Fix guide
Fix "Replacement Transaction Underpriced" on Polygon
The replacement transaction underpriced error occurs when an EVM client attempts to replace an unmined transaction without paying the required minimum gas fee increase mandated by the Bor mempool.
Root Causes on Polygon PoS
Submitting a replacement transaction where maxFeePerGas or maxPriorityFeePerGas was not bumped by at least 10–12% above the prior pending transaction.
Bor execution client enforces a strict txpool replacement threshold (typically 10% bump) to prevent mempool spam.
Local nonce desynchronization: trying to broadcast a new trade with a nonce already reserved by an unmined transaction.
Base fee surges on Polygon PoS causing rapid gas price escalation while the replacement was in flight.
Diagnostic & Resolution Steps
Inspect the pending transaction in the mempool: `eth_getTransactionByHash(txHash)` to see its exact maxPriorityFeePerGas.
Apply the 12% fee bump formula: `newMaxFee = oldMaxFee * 112 / 100` and `newPriorityFee = oldPriorityFee * 112 / 100` with the exact same nonce.
Query `eth_getTransactionCount(senderAddress, 'pending')` to ensure you are not submitting an overlapping nonce.
Route transactions through a dedicated bare-metal node with a local Bor txpool to guarantee instant propagation to validator peers.
// EIP-1559 Replacement Gas Formula (Polygon Bor):
const newPriorityFee = (oldPriorityFee * 115n) / 100n; // Minimum +15% bump
const newMaxFee = (oldMaxFee * 115n) / 100n;
When Does RPC Infrastructure Matter?
Standard RPC is adequate when:
- ✓ submitting manual transfers via MetaMask
- ✓ transaction speed-up button handles fee bumping in wallet UI
- ✓ low volume transaction frequency (<1 tx/hr)
- ✓ slight inclusion delay does not risk financial loss
Dedicated bare-metal RPC is required when:
- ✓ running automated MEV, DEX arbitrage, or liquidation bots
- ✓ transaction queue must clear in the immediate next 2-second Bor block
- ✓ dynamic gas bumping must be automated with microsecond precision
- ✓ nonce collisions crash downstream trading or execution threads