> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cosmos.network/llms.txt
> Use this file to discover all available pages before exploring further.

# Metaprotocol

> The x/metaprotocol module adds support for encoding and decoding additional fields attached to transactions.

The `x/metaprotocol` module adds support for encoding and decoding additional fields attached to transactions.

`extension_options` and `non_critical_extension_options` are optional fields that can be used to attach data to valid transactions. The fields are validated by the blockchain, but they are not used in any way. The fields pass validation if they are provided as empty lists (`[ ]`) or they use a list of `ExtensionData` types.

The application does not use the attached data but it does ensure that the correct type is provided and that it can be successfully unmarshalled. The attached data will be part of a block.

<Tip>
  Txs where `extension_options` or `non_critical_extension_options` are populated with a type other than `/gaia.metaprotocols.ExtensionData` are considered invalid and will be rejected.
</Tip>

Here is an example of a correctly formed `non_critical_extension_options` field:

```json theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
{
  "@type": "/gaia.metaprotocols.ExtensionData", // must be this exact string
  "protocol_id": "some-protocol",
  "protocol_version": "1",
  "data": "<base64 encoded bytes>"
}
```

Here is an example of a correctly populated `non_critical_extension_options` on a `bank.MsgSend` transaction:

```json expandable theme={"theme":{"light":"github-light-high-contrast","dark":"github-dark-high-contrast"}}
{
  "body": {
    "messages": [
      {
        "@type": "/cosmos.bank.v1beta1.MsgSend",
        "from_address": "cosmos1ehpqg9sj09037uhe56sqktk30asn47asthyr22",
        "to_address": "cosmos1ehpqg9sj09037uhe56sqktk30asn47asthyr22",
        "amount": [
          {
            "denom": "uatom",
            "amount": "100"
          }
        ]
      }
    ],
    "memo": "memo_smaller_than_512_bytes",
    "timeout_height": "0",
    "extension_options": [],
    "non_critical_extension_options": [
      {
        "@type": "/gaia.metaprotocols.ExtensionData",
        "protocol_id": "some-protocol",
        "protocol_version": "1",
        "data": "<base64 encoded bytes>"
      }
    ]
  },
  "auth_info": {
    "signer_infos": [],
    "fee": {
      "amount": [],
      "gas_limit": "200000",
      "payer": "",
      "granter": ""
    },
    "tip": null
  },
  "signatures": []
}
```