Tag: u

Overview

The u tag represents a URL in the Nostr protocol. It is used to specify a URL to a resource, such as a mint URL in Nutzaps (NIP-61) or the absolute request URL in HTTP Auth (NIP-98).

Specification

Property	Value
Tag Name	u
Defined in	NIP-61, NIP-98
Required for	Kind 9321 (Nutzap), Kind 27235 (HTTP Auth)

Format

["u", "<url>"]

Usage Description

The u tag contains a URL string value. The specific use depends on the event kind:

1. In Kind 9321 (Nutzap events, NIP-61): The u tag specifies the URL of the mint exactly as specified by the recipient's kind:10019 event.

2. In Kind 27235 (HTTP Auth events, NIP-98): The u tag must contain exactly the same absolute request URL as the one being authenticated (including query parameters).

Examples

Example in Nutzap (NIP-61)

["u", "https://stablenut.umint.cash"]

Example in HTTP Auth (NIP-98)

["u", "https://api.snort.social/api/v1/n5sp/list"]

Client Behavior

• In Nutzap context (NIP-61):

  • When creating a Nutzap event, clients MUST include the mint URL exactly as specified in the recipient's kind:10019 event.
  • When displaying Nutzap events, clients MAY use the u tag to show which mint the Nutzap tokens were minted at.

• In HTTP Auth context (NIP-98):

  • When generating an auth event, clients MUST include the exact absolute URL of the requested resource, including all query parameters.

Relay Behavior

Relays generally do not need to perform any special validation on the u tag itself, but the tag is essential for the functionality of the events that use it.

References

• NIP-61: Nutzaps
• NIP-98: HTTP Auth

Related Tags

• p tag - Used in conjunction with the u tag in Nutzaps to specify the recipient
• proof tag - Used in Nutzaps to provide the Cashu proofs
• method tag - Used in HTTP Auth to specify the HTTP method

Notes

• The u tag is case-sensitive.
• In HTTP Auth (NIP-98), servers MUST verify that the u tag exactly matches the requested URL.
• In Nutzaps (NIP-61), the mint URL must exactly match what the recipient has specified in their kind:10019 event.