From 10545299f88c6e38233bdceff173610ebbb12f5e Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Sun, 11 Oct 2026 08:25:15 +0000
Subject: [PATCH] Add a2a module: agent card, tasks, streaming, input-required
 and agent-to-agent calls on the A2A Java SDK, served over the JDK HTTP server

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G8ikz8xdWuTP5yun8DZ1hk
---
 README.md                                     |   6 +-
 a2a/output/01-agent-card.txt                  |  40 ++++
 a2a/output/02-raw-send-message.txt            |  61 +++++
 a2a/output/03-streaming.txt                   |  15 ++
 a2a/output/04-input-required.txt              |   9 +
 a2a/output/05-agent-to-agent.txt              |  17 ++
 a2a/output/06-dependencies.txt                |  42 ++++
 a2a/output/07-blocking.txt                    |   5 +
 a2a/pom.xml                                   |  54 +++++
 .../java/com/ankurm/agents/a2a/A2aCaller.java | 160 +++++++++++++
 .../com/ankurm/agents/a2a/A2aHttpServer.java  | 224 ++++++++++++++++++
 .../com/ankurm/agents/a2a/BriefAgent.java     |  79 ++++++
 .../com/ankurm/agents/a2a/GlossaryAgent.java  |  70 ++++++
 .../a2a/JdkJsonRpcTransportMetadata.java      |  16 ++
 .../services/io.a2a.server.TransportMetadata  |   1 +
 .../java/com/ankurm/agents/a2a/A2aTest.java   | 203 ++++++++++++++++
 .../com/ankurm/agents/a2a/Transcript.java     |  39 +++
 .../test/resources/simplelogger.properties    |   1 +
 pom.xml                                       |   2 +
 19 files changed, 1042 insertions(+), 2 deletions(-)
 create mode 100644 a2a/output/01-agent-card.txt
 create mode 100644 a2a/output/02-raw-send-message.txt
 create mode 100644 a2a/output/03-streaming.txt
 create mode 100644 a2a/output/04-input-required.txt
 create mode 100644 a2a/output/05-agent-to-agent.txt
 create mode 100644 a2a/output/06-dependencies.txt
 create mode 100644 a2a/output/07-blocking.txt
 create mode 100644 a2a/pom.xml
 create mode 100644 a2a/src/main/java/com/ankurm/agents/a2a/A2aCaller.java
 create mode 100644 a2a/src/main/java/com/ankurm/agents/a2a/A2aHttpServer.java
 create mode 100644 a2a/src/main/java/com/ankurm/agents/a2a/BriefAgent.java
 create mode 100644 a2a/src/main/java/com/ankurm/agents/a2a/GlossaryAgent.java
 create mode 100644 a2a/src/main/java/com/ankurm/agents/a2a/JdkJsonRpcTransportMetadata.java
 create mode 100644 a2a/src/main/resources/META-INF/services/io.a2a.server.TransportMetadata
 create mode 100644 a2a/src/test/java/com/ankurm/agents/a2a/A2aTest.java
 create mode 100644 a2a/src/test/java/com/ankurm/agents/a2a/Transcript.java
 create mode 100644 a2a/src/test/resources/simplelogger.properties

diff --git a/README.md b/README.md
index 63b7281..0e30b50 100644
--- a/README.md
+++ b/README.md
@@ -7,12 +7,14 @@ facts, so the build fails when a claim stops being true.
 | Module | Post | What it shows |
 |---|---|---|
 | `embabel` | Embabel: Goal-Oriented AI Agents on the JVM | actions, goals, conditions and cost-based planning with GOAP, compared with plain Spring AI |
+| `a2a` | A2A Protocol in Java: Agents That Talk to Each Other | agent card, tasks, streaming, input-required and agent-to-agent calls with the A2A Java SDK |
 
 ## Versions (verified on Maven Central, 2026-10-11)
 
 | Component | Version | Note |
 |---|---|---|
 | Embabel `embabel-agent-api` | 1.5.3 | brings Spring AI 2.0.1 and Spring Boot 4.1.1 transitively |
+| A2A Java SDK `io.github.a2asdk` | 1.0.0.Alpha3 | alpha; targets A2A protocol 1.0. The newest stable release, 0.3.3.Final, targets the 0.3 protocol and its API differs |
 | JDK | 25 LTS | |
 | JUnit | 6.1.3 | |
 
@@ -21,9 +23,9 @@ facts, so the build fails when a claim stops being true.
 ```bash
 export JAVA_HOME=/path/to/jdk-25
 mvn test                      # everything
-mvn test -pl embabel -am      # one module
+mvn test -pl a2a -am          # one module
 ```
 
-No API key is needed. The Embabel tests use `ScriptedLlmOperations`, the scripted stand-in shipped inside
+No API key is needed. The A2A agents contain no model call, because the protocol is the subject. The Embabel tests use `ScriptedLlmOperations`, the scripted stand-in shipped inside
 `embabel-agent-api`, and the Spring AI comparison uses a small scripted `ChatModel`.
