Native library reference. Extended CLI examples use jacs-compat. This source reference can include unreleased changes; see the package guide for installation and MCP profiles for available tools.

Serve Your Agent Card

By default, these discovery helpers export the legacy A2A 0.4.0 card shape. For the explicit A2A 1.0 options and their limits, see the current quickstart.

Source and published packages: Version 0.16.1 has a recorded release inventory. Version 0.16.0 is the first release whose source includes the native PQ DID/JWK and Agent Card behavior. See the identity integration guide.

Make your JACS agent discoverable by other A2A agents.

These helpers serve discovery documents only. They do not implement message/task handlers or the legacy optional /jacs/sign and /jacs/verify host examples in the extension descriptor. Configure the signed public interface for a service your host actually implements; a local discovery listener does not create that service or grant remote signing access.

Prerequisites: The current-source PQ examples require source-built bindings, plus Python's a2a-server extra or Express for Node.js. Registry installation alone does not establish that these newer exports are available. Node.js snippets below run inside an async function.

from jacs.a2a import JACSA2AIntegration

JACSA2AIntegration.quickstart(name="my-agent", domain="my-agent.example.com").serve(port=8080)

from fastapi import FastAPI
from jacs.client import JacsClient
from jacs.a2a_server import jacs_a2a_routes

app = FastAPI()
client = JacsClient.quickstart(name="my-agent", domain="my-agent.example.com")
router = jacs_a2a_routes(client)
app.include_router(router)

const express = require('express');
const { JacsClient } = require('@hai.ai/jacs/client');
const { jacsA2AMiddleware } = require('@hai.ai/jacs/a2a-server');

const client = await JacsClient.quickstart({
  name: 'my-agent',
  domain: 'my-agent.example.com',
});
const app = express();
app.use(jacsA2AMiddleware(client));
app.listen(8080);

What Gets Served

The native generator returns five public discovery documents by default. Explicit ES256 compatibility adds the sixth, a delegated-key binding:

The Agent Card includes the urn:jacs:provenance-v1 extension in capabilities.extensions, signaling to other JACS agents that your agent supports cryptographic provenance.

Current source signs the card with the persisted ML-DSA-87 root and publishes its AKP JWK. The key remains stable across calls and restarts; randomized signatures can differ. This path needs no compatibility key or binding. For a peer requiring ES256, select a2a_algorithm="ES256" in the Python jacs_a2a_routes / create_a2a_app options or a2aAlgorithm: "ES256" in Node's jacsA2AMiddleware options. The existing compatibility path uses a persisted ES256 key and a native-root-signed binding. Provision that key with jacs-compat agent add-compat-key when needed; loading alone never creates it.

Strict verification of a direct root card requires the exact explicitly trusted native key. For ES256 it additionally verifies the scoped compatibility binding referenced by fixed path and content hash. Local loopback trust tests require JACS_ALLOW_PRIVATE_JWKS=true; private-address JWKS fetching is otherwise denied.

Mounted routers refresh complete snapshots lazily after six days: measured from local generation for direct root cards, or signed issuedAt for ES256 bindings. The seven-day snapshot serving limit for a direct root card is a local cache policy, not a signed expiry or revocation signal. For ES256, strict verifiers accept a binding for at most seven days after issuedAt, with five minutes of future skew. Generation refreshes an authentic binding after six days while preserving its scopes and explicit expiresAt.

HTTP freshness ends by renewal or earlier expiry; failed renewal can serve a still-valid snapshot with no-store, while hard expiry returns 503. An explicit binding expiry requires renewed authorization and remounting. Separate resource fetches can straddle replacement; verify related documents together and refetch if their references differ.

Next Steps

• Discover & Trust Remote Agents -- Find other agents and assess their trustworthiness
• Exchange Signed Artifacts -- Sign and verify A2A artifacts
• A2A Interoperability Reference -- Full API reference