BIP 34

BIP-34 requires every Bitcoin block's coinbase transaction to encode the block height as the first push in its input script. Activated as a soft fork in March 2013, it's a structural cleanup that solved several niggling issues at once.

What the rule actually does:

coinbase input script must begin with: <block_height_serialized>

For block 800,000, the coinbase input starts with the byte sequence encoding the integer 800,000. Simple, mechanical, easy to verify.

What it buys:

• Uniqueness. Every coinbase transaction now has a unique input that differs from every other block's coinbase, so two blocks can't accidentally produce the same coinbase txid. This is the structural fix that makes the BIP-30 "no duplicate txids" rule trivially satisfiable forever.
• Self-referential blocks. A block now explicitly states its own height. This made certain validation logic cleaner and provided a sanity-check anchor for nodes resyncing or recovering from corruption.
• Soft-fork activation precedent. BIP-34 was one of the first non-trivial soft forks to use a clean miner-signaling activation method (predating the more formal BIP-9). It established patterns that later activations refined.

You'll see BIP-34 referenced in node logs, in protocol documentation, and occasionally in mining-pool discussions, but most users never directly interact with it. It's part of the "things that just work because they were carefully designed in 2013" infrastructure of Bitcoin.