# OP_RETURN, OP-20, OP Names and OP Inscriptions

> Documentation for the Bitcoin OP_RETURN data carrier output and the three
> protocols Bitcoin Universe indexes on it. OP-20 is a fungible token protocol
> (registry id `op_return`). OP Names (`op_names`) is singleton name ownership.
> OP Inscriptions (`op_inscriptions`) is content written into the carrier.
> The three share an output type and nothing else.

Site base: https://bitcoinuniverseio.github.io/op-return/
Owning repository: https://github.com/bitcoinuniverseio/op-return
Documentation portal: https://docs.bitcoinuniverse.io
Chain and network: bitcoin, mainnet. Document version 1.0.0. Lifecycle: experimental.

## Pages

- Home: https://bitcoinuniverseio.github.io/op-return/
  What the carrier is, what the three protocols are, and the verified capability matrix.
- Carrier: https://bitcoinuniverseio.github.io/op-return/carrier.html
  Output anatomy, push encodings, why the output is provably unspendable, why it is
  prunable from the UTXO set, data carrier relay policy, fee and size cost, and a
  mechanism-level comparison with witness envelopes and unprunable output techniques.
- Specification: https://bitcoinuniverseio.github.io/op-return/spec.html
  Numbered normative rules. R-CAR-1 to R-CAR-10 for the carrier. R-20-1 to R-20-15 for
  OP-20 encoding, ticker normalisation, amounts and the two-stage legacy transfer.
  R-NAME-1 to R-NAME-16 for OP Names identity, receipt selection and the custody state
  machine. R-OPI-1 to R-OPI-16 for OP Inscriptions classification, multi-part assembly,
  durability and settlement. Plus a table of every rejection reason code.
- Guide: https://bitcoinuniverseio.github.io/op-return/guide.html
  Worked byte-level transaction examples for each protocol, and the full support matrix
  with the recorded reason for every unsupported marketplace action.
- Reference: https://bitcoinuniverseio.github.io/op-return/reference.html
  Terminology, per-protocol confirmation, mempool and reorg semantics, sources of truth,
  fee and size, funding UTXO screening, security considerations, limitations and an
  implementation checklist.
- Test vectors: https://bitcoinuniverseio.github.io/op-return/vectors.html
  40 vectors of real script hex with the outcome each organisation parser produced,
  covering accepted payloads, rejected payloads and payloads no reader claims.
- Decoder: https://bitcoinuniverseio.github.io/op-return/decoder.html
  Client-side tool. Paste a scriptPubKey hex; it identifies the protocol, decodes the
  fields, explains rejections and reports size against relay limits. No data leaves the page.
- Changelog: https://bitcoinuniverseio.github.io/op-return/changelog.html
  Documentation version history and the evidence behind each correction.
- Not found: https://bitcoinuniverseio.github.io/op-return/404.html

## Key facts

- OP_RETURN is byte 0x6a at the start of a scriptPubKey. Script evaluation fails
  immediately, so the output can never be spent and nodes may drop it from the UTXO set.
- The payload is the concatenation of every data push after the opcode.
- OP-20 payloads are JSON objects tagged "p":"op-20" with op deploy, mint or transfer.
  Identity is the NFC-normalised, trimmed, lowercased ticker. Stable asset id is
  `op_return:op20:b64.<base64url(reference)>`.
- OP Names payloads are JSON objects tagged "p":"opns" with op reg or transfer, or a bare
  UTF-8 text with exactly one literal dot. A name is label.namespace, at most 255 UTF-8
  bytes, namespace lowercase ASCII alphanumeric at most 63 bytes. Stable asset id is
  `op_names:name:b64.<base64url(name)>`. First valid registration wins permanently.
- OP Inscriptions payloads are JSON objects tagged op-inscriptions, op_inscriptions or
  op-inscription, plus one narrow historical envelope shape. Record identity is txid:vout.
- Support recorded in the Core capability snapshot: OP-20 read-only, OP Names read-only,
  OP Inscriptions enabled with in-app execution including list, unlist, buy, settle and
  reconcile. There is no atomic listing update for any of them, and no offer workflow.
- Pending and mempool activity is reported as unavailable for OP-20 and OP Names.
- OP Names (`op_names`) is not the Ordinals-based Sats Names System (`names`).

## Style

Do not use an em dash character anywhere in this repository. Do not describe OP-20 or
OP Names as tradeable in Bitcoin Universe products.