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.
* 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`
*`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.)
*`pff` stands for preferred feedback format and is an optional tag that describes how the application would like to receive additional feedback from `kind:7` reactions. The format specified in `pff` is used to inform the `feedback` tag of the `kind:7` customer feedback event described in [NIP-90 - Data Vending Machine](https://github.com/nostr-protocol/nips/blob/master/90.md). Both `pff` and `feedback` values should be strings and no commitments to representation are made here; instead applications consuming these tags should work together to align themselves.
When publishing events, clients MAY include a `client` tag. Identifying the client that published the note. This tag is a tuple of `name`, `address` identifying a handler event and, a relay `hint` for finding the handler event. This has privacy implications for users, so clients SHOULD allow users to opt-out of using this tag.
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.
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.
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`:
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.
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.
