[Arc Testnet quirk] eth_gasPrice returns stale baseline — back-to-back txs underpriced without ≥30% premium

[osr21]

Symptom

When submitting two or more transactions in quick succession (approve → burn, or approve → deposit), the second transaction is rejected with:

replacement transaction underpriced

This happens even when each tx uses the same gas price returned by eth_gasPrice, because Arc Testnet's RPC returns a stale/cached baseline that the node's mempool considers already "used."

Root cause

Arc's eth_gasPrice endpoint returns a fixed or slowly-updating value. The mempool enforces a minimum increment between successive transactions from the same sender, but the RPC doesn't reflect this. Standard clients that trust eth_gasPrice will always underprice the second tx.

Workaround

Apply a 30% upward premium to the gas price returned by the RPC and reuse the same bumped price for all transactions in a batch:

const feeData = await provider.getFeeData();
const gasPrice = (feeData.gasPrice * 130n) / 100n; // +30%

const approveTx = await usdc.approve(spender, amount, { gasPrice });
await approveTx.wait();
const burnTx = await messenger.depositForBurn(...args, { gasPrice }); // same bumped price

References

• Tracked in arc-node#87
• Fix in this repo: getBumpedGasPrice() helper in src/lib/bridge.ts