Bitcoin Consensus

Bitcoin consensus = the set of rules every full node enforces to decide if a block / transaction is valid for inclusion in the chain. Distinct from mempool policy (which is a node-local default for what tx to accept and relay).

Block validation (high-level)

1. Decoded block header is well-formed (80 bytes).
2. prev_block exists and points to current best chain (or triggers reorg).
3. Proof-of-work: SHA256d(header) ≤ target where target is derived from the difficulty re-target window (every 2016 blocks).
4. Block timestamp > median of last 11 blocks (MTP) and ≤ network-adjusted time + 2 hours.
5. Coinbase is the first tx, has 1 input with null prevout, scriptSig length in [2,100], and includes BIP34 height in scriptSig (active since block 227,931).
6. Witness commitment in coinbase's OP_RETURN matches the merkle root of wtxids (BIP141).
7. Merkle root of txids matches header.
8. All transactions are individually consensus-valid (see below).
9. Total fees + subsidy ≥ coinbase value.

Transaction validation

1. Non-empty vin and vout.
2. No negative or > MAX_MONEY (21M × 1e8 sats) output amount.
3. No duplicate inputs (within the tx).
4. Coinbase input present iff this is the coinbase tx.
5. For each non-coinbase input: prevout exists in UTXO set and is unspent.
6. Script execution succeeds for scriptSig + scriptPubKey and (post-segwit) the witness stack.
7. Sum(inputs) ≥ Sum(outputs).
8. nLockTime / nSequence rules (BIP65, BIP112) — see proposals/.

Soft-fork activation

Thresholds are per deployment, not a BIP9 constant: BIP9 specifies 1916/2016 (95%) on mainnet and SegWit used it; Taproot's Speedy Trial used 1815/2016 (90%), which is still Core's mainnet default.

BIP110 is the most recent mainnet attempt. Its mandatory-signaling window opened at block 961,632 (2026-08-08) and mainnet did not signal bit 4, so enforcing nodes split onto a branch that mined two blocks in ~8 hours; the BIPs repo moved BIP110 to Closed on 2026-08-10. No BIP110 rule is in force as of September 2026.

nLockTime / nSequence semantics (key for layer-2)

• nLockTime: tx valid only after specified block-height or unix-time.
• nSequence per-input:
  • 0xFFFFFFFF → final, disables nLockTime for that input.
  • 0xFFFFFFFE → does not signal opt-in RBF (BIP125). Signalling is moot for relay: full-RBF is the Core default since v28.0 (Oct 2024).
  • < 0xFFFFFFFE → signals opt-in RBF.
  • BIP68 relative locktime when input's tx version ≥ 2:
    • bit 31 = 0 → relative time enforced
    • bit 22 = 0 → block-height units, =1 → 512s units
    • low 16 bits = the lock value

Consensus vs Policy

A tx that is non-standard but consensus-valid can be mined (via direct submission to a miner like mempool.space accelerator) and is valid in a block.

Recent additions

• Cluster mempool (Bitcoin Core 31.0, April 2026) — the mempool no longer enforces ancestor/descendant size or count limits. Connected components ("clusters") are capped instead: 64 txs and 101 kvB (-limitclustercount / -limitclustersize). RBF now requires the resulting mempool feerate diagram to be strictly better; the CPFP carveout is gone (use TRUC + sibling eviction). Still current in 31.1 (July 2026).
• Opportunistic 1p1c relay (Bitcoin Core 28.0, October 2024) — a parent below the mempool min feerate is paired with one child and evaluated as a package over the existing tx-relay protocol; submitpackage (26.0, December 2023) does the same locally. BIP331 (“Ancestor Package Relay”) is still Status: Draft as of September 2026 and its sendpackages / pkgtxns P2P messages have never shipped — see package-relay/SKILL.md.
• TRUC v3 transactions (BIP431) — version=3, ≤1 unconfirmed ancestor, ≤ 10kvB, sibling-eviction, child can pay 0 fee. Pairs with ephemeral anchors (output value = 0, must be spent in same package).

See also

• transactions/SKILL.md — tx structure, sighash
• scripts/SKILL.md — Script language
• proposals/SKILL.md — covenants, unactivated forks

Reference

• Bitcoin Core source: src/consensus/, src/validation.cpp
• BIPs: 9, 8, 16, 30, 34, 65, 68, 112, 113, 141, 143, 144, 147, 341, 342, 431