 Each module writes its transcripts to `<module>/output/` when you run the tests.
diff --git a/a2a/output/01-agent-card.txt b/a2a/output/01-agent-card.txt
new file mode 100644
index 0000000..960aa82
--- /dev/null
+++ b/a2a/output/01-agent-card.txt
@@ -0,0 +1,40 @@
+GET /.well-known/agent-card.json
+
+{
+  "name": "Glossary Agent",
+  "description": "Defines Java terms",
+  "version": "1.0.0",
+  "capabilities": {
+    "streaming": true,
+    "pushNotifications": false,
+    "extendedAgentCard": false
+  },
+  "defaultInputModes": [
+    "text/plain"
+  ],
+  "defaultOutputModes": [
+    "text/plain"
+  ],
+  "skills": [
+    {
+      "id": "define",
+      "name": "Define a term",
+      "description": "Returns a one-sentence definition",
+      "tags": [
+        "java",
+        "glossary"
+      ],
+      "examples": [
+        "record"
+      ]
+    }
+  ],
+  "supportedInterfaces": [
+    {
+      "protocolBinding": "JSONRPC",
+      "url": "http://127.0.0.1:PORT",
+      "tenant": "",
+      "protocolVersion": "1.0"
+    }
+  ]
+}
diff --git a/a2a/output/02-raw-send-message.txt b/a2a/output/02-raw-send-message.txt
new file mode 100644
index 0000000..286853d
--- /dev/null
+++ b/a2a/output/02-raw-send-message.txt
@@ -0,0 +1,61 @@
+POST /
+{
+  "jsonrpc": "2.0",
+  "id": "1",
+  "method": "SendMessage",
+  "params": {
+    "message": {
+      "messageId": "m-1",
+      "role": "ROLE_USER",
+      "parts": [
+        {
+          "text": "record"
+        }
+      ]
+    },
+    "configuration": {
+      "blocking": true
+    }
+  }
+}
+
+Response:
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "result": {
+    "task": {
+      "id": "<id-1>",
+      "contextId": "<id-2>",
+      "status": {
+        "state": "TASK_STATE_COMPLETED",
+        "timestamp": "<time>"
+      },
+      "artifacts": [
+        {
+          "artifactId": "definition",
+          "name": "definition",
+          "description": "",
+          "parts": [
+            {
+              "text": "A record is a final class that is a transparent",
+              "metadata": {},
+              "filename": "",
+              "mediaType": ""
+            },
+            {
+              "text": " carrier for its components.",
+              "metadata": {},
+              "filename": "",
+              "mediaType": ""
+            }
+          ],
+          "metadata": {},
+          "extensions": []
+        }
+      ],
+      "history": [],
+      "metadata": {}
+    }
+  }
+}
diff --git a/a2a/output/03-streaming.txt b/a2a/output/03-streaming.txt
new file mode 100644
index 0000000..0f6a265
--- /dev/null
+++ b/a2a/output/03-streaming.txt
@@ -0,0 +1,15 @@
+Same question, "record", asked twice.
+
+Blocking (streaming off): 1 event
+  TaskEvent         state=COMPLETED artifacts=1
+  answer: A record is a final class that is a transparent carrier for its components.
+
+Streaming: 5 events
+  TaskStatusUpdate  state=SUBMITTED final=false
+  TaskStatusUpdate  state=WORKING final=false
+  TaskArtifactUpdate append=false lastChunk=false text="A record is a final class that is a transparent"
+  TaskArtifactUpdate append=true lastChunk=true text=" carrier for its components."
+  TaskStatusUpdate  state=COMPLETED final=true
+  answer: A record is a final class that is a transparent carrier for its components.
+
+GetTask for the streamed task afterwards: state TASK_STATE_COMPLETED, artifacts 1, artifact text "A record is a final class that is a transparent carrier for its components."
diff --git a/a2a/output/04-input-required.txt b/a2a/output/04-input-required.txt
new file mode 100644
index 0000000..021bd86
--- /dev/null
+++ b/a2a/output/04-input-required.txt
@@ -0,0 +1,9 @@
+Turn 1: message with no term.
+  task <id-1> state INPUT_REQUIRED
+  agent says: Which term should I define?
+
+Turn 2: message "sealed class" sent with taskId <id-1>.
+  task <id-1> state COMPLETED
+  answer: A sealed class restricts which other classes may extend it.
+
+Same task id both turns: true
diff --git a/a2a/output/05-agent-to-agent.txt b/a2a/output/05-agent-to-agent.txt
new file mode 100644
index 0000000..b1ed021
--- /dev/null
+++ b/a2a/output/05-agent-to-agent.txt
@@ -0,0 +1,17 @@
+Caller -> Brief Agent -> Glossary Agent.
+
+Request: "record, sealed class"
+  Brief Agent task state: COMPLETED
+  result:
+    - record: A record is a final class that is a transparent carrier for its components.
+    - sealed class: A sealed class restricts which other classes may extend it.
+  Calls the Brief Agent made to the Glossary Agent:
+    SendMessage "record" -> COMPLETED "A record is a final class that is a transparent carrier for its components."
+    SendMessage "sealed class" -> COMPLETED "A sealed class restricts which other classes may extend it."
+
+Request: "record, monad"
+  Brief Agent task state: FAILED
+  message: Glossary Agent could not help: I do not know the term 'monad'.
+  Calls the Brief Agent made to the Glossary Agent:
+    SendMessage "record" -> COMPLETED "A record is a final class that is a transparent carrier for its components."
