How Should Custom Emoji Be Handled in WordPress ActivityPub?

[Jiwoon-Kim]

📌 Issue Title:

Discussion: How Should Custom Emoji Be Handled in WordPress ActivityPub?
https://developer.wordpress.org/reference/functions/_wp_emoji_list/

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📄 Issue Body:

I’d like to open a discussion about how the WordPress ActivityPub plugin should handle custom emoji.

Currently:

• Self-hosted WordPress ActivityPub instances render emoji pulled from the .org side.
• WordPress.com blogs, the Jetpack mobile app, and the WordPress Reader fetch emoji assets via .com infrastructure.

If we plan to support custom emoji (like those on Mastodon or Misskey), this separation could lead to inconsistencies in rendering across platforms.
So I did a simple investigation on how other federated platforms handle this:

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Example: My Misskey Profile

• Profile with a :misskey: custom emoji
  Profile: Misskey Profile
  Emoji asset:
  https://files.mastodon.social/cache/custom_emojis/images/000/273/113/original/b29393e466430baf.png

Also accessible via proxy:
https://hoto.moe/proxy/image.webp?url=https%3A%2F%2Fmedia.misskeyusercontent.com%2Femoji%2Fmisskey.png&emoji=1

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Example: WordPress Profiles on Mastodon / Misskey / Hoto

• [@[email protected]](https://mastodon.social/@[email protected])
  :wordpress: emoji on Mastodon:
  https://files.mastodon.social/cache/custom_emojis/images/000/465/125/original/3b01c56fdd6c225c.png

  On Misskey:
  https://proxy.misskeyusercontent.jp/image/files.wptoots.social%2Fcustom_emojis%2Fimages%2F000%2F000%2F007%2Foriginal%2Ff227660a4022c9cc.png?emoji=1

  On Hoto.moe:
  https://hoto.moe/emoji/[email protected]
  https://hoto.moe/proxy/image.webp?url=https%3A%2F%2Ffiles.wptoots.social%2Fcustom_emojis%2Fimages%2F000%2F000%2F007%2Foriginal%2Ff227660a4022c9cc.png&emoji=1

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Thoughts & Questions:

• Should we unify emoji rendering through a single proxy server for consistency?
• Should WordPress/Automattic officially distribute a set of custom emoji (maybe even offer premium emoji packs)?
• Should we allow custom emoji reactions and interoperability with other platforms like Slack, Misskey, and GitHub?
• Should we adopt something like Misskey’s custom emoji system, even though it’s technically non-standard in ActivityPub?

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Would love to hear everyone’s thoughts and ideas on this!

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📌 Current State of Custom Emoji Handling in the Fediverse

1️⃣ Mastodon

• Each instance has its own custom_emoji list, embedded in the actor’s JSON profile or note (activity).
• Image URLs point to the instance’s CDN (e.g. files.mastodon.social/...).

2️⃣ Misskey

• Similar, but with a different emoji call and proxy system.
• Uses :emoji_name: syntax with optional media proxy.
• Non-standard, but has independent extensions.

3️⃣ Slack / Discord / GitHub

• Supports custom emoji for reactions and messages (instance-hosted or global).
• Handled via dedicated fields in JSON (e.g. emoji: { name: "🔥", url: "https://..." })

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📌 How It Works in WordPress ActivityPub (Currently)

• No native support for custom emojis
• No customEmoji list in ActivityPub JSON.
• Cannot embed via OpenGraph or HTML meta.
• Reader apps (WordPress Reader, Jetpack app) don’t support it either.

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📌 Possible Future Approaches

✅ Option A: Extend ActivityPub Object Schema

• Add customEmoji property to ActivityPub JSON objects.
• Include emoji name and image URL in the actor profile JSON.

"emoji": {
  "misskey": "https://example.com/misskey.png",
  "wordpress": "https://example.com/wordpress.png"
}

• Caveat: Mastodon mostly ignores external emoji URLs — would likely only work for Misskey and custom apps.

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✅ Option B: Centralized Proxy Server

• Automattic, WordPress.com, Jetpack, or a community-hosted proxy like:
  https://wp-fediverse-emoji-proxy.net/custom_emoji/{emoji_name}.webp
• Use this proxy URL for all emoji in ActivityPub objects.
• Solves cross-instance rendering issues.
• Bonus: Potential for a paid emoji marketplace (like “WordPress.com Premium Emoji Pack”)

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✅ Option C: OpenGraph / HTML Meta Emoji Declaration

• Use <meta name="fediverse:emoji-misskey" content="https://..." />
• Mastodon likely won’t parse this, but Misskey or custom readers might.

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📌 What About Emoji Reactions?

• ActivityPub spec defines reactions, but not emoji-based reactions officially.
• Misskey extends this via Reaction objects:

{
  "type": "Reaction",
  "actor": "...",
  "object": "...",
  "content": ":misskey:"
}

• Could implement this in the WordPress ActivityPub plugin, though Mastodon won’t recognize it.

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📌 Conclusion

Should WordPress/Automattic distribute official custom emoji? → Possible.

• Could sell premium emoji packs via Jetpack (like Slack)
• Adding a proxy system to Reader apps and Jetpack ActivityPub integration would solve most rendering issues.

Should we adopt Misskey’s non-standard system?
→ Honestly, ActivityPub is a loose protocol and community extensions are common.
(See IndieWeb, ForgeFed, etc.)

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🔥 Recommended Next Step

• Propose this as an upstream issue on the ActivityPub plugin repo.
• Suggest adding customEmoji property to ActivityPub JSON.
• Propose establishing a Jetpack/WordPress.com emoji proxy server.