Expose any agent as a spec-compliant A2A protocol endpoint (v1.0).
from langchain_adk import LlmAgent, InMemorySessionService
from langchain_adk.a2a import A2AServer, AgentSkill
agent = LlmAgent(name="MyAgent", llm=llm, tools=[...])
server = A2AServer(
agent=agent,
session_service=InMemorySessionService(),
app_name="my-agent-service",
skills=[
AgentSkill(
id="qa", name="Q&A",
description="Answers general questions.",
tags=["general"],
),
],
)
app = server.as_fastapi_app()
# uvicorn my_module:app --host 0.0.0.0 --port 8000
Endpoints
| Method | Endpoint | Description |
|---|
GET | /.well-known/agent-card.json | Agent Card discovery |
POST | / | JSON-RPC 2.0 dispatch |
JSON-RPC methods (v1.0)
| Method | Description |
|---|
SendMessage | Send message, receive completed Task |
SendStreamingMessage | Send message, receive SSE stream |
GetTask | Retrieve task by ID |
CancelTask | Cancel a running task |
The server also accepts the legacy v0.x method names (message/send, message/stream, tasks/get, tasks/cancel) for backwards compatibility.
Example requests
Discover
Send (blocking)
Stream (SSE)
curl http://localhost:8000/.well-known/agent-card.json
curl -X POST http://localhost:8000/ \
-H "Content-Type: application/json" \
-H "A2A-Version: 1.0" \
-d '{
"jsonrpc": "2.0", "id": "1",
"method": "SendMessage",
"params": {
"message": {
"role": "ROLE_USER",
"parts": [{"text": "What is 2+2?", "mediaType": "text/plain"}]
}
}
}'
curl -N -X POST http://localhost:8000/ \
-H "Content-Type: application/json" \
-H "A2A-Version: 1.0" \
-d '{
"jsonrpc": "2.0", "id": "2",
"method": "SendStreamingMessage",
"params": {
"message": {
"role": "ROLE_USER",
"parts": [{"text": "Tell me a story", "mediaType": "text/plain"}]
}
}
}'
A2A Client (A2AAgent)
Use A2AAgent to call a remote A2A server from within your agent tree:
from langchain_adk.agents.a2a_agent import A2AAgent
remote = A2AAgent(
name="RemoteAgent",
url="http://localhost:8000",
description="A remote agent via A2A protocol.",
)
# Use as a sub-agent or wrap with AgentTool
async for event in remote.astream("What is the weather?"):
if event.is_final_response():
print(event.text)
v1.0 Protocol Details
A2A v1.0 uses:
- PascalCase JSON-RPC method names (
SendMessage, not message/send)
- SCREAMING_SNAKE_CASE enum values (
ROLE_USER, TASK_STATE_COMPLETED)
- Unified Part type with oneof content fields (
text, raw, url, data) plus mediaType
supportedInterfaces on AgentCard (replaces top-level url)
