Ag2 A2a

A2A (Agent-to-Agent) integration

Expose an AG2 Agent over the A2A protocol — a standard JSON-RPC / REST / gRPC contract plus a discoverable AgentCard — so other systems can call your agent over the network. The same module also lets an AG2 Agent consume a remote A2A agent by using A2AConfig as the agent's model config.

When to use

• You want an AG2 agent reachable as a standard networked service that non-AG2 A2A clients can call (interop), not just from Python.
• You need a discoverable agent: a published AgentCard at /.well-known/agent-card.json describing the agent's skills, transports, auth, and capabilities.
• You want a choice of transport (JSON-RPC, REST, or gRPC) — possibly several at once from one server.
• You need declared auth schemes (bearer / API key / OAuth2 / OIDC / mTLS), push notifications, or multi-tenancy surfaced on the card.
• Conversely, you want one AG2 agent to call a remote A2A agent as if it were an LLM provider — use A2AConfig as the calling agent's config.

If you only need a web frontend (React/CopilotKit) in front of an agent, use the AG-UI skill instead. If two AG2 agents simply need to talk inside one process/cluster, the AG2 network is the standard multi-agent pattern.

Installation

pip install "ag2[a2a]"

> Required. The a2a extra pulls in a2a-sdk (with the HTTP server). gRPC needs the a2a-sdk[grpc] extra in addition. If you cannot run commands, state the exact pip install command.

Public API (from ag2.a2a import ...): A2AConfig, A2AServer, build_card.

60-second recipe — serve an agent over A2A

A2AServer.build_jsonrpc(...) returns a ready-to-serve Starlette ASGI app. Run it directly with uvicorn:

```python title="serve_a2a.py" from ag2 import Agent from ag2.a2a import A2AServer from ag2.config import OpenAIConfig

agent = Agent( name="weather_bot", prompt="You answer questions about the weather.", config=OpenAIConfig(model="gpt-4o-mini"), )

server = A2AServer(agent)

returns a Starlette ASGI app; url is the public URL written into the card

app = server.build_jsonrpc(url="http://localhost:8000")

```bash
uvicorn serve_a2a:app --host 0.0.0.0 --port 8000

The agent card is then served at http://localhost:8000/.well-known/agent-card.json and JSON-RPC requests at /.

To combine with other routes, mount the returned app under a parent Starlette (or FastAPI) app:

from starlette.applications import Starlette
from starlette.routing import Mount

a2a_app = server.build_jsonrpc(url="http://localhost:8000/a2a")
parent = Starlette(routes=[Mount("/a2a", app=a2a_app)])

> A2A has no middleware spec — attach CORS / auth / tracing to the returned Starlette app yourself.

Calling a served agent from another AG2 agent

A2AConfig is a ModelConfig: point an Agent at a remote card URL and ask() it like any agent. The client fetches the card from {card_url}/.well-known/agent-card.json, picks a transport, and runs the task.

from ag2 import Agent
from ag2.a2a import A2AConfig

remote = Agent(
    "remote",
    config=A2AConfig(card_url="http://localhost:8000"),
)
reply = await remote.ask("What is the weather in Paris?")
print(await reply.content())

Key A2AConfig options (all keyword, with defaults):

| Option | Default | Purpose | |---|---|---| | card_url | (required) | Base URL; card fetched from {card_url}/.well-known/agent-card.json. | | prefer | None | Force a transport when the card lists several: "jsonrpc", "rest", or "grpc". None auto-picks. | | streaming | True | Stream the task; set False to poll get_task every polling_intervals instead. | | headers | None | Extra HTTP headers sent on every request (e.g. an auth token). | | timeout | 60.0 | Per-request timeout (seconds). | | polling_interval | 0.5 | Poll interval (s) when streaming is off / unsupported. | | input_required_timeout | None | Cap on waiting for the HITL hook on TASK_STATE_INPUT_REQUIRED; None = wait forever. | | tenant | None | Scope every request to a server-side tenant (override per-call via context.variables["a2a:tenant"]). | | history_length | None | Server-side hint: truncate echoed Task.history to the most recent N messages. | | httpx_client_factory | None | Supply a custom httpx.AsyncClient factory (used by the in-process test helpers below). | | preset_card | None | Use an already-fetched AgentCard to skip the discovery round-trip (see A2AConfig.from_card(...)). |

