forge verify-contract failing on Basescan even though constructor args look correct

[sigittt1998]

Deployed a contract to Base via forge script --broadcast and trying to verify on Basescan:

forge verify-contract 0x1234...abcd src/SimpleVault.sol:SimpleVault \
  --chain base \
  --constructor-args $(cast abi-encode "constructor(address,string,string,address)" 0xUSDC... "Vault" "vTKN" 0xFEE...)

Keeps failing with "Unable to verify" but no specific error. The contract definitely deployed successfully and works fine.

Things I've tried:

• Double-checked compiler version matches foundry.toml
• Verified optimizer settings match
• Used --watch flag to wait longer
• Tried with and without --etherscan-api-key

Is Basescan just flaky or am I encoding the constructor args wrong? The same flow worked fine on Ethereum mainnet last month.

[mehvetero]

This one almost always comes down to constructor args encoding or compiler metadata, not Basescan being flaky.

A few things to check in order of likelihood:

1. Constructor args encoding — cast abi-encode with the word constructor in the signature is wrong. Drop it:

cast abi-encode "f(address,string,string,address)" 0xUSDC... "Vault" "vTKN" 0xFEE...

The function name doesn't matter (Foundry ignores it), but constructor(...) can trip up some versions. If you want to be safe, just pass the raw hex directly with --constructor-args-path.

2. Metadata hash mismatch — even if the optimizer settings match, Solidity appends a CBOR-encoded metadata hash at the end of the bytecode. If anything differs between your local compilation and what you deployed (different file path, different solc binary, different OS), the hash won't match. Try adding --via-ir if you compiled with it, or check cbor_metadata = false in foundry.toml (this strips the hash entirely — makes verification easier at the cost of losing source mapping).

3. Basescan API key — you need one. Without it, Basescan rate-limits you silently and returns generic errors. Get one from basescan.org (free tier is fine).

4. Try forge verify-contract with --show-standard-json-input — this dumps exactly what Foundry sends to the verifier. You can paste that JSON into Basescan's manual verification page and it'll give you a much more specific error message than the CLI does.

The fact that it worked on Ethereum mainnet but not Base suggests the compiler metadata hash — you might have recompiled between deploys.