A2A extension: bearer issuance, v1

https://mdesalvo.github.io/Morgana/a2a/extensions/bearer-issuance/v1/

An A2A agent card already carries the two halves of a transport contract: where an agent answers (supportedInterfaces) and which authentication scheme it requires (securitySchemes / securityRequirements). For an HTTP bearer scheme, however, the standard stops at the word bearer: it says a token is required, never how a caller is supposed to produce one. A caller holding a shared secret still has to be told, out of band, which iss to sign under and which aud to name — and out of band means configured on the calling side, which is precisely the coupling a discovery document exists to remove.

This extension closes that gap and nothing else. It declares the two claim values the publishing agent will validate, so a caller that already holds the shared secret needs no configuration beyond that secret.

Declaration

The extension is declared in the agent card’s capabilities.extensions:

{
  "capabilities": {
    "extensions": [
      {
        "uri": "https://mdesalvo.github.io/Morgana/a2a/extensions/bearer-issuance/v1",
        "description": "Issuer and audience a caller must mint its bearer token under.",
        "required": false,
        "params": {
          "issuer": "morgana",
          "audience": "morgana.ai"
        }
      }
    ]
  },
  "securitySchemes": {
    "morgana-bearer": {
      "httpAuthSecurityScheme": {
        "scheme": "bearer",
        "bearerFormat": "JWT"
      }
    }
  },
  "securityRequirements": [
    { "schemes": { "morgana-bearer": { "list": [] } } }
  ]
}

The security scheme is written as A2A defines it — a discriminated union whose member is named by its own field, not by a type discriminator — and the requirement in the form the reference .NET implementation emits. Neither belongs to this extension: they are shown only so the excerpt above is a card that exists, rather than a fragment assembled by hand.

params

Field	Type	Required	Meaning
issuer	string	yes	Value the publisher expects in the token’s iss claim.
audience	string	yes	Value the publisher expects in the token’s aud claim.

Both are opaque identifiers, compared for equality. Neither is required to be a URL, a hostname, or a resource anybody owns.

required is false, deliberately

The requirement to authenticate is already stated in standard form by securityRequirements, which every A2A consumer reads. This extension only says how to mint a token, which is a convenience for a caller that can mint one — not a condition for talking to the agent. A consumer that already holds a token issued out of band, or one that ignores the extension entirely, is unaffected: it is held to the standard requirement and nothing more.

Consequently, an implementation that does not understand this extension MUST NOT refuse the card.

What it does not carry

• The secret. The card is served unauthenticated — discovery is what tells a caller how to authenticate, so it cannot itself require authentication — and a public document is no place for a key. The shared secret is exchanged out of band, once and is the only thing a caller configures.
• The algorithm. bearerFormat: "JWT" on the security scheme says the token is a JWT; which signature algorithm the publisher accepts is a property of the credential it handed out, not of its public description.
• Authorization. The extension says how a caller is identified, never what it may then ask for.

Token expectations

A publisher declaring this extension validates, at minimum, iss and aud against the values above plus the token’s own lifetime. Callers SHOULD mint short-lived tokens — a single request-response exchange needs minutes, not hours — and SHOULD carry the calling identity in sub.

Implementation

Morgana publishes this extension on every agent card it serves and reads it on every card it consults. See the Morgana Handbook and ConfigurationAgentDirectoryService.