Anvil implicitly defaults to chain id 1 when not specified in the chain config in the genesis.json

[mmv08]

Component

Anvil

Have you ensured that all of these are up to date?

• Foundry
• Foundryup

What version of Foundry are you on?

forge Version: 1.0.0-nightly Commit SHA: 7e903eb Build Timestamp: 2025-03-12T06:02:03.811377000Z (1741759323) Build Profile: maxperf

What version of Foundryup are you on?

foundryup installed - forge Version: 1.0.0-nightly Commit SHA: 7e903eb Build Timestamp: 2025-03-12T06:02:03.811377000Z (1741759323) Build Profile: maxperf
foundryup: installed - cast Version: 1.0.0-nightly Commit SHA: 7e903eb Build Timestamp: 2025-03-12T06:02:03.811377000Z (1741759323) Build Profile: maxperf
foundryup: installed - anvil Version: 1.0.0-nightly Commit SHA: 7e903eb Build Timestamp: 2025-03-12T06:02:03.811377000Z (1741759323) Build Profile: maxperf
foundryup: installed - chisel Version: 1.0.0-nightly Commit SHA: 7e903eb Build Timestamp: 2025-03-12T06:02:03.811377000Z (1741759323) Build Profile: maxperf

What command(s) is the bug in?

No response

Operating System

macOS (Apple Silicon)

Describe the bug

Base command:

echo '{  "nonce": "0xc0ffee",  "timestamp": "0x0",  "extraData": "0x0000000000000000000000000000000000000000000000000000000000000000",  "gasLimit": "0x80000000",  "difficulty": "0x20000",  "mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000",  "coinbase": "0x0000000000000000000000000000000000000000",  "number": "0x0",  "gasUsed": "0x0",  "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000"}' > test-genesis.json && anvil --init ./test-genesis.json

This command works as expected and the chain id is displayed correctly in the initial message;

Chain ID

1

If we pass a chain_id option, things get more interesting:

echo '{  "nonce": "0xc0ffee",  "timestamp": "0x0",  "extraData": "0x0000000000000000000000000000000000000000000000000000000000000000",  "gasLimit": "0x80000000",  "difficulty": "0x20000",  "mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000",  "coinbase": "0x0000000000000000000000000000000000000000",  "number": "0x0",  "gasUsed": "0x0",  "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000"}' > test-genesis.json && anvil --init  ./test-genesis.json --chain-id 100200300

Chain id from the initial message:

Chain ID

100200300

However, if you do an RPC call to the node, it will respond with chain id 1:

curl --location 'http://localhost:8545/' \              
--header 'Content-Type: application/json' \
--data '{
    "jsonrpc": "2.0",
    "id": 1337,
    "method": "eth_chainId",
    "params": []
}'

{"jsonrpc":"2.0","id":1337,"result":"0x1"}

From what I tested, the node always uses the chain id from the genesis.json file (and defaults to 1 if not specified) and ignores the --chain_id option. This is different from other options, such as --timestamp which takes precedence over what's specified in the genesis file