(Translated and adapted from the Ethereum Foundation's original article.)
The Ethereum transition to Proof of Stake (PoS), known as The Merge, is approaching. The developer community has already established a Devnet, finalized specifications, and is eager to share updates. The Merge is designed to minimize disruptions for end-users, smart contracts, and decentralized applications (dApps). However, some minor changes deserve attention.
Before diving deeper, familiarize yourself with these foundational resources:
This article assumes readers already understand the basics. For deeper insights, refer to the full specifications:
Block Structure Post-Merge
After The Merge, Proof-of-Work (PoW) blocks will no longer exist on the network. Instead, historical PoW data will become components of Beacon Chain blocks, Ethereum’s new PoS consensus layer, replacing PoW. Beacon Chain blocks will include ExecutionPayloads, equivalent to post-Merge blocks on the current PoW chain.
For end-users and developers, ExecutionPayloads represent the interaction layer with Ethereum. Transactions here remain processed by execution-layer clients (e.g., Besu, Erigon, Geth, Nethermind). The key advantage? Execution-layer stability means minimal breaking changes.
Deprecated Mining & Ommer Block Fields
Post-Merge, many PoW-related block header fields will become obsolete. To reduce disruption to tools and infrastructure, these fields will be set to 0 or their data structure equivalents, rather than removed entirely. Full changes are outlined in EIP-3676.
Since PoS doesn’t naturally produce Ommers (uncle blocks), the ommers list in each block will be empty, and its hash will encode an RLP (Recursive Length Prefix) hash of an empty list. Similarly:
difficulty→ 0nonce→ 0 (byte-formatted)mixHash→ Replaced by the Beacon Chain’s RANDAO value (details below).
Changes to BLOCKHASH & DIFFICULTY Opcodes
Post-Merge:
- BLOCKHASH retains functionality but loses much of its pseudorandomness due to the absence of PoW hashing.
- DIFFICULTY opcode (0x44) is renamed RANDOM and returns a random beacon output from the Beacon Chain.
The RANDOM value is stored in the ExecutionPayload’s mixHash field (renamed to random). This change, proposed in EIP-4399, allows smart contracts to detect The Merge:
"If the DIFFICULTY opcode returns a value > 2^64, the transaction executes in a PoS block."
Block Time Adjustments
Average block time decreases slightly:
| Consensus | Avg. Block Time |
|-----------|----------------|
| PoW | ~13 sec |
| PoS | 12 sec (fixed) |
Implications:
- Smart contracts relying on specific block intervals must account for the ~1-second reduction.
- Missed slots (due to offline validators) occur <1% of the time.
Safe Head Blocks & Finalized Blocks
Post-Merge, concepts like reorgs (reorganizations) and block confirmations evolve:
Finalized Blocks
- Accepted by >2/3 of validators.
- Reversing requires destroying ≥1/3 of total staked ETH (~$10B+ at press time).
Safe Head Blocks
- Expected to remain canonical under normal conditions.
- JSON-RPC defaults to returning the safe head (use
unsafeflag for the latest block).
👉 Learn how PoS finality enhances security
Next Steps
The Merge marks a monumental shift for Ethereum. Developers should:
- Test on upcoming testnets.
- Join community calls for infrastructure/tooling updates.
Ethereum’s transition to PoS promises scalability, security, and sustainability—stay tuned for further updates!
FAQs
Q: Will existing dApps break post-Merge?
A: No—The Merge prioritizes backward compatibility. Minor adjustments may be needed for opcode-dependent contracts.
Q: How does PoS impact gas fees?
A: The Merge itself doesn’t reduce fees. Future upgrades (e.g., sharding) target scalability.
Q: Can validators influence RANDOM outputs?
A: No—RANDAO values are cryptographically secure and resistant to manipulation.