NIP-89
======

Recommended Application Handlers
--------------------------------

`draft` `optional`

This NIP describes `kind:31989` and `kind:31990`: a way to discover applications that can handle unknown event-kinds.

## Rationale

Nostr's discoverability and transparent event interaction is one of its most interesting/novel mechanics.
This NIP provides a simple way for clients to discover applications that handle events of a specific kind to ensure smooth cross-client and cross-kind interactions.

### Parties involved

There are three actors to this workflow:

* application that handles a specific event kind (note that an application doesn't necessarily need to be a distinct entity and it could just be the same pubkey as user A)
    * Publishes `kind:31990`, detailing how apps should redirect to it
* user A, who recommends an app that handles a specific event kind
    * Publishes `kind:31989`
* user B, who seeks a recommendation for an app that handles a specific event kind
    * Queries for `kind:31989` and, based on results, queries for `kind:31990`

## Events

### Recommendation event
```json
{
  "kind": 31989,
  "pubkey": <recommender-user-pubkey>,
  "tags": [
    ["d", <supported-event-kind>],
    ["a", "31990:app1-pubkey:<d-identifier>", "wss://relay1", "ios"],
    ["a", "31990:app2-pubkey:<d-identifier>", "wss://relay2", "web"]
  ]
}
```

The `d` tag in `kind:31989` is the supported event kind this event is recommending.

Multiple `a` tags can appear on the same `kind:31989`.

The second value of the tag SHOULD be a relay hint.
The third value of the tag SHOULD be the platform where this recommendation might apply.

## Handler information
```json
{
  "kind": 31990,
  "pubkey": "<application-pubkey>",
  "content": "<optional-kind:0-style-metadata>",
  "tags": [
    ["d", <random-id>],
    ["k", <supported-event-kind>],
    ["web", "https://..../a/<bech32>", "nevent"],
    ["web", "https://..../p/<bech32>", "nprofile"],
    ["web", "https://..../e/<bech32>"],
    ["ios", ".../<bech32>"]
  ]
}
```

* `content` is an optional `metadata`-like stringified JSON object, as described in NIP-01. This content is useful when the pubkey creating the `kind:31990` is not an application. If `content` is empty, the `kind:0` of the pubkey should be used to display application information (e.g. name, picture, web, LUD16, etc.)
* `k` tags' value is the event kind that is supported by this `kind:31990`.
Using a `k` tag(s) (instead of having the kind of the `d` tag) provides:
    * Multiple `k` tags can exist in the same event if the application supports more than one event kind and their handler URLs are the same.
    * The same pubkey can have multiple events with different apps that handle the same event kind.
* `bech32` in a URL MUST be replaced by clients with the NIP-19-encoded entity that should be loaded by the application.

Multiple tags might be registered by the app, following NIP-19 nomenclature as the second value of the array.

A tag without a second value in the array SHOULD be considered a generic handler for any NIP-19 entity that is not handled by a different tag.

## Client tag
When publishing events, clients MAY include a `client` tag in the same format as the recommendation event's `a` tags. This has privacy implications for users, so clients SHOULD allow users to opt-out of using this tag.

```json
{
  "kind": 1,
  "tags": [
    ["client", "31990:app1-pubkey:<d-identifier>", "wss://relay1", "ios"]
  ]
  ...
}
```

## User flow
A user A who uses a non-`kind:1`-centric nostr app could choose to announce/recommend a certain kind-handler application.

When user B sees an unknown event kind, e.g. in a social-media centric nostr client, the client would allow user B to interact with the unknown-kind event (e.g. tapping on it).

The client MIGHT query for the user's and the user's follows handler.

## Example

### User A recommends a `kind:31337`-handler
User A might be a user of Zapstr, a `kind:31337`-centric client (tracks). Using Zapstr, user A publishes an event recommending Zapstr as a `kind:31337`-handler.

```json
{
  "kind": 31989,
  "tags": [
    ["d", "31337"],
    ["a", "31990:1743058db7078661b94aaf4286429d97ee5257d14a86d6bfa54cb0482b876fb0:abcd", <relay-url>, "web"]
  ],
  ...
}
```

### User B interacts with a `kind:31337`-handler
User B might see in their timeline an event referring to a `kind:31337` event (e.g. a `kind:1` tagging a `kind:31337`).

User B's client, not knowing how to handle a `kind:31337` might display the event using its `alt` tag (as described in NIP-31). When the user clicks on the event, the application queries for a handler for this `kind`:

```json
["REQ", <id>, '[{ "kinds": [31989], "#d": ["31337"], 'authors': [<user>, <users-contact-list>] }]']
```

User B, who follows User A, sees that `kind:31989` event and fetches the `a`-tagged event for the app and handler information.

User B's client sees the application's `kind:31990` which includes the information to redirect the user to the relevant URL with the desired entity replaced in the URL.

### Alternative query bypassing `kind:31989`
Alternatively, users might choose to query directly for `kind:31990` for an event kind. Clients SHOULD be careful doing this and use spam-prevention mechanisms or querying high-quality restricted relays to avoid directing users to malicious handlers.

```json
["REQ", <id>, '[{ "kinds": [31990], "#k": [<desired-event-kind>], 'authors': [...] }]']
```