contract-verification

name	contract-verification
description	Use when verifying deployed contracts on block explorers. Covers forge verify-contract, Etherscan/Basescan/Arbiscan APIs, Blockscout verification, constructor args encoding, flattening, proxy verification, and multi-file verification.

Contract Verification

Why Verify

Verified contracts show source code on block explorers, enabling:

• Public auditability
• Etherscan read/write UI for interaction
• Trust from users and integrators
• Required for many DeFi integrations

forge verify-contract

# Basic verification
forge verify-contract 0xContractAddress src/MyContract.sol:MyContract \
  --chain-id 1 \
  --etherscan-api-key $ETHERSCAN_API_KEY

# With constructor arguments
forge verify-contract 0xContractAddress src/MyToken.sol:MyToken \
  --chain-id 1 \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  --constructor-args $(cast abi-encode "constructor(string,string,uint256)" "MyToken" "MTK" 1000000000000000000000000)

# With compiler settings matching deployment
forge verify-contract 0xContractAddress src/MyContract.sol:MyContract \
  --chain-id 1 \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  --compiler-version v0.8.28 \
  --num-of-optimizations 200

# Watch verification status
forge verify-contract 0xContractAddress src/MyContract.sol:MyContract \
  --chain-id 1 \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  --watch

Chain-Specific Verification

# Mainnet (Etherscan)
forge verify-contract $ADDR MyContract \
  --chain-id 1 --etherscan-api-key $ETHERSCAN_API_KEY

# Base (Basescan)
forge verify-contract $ADDR MyContract \
  --chain-id 8453 \
  --verifier etherscan \
  --verifier-url https://api.basescan.org/api \
  --etherscan-api-key $BASESCAN_API_KEY

# Optimism (Optimistic Etherscan)
forge verify-contract $ADDR MyContract \
  --chain-id 10 \
  --verifier etherscan \
  --verifier-url https://api-optimistic.etherscan.io/api \
  --etherscan-api-key $OPSCAN_API_KEY

# Arbitrum (Arbiscan)
forge verify-contract $ADDR MyContract \
  --chain-id 42161 \
  --verifier etherscan \
  --verifier-url https://api.arbiscan.io/api \
  --etherscan-api-key $ARBISCAN_API_KEY

Blockscout Verification

# Blockscout uses a different verifier
forge verify-contract $ADDR MyContract \
  --chain-id 8453 \
  --verifier blockscout \
  --verifier-url https://base.blockscout.com/api/

# Blockscout doesn't require an API key for most instances

Constructor Arguments

The most common verification failure is incorrect constructor args:

# Encode constructor args
cast abi-encode "constructor(address,uint256,string)" \
  0xAdminAddress 1000000 "MyToken"

# For complex types (arrays, structs)
cast abi-encode "constructor(address[])" \
  "[0xAddr1,0xAddr2,0xAddr3]"

# If constructor takes no args, omit --constructor-args entirely

Flattening

When verification tools fail with import resolution, flatten the contract:

# Flatten to single file
forge flatten src/MyContract.sol > MyContract.flat.sol

# Verify using flattened source (manual Etherscan upload)
# 1. Go to etherscan.io/verifyContract
# 2. Select "Solidity (Single file)"
# 3. Paste flattened source
# 4. Set compiler version and optimizer settings to match deployment

Proxy Verification

Verify Implementation

# Verify the implementation contract first
forge verify-contract $IMPL_ADDR src/MyContract.sol:MyContract \
  --chain-id 1 --etherscan-api-key $ETHERSCAN_API_KEY

Mark Proxy on Etherscan

After verifying the implementation, mark the proxy:

# Using Etherscan API to mark as proxy
curl "https://api.etherscan.io/api?module=contract&action=verifyproxycontract&apikey=$ETHERSCAN_API_KEY" \
  -d "address=$PROXY_ADDR"

# Check proxy verification status
curl "https://api.etherscan.io/api?module=contract&action=checkproxyverification&guid=$GUID&apikey=$ETHERSCAN_API_KEY"

Verify ERC-1967 Proxy

# Verify the proxy contract itself (usually OpenZeppelin's)
forge verify-contract $PROXY_ADDR \
  lib/openzeppelin-contracts/contracts/proxy/ERC1967/ERC1967Proxy.sol:ERC1967Proxy \
  --chain-id 1 \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  --constructor-args $(cast abi-encode "constructor(address,bytes)" $IMPL_ADDR $INIT_DATA)

Verification in Forge Scripts

# Verify during deployment (recommended)
forge script script/Deploy.s.sol \
  --rpc-url $ETH_RPC_URL \
  --broadcast \
  --verify \
  --etherscan-api-key $ETHERSCAN_API_KEY

# This auto-verifies all deployed contracts with correct constructor args

foundry.toml Etherscan Config

[etherscan]
mainnet = { key = "${ETHERSCAN_API_KEY}" }
sepolia = { key = "${ETHERSCAN_API_KEY}" }
base = { key = "${BASESCAN_API_KEY}", url = "https://api.basescan.org/api" }
optimism = { key = "${OPSCAN_API_KEY}", url = "https://api-optimistic.etherscan.io/api" }
arbitrum = { key = "${ARBISCAN_API_KEY}", url = "https://api.arbiscan.io/api" }

With this config, forge verify-contract auto-selects the right API based on --chain-id.

Troubleshooting

"Compiler version mismatch"

# Check what version was used to compile
forge build --force
# Look in out/MyContract.sol/MyContract.json → metadata → compiler

# Specify exact version
forge verify-contract $ADDR MyContract --compiler-version v0.8.28+commit.7893614a

"Constructor arguments mismatch"

# Get constructor args from deployment tx
cast tx $DEPLOY_TX_HASH input --rpc-url $ETH_RPC_URL
# The constructor args are appended after the bytecode
# Compare with: cast abi-encode "constructor(...)" args...

"Bytecode mismatch"

Ensure identical compiler settings:

• Same solc version
• Same optimizer settings (enabled, runs)
• Same EVM version
• Same via-ir setting
• Same remappings

# Rebuild with exact settings and compare
forge build --force
cast code $ADDR --rpc-url $ETH_RPC_URL > deployed.hex
# Compare with out/MyContract.sol/MyContract.json → deployedBytecode

Verification Checklist

• Implementation contract verified first (for proxies)
• Constructor arguments correctly encoded
• Compiler version matches deployment exactly
• Optimizer settings match (runs, via-ir, evm-version)
• All imports are resolvable (check remappings)
• Proxy marked on Etherscan after implementation verified
• Verification confirmed on block explorer (green checkmark)
• Read/Write contract tabs work on explorer