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+---
+slug: "9098"
+authors: silverpill <@silverpill@mitra.social>
+type: implementation
+status: DRAFT
+dateReceived: 2025-07-06
+discussionsTo: https://codeberg.org/silverpill/feps/issues
+---
+# FEP-9098: Custom emojis
+
+## Summary
+
+This document describes how custom emojis are implemented in [ActivityPub] network.
+
+## History
+
+Custom emojis were introduced by Pleroma ([commits](https://gitgud.io/lambadalambda/pleroma/-/compare/c17c8ce36d35db03a007a264cc2e507afa9b803e...7c82b8219734102ff24d9dc24226c08351e608cc)) and Mastodon ([PR](https://github.com/mastodon/mastodon/pull/4988)) in 2017.
+
+## Requirements
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119].
+
+## Emoji object
+
+Custom emojis are represented as `Emoji` objects (the full type IRI is `http://joinmastodon.org/ns#Emoji`). The `Emoji` object has the following properties:
+
+- `id` (RECOMMENDED): the unique identifier of the custom emoji. Some implementations omit this property (i.e. the object is anonymous).
+- `type` (REQUIRED): the `Emoji` string.
+- `name` (REQUIRED): the [shortcode][Shortcode] of the custom emoji (the name of the emoji, enclosed in colons).
+- `updated` (OPTIONAL): the date when custom emoji was updated, formatted as [RFC-3339] timestamp.
+- `icon` (REQUIRED): `Image` object describing emoji image.
+  - `type` (REQUIRED): the `Image` string.
+  - `url` (REQUIRED): the URL of the emoji image.
+
+Example:
+
+```json
+{
+  "id": "https://social.example/emoji/blobcat",
+  "type": "Emoji",
+  "name": ":blobcat:",
+  "updated": "1970-01-01T00:00:00Z",
+  "icon": {
+    "type": "Image",
+    "url": "https://social.example/media/blobcat.png"
+  }
+}
+```
+
+### Uniqueness
+
+The primary unique identifier of a custom emoji is a combination of its name and the domain name. The domain name can be extracted from the `id` of `Emoji` object or from the `id` of the object within which it is embedded.
+
+If `id` is not guaranteed to be globally unique, it MUST be omitted.
+
+### Accessibility
+
+The short description of a custom emoji can be specified with the `alternateName` property (the full IRI is `http://schema.org/alternateName`).
+
+### Compatibility
+
+In order to be compatible with as many servers as possible, implementations SHOULD comply with the following additional requirements:
+
+- Name contains at least two characters ([Mastodon](https://github.com/mastodon/mastodon/blob/v4.3.7/app/models/custom_emoji.rb#L28)).
+- Name matches the regular expression `^[a-zA-Z0-9_]+$` ([Mastodon](https://github.com/mastodon/mastodon/blob/v4.3.7/app/models/custom_emoji.rb#L30)).
+- Image media type is `image/png`, `image/gif` or `image/webp` ([Mastodon](https://github.com/mastodon/mastodon/blob/v4.3.7/app/models/custom_emoji.rb#L37)).
+- Image size is not greater than 256 KB ([Mastodon](https://github.com/mastodon/mastodon/blob/v4.3.7/app/models/custom_emoji.rb#L27)).
+- Image is a square (some clients may display non-squre emojis with a wrong aspect ratio).
+
+## Using custom emojis
+
+### Microsyntax
+
+Custom emojis can be inserted into textual representations of an object using [shortcodes][Shortcode] (shortcode is a name of an emoji enclosed in colons: `:emojiname:`). These textual representations are often specified with `name`, `summary` and `content` properties.
+
+The corresponding `Emoji` objects are added to the `tag` array of the object.
+
+Example:
+
+```json
+{
+  "type": "Note",
+  "content": "<p>:blobcat:</p>",
+  "tag": [
+    {
+      "id": "https://social.example/emoji/blobcat",
+      "type": "Emoji",
+      "name": ":blobcat:",
+      "icon": {
+        "type": "Image",
+        "url": "https://social.example/media/blobcat.png"
+      }
+    }
+  ]
+}
+```
+
+### Rendering
+
+Applications typically replace custom emoji shortcodes with `<img>` HTML tags and render the output as HTML.
+
+To prevent [Cross-Site Scripting (XSS)][XSS] attacks, implementers MUST ensure that:
+
+- Emoji names, descriptions, URLs and other strings that are used in replacements do not contain reserved HTML characters.
+- When HTML content (e.g. `summary`, `content`) is processed, shortcodes are replaced only inside [Text][DOM-Text] nodes.
+- Reserved HTML characters in text are escaped before replacing shortcodes.
+
+## Implementations
+
+This document is based on implementations of custom emojis in Pleroma, Mastodon, Misskey and Fedibird.
+
+## References
+
+- Christine Lemmer-Webber, Jessica Tallon, Erin Shepherd, Amy Guy, Evan Prodromou, [ActivityPub], 2018
+- S. Bradner, [Key words for use in RFCs to Indicate Requirement Levels][RFC-2119], 1997
+- G. Klyne, C. Newman, [Date and Time on the Internet: Timestamps][RFC-3339], 2002
+
+[ActivityPub]: https://www.w3.org/TR/activitypub/
+[RFC-2119]: https://tools.ietf.org/html/rfc2119.html
+[RFC-3339]: https://www.rfc-editor.org/rfc/rfc3339
+[Shortcode]: https://emojipedia.org/shortcodes
+[XSS]: https://owasp.org/www-community/attacks/xss/
+[DOM-Text]: https://developer.mozilla.org/en-US/docs/Web/API/Text
+
+## Copyright
+
+CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
+
+To the extent possible under law, the authors of this Fediverse Enhancement Proposal have waived all copyright and related or neighboring rights to this work.