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+# MSC0000: Adding User-Defined Custom Fields to User Profiles
+
+*This proposal depends on [MSC4133: Extending User Profile API](https://github.com/matrix-org/matrix-spec-proposals/pull/4133). It introduces user-defined custom fields in the `u.*` namespace for user profiles.*
+
+## Proposal
+
+This proposal aims to enable users to add arbitrary custom key:value pairs to their global user profiles within the Matrix ecosystem. By leveraging the extended profile API introduced in [MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133), users can enrich their profiles with additional public information such as preferred languages, organisational roles, or other relevant details.
+
+### Key/Namespace Requirements
+
+As per [MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133), the profile API is extended to allow additional fields beyond `avatar_url` and `displayname`. This MSC defines the use of the `u.*` namespace for user-defined custom fields.
+
+- **Namespace `u.*`**: Reserved for user-defined custom fields. The portion of the key name after the `u.` defines the display name of this field (e.g., `u.bio`). The values in this namespace must always be UTF-8 strings with content not exceeding 512 bytes.
+
+### Client-Server API Changes
+
+#### Setting a Custom Profile Field
+
+To set or update a custom profile field, clients can use the `PUT` endpoint defined in [MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133):
+
+- **Endpoint**: `PUT /_matrix/client/v3/profile/{userId}/{key_name}`
+- **Description**: Set or update the value of a specified `key_name` in the user's profile, if permitted by the homeserver.
+- **Request Body**:
+
+```json
+{
+  "key_name": "new_value"
+}
+```
+
+*Example*: Setting the custom field `u.bio` for user `@alice:matrix.org`:
+
+```http
+PUT /_matrix/client/v3/profile/@alice:matrix.org/u.bio
+```
+
+With body:
+
+```json
+{
+  "u.bio": "I love Matrix!"
+}
+```
+
+#### Retrieving Custom Profile Fields
+
+To retrieve custom fields from a user's profile, clients use the `GET` endpoint:
+
+- **Endpoint**: `GET /_matrix/client/v3/profile/{userId}/{key_name}`
+- **Description**: Retrieve the value of a specified `key_name` from a user's profile.
+- **Response**:
+
+```json
+{
+  "key_name": "field_value"
+}
+```
+
+*Example*: Retrieving the custom field `u.bio` for user `@alice:matrix.org`:
+
+```http
+GET /_matrix/client/v3/profile/@alice:matrix.org/u.bio
+```
+
+Response:
+
+```json
+{
+  "u.bio": "I love Matrix!"
+}
+```
+
+#### Deleting a Custom Profile Field
+
+To delete a custom profile field:
+
+- **Endpoint**: `DELETE /_matrix/client/v3/profile/{userId}/{key_name}`
+- **Description**: Remove a specified `key_name` and its value from the user's profile, if permitted by the homeserver.
+
+#### Retrieving All Profile Fields
+
+Clients can retrieve all profile fields for a user, including custom fields:
+
+- **Endpoint**: `GET /_matrix/client/v3/profile/{userId}`
+- **Description**: Retrieve all profile fields for a user.
+- **Response**:
+
+```json
+{
+  "avatar_url": "mxc://matrix.org/MyC00lAvatar",
+  "displayname": "Alice",
+  "u.bio": "I love Matrix!",
+  "u.location": "London"
+}
+```
+
+### Capabilities
+
+A new capability `m.custom_profile_fields` is introduced to control the ability to set custom profile fields. It is advertised on the `GET /_matrix/client/v3/capabilities` endpoint. Clients should check for this capability before attempting to create or modify custom fields.
+
+- **Capability Structure**:
+
+```json
+{
+  "capabilities": {
+    "m.custom_profile_fields": {
+      "enabled": true,
+      "allowed": ["u.*"],
+      "disallowed": []
+    }
+  }
+}
+```
+
+- **Behaviour**:
+  - **When capability is missing**: Clients should assume that custom profile fields are not supported and refrain from providing options to set them.
+  - **When `enabled` is `false`**: Clients should not allow users to create or update custom fields. Any attempt to do so should result in a `403 Forbidden` error.
+  - **When `enabled` is `true`**: Clients should allow users to create or update custom fields in the `u.*` namespace, except those listed in the `disallowed` array.
+
+### Error Handling
+
+Consistent error codes and messages ensure clear communication of issues. Homeservers should use the following error codes:
+
+- **`M_KEY_NOT_ALLOWED`**: The specified key is not allowed to be set.
+
+```json
+{
+  "errcode": "M_KEY_NOT_ALLOWED",
+  "error": "The profile key 'u.bio' is not allowed to be set."
+}
+```
+
+- **`M_TOO_LARGE`**: The value provided exceeds the maximum allowed length.
+
+```json
+{
+  "errcode": "M_TOO_LARGE",
+  "error": "The value exceeds the maximum allowed length of 512 bytes."
+}
+```
+
+### Client Considerations
+
+- Clients SHOULD provide a UI for users to view and edit their own custom fields in the `u.*` namespace.
+- When displaying other users' profiles, clients SHOULD retrieve and display custom fields in the `u.*` namespace.
+- Clients SHOULD be cautious about the amount of data displayed and provide options to limit or filter the display of custom fields.
+
+### Server Considerations
+
+- Homeservers MAY impose limits on the size and number of custom fields.
+- Homeservers SHOULD validate that keys in the `u.*` namespace conform to the required format and enforce size limits on values.
+- Homeservers MAY restrict certain keys or values based on server policies.
+
+## Potential Issues
+
+- **Privacy Concerns**: Users need to be aware that custom profile fields are public and visible to anyone who can access their profile.
+- **Abuse Potential**: As with any user-generated content, there is potential for misuse. Servers and clients need to be prepared to handle offensive or inappropriate content.
+- **Interoperability**: Until widespread adoption occurs, not all clients may support displaying custom fields, leading to inconsistent user experiences.
+
+## Security Considerations
+
+- **Content Moderation**: Homeservers SHOULD provide mechanisms to report and handle abusive content in custom profile fields.
+- **Data Protection**: Homeservers MUST ensure compliance with data protection regulations, especially regarding user consent and data retention.
+
+## Unstable Prefixes
+
+### Unstable Namespace
+
+Until this proposal is stabilised, custom fields should use an unstable prefix in their keys:
+
+- **Namespace `uk.tcpip.msc0000.u.*`**: For example, `uk.tcpip.msc0000.u.bio`.
+
+### Unstable Capability
+
+The capability should be advertised with an unstable prefix:
+
+```json
+{
+  "capabilities": {
+    "uk.tcpip.msc0000.custom_profile_fields": {
+      "enabled": true,
+      "allowed": ["uk.tcpip.msc0000.u.*"]
+    }
+  }
+}
+```
+
+## Dependencies
+
+This proposal depends on [MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133), which introduces the extended profile API to allow additional fields in user profiles.
+
+## Conclusion
+
+By introducing user-defined custom fields in the `u.*` namespace, this proposal allows users to enrich their profiles with additional information, enhancing interactions within the Matrix ecosystem while maintaining simplicity and compatibility with existing infrastructure.