Agent-to-Agent Protocol (A2A)

https://{server_domain}/.well-known/agent-card.json

{
  "name": "DocsAssistant Reviewer",
  "description": "Reviews a proposed documentation fix for factual accuracy.",
  "supportedInterfaces": [
    {"url": "http://localhost:9999/a2a", "protocolBinding": "JSONRPC", "protocolVersion": "1.0"}
  ],
  "version": "1.0.0",
  "capabilities": {"streaming": false, "pushNotifications": false},
  "defaultInputModes": ["text/plain"],
  "defaultOutputModes": ["text/plain"],
  "skills": [
    {
      "id": "review-doc-fix",
      "name": "Review a documentation fix",
      "description": "Checks a proposed change to a docs page against the page's sources.",
      "tags": ["docs", "review"]
    }
  ]
}

from fastapi import FastAPI

from a2a.helpers import get_message_text, new_task_from_user_message, new_text_part
from a2a.server.agent_execution import AgentExecutor, RequestContext
from a2a.server.events import EventQueue
from a2a.server.request_handlers import DefaultRequestHandler
from a2a.server.routes import (
    add_a2a_routes_to_fastapi,
    create_agent_card_routes,
    create_jsonrpc_routes,
)
from a2a.server.tasks import InMemoryTaskStore, TaskUpdater
from a2a.types import AgentCapabilities, AgentCard, AgentInterface, AgentSkill


def review_page(text: str) -> str:
    """Stand-in for the reviewer agent's own LLM loop."""
    return f"Reviewed: {text[:60]}... -- no factual issues found."


class ReviewerExecutor(AgentExecutor):
    async def execute(self, context: RequestContext, event_queue: EventQueue) -> None:
        task = context.current_task or new_task_from_user_message(context.message)
        await event_queue.enqueue_event(task)
        updater = TaskUpdater(event_queue, task.id, task.context_id)
        await updater.start_work()
        verdict = review_page(get_message_text(context.message))
        await updater.add_artifact([new_text_part(verdict)], name="review")
        await updater.complete()

    async def cancel(self, context: RequestContext, event_queue: EventQueue) -> None:
        raise NotImplementedError("reviews are short; cancellation is not supported")


card = AgentCard(
    name="DocsAssistant Reviewer",
    description="Reviews a proposed documentation fix for factual accuracy.",
    version="1.0.0",
    supported_interfaces=[
        AgentInterface(
            url="http://localhost:9999/a2a",
            protocol_binding="JSONRPC",
            protocol_version="1.0",
        )
    ],
    capabilities=AgentCapabilities(streaming=False, push_notifications=False),
    default_input_modes=["text/plain"],
    default_output_modes=["text/plain"],
    skills=[
        AgentSkill(
            id="review-doc-fix",
            name="Review a documentation fix",
            description="Checks a proposed change to a docs page against the page's sources.",
            tags=["docs", "review"],
        )
    ],
)

handler = DefaultRequestHandler(
    agent_executor=ReviewerExecutor(),
    task_store=InMemoryTaskStore(),
    agent_card=card,
)
app = FastAPI()
add_a2a_routes_to_fastapi(
    app,
    agent_card_routes=create_agent_card_routes(card),
    jsonrpc_routes=create_jsonrpc_routes(handler, rpc_url="/a2a"),
)

import dev.langchain4j.agentic.Agent;
import dev.langchain4j.agentic.AgenticServices;
import dev.langchain4j.agentic.UntypedAgent;
import dev.langchain4j.service.V;

public interface RemoteReviewer {
    @Agent("Reviews a proposed documentation fix against official sources")
    String review(@V("proposedFix") String proposedFix);
}

RemoteReviewer reviewer = AgenticServices
        .a2aBuilder("http://localhost:9999", RemoteReviewer.class)
        .outputKey("review")
        .build();

UntypedAgent draftThenReview = AgenticServices.sequenceBuilder()
        .subAgents(fixDrafter, reviewer)          // fixDrafter writes "proposedFix"
        .outputKey("review")
        .build();

import io.a2a.server.agentexecution.AgentExecutor;
import io.a2a.spec.AgentCapabilities;
import io.a2a.spec.AgentCard;
import io.a2a.spec.AgentSkill;
import org.springaicommunity.a2a.server.executor.DefaultAgentExecutor;
import org.springframework.ai.chat.client.ChatClient;

@Configuration
class ReviewerA2aConfig {

    @Bean
    AgentCard agentCard(@Value("${server.port:8080}") int port) {
        return new AgentCard.Builder()
                .name("DocsAssistant Reviewer")
                .description("Reviews a proposed documentation fix for factual accuracy.")
                .url("http://localhost:" + port + "/a2a/")
                .version("1.0.0")
                .capabilities(new AgentCapabilities.Builder().streaming(false).build())
                .defaultInputModes(List.of("text"))
                .defaultOutputModes(List.of("text"))
                .skills(List.of(new AgentSkill.Builder()
                        .id("review-doc-fix").name("Review a documentation fix")
                        .description("Checks a proposed change to a docs page against its sources.")
                        .tags(List.of("docs", "review")).build()))
                .protocolVersion("0.3.0")
                .build();
    }

    @Bean
    AgentExecutor agentExecutor(ChatClient.Builder builder, DocsTools docsTools) {
        ChatClient reviewer = builder.clone()
                .defaultSystem("Review the proposed documentation fix against the official sources. "
                        + "Reply APPROVE or REJECT with one line per problem.")
                .defaultTools(docsTools)
                .build();
        return new DefaultAgentExecutor(reviewer, (chat, ctx) -> chat.prompt()
                .user(DefaultAgentExecutor.extractTextFromMessage(ctx.getMessage()))
                .call()
                .content());
    }
}