Which coins does your wallet spend? Core's coin selection and the waste metric

[windpath]

A new chapter on coin selection, written against Bitcoin Core's source and Murch's 2016 thesis: Which Coins Does Your Wallet Spend?

What's in it:

• The three costs of a pick with the real sizes: a native-segwit input is 68 vbytes, a change output costs 31 vbytes now and 68 later, and Core prices the cost of change as the output at the effective feerate plus its future spend at the discard feerate (235 sats at 1 sat/vB).
• The thesis receipts: on 11,860 real outgoing payments the old Core picker found a changeless match 0.57% of the time, Branch and Bound 30.47%, with a 35 to 37% smaller pool.
• The four algorithms with their gates (BnB's window and 100,000 tries, Knapsack's random change target since 24.0.1, SRD's 50,000-sat change floor, CoinGrinder strictly above 3x -consolidatefeerate) and the waste metric with a worked example at 1 and 100 sat/vB.
• Change detection from the other side: Meiklejohn 2013 and Moser-Narayanan 2022, and what Core does about it (random position, matched change type, single-type input sets, randomized amounts).
• Consolidation: Core's own rule (below 10 sat/vB every extra input lowers the score), the 2023 to 2026 fee record, and the privacy price. Fee bands from ChainQuery's own node: 99.3% of the last year's blocks in its lowest band, which tops out at 5 sat/vB.
• The honest limits, including avoid_reuse only filtering sendtoaddress (confirmed on a 31.1 regtest node) and the CoinGrinder PR's own line about missing privacy metrics.

If any number, line reference or quote is off, say so here and it gets fixed and credited.

[OcTradism]

A new chapter on coin selection, written against Bitcoin Core's source and Murch's 2016 thesis: Which Coins Does Your Wallet Spend?

What's in it:

What's behind the Automatic coin selection from a wallet algorithm is too complicated and I believe that it's not important for most Bitcoin users to understand about it just like they don't need to understand every code line of Satoshi Nakamoto when the founder created Bitcoin.

With users, they only need to know importance of privacy, and how mastering Coin selection for their wallets, UTXOs and transactions will help them to improve and maintain better privacy.

It can help them to freeze UTXOs such as dust UTXOs.
Dust Attack, what it is, why it is dangerous and how to prevent falling to it.
Guideline for Bitcoin transaction privacy.

[Antidote47k]

Nice work! Bitcoin core's coin selection primary objective is an economical transaction and that may sometimes clash with privacy. Suppose you have three sets of UTXO in a wallet:
UTXO A: 40,000 sats
UTXO B: 35,000 sats
UTXO C: 80,000 sats
Your plan is to spend 60,000 sats, the wallet may choose UTXO C alone or it may decided to combine UTXO A and B depending on the transaction fees and cost of creating and later spending change, why does the selection matter? Combing UTXO A and B can reveal to a blockchain observer that the two inputs are likely controlled by the same wallet together and be able to link their transaction history,  although using one input alone doesn’t always mean more private in every situation, other patterns like adress history also matters. The point being that a lower waste selection isn’t necessarily the most private selection cause the waste metric evaluates economic cost rather than privacy itself.

[LFC_Bitcoin]

I’ve learnt how to operate coin control when using Bitcoin Core. Those of us based in Europe need to know how to use it.

When sending large amounts of Bitcoin to mainstream exchanges in Europe, they make you prove where the coins have come from, to prove that you control the address they have been sent from, it’s very frustrating. It’s called The Travel Rule.

Just like in the old days you send Bitcoin to for example Kraken. Your deposit is credited to your Kraken balance but you can’t trade until you’ve passed The Travel Rule.

I had Bitcoin sitting there during the last bull run in 2025, unable to sell for over 12 hours until some jeet on Kraken verified my deposit. It happened again with another semi large amount of Bitcoin in 2025 but that was about 4 hours.

Once you have verified your sending address you can use that address again without verification but that’s not good practise and secondly, Bitcoin Core auto sends your unspent coins to another address once you have spent from an address.

It’s really annoying, I'm not the most tech gifted guy on the forum so I had to research how to operate the built in, coin control feature.