When the wallet funds a transaction, it needs to add inputs until all output amounts as well as the fee are satisfied. The feerate is either set explicitly by the user or estimated.

To aid debugging, the fee reason should be reported to the user. For most wallet RPCs this is a breaking API change, so I suggest to make it an option like verbose=1 (default=0).

Useful skills:

Bitcoin Core wallet, RPC interface, functional tests.

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