AgentCard v0.3 vs v1.0

A2A has two spec versions in active use. The AgentCard — the JSON your agent serves at /.well-known/agent.json — differs in shape between them. Each A2XAgent instance is bound to one wire format, chosen at construction time via protocolVersion (default '1.0').

You usually don't pick. toA2x() and DefaultRequestHandler.getAgentCard() render whatever protocolVersion was configured. The card shape, the JSON-RPC response shape, and the pushNotificationConfig param shape always match — so clients reading the card get a faithful contract for the wire underneath.

The key differences

A2X models everything internally in a version-neutral shape and renders it through the configured wire format. This means:

• Every declaration you make on A2XAgent (skills, security, default URL) works for both versions.
• The card and the wire are always consistent — the SDK never publishes a card whose protocolVersion disagrees with what the server actually emits.
• You don't write version branches in your own code.

Picking the wire format

Pass protocolVersion to the constructor. The choice is fixed for the life of the agent:

const a2xAgentV10 = new A2XAgent({ taskStore, executor }); // v1.0 (default)
const a2xAgentV03 = new A2XAgent({ taskStore, executor, protocolVersion: '0.3' });
 
a2xAgentV10.getAgentCard(); // → v1.0 card
a2xAgentV03.getAgentCard(); // → v0.3 card

To serve both versions from one deployment, run two A2XAgent instances behind separate URLs and advertise each in the other's additionalInterfaces. One instance cannot lie about its wire format.

When this matters to you

Three scenarios where you actually need to care:

1. Deploying alongside legacy v0.3 clients

If you have CLI tools or third-party agents still pinned to v0.3, they read url and preferredTransport — not supportedInterfaces. Construct the agent with protocolVersion: '0.3' and the SDK will speak v0.3 end-to-end (card + wire). For deployments that need to serve both at once, see Multi-transport agents below.

2. Emitting a non-standard scheme on v0.3

OAuth 2.0 Device Code is a v1.0-native security flow. A2X extends it onto v0.3 cards as a non-standard extension so headless clients can still negotiate it. See Protocol Extensions for details.

Advertising the authenticated extended card

When you call a2xAgent.setAuthenticatedExtendedCardProvider(...), A2X flips the version-specific capability flag on the base card automatically:

• v0.3: top-level supportsAuthenticatedExtendedCard: true
• v1.0: capabilities.extendedAgentCard: true

Conforming clients read this to decide whether to call agent/getAuthenticatedExtendedCard. You don't set either flag by hand — registering the provider is the only signal you need. See Authenticated Extended AgentCard.

3. Multi-transport agents

If you plan to expose JSON-RPC over HTTP and another transport (e.g. gRPC), only v1.0 expresses this cleanly. Legacy v0.3 consumers will only see the first/primary interface.

Consuming: what A2XClient does

On the client side, A2XClient reads the remote card's protocolVersion field (or infers v0.3 when absent) and routes calls accordingly. You usually don't need to pick a version yourself:

const resolved = await client.resolveAgentCard();
console.log(resolved.version);   // '0.3' | '1.0'
console.log(resolved.card);      // the parsed card for that version

To force a preference when the remote supports both:

const client = new A2XClient(url, { preferredVersion: '1.0' });

Recommendation

• Serve v1.0 as primary — the constructor default. New clients should target v1.0.
• Pin your own clients to v1.0 when calling agents that support both. Future-facing.
• Spin up a dedicated v0.3 instance only when a deployment target needs it. Configure it with protocolVersion: '0.3' rather than trying to coax a v1.0 agent into pretending.

Beyond the AgentCard: push notification authentication

tasks/pushNotificationConfig/{set,get,list} carries an optional authentication block whose shape diverges between specs:

A2X stores the v0.3 shape internally and translates at the v1.0 boundary: scheme = schemes[0] outbound, schemes = [scheme] inbound. The conversion is lossy when v0.3 lists more than one scheme — only the first is preserved on a v1.0 wire. Configure your agent's protocolVersion to match the wire your clients speak; the SDK handles the rest.