Consensus rules and validation

Wonder

1. What stops a miner with the most power from printing infinite coins?
2. Why can a transaction your node refuses to relay still end up in a valid block?
3. If consensus code changes, does everyone have to upgrade at the exact same second?

Explore

1. Consensus rules are the strict criteria every full node uses to validate blocks. They define what is technically possible: the issuance schedule, the block weight limit of 4,000,000 units, proof-of-work difficulty, and signature validity. A block that breaks any single rule is rejected by every honest node, regardless of how much hashing power produced it. Miners propose blocks; nodes decide whether they count. This separation ensures no entity can unilaterally alter the ledger’s state.
2. Policy rules govern what a node relays to its peers before mining. They include dust thresholds, minimum relay fees, and standard script types. A transaction that violates policy is dropped by most nodes but remains valid if a miner includes it in a block. This distinction matters: relying on policy rejection for security is risky because a determined actor can bypass peer filters and submit directly to miners, forcing the network to accept non-standard transactions as long as they satisfy consensus.
3. The code is the specification. The rules are whatever widely run node software actually enforces, edge cases included, which is why consensus code changes slowly and is reviewed more intensely than anything else in Bitcoin Core. Consensus bugs are handled as emergencies: the 2010 value-overflow bug and the 2018 inflation bug were both fixed under pressure, with coordinated releases.
4. Changes to these rules require coordinated adoption. Soft forks tighten the rules, so un-upgraded nodes still accept the new blocks; hard forks loosen or change them, so every node must upgrade or split off. A later lesson covers both in detail. SegWit (August 2017) was a soft fork; Bitcoin Cash (August 2017) resulted from a hard fork dispute over block size limits.
5. Producing a block is expensive; checking one is cheap. A miner who produces an invalid block loses the reward and the energy spent, because every validating node will discard it, which makes rule-breaking a costly gamble. This cost reinforces the strictness of validation: the network’s security relies on thousands of independent nodes checking every header and transaction, ensuring that no single point of failure can corrupt the shared state.
6. Open a block explorer like mempool.space or blockchain.com. Select any recent block and examine its ‘weight’ field. Compare this number to the 4,000,000 weight unit limit defined in BIP141. Observe how close modern blocks run to this ceiling, reflecting miners maximizing revenue per byte while staying within the consensus constraints that every node enforces.

Key facts

Exercise nine, six, three, one

Fact check

Dates, figures and claims checked against the published sources listed below.

• A. M. Antonopoulos & D. A. Harding, Mastering Bitcoin, 3rd ed. (2023), ch. 12
• E. Lombrozo, J. Lau & P. Wuille, BIP141: Segregated Witness (Consensus layer) (2015)
• Bitcoin Core source code: validation.cpp and policy.cpp
• Satoshi Nakamoto, Bitcoin: A Peer-to-Peer Electronic Cash System, section 5