+    SendMessage "monad" -> FAILED "I do not know the term 'monad'."
diff --git a/a2a/output/06-dependencies.txt b/a2a/output/06-dependencies.txt
new file mode 100644
index 0000000..3176859
--- /dev/null
+++ b/a2a/output/06-dependencies.txt
@@ -0,0 +1,42 @@
+mvn dependency:tree for the a2a module (compile scope):
+com.ankurm.agents:a2a:jar:1.0.0
+io.github.a2asdk:a2a-java-sdk-server-common:jar:1.0.0.Alpha3:compile
+io.github.a2asdk:a2a-java-sdk-spec:jar:1.0.0.Alpha3:compile
+io.github.a2asdk:a2a-java-sdk-jsonrpc-common:jar:1.0.0.Alpha3:compile
+io.github.a2asdk:a2a-java-sdk-spec-grpc:jar:1.0.0.Alpha3:compile
+com.google.protobuf:protobuf-java:jar:4.33.1:compile
+com.google.api.grpc:proto-google-common-protos:jar:2.63.1:compile
+io.github.a2asdk:a2a-java-sdk-http-client:jar:1.0.0.Alpha3:compile
+io.smallrye.reactive:mutiny-zero:jar:1.1.1:compile
+jakarta.enterprise:jakarta.enterprise.cdi-api:jar:4.1.0:compile
+jakarta.enterprise:jakarta.enterprise.lang-model:jar:4.1.0:compile
+jakarta.annotation:jakarta.annotation-api:jar:3.0.0:compile
+jakarta.el:jakarta.el-api:jar:6.0.0:compile
+jakarta.interceptor:jakarta.interceptor-api:jar:2.2.0:compile
+jakarta.inject:jakarta.inject-api:jar:2.0.1:compile
+org.slf4j:slf4j-api:jar:2.0.17:compile
+io.github.a2asdk:a2a-java-sdk-transport-jsonrpc:jar:1.0.0.Alpha3:compile
+com.google.protobuf:protobuf-java-util:jar:4.32.1:compile
+com.google.code.findbugs:jsr305:jar:3.0.2:runtime
+com.google.errorprone:error_prone_annotations:jar:2.18.0:compile
+com.google.guava:guava:jar:32.0.1-jre:runtime
+com.google.guava:failureaccess:jar:1.0.1:runtime
+com.google.guava:listenablefuture:jar:9999.0-empty-to-avoid-conflict-with-guava:runtime
+com.google.j2objc:j2objc-annotations:jar:2.8:runtime
+com.google.code.gson:gson:jar:2.13.2:compile
+io.github.a2asdk:a2a-java-sdk-client:jar:1.0.0.Alpha3:compile
+io.github.a2asdk:a2a-java-sdk-client-transport-spi:jar:1.0.0.Alpha3:compile
+io.github.a2asdk:a2a-java-sdk-common:jar:1.0.0.Alpha3:compile
+io.github.a2asdk:a2a-java-sdk-client-transport-jsonrpc:jar:1.0.0.Alpha3:compile
+org.slf4j:slf4j-simple:jar:2.0.17:test
+org.junit.jupiter:junit-jupiter:jar:6.1.3:test
+org.junit.jupiter:junit-jupiter-api:jar:6.1.3:test
+org.opentest4j:opentest4j:jar:1.3.0:test
+org.junit.platform:junit-platform-commons:jar:6.1.3:test
+org.apiguardian:apiguardian-api:jar:1.1.2:test
+org.jspecify:jspecify:jar:1.0.0:test
+org.junit.jupiter:junit-jupiter-params:jar:6.1.3:test
+org.junit.jupiter:junit-jupiter-engine:jar:6.1.3:test
+org.junit.platform:junit-platform-engine:jar:6.1.3:test
+org.assertj:assertj-core:jar:3.27.6:test
+net.bytebuddy:byte-buddy:jar:1.17.7:test
diff --git a/a2a/output/07-blocking.txt b/a2a/output/07-blocking.txt
new file mode 100644
index 0000000..1cc9900
--- /dev/null
+++ b/a2a/output/07-blocking.txt
@@ -0,0 +1,5 @@
+The agent accepts the task at once, then works for 400 ms before finishing.
