# Chain Idiosyncrasies
Differences in chain design inevitably lead to some unique chain-specific considerations. Below are idiosyncrasies we have identified, though it may not be comprehensive and we are interested in hearing if you encounter others.
# config.json - chains field
The idiosyncrasies below relate to the chains field of config.json. For the
corresponding Airnode configuration guide section
see here
and for the corresponding reference section
see here.
# Avalanche
Gas fees on Avalanche mainnet are often underestimated when using legacy as a
txType. The error manifests as the max fee per gas being set to less than the
block base fee, resulting in unfulfilled requests. A solution to this is to set
gasPriceMultiplier to slightly greater than 1 e.g. 1.1.
# Arbitrum
Execution costs on Arbitrum are calculated slightly differently than Ethereum,
which impacts the gas required to fulfill requests. To account for this, we
recommend a minimum value of 5000000 for fulfillmentGasLimit when using both
Arbitrum mainnet and testnet. For more on ArbGas
see here (opens new window).
# Metis
On the Metis testnet Stardust, though not on the Metis mainnet Andromeda, we
recommend a fulfillmentGasLimit of at least 5000000.
# Optimism
Use legacy as the txType rather than eip1559.
As a L2 scaling solution, Optimism has an L1 data fee and an L2 execution fee,
which are
accounted for separately (opens new window).
To cover the L1 data fee when a sponsor requests a
withdrawal, an amount has to be subtracted
from the funds returned to the sponsor. The withdrawalRemainder parameter has
been introduced specifically for this reason, though the value required will
differ between Optimism mainnet and testnet due to differences in L1 gas fees.
For Optimism testnet, a withdrawalRemainder of 1 gwei should suffice, while
for Optimism mainnet, a value as high as 2.4 finney (2400000 gwei) may be
required in order to cover an L1 gas price of 300 gwei and 8000 L1 gas used
by the transaction.