bitcoin/doc/release-notes-34075.md

Updated RPCs

• The estimatesmartfee RPC now combines two fee rate estimators: the existing block policy fee rate estimator and a new mempool fee rate estimator.

• The new mempool fee rate estimator produces conservative and economical fee rate estimates from the current contents of the mempool. It only produces a fee rate estimate when recent blocks indicate a healthy mempool, and falls back to the higher of the minimum relay fee rate and the current mempool minimum fee rate when the mempool is too sparse. Its statistics are persisted to fees/mempool_policy_estimator.dat and reloaded on startup.

  estimatesmartfee returns the lower of the two fee rate estimators' results, so the mempool fee rate estimator can only lower the block policy fee rate estimate.

• The combined estimate requires both estimators to succeed. If the mempool fee rate estimator cannot produce an estimate, for example, while the mempool is still loading, when too few recent blocks have been observed, or when the mempool is suspected to be unhealthy, an error is returned.

• estimatesmartfee accepts an options object with fee_rate_estimator. Recognized values are "none" (the default, combined behavior described above), "block_policy" (use only the block policy fee rate estimator), and "mempool_policy" (use only the mempool fee rate estimator). All unknown values are treated as "none". Users who want the previous behavior can select the block policy fee rate estimator explicitly.

• The options object also accepts verbosity. A verbosity of 2 or higher also returns mempool_health_statistics.

• When fee_rate_estimator is "none" and the estimate succeeds, the response also includes an estimator field identifying which fee rate estimator produced the result.

• Block policy fee estimator data is now stored in fees/block_policy_estimates.dat. If the new file does not exist, the legacy fee_estimates.dat file is moved to the new path during startup. If both files exist, the legacy file is removed.

• Wallet fee rate estimation uses the default combined estimate.

Wallet

• The fee_reason field returned by wallet transaction creation RPCs now reports the reason the wallet selected the fee rate (fee rate estimator, mempool minimum, fallback, or minimum required) instead of the block policy fee rate estimator's internal threshold details. Those details remain available in the block policy fee rate estimator debug log.