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Creating simple casts is covered in the messages tutorial. This tutorial covers advanced topics like mentions, embeds, emojis, replies and channels.

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Setup

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Mentions

Users can be tagged in a cast with an @username mention (e.g. “Hello @bob!”) which causes clients to send notifications. A mention is not included in the text of the cast. The target fid and the position of the mention in the text are specified in the mentions and mentionPositions array, and are populated into the cast by clients at render-time. 
/**
 * "@dwr and @v are big fans of @farcaster"
 */
const castWithMentions = await makeCastAdd(
  {
    text: ' and  are big fans of ',
    embeds: [],
    embedsDeprecated: [],
    mentions: [3, 2, 1],
    mentionsPositions: [0, 5, 22], // The position in bytes (not characters)
  },
  dataOptions,
  ed25519Signer
);

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Embeds

URLs can be embedded in the cast which instructs clients to render a preview. A cast can have up to 2 embeds which can each be up to 256 bytes long. No other validation is performed on embeds. 
/**
 * A cast with a mention and an attachment
 *
 * "Hey @dwr, check this out!"
 */
const castWithMentionsAndAttachment = await makeCastAdd(
  {
    text: 'Hey, check this out!',
    embeds: [{ url: 'https://farcaster.xyz' }],
    embedsDeprecated: [],
    mentions: [3],
    mentionsPositions: [4],
  },
  dataOptions,
  ed25519Signer
);

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Emoji

Emojis can be included directly in the text of a cast and be rendered by clients. Since an emoji character often takes up multiple bytes but renders as a single character, it has an impact on how mention positions and cast length should be calculated. 
/**
 * A cast with emojis and mentions
 *
 * "🤓@farcaster can mention immediately after emoji"
 */
const castWithEmojiAndMentions = await makeCastAdd(
  {
    text: '🤓 can mention immediately after emoji',
    embeds: [],
    embedsDeprecated: [],
    mentions: [1],
    mentionsPositions: [4],
  },
  dataOptions,
  ed25519Signer
);

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Reply

A cast can be a reply to another cast, URL or NFT. A reply to another cast is treated as a thread, while a reply to a URL or NFT can be treated as a comment or a channel. The optional parentUrl property can be set to a URL or to an fid-hash value of the cast being replied to, as shown in th example below. See FIP-2 for more details on reply types. 
/**
 * A cast that replies to a URL
 *
 * "I think this is a great protocol 🚀"
 */
const castReplyingToAUrl = await makeCastAdd(
  {
    text: 'I think this is a great protocol 🚀',
    embeds: [],
    embedsDeprecated: [],
    mentions: [],
    mentionsPositions: [],
    parentUrl: 'https://www.farcaster.xyz/',
  },
  dataOptions,
  ed25519Signer
);

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Channels

A cast can be sent into a channel, which instructs clients that it is related to a specific topic. Clients may choose to use this metadata in different ways, like grouping channel casts together or recommending them to certain users. A channel is simply a parentURL that is either a URL or NFT, which all clients recognize as a channel by loose consensus. There is no protocol level agreement on the list of channels yet, but the casts table in the replicator database can be used to get a list of commonly used channels. 
/* Query for a list of channels */
select parent_url,
       count(*) as count
from casts
where parent_url is not null
group by parent_url
order by count desc;

// Cast into the Farcaster channel
const channelCast = await makeCastAdd(
  {
    text: 'I love farcaster',
    embeds: [],
    embedsDeprecated: [],
    mentions: [],
    mentionsPositions: [],
    parentUrl: 'https://www.farcaster.xyz/', // This is illustrative, and is not an actual channel URL
  },
  dataOptions,
  ed25519Signer
);