Etch a message onto Bitcoin

An OP_RETURN output lets you attach a small piece of arbitrary data to a Bitcoin transaction. Miners record it in the blockchain like any other transaction — which means once it confirms, it is public and permanent.

⚠ BEFORE YOU DO THIS

There is no undo. Anything you write is public forever, tied to your transaction, and costs a real fee. Never include anything private, illegal, or that identifies you unless you intend to.

Understand the tradeoff

OP_RETURN attaches data to a provably-unspendable output. Bitcoin Core 30.0 (2025) raised the default data-carrier limit from 83 bytes to 100,000, so larger payloads and multiple OP_RETURN outputs now relay and confirm on nodes running default settings. That is policy, not consensus, and some nodes keep the old 83-byte limit. It is cheap but not free — you pay a fee that scales with size — and it is immutable once mined.

Use a wallet that supports it

Bitcoin Core via bitcoin-cli (documented below), or Electrum, whose Send tab and console accept a script such as OP_RETURN <hex> in place of an address. Sparrow Wallet has no native OP_RETURN field yet (the feature request, issue #97, is still open with a pull request pending), but its transaction editor can load and sign a PSBT built elsewhere. Custodial and exchange wallets will not let you.

Write your message

Plain UTF-8 text. Default nodes now relay up to 100,000 bytes across all OP_RETURN outputs, but bigger data costs a higher fee and some nodes keep the old 83-byte limit — keep it short for reliability, or split it across outputs. Then encode it to hex.

Build the transaction

Add one OP_RETURN output carrying your data (0 sats) plus a change output back to yourself, and set a fee rate from mempool.space.

# Bitcoin Core — attach arbitrary data (80+ bytes now relay by default) DATA=$(printf 'gm, permanent record' | xxd -p -c 999) bitcoin-cli -named createrawtransaction \ inputs='[{"txid":"<your-utxo>","vout":0}]' \ outputs='[{"data":"'$DATA'"},{"<change-addr>":0.0009}]' # then: signrawtransactionwithwallet + sendrawtransaction

Broadcast and wait

Sign, broadcast, and watch it hit the mempool. Once a block confirms it, it lives on-chain forever. Send it to an address we monitor and it shows up in the feed here.

This tool only reads the chain — it never asks for your keys and cannot send anything for you.

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