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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

{
  "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"]
  ],
  // other fields...
}

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

{
  "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>"]
  ],
  // other fields...
}
  • 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. 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.

{
  "kind": 1,
  "tags": [
    ["client", "My Client", "31990:app1-pubkey:<d-identifier>", "wss://relay1"]
  ]
  // other fields...
}

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.

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

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:

["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.

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