debug_traceTransaction returns null for transactions preceded by a gasless (zero-fee) tx in the same block

[AntonieDavid]

Summary

debug_traceTransaction returns {"result": null} (HTTP 200) for a large fraction of X Layer mainnet transactions, even though the transactions exist, succeeded on-chain, and are traceable via other methods. Through some investigation this appears to be caused by the gasless (zero-fee) transaction feature interacting badly with the transaction-replay path used by the RPC tracer.

Environment

xlayer-reth-node Version: 0.1.0-dev (dbc1495)   # tag v0.0.6.3
Upstream Reth Version: 5101851b

Observed behavior

For a transaction that is not the first non-deposit transaction in its block, debug_traceTransaction returns null:

# Example on X Layer mainnet: tx at block 64,564,751, index 5
curl -s -X POST "$RPC" -H 'Content-Type: application/json' -d '{
  "jsonrpc":"2.0","id":1,"method":"debug_traceTransaction",
  "params":["0xfeba105bc2c9a813712fe78f6619be7bea4601ddc8df99943f843737ffe8cd06",
            {"tracer":"callTracer"}]
}'
# -> {"jsonrpc":"2.0","id":1,"result":null}

The transaction clearly exists and succeeded:

• eth_getTransactionByHash / eth_getTransactionReceipt return it (status: 0x1, transactionIndex: 0x5).
• debug_traceBlockByNumber on the same block returns a full, correct trace for all transactions in the block, including this one.
• The parity-style trace_transaction on the same hash does not return null but surfaces an error instead:

{"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"max fee per gas less than block base fee"}}

So the state and history are present; only the single-transaction trace path fails.

Reproduction pattern (root cause)

Tracing each transaction in block 64,564,751 individually with debug_traceTransaction shows a clear boundary:

index	tx type	effective gas price	debug_traceTransaction
0	126 (deposit)	0	OK
1	2 (EIP-1559)	maxFeePerGas = 0	OK (traced directly)
2..8	mixed	—	null

Index 1 is a gasless transaction: a type 0x2 tx with maxFeePerGas = 0 / maxPriorityFeePerGas = 0, included in a block whose baseFeePerGas = 20000000.

• Tracing index 1 directly works, because the tracer disables the base-fee check for the target transaction.
• Tracing any transaction at index ≥ 2 fails, because debug_traceTransaction first replays the preceding transactions in the block to reconstruct the pre-state — and replaying the gasless index-1 tx goes through the normal execution path with base-fee validation enabled. The zero-fee tx is rejected with max fee per gas less than block base fee, the replay aborts, and debug_traceTransaction swallows the error and returns null.

In other words: any transaction that is preceded in its block by a gasless (zero-fee) transaction cannot be traced with debug_traceTransaction / trace_transaction / trace_get / trace_replayTransaction. Since gasless transactions are common, this affects a large share of trace requests. Block-level tracers (debug_traceBlockByNumber / debug_traceBlockByHash) are unaffected because they execute the block through the block executor, which already applies the gasless base-fee relaxation.

Suggested cause / fix

The gasless base-fee relaxation is applied in the block executor (OpBlockExecutor::execute_transaction_without_commit) and is also re-implemented for the flashblocks builder. The builder code even documents this explicitly — see crates/builder/src/flashblocks/context.rs, transact_maybe_gasless / is_gasless:

"Mirrors the gasless detection in the upstream block executor … The flashblocks builder executes pool transactions directly via Evm::transact rather than through the block executor, so the detection and base-fee relaxation have to be replicated here, otherwise zero-priced (whitelisted) transactions would be rejected by base-fee validation even when gasless is enabled."

The RPC tracing/replay path (the eth/debug/trace namespaces, delegated to the upstream OpEthApi replay_transactions_until) is a third place that executes transactions via Evm::transact outside the block executor, and it does not replicate the gasless detection. As a result, preceding gasless transactions are rejected during replay.

Two possible directions:

1. Apply the same gasless detection (set_gasless(true) when the tx is a whitelisted zero-fee tx) when replaying preceding transactions in the RPC tracing path, mirroring transact_maybe_gasless.
2. Disable base-fee validation for all transactions replayed during tracing (the tracer already disables it for the target transaction; extending this to the replayed predecessors would also resolve it and is arguably correct for tracing in general).

Impact

debug_traceTransaction, trace_transaction, trace_get, and trace_replayTransaction return null / error for any transaction positioned after a gasless transaction in the same block, despite the data being fully available on the node.