+
+SendMessage with configuration.blocking = false: task state TASK_STATE_WORKING, returned in under 400 ms: true
+SendMessage with configuration.blocking = true: task state TASK_STATE_COMPLETED, returned in at least 400 ms: true
+SendMessage with no configuration at all: task state TASK_STATE_WORKING, returned in under 400 ms: true
diff --git a/a2a/pom.xml b/a2a/pom.xml
new file mode 100644
index 0000000..d59eaae
--- /dev/null
+++ b/a2a/pom.xml
@@ -0,0 +1,54 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>com.ankurm.agents</groupId>
+    <artifactId>java-ai-agents</artifactId>
+    <version>1.0.0</version>
+  </parent>
+
+  <artifactId>a2a</artifactId>
+
+  <dependencies>
+    <dependency>
+      <groupId>io.github.a2asdk</groupId>
+      <artifactId>a2a-java-sdk-server-common</artifactId>
+      <version>${a2a.sdk.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>io.github.a2asdk</groupId>
+      <artifactId>a2a-java-sdk-transport-jsonrpc</artifactId>
+      <version>${a2a.sdk.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>io.github.a2asdk</groupId>
+      <artifactId>a2a-java-sdk-client</artifactId>
+      <version>${a2a.sdk.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>io.github.a2asdk</groupId>
+      <artifactId>a2a-java-sdk-client-transport-jsonrpc</artifactId>
+      <version>${a2a.sdk.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>org.slf4j</groupId>
+      <artifactId>slf4j-simple</artifactId>
+      <version>${slf4j.version}</version>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+      <version>${assertj.version}</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>
diff --git a/a2a/src/main/java/com/ankurm/agents/a2a/A2aCaller.java b/a2a/src/main/java/com/ankurm/agents/a2a/A2aCaller.java
new file mode 100644
index 0000000..3aa9c2b
--- /dev/null
+++ b/a2a/src/main/java/com/ankurm/agents/a2a/A2aCaller.java
@@ -0,0 +1,160 @@
+package com.ankurm.agents.a2a;
+
+import io.a2a.A2A;
+import io.a2a.client.Client;
+import io.a2a.client.ClientEvent;
+import io.a2a.client.MessageEvent;
+import io.a2a.client.TaskEvent;
+import io.a2a.client.TaskUpdateEvent;
+import io.a2a.client.config.ClientConfig;
+import io.a2a.client.transport.jsonrpc.JSONRPCTransport;
+import io.a2a.client.transport.jsonrpc.JSONRPCTransportConfigBuilder;
+import io.a2a.spec.AgentCard;
+import io.a2a.spec.Artifact;
+import io.a2a.spec.Message;
+import io.a2a.spec.Part;
+import io.a2a.spec.Task;
+import io.a2a.spec.TaskArtifactUpdateEvent;
+import io.a2a.spec.TaskState;
+import io.a2a.spec.TaskStatusUpdateEvent;
+import io.a2a.spec.TextPart;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.UUID;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+
+/** One call to a remote A2A agent: fetch its card, send a message, collect what comes back until the task settles. */
+public final class A2aCaller {
+
+    /** What came back: every event in order, and the last task snapshot seen. */
+    public record Reply(List<ClientEvent> events, Task task, Message message) {
+
+        public String text() {
+            if (task != null && task.artifacts() != null && !task.artifacts().isEmpty()) {
+                StringBuilder sb = new StringBuilder();
+                for (Artifact a : task.artifacts()) {
+                    for (Part<?> p : a.parts()) {
+                        if (p instanceof TextPart t) sb.append(t.text());
+                    }
+                }
+                return sb.toString();
+            }
+            if (task != null && task.status().message() != null) {
+                return textOf(task.status().message());
+            }
+            return message == null ? "" : textOf(message);
+        }
+    }
+
+    private A2aCaller() {
+    }
+
+    public static AgentCard fetchCard(String agentUrl) throws Exception {
+        return A2A.getAgentCard(agentUrl);
+    }
+
+    public static Reply send(String agentUrl, String text, String continueTaskId, boolean streaming) throws Exception {
+        AgentCard card = fetchCard(agentUrl);
+        List<ClientEvent> events = Collections.synchronizedList(new ArrayList<>());
+        AtomicReference<Task> task = new AtomicReference<>();
+        AtomicReference<Message> message = new AtomicReference<>();
+        AtomicReference<Throwable> error = new AtomicReference<>();
+        CountDownLatch settled = new CountDownLatch(1);
+        // The client cancels the SSE request itself once it sees a final state; that cancel comes back as an
+        // error, so an error only counts when no final state was seen first.
+        java.util.concurrent.atomic.AtomicBoolean seenFinal = new java.util.concurrent.atomic.AtomicBoolean();
+
+        Client client = Client.builder(card)
+                .withTransport(JSONRPCTransport.class, new JSONRPCTransportConfigBuilder())
+                .clientConfig(ClientConfig.builder().setStreaming(streaming).build())
+                .addConsumer((event, c) -> {
+                    events.add(event);
+                    if (event instanceof TaskEvent te) {
+                        task.set(te.getTask());
+                        if (stops(te.getTask().status().state())) { seenFinal.set(true); settled.countDown(); }
+                    } else if (event instanceof TaskUpdateEvent tu) {
+                        task.set(tu.getTask());
