blob transaction

A blob transaction is a new kind of Ethereum transaction (type 3, introduced by EIP-4844) that carries one or more large data blobs alongside the usual fields. A blob is a fixed 128 KiB chunk of data — technically 4096 field elements of 32 bytes each — meant to hold a rollup's compressed batch of transactions. The defining feature is that the blob data is available to the network but not accessible to the EVM: smart contracts can never read the bytes of a blob, only a short cryptographic commitment to it. This is the deliberate design choice that makes blobs so much cheaper than calldata.

Two things keep blobs cheap. First, they are priced in a separate blob-gas market with its own EIP-1559-style base fee that rises and falls independently of execution gas, so a busy DeFi day does not directly inflate rollup data costs and vice versa. Second, the data is ephemeral: consensus nodes guarantee blob availability for roughly 18 days (4096 epochs) and then prune it, because that window is long enough for anyone to download the data and for any dispute to resolve. Permanent history is left to whoever wants to keep it off-chain.

What the chain keeps forever is not the blob but a versioned hash of its KZG commitment. A contract can read that commitment via the BLOBHASH opcode, and can prove facts about the blob's contents using the point-evaluation precompile — which lets a rollup's verifier confirm that a specific value sits at a specific position in the blob without the blob ever entering the EVM. This is how an optimistic rollup's fraud proof or a ZK rollup's validity check can reference data that the execution layer itself cannot see, while paying blob prices rather than calldata prices.