOES

Legacy Contract Compatibility Shim Deployer helps you create and deploy shim contracts that bridge the gap between legacy contract interfaces and new implementations. You provide a legacy contract address, a new contract address, and a list of function signature mappings. The tool generates a Solidity shim contract that routes calls from the legacy interface to the new contract using delegatecall. The shim contract includes a fallback function that determines which contract to call based on the function selector, allowing existing applications that depend on the legacy ABI to continue working while the underlying implementation migrates to a new contract. This is essential for teams deprecating old contracts, upgrading to new versions with different interfaces, or maintaining backward compatibility during migration periods.

HOW TO USE

Connect your wallet to access the EVM network. Enter a shim name, the legacy contract address, and the new contract address. Define function mappings—one per line—in the format "legacySignature → newSignature". For example: "transfer(address,uint256) → transfer(address,uint256)" or "balanceOf(address) → balanceOf(address)". Click "Prepare Shim Deployment." The tool validates the mappings and generates a complete Solidity shim contract with a fallback function that routes based on function selectors. Review the generated shim source code and the mapping list. Click "Deploy Shim Contract" to deploy the shim to the selected network. The tool will display the deployed shim address and transaction hash upon success.

THE REAL MECHANISM

The tool uses a pure JavaScript template engine to generate Solidity source code. It parses the user-provided function mappings, extracting the legacy and new function signatures. For each mapping, it generates a condition in the fallback function that checks if the incoming call's function selector matches the legacy function using `msg.sig == this.functionName.selector`. If it matches, it delegatecalls to the new contract with the same calldata. If no mapping matches, it falls back to delegatecalling the legacy contract, ensuring that unmapped functions still work. The shim contract also exposes each legacy function as an external view function so the selector can be accessed on-chain. The generated source is compiled and deployed using ethers.js ContractFactory. All processing is done client-side with no external dependencies beyond ethers.js.

WHAT IT CANNOT SEE

This tool cannot automatically generate shim code for arbitrary contract interfaces without user-defined mappings. It cannot detect if the shim correctly emulates the legacy contract behavior or if the mappings are semantically correct. It cannot deploy shims for non-EVM contracts. It cannot verify that the shim matches the legacy ABI exactly or that all functions are correctly mapped. It cannot test the shim before deployment—you must test it yourself after deployment. It cannot automatically map all function signatures—you must provide each mapping manually. The tool does not validate that the new contract actually implements the mapped functions.

PLEASE NOTE

Legacy Contract Compatibility Shim Deployer works exclusively with EVM-compatible chains. It requires a connected wallet with sufficient gas funds to deploy the shim contract. The generated shim uses delegatecall, which means the new contract's storage layout must be compatible with the legacy contract's expectations. All processing occurs client-side in your browser; no data is sent to any external server. The tool is free to use and does not require authentication. Use with caution—deploying a shim contract is irreversible and should be thoroughly tested in a staging environment first.