+                        if (stops(tu.getTask().status().state())) { seenFinal.set(true); settled.countDown(); }
+                    } else if (event instanceof MessageEvent me) {
+                        message.set(me.getMessage());
+                        seenFinal.set(true);
+                        settled.countDown();
+                    }
+                })
+                .streamingErrorHandler(t -> {
+                    error.set(t);
+                    settled.countDown();
+                })
+                .build();
+        try {
+            Message.Builder m = Message.builder().role(Message.Role.ROLE_USER)
+                    .parts(new TextPart(text)).messageId(UUID.randomUUID().toString());
+            if (continueTaskId != null) {
+                m.taskId(continueTaskId);
+            }
+            client.sendMessage(m.build());
+            if (!settled.await(20, TimeUnit.SECONDS)) {
+                throw new IllegalStateException("Timed out waiting for " + agentUrl);
+            }
+            if (error.get() != null && !seenFinal.get()) {
+                throw new IllegalStateException("Streaming failed", error.get());
+            }
+            return new Reply(List.copyOf(events), task.get(), message.get());
+        } finally {
+            client.close();
+        }
+    }
+
+    /** Asks the remote agent for a task it already knows about. */
+    public static Task getTask(String agentUrl, String taskId) throws Exception {
+        Client client = Client.builder(fetchCard(agentUrl))
+                .withTransport(JSONRPCTransport.class, new JSONRPCTransportConfigBuilder())
+                .build();
+        try {
+            return client.getTask(new io.a2a.spec.TaskQueryParams(taskId), null);
+        } finally {
+            client.close();
+        }
+    }
+
+    private static boolean stops(TaskState s) {
+        return s.isFinal() || s == TaskState.TASK_STATE_INPUT_REQUIRED || s == TaskState.TASK_STATE_AUTH_REQUIRED;
+    }
+
+    static String textOf(Message m) {
+        StringBuilder sb = new StringBuilder();
+        for (Part<?> p : m.parts()) {
+            if (p instanceof TextPart t) sb.append(t.text());
+        }
+        return sb.toString();
+    }
+
+    /** A one-line description of an event, for transcripts. */
+    public static String describe(ClientEvent e) {
+        if (e instanceof TaskEvent te) {
+            return "TaskEvent         state=" + short_(te.getTask().status().state()) + " artifacts=" + te.getTask().artifacts().size();
+        }
+        if (e instanceof MessageEvent me) {
+            return "MessageEvent      text=\"" + textOf(me.getMessage()) + "\"";
+        }
+        if (e instanceof TaskUpdateEvent tu) {
+            if (tu.getUpdateEvent() instanceof TaskStatusUpdateEvent s) {
+                return "TaskStatusUpdate  state=" + short_(s.status().state()) + " final=" + s.isFinal();
+            }
+            if (tu.getUpdateEvent() instanceof TaskArtifactUpdateEvent a) {
+                StringBuilder sb = new StringBuilder();
+                for (Part<?> p : a.artifact().parts()) if (p instanceof TextPart t) sb.append(t.text());
+                return "TaskArtifactUpdate append=" + a.append() + " lastChunk=" + a.lastChunk() + " text=\"" + sb + "\"";
+            }
+        }
+        return e.getClass().getSimpleName();
+    }
+
+    static String short_(TaskState s) {
+        return s.name().replace("TASK_STATE_", "");
+    }
+}
diff --git a/a2a/src/main/java/com/ankurm/agents/a2a/A2aHttpServer.java b/a2a/src/main/java/com/ankurm/agents/a2a/A2aHttpServer.java
new file mode 100644
index 0000000..2e09814
--- /dev/null
+++ b/a2a/src/main/java/com/ankurm/agents/a2a/A2aHttpServer.java
@@ -0,0 +1,224 @@
+package com.ankurm.agents.a2a;
+
+import com.sun.net.httpserver.HttpExchange;
+import com.sun.net.httpserver.HttpServer;
+import io.a2a.grpc.utils.JSONRPCUtils;
+import io.a2a.grpc.utils.ProtoUtils;
+import io.a2a.jsonrpc.common.json.JsonUtil;
+import io.a2a.jsonrpc.common.wrappers.*;
+import io.a2a.server.ServerCallContext;
+import io.a2a.server.agentexecution.AgentExecutor;
+import io.a2a.server.auth.UnauthenticatedUser;
+import io.a2a.server.events.InMemoryQueueManager;
+import io.a2a.server.events.MainEventBus;
+import io.a2a.server.events.MainEventBusProcessor;
+import io.a2a.server.requesthandlers.DefaultRequestHandler;
+import io.a2a.server.tasks.BasePushNotificationSender;
+import io.a2a.server.tasks.InMemoryPushNotificationConfigStore;
+import io.a2a.server.tasks.InMemoryTaskStore;
+import io.a2a.server.util.sse.SseFormatter;
+import io.a2a.spec.A2AError;
+import io.a2a.spec.AgentCard;
+import io.a2a.spec.InternalError;
+import io.a2a.spec.TransportProtocol;
+import io.a2a.transport.jsonrpc.handler.JSONRPCHandler;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.net.InetSocketAddress;
+import java.nio.charset.StandardCharsets;
+import java.util.HashMap;
+import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Flow;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Function;
+
+/**
+ * Serves one A2A agent over JSON-RPC using only the JDK's built-in HTTP server. The SDK ships the protocol
+ * logic (tasks, queues, streaming) and a Quarkus binding; this class is the small binding that replaces Quarkus.
+ * The request routing follows the SDK's own Quarkus route class.
