Bitcoin RPC Error -26: min relay fee not met CPFP & Fee Bump
Bitcoin RPC error -26 "min relay fee not met" (or "mempool min fee not met") occurs when the fee rate (satoshis per virtual byte, sat/vB) of a submitted transaction is lower than the node's minimum relay fee threshold. While the static protocol default is 1 sat/vB, during severe mempool congestion nodes dynamically increase their rejection threshold to evict low-paying transactions. Increase fee rate above current dynamic mempool levels.
# Root Cause & Technical Architecture
Bitcoin Core enforces `minrelaytxfee` (default 1,000 satoshis/kB or 1 sat/vB). However, when the node mempool memory usage exceeds `maxmempool` (default 300MB), the node dynamically increments its internal `mempoolminfee` threshold to shed the lowest-fee transactions. Any incoming transaction with `feerate < mempoolminfee` is immediately rejected with RPC error code `-26: min relay fee not met`.
# 4-Step Actionable Diagnostic Protocol
Step-by-Step ResolutionCheck Active Dynamic Mempool Minimum Fee Rate
Execute bitcoin-cli getmempoolinfo. Inspect the mempoolminfee property to determine the current dynamic admission threshold in BTC/kB.
Recalculate Fee Rate in Satoshis per Virtual Byte (sat/vB)
Ensure total fee divided by transaction virtual size (vSize) exceeds the node minimum. Aim for at least 2 sat/vB above mempoolminfee.
Accelerate Unconfirmed Parent with CPFP
If an unconfirmed parent transaction has a low fee, spend its change output with a high-fee child transaction (Child-Pays-For-Parent).
Bump Fee via BIP-125 Opt-In RBF
If the transaction signaled RBF (sequence < 0xfffffffe), construct a replacement transaction with an increased fee rate and broadcast.
# Diagnostic Verification Code / Command
bash# Query dynamic mempool minimum fee on local node
INFO=$(bitcoin-cli getmempoolinfo)
echo "$INFO" | grep "mempoolminfee"
# Calculate sat/vB equivalent:
# Example: "mempoolminfee": 0.00003500 BTC/kB = 3.5 sat/vB
# Any transaction submitted below 3.5 sat/vB fails with Error -26!Launch Derivation Path Inspector
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# Frequently Asked Questions
FAQ SchemaWhy does Bitcoin Core reject 1 sat/vB transactions during congestion?
When the 300MB mempool cache fills up, Bitcoin Core increases its minimum admission threshold above 1 sat/vB to prioritize higher fee transactions.
What is the difference between RBF and CPFP?
RBF replaces the original transaction with a higher fee version. CPFP creates a new child transaction with a large fee to incentivize mining both.
Can miners still include a transaction that failed relay?
If submitted directly to a mining pool (via private transaction accelerators), miners can include transactions below standard relay thresholds.