Bitcoin Core fundrawtransaction RPC: Coin-Selection Guide

This guide explains Bitcoin Core fundrawtransaction RPC in plain English. It shows what the command does, what its result means and what it cannot prove.

Existing and added inputs

The method preserves supplied inputs and selects more from the wallet as needed. Every existing input’s previous transaction had to be known to the wallet, and selected scripts needed supported forms. Verify ownership, confirmation, coin-control restrictions and solvable status before allowing automatic selection, especially across customer or mining-payout pools.

Change controls

changeAddress, changePosition or change_type influence the added change output, while a random position is the default. Supplying an address disables change_type. Decode the funded result to identify actual change by script ownership rather than trusting position alone, and verify legacy, wrapped or native type matches downstream backup and monitoring support.

Fee rate and subtraction

feeRate in the historical options is BTC per kilobyte. SubtractFeeFromOutputs deducts the fee equally from listed pre-change output indexes, meaning those recipients receive less. Require explicit commercial approval for subtraction, validate indexes before funding and compare every original and final output in integer satoshis.

Watch-only and scripts

includeWatching can select watch-only inputs, but funding does not provide their signatures. P2SH scripts had to be present through importaddress or addmultisigaddress for fee calculation, and supported watch-only forms were limited. Prove the signer set can complete every selected input before reserving funds or presenting a transaction for approval.

UTXO locks and concurrency

lockUnspents can reserve selected outputs against competing wallet activity. Record and release locks through an idempotent failure path. Without locks, parallel funders can choose the same coins; with abandoned locks, available balance can appear lower indefinitely. Test crashes, timeouts and restart behaviour.

Replaceability and witness mode

replaceable controls BIP 125 signalling, while iswitness overrides heuristic decoding of the supplied transaction. Choose both explicitly when the serialisation and payment policy are known. Added inputs or outputs mean pre-existing signatures may no longer be valid, so always sign the final funded hex afresh.

Final transaction review

Decode the result, fetch every previous output and verify external recipients, exact amounts, change, fee, vsize, locktime, sequences and replaceability. Compare returned fee in BTC against independent satoshi arithmetic and enforce total and rate ceilings. Only then send the candidate to signers under a separate approval.

Conclusion

fundrawtransaction automates coin selection and change, but it can alter signatures and recipient amounts under explicit options. Safe use verifies solvability, locks, fee units and every output before the final candidate reaches a signer.