+ */
+public final class A2aHttpServer implements AutoCloseable {
+
+    private final HttpServer http;
+    private final JSONRPCHandler rpc;
+    private final AgentCard card;
+    private final ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
+    private final MainEventBusProcessor processor;
+
+    private A2aHttpServer(HttpServer http, JSONRPCHandler rpc, AgentCard card, MainEventBusProcessor processor) {
+        this.http = http;
+        this.processor = processor;
+        this.rpc = rpc;
+        this.card = card;
+    }
+
+    /** Starts on a free port. The card is built after the port is known, so it can carry the real URL. */
+    public static A2aHttpServer start(Function<String, AgentCard> cardForUrl, AgentExecutor agent) throws IOException {
+        HttpServer http = HttpServer.create(new InetSocketAddress("127.0.0.1", 0), 0);
+        String url = "http://127.0.0.1:" + http.getAddress().getPort();
+        AgentCard card = cardForUrl.apply(url);
+
+        ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
+        MainEventBus bus = new MainEventBus();
+        InMemoryTaskStore taskStore = new InMemoryTaskStore();
+        InMemoryQueueManager queues = new InMemoryQueueManager(taskStore, bus);
+        InMemoryPushNotificationConfigStore pushConfigs = new InMemoryPushNotificationConfigStore();
+        MainEventBusProcessor processor = new MainEventBusProcessor(bus, taskStore, new BasePushNotificationSender(pushConfigs), queues);
+        callPackagePrivate(processor, "start");
+        DefaultRequestHandler handler = DefaultRequestHandler.create(agent, taskStore, queues, pushConfigs, processor, executor, executor);
+        JSONRPCHandler rpc = new JSONRPCHandler(card, handler, executor);
+
+        A2aHttpServer server = new A2aHttpServer(http, rpc, card, processor);
+        http.createContext("/", server::route);
+        http.setExecutor(server.executor);
+        http.start();
+        return server;
+    }
+
+    public String url() {
+        return "http://127.0.0.1:" + http.getAddress().getPort();
+    }
+
+    public AgentCard card() {
+        return card;
+    }
+
+    private void route(HttpExchange x) throws IOException {
+        try (x) {
+            String path = x.getRequestURI().getPath();
+            if (x.getRequestMethod().equals("GET") && path.equals("/.well-known/agent-card.json")) {
+                reply(x, JsonUtil.toJson(card));
+            } else if (x.getRequestMethod().equals("POST") && path.equals("/")) {
+                jsonRpc(x, new String(x.getRequestBody().readAllBytes(), StandardCharsets.UTF_8));
+            } else {
+                x.sendResponseHeaders(404, -1);
+            }
+        } catch (Exception e) {
+            throw new IOException(e);
+        }
+    }
+
+    private void jsonRpc(HttpExchange x, String body) throws Exception {
+        ServerCallContext context = new ServerCallContext(UnauthenticatedUser.INSTANCE,
+                new HashMap<>(java.util.Map.of(ServerCallContext.TRANSPORT_KEY, TransportProtocol.JSONRPC)), Set.of());
+        A2AResponse<?> response = null;
+        Flow.Publisher<? extends A2AResponse<?>> stream = null;
+        try {
+            A2ARequest<?> request = JSONRPCUtils.parseRequestBody(body, "");
+            if (request instanceof SendMessageRequest r) {
+                response = rpc.onMessageSend(r, context);
+            } else if (request instanceof GetTaskRequest r) {
+                response = rpc.onGetTask(r, context);
+            } else if (request instanceof ListTasksRequest r) {
+                response = rpc.onListTasks(r, context);
+            } else if (request instanceof CancelTaskRequest r) {
+                response = rpc.onCancelTask(r, context);
+            } else if (request instanceof SendStreamingMessageRequest r) {
+                stream = rpc.onMessageSendStream(r, context);
+            } else if (request instanceof SubscribeToTaskRequest r) {
+                stream = rpc.onSubscribeToTask(r, context);
+            } else {
+                response = new A2AErrorResponse(request.getId(), new io.a2a.spec.UnsupportedOperationError());
+            }
+        } catch (A2AError e) {
+            response = new A2AErrorResponse(e);
+        } catch (Exception e) {
+            response = new A2AErrorResponse(new InternalError(e.getMessage()));
+        }
+        if (stream != null) {
+            sse(x, stream);
+        } else {
+            reply(x, serialize(response));
+        }
+    }
+
+    private static String serialize(A2AResponse<?> response) {
+        if (response instanceof A2AErrorResponse error) {
+            return JSONRPCUtils.toJsonRPCErrorResponse(error.getId(), error.getError());
+        }
+        if (response.getError() != null) {
+            return JSONRPCUtils.toJsonRPCErrorResponse(response.getId(), response.getError());
+        }
+        com.google.protobuf.MessageOrBuilder proto;
+        if (response instanceof GetTaskResponse r) {
+            proto = ProtoUtils.ToProto.task(r.getResult());
+        } else if (response instanceof CancelTaskResponse r) {
+            proto = ProtoUtils.ToProto.task(r.getResult());
+        } else if (response instanceof SendMessageResponse r) {
+            proto = ProtoUtils.ToProto.taskOrMessage(r.getResult());
+        } else if (response instanceof ListTasksResponse r) {
+            proto = ProtoUtils.ToProto.listTasksResult(r.getResult());
+        } else {
+            throw new IllegalArgumentException("Unsupported response: " + response.getClass().getName());
+        }
+        return JSONRPCUtils.toJsonRPCResultResponse(response.getId(), proto);
+    }
+
+    private void sse(HttpExchange x, Flow.Publisher<? extends A2AResponse<?>> publisher) throws IOException, InterruptedException {
+        x.getResponseHeaders().add("Content-Type", "text/event-stream");
+        x.sendResponseHeaders(200, 0);
+        OutputStream out = x.getResponseBody();
+        CountDownLatch done = new CountDownLatch(1);
+        AtomicLong id = new AtomicLong();
+        publisher.subscribe(new Flow.Subscriber<A2AResponse<?>>() {
+            @Override
+            public void onSubscribe(Flow.Subscription s) {
+                s.request(Long.MAX_VALUE);
+            }
+
+            @Override
+            public void onNext(A2AResponse<?> item) {
+                try {
+                    out.write(SseFormatter.formatResponseAsSSE(item, id.getAndIncrement()).getBytes(StandardCharsets.UTF_8));
+                    out.flush();
+                } catch (IOException e) {
+                    done.countDown();
+                }
+            }
+
+            @Override
+            public void onError(Throwable t) {
+                done.countDown();
+            }
+
+            @Override
+            public void onComplete() {
+                done.countDown();
+            }
+        });
+        done.await();
+    }
+
+    private static void reply(HttpExchange x, String json) throws IOException {
+        byte[] bytes = json.getBytes(StandardCharsets.UTF_8);
+        x.getResponseHeaders().add("Content-Type", "application/json");
+        x.sendResponseHeaders(200, bytes.length);
+        x.getResponseBody().write(bytes);
+    }
+
+    /**
+     * The SDK starts its event-bus thread from a CDI lifecycle callback, and the public ensureStarted() does nothing
+     * outside CDI. Without this thread every event is accepted and none is delivered, and the client gets
+     * "Could not find a Task/Message". start() and stop() are package-private, so this calls them by reflection.