The agent card — build_card

A2AServer builds a card automatically, but call build_card directly to customise it and pass the result via card= to a build_* method.

from a2a.types import AgentProvider, AgentSkill

from ag2.a2a import build_card

card = build_card(
    agent,
    url="http://localhost:8000",
    transports=("jsonrpc",),          # one AgentInterface per binding
    version="2.1.0",
    description="Customer support agent",
    push_notifications=True,
    skills=[
        AgentSkill(
            id="refunds",
            name="Process refunds",
            description="Issue refunds for orders",
            tags=["billing"],
        ),
    ],
    provider=AgentProvider(organization="Acme", url="https://acme.example"),
    documentation_url="https://acme.example/docs",
)

Card behaviour worth knowing:

• Skills: if you omit skills, build_card auto-detects any SkillsToolkit on agent.tools and publishes its local skills; if there are none it falls back to a single skill derived from agent.name / the system prompt, so the card stays spec-compliant.
• Default description: when description is omitted it uses the agent's system prompt.
• Always declares the urn:ag2:client-tools:v1 extension as required=False (lets AG2 clients run client-side tools, falling back to plain text for non-AG2 clients).
• transports= accepts any of ("jsonrpc", "rest", "grpc"). For "grpc" you must also pass grpc_url=.

Auth — declaring security schemes

Build schemes with the factory helpers in ag2.a2a.security, group them into require(...) requirements, and pass security=[...] to build_card. The card's security_schemes are auto-derived from the schemes you reference — no duplicate declarations.

from ag2.a2a import build_card
from ag2.a2a.security import api_key_scheme, bearer_scheme, require

bearer = bearer_scheme(name="bearer")                               # Authorization: Bearer 
apikey = api_key_scheme(name="apikey", key_name="X-API-Key", location="header")

card = build_card(
    agent,
    url="http://localhost:8000",
    # list is OR-ed: present a bearer token OR an API key
    security=[require(bearer), require(apikey)],
)

Schemes in a single require(...) are AND-ed (all must be presented); multiple require(...) entries are OR-ed (any one suffices). OAuth2 / OIDC scopes attach via .with_scopes(...):

from a2a.types import AuthorizationCodeOAuthFlow, OAuthFlows

from ag2.a2a.security import bearer_scheme, oauth2_scheme, require

flows = OAuthFlows(
    authorization_code=AuthorizationCodeOAuthFlow(
        authorization_url="https://auth.example/authorize",
        token_url="https://auth.example/token",
        scopes={"read": "Read access", "write": "Write access"},
    ),
)
oauth = oauth2_scheme(name="oauth", flows=flows)
bearer = bearer_scheme(name="bearer")

card = build_card(
    agent,
    url="http://localhost:8000",
    security=[require(bearer, oauth.with_scopes("read", "write"))],  # AND, scoped
)

Available scheme factories: bearer_scheme, http_auth_scheme (basic/digest/...), api_key_scheme, oauth2_scheme, open_id_connect_scheme, mtls_scheme. The card only declares the scheme — A2A defines no middleware, so you must enforce auth on the returned Starlette/gRPC object (e.g. a Starlette middleware that validates the header).

Multiple transports from one server

One A2AServer shares a single task store across transports, so the same agent can be exposed several ways simultaneously:

server = A2AServer(agent)
jsonrpc_app = server.build_jsonrpc(url="http://localhost:8000")
rest_app    = server.build_rest(url="http://localhost:8001", path_prefix="/v1")
grpc_srv    = server.build_grpc(bind="0.0.0.0:50051", grpc_url="localhost:50051")  # needs ag2[a2a] grpc extra

• build_jsonrpc(url=...) / build_rest(url=..., path_prefix=...) → Starlette ASGI apps (serve with uvicorn).
• build_grpc(bind=..., grpc_url=...) → a grpc.aio.Server you start()/await yourself (bind = listener address, grpc_url = public URL in the card; insecure binding only).

To consolidate the card into a single multi-binding card, call build_card(agent, url=..., transports=("jsonrpc","rest","grpc"), grpc_url=...) and pass it via card= to each builder.