+     */
+    private static void callPackagePrivate(MainEventBusProcessor processor, String method) {
+        try {
+            var m = MainEventBusProcessor.class.getDeclaredMethod(method);
+            m.setAccessible(true);
+            m.invoke(processor);
+        } catch (ReflectiveOperationException e) {
+            throw new IllegalStateException(e);
+        }
+    }
+
+    @Override
+    public void close() {
+        callPackagePrivate(processor, "stop");
+        http.stop(0);
+        executor.shutdownNow();
+    }
+}
diff --git a/a2a/src/main/java/com/ankurm/agents/a2a/BriefAgent.java b/a2a/src/main/java/com/ankurm/agents/a2a/BriefAgent.java
new file mode 100644
index 0000000..a587b0f
--- /dev/null
+++ b/a2a/src/main/java/com/ankurm/agents/a2a/BriefAgent.java
@@ -0,0 +1,79 @@
+package com.ankurm.agents.a2a;
+
+import io.a2a.server.agentexecution.AgentExecutor;
+import io.a2a.server.agentexecution.RequestContext;
+import io.a2a.server.tasks.AgentEmitter;
+import io.a2a.spec.A2AError;
+import io.a2a.spec.AgentCapabilities;
+import io.a2a.spec.AgentCard;
+import io.a2a.spec.AgentInterface;
+import io.a2a.spec.AgentSkill;
+import io.a2a.spec.Part;
+import io.a2a.spec.TaskState;
+import io.a2a.spec.TextPart;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+/**
+ * Agent B: turns a comma-separated list of terms into a short brief. It does not know the definitions. For each
+ * term it asks Agent A (the glossary) over A2A, so one agent is the server for its caller and the client of another.
+ */
+public final class BriefAgent implements AgentExecutor {
+
+    private final String glossaryUrl;
+    /** What this agent sent to the glossary and what came back, for the transcript. */
+    public final List<String> callLog = new CopyOnWriteArrayList<>();
+
+    public BriefAgent(String glossaryUrl) {
+        this.glossaryUrl = glossaryUrl;
+    }
+
+    public static AgentCard card(String url) {
+        return AgentCard.builder()
+                .name("Brief Agent")
+                .description("Writes a short brief from a list of Java terms by asking the Glossary Agent")
+                .version("1.0.0")
+                .supportedInterfaces(List.of(new AgentInterface("JSONRPC", url)))
+                .capabilities(AgentCapabilities.builder().streaming(true).build())
+                .defaultInputModes(List.of("text/plain"))
+                .defaultOutputModes(List.of("text/plain"))
+                .skills(List.of(AgentSkill.builder()
+                        .id("brief").name("Brief on several terms").description("Comma-separated terms in, bullet list out")
+                        .tags(List.of("java", "summary")).examples(List.of("record, sealed class")).build()))
+                .build();
+    }
+
+    @Override
+    public void execute(RequestContext context, AgentEmitter emitter) throws A2AError {
+        emitter.submit();
+        emitter.startWork();
+        List<String> lines = new ArrayList<>();
+        for (String term : context.getUserInput(" ").split(",")) {
+            term = term.strip();
+            try {
+                A2aCaller.Reply reply = A2aCaller.send(glossaryUrl, term, null, false);
+                TaskState state = reply.task().status().state();
+                callLog.add("SendMessage \"" + term + "\" -> " + A2aCaller.short_(state) + " \"" + reply.text() + "\"");
+                if (state != TaskState.TASK_STATE_COMPLETED) {
+                    emitter.fail(emitter.newAgentMessage(
+                            List.<Part<?>>of(new TextPart("Glossary Agent could not help: " + reply.text())), Map.of()));
+                    return;
+                }
+                lines.add("- " + term + ": " + reply.text());
+            } catch (Exception e) {
+                emitter.fail(emitter.newAgentMessage(List.<Part<?>>of(new TextPart("Glossary Agent unreachable: " + e.getMessage())), Map.of()));
+                return;
+            }
+        }
+        emitter.addArtifact(List.<Part<?>>of(new TextPart(String.join("\n", lines))), "brief", "brief", Map.of(), false, true);
+        emitter.complete();
+    }
+
+    @Override
+    public void cancel(RequestContext context, AgentEmitter emitter) throws A2AError {
+        emitter.cancel();
+    }
+}
diff --git a/a2a/src/main/java/com/ankurm/agents/a2a/GlossaryAgent.java b/a2a/src/main/java/com/ankurm/agents/a2a/GlossaryAgent.java
new file mode 100644
index 0000000..2aa7adb
--- /dev/null
+++ b/a2a/src/main/java/com/ankurm/agents/a2a/GlossaryAgent.java
@@ -0,0 +1,70 @@
+package com.ankurm.agents.a2a;
+
+import io.a2a.server.agentexecution.AgentExecutor;
+import io.a2a.server.agentexecution.RequestContext;
+import io.a2a.server.tasks.AgentEmitter;
+import io.a2a.spec.A2AError;
+import io.a2a.spec.AgentCapabilities;
+import io.a2a.spec.AgentCard;
+import io.a2a.spec.AgentInterface;
+import io.a2a.spec.AgentSkill;
+import io.a2a.spec.Part;
+import io.a2a.spec.TextPart;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Agent A: defines Java terms. It does no model call, because the point is the protocol. A real agent
+ * would put an LLM or a database behind execute().