Push notifications

Pass a PushNotificationConfigStore to the server; the card's capabilities.push_notifications flips on automatically.

from a2a.server.tasks import InMemoryPushNotificationConfigStore

from ag2.a2a import A2AServer

server = A2AServer(agent, push_config_store=InMemoryPushNotificationConfigStore())
app = server.build_jsonrpc(url="http://localhost:8000")

Client side (consuming AG2 agent), register a webhook for a task via the helpers in ag2.a2a.push:

from ag2.a2a import A2AConfig
from ag2.a2a.push import A2APushConfig, create_push_notification_config

config = A2AConfig(card_url="http://localhost:8000")
await create_push_notification_config(
    config,
    task_id="task-123",
    push_config=A2APushConfig(url="https://my-app.example/webhook", token="secret"),
)

Companion helpers: get_push_notification_config, list_push_notification_configs, delete_push_notification_config. Task inspection helpers live in ag2.a2a.tasks: get_task, list_tasks (returns a ListedTasks with pagination metadata), cancel_task.

> These client helpers require a running, reachable A2A server (real network round-trips), so they aren't exercised by the in-process tests below.

Testing — in-process, no network, no API keys

ag2.a2a.testing ships ASGI helpers that dispatch directly into the server app via httpx.ASGITransport — no socket, no port, no SSE proxy. Plug the factory into A2AConfig.httpx_client_factory and do a full round trip:

import asyncio

from ag2 import Agent
from ag2.a2a import A2AConfig, A2AServer
from ag2.a2a.testing import make_test_client_factory
from ag2.testing import TestConfig  # mock the served agent's LLM

async def main():
    served = Agent(
        name="weather_bot",
        prompt="You answer questions about the weather.",
        config=TestConfig("It is sunny in Paris."),  # no real LLM call
    )
    server = A2AServer(served)

    factory = make_test_client_factory(server, url="http://test")
    remote = Agent("remote", config=A2AConfig(card_url="http://test", httpx_client_factory=factory))

    reply = await remote.ask("What is the weather in Paris?")
    print(await reply.content())  # -> "It is sunny in Paris."

asyncio.run(main())

For REST use make_test_rest_client_factory and prefer="rest":

from ag2.a2a.testing import make_test_rest_client_factory

factory = make_test_rest_client_factory(server, url="http://test")
remote = Agent("remote", config=A2AConfig(card_url="http://test", prefer="rest", httpx_client_factory=factory))

For gRPC (which has no in-process ASGI equivalent), testing.pick_free_port() finds a free TCP port to bind a real gRPC server in a test.

A runnable test that exercises every snippet in this skill lives at assets/test_snippets.py:

.venv/bin/python skills/ag2-a2a/assets/test_snippets.py

Common pitfalls

• Missing a2a extra — pip install "ag2[a2a]"; without it from ag2.a2a import A2AServer raises a "missing additional dependency" error. gRPC additionally needs the a2a-sdk[grpc] extra.
• No FastAPI required — build_jsonrpc/build_rest already return a Starlette ASGI app; serve it with uvicorn directly or mount it. Don't wrap an agent in a hand-rolled FastAPI route.
• url= is the public URL, not the bind address — it is written into the AgentCard interface entries, so clients must be able to reach it. For gRPC, bind is the listener and grpc_url is what clients connect to.
• Auth is declared, not enforced — security=[...] only advertises schemes on the card. A2A has no middleware spec; enforce auth on the returned Starlette/gRPC object yourself.
• Stateless executor — each request is a self-contained turn; the AG2 client sends its full history on every call. No sticky sessions are needed, but don't assume server-side per-task memory between turns.
• build_grpc needs grpc_url — and build_card(..., transports=("grpc",)) raises ValueError if you forget grpc_url=.

Going deeper (source of truth)

• ag2/a2a/server.py — A2AServer, the build_jsonrpc / build_rest / build_grpc builders.
• ag2/a2a/card.py — build_card and skill/security/interface resolution.
• ag2/a2a/config.py — A2AConfig, from_card, all client options.
• ag2/a2a/security.py — scheme factories and require.
• ag2/a2a/push.py / tasks.py — client-side push + task helpers.
• ag2/a2a/testing.py — in-process test factories.
• ag2/a2a/events.py — A2AEvent family for observing wire events on the AG2 stream.
• A2A protocol: https://a2a-protocol.org/

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: ag2ai
• Source: ag2ai/ag2-skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.