+ */
+public final class GlossaryAgent implements AgentExecutor {
+
+    static final Map<String, String> TERMS = Map.of(
+            "record", "A record is a final class that is a transparent carrier for its components.",
+            "sealed class", "A sealed class restricts which other classes may extend it.",
+            "virtual thread", "A virtual thread is a lightweight thread scheduled by the JVM, not the OS.");
+
+    public static AgentCard card(String url) {
+        return AgentCard.builder()
+                .name("Glossary Agent")
+                .description("Defines Java terms")
+                .version("1.0.0")
+                .supportedInterfaces(List.of(new AgentInterface("JSONRPC", url)))
+                .capabilities(AgentCapabilities.builder().streaming(true).build())
+                .defaultInputModes(List.of("text/plain"))
+                .defaultOutputModes(List.of("text/plain"))
+                .skills(List.of(AgentSkill.builder()
+                        .id("define").name("Define a term").description("Returns a one-sentence definition")
+                        .tags(List.of("java", "glossary")).examples(List.of("record")).build()))
+                .build();
+    }
+
+    @Override
+    public void execute(RequestContext context, AgentEmitter emitter) throws A2AError {
+        if (context.getTask() == null) {
+            emitter.submit();
+        }
+        String term = context.getUserInput(" ").strip().toLowerCase();
+        if (term.isEmpty()) {
+            emitter.requiresInput(emitter.newAgentMessage(List.<Part<?>>of(new TextPart("Which term should I define?")), Map.of()));
+            return;
+        }
+        emitter.startWork();
+        String definition = TERMS.get(term);
+        if (definition == null) {
+            emitter.fail(emitter.newAgentMessage(List.<Part<?>>of(new TextPart("I do not know the term '" + term + "'.")), Map.of()));
+            return;
+        }
+        // Two chunks of one artifact, so a streaming client sees the answer arrive in pieces.
+        int cut = definition.indexOf(' ', definition.length() / 2);
+        emitter.addArtifact(List.<Part<?>>of(new TextPart(definition.substring(0, cut))), "definition", "definition", Map.of(), false, false);
+        emitter.addArtifact(List.<Part<?>>of(new TextPart(definition.substring(cut))), "definition", "definition", Map.of(), true, true);
+        emitter.complete();
+    }
+
+    @Override
+    public void cancel(RequestContext context, AgentEmitter emitter) throws A2AError {
+        emitter.cancel();
+    }
+}
diff --git a/a2a/src/main/java/com/ankurm/agents/a2a/JdkJsonRpcTransportMetadata.java b/a2a/src/main/java/com/ankurm/agents/a2a/JdkJsonRpcTransportMetadata.java
new file mode 100644
index 0000000..57197c6
--- /dev/null
+++ b/a2a/src/main/java/com/ankurm/agents/a2a/JdkJsonRpcTransportMetadata.java
@@ -0,0 +1,16 @@
+package com.ankurm.agents.a2a;
+
+import io.a2a.server.TransportMetadata;
+import io.a2a.spec.TransportProtocol;
+
+/**
+ * Tells the SDK that this application serves JSON-RPC. The SDK checks an agent card against the transports it
+ * can discover, and the Quarkus module normally registers one; since this demo has no Quarkus, it registers its own.
+ */
+public class JdkJsonRpcTransportMetadata implements TransportMetadata {
+
+    @Override
+    public String getTransportProtocol() {
+        return TransportProtocol.JSONRPC.asString();
+    }
+}
diff --git a/a2a/src/main/resources/META-INF/services/io.a2a.server.TransportMetadata b/a2a/src/main/resources/META-INF/services/io.a2a.server.TransportMetadata
new file mode 100644
index 0000000..1d2a16d
--- /dev/null
+++ b/a2a/src/main/resources/META-INF/services/io.a2a.server.TransportMetadata
@@ -0,0 +1 @@
+com.ankurm.agents.a2a.JdkJsonRpcTransportMetadata
diff --git a/a2a/src/test/java/com/ankurm/agents/a2a/A2aTest.java b/a2a/src/test/java/com/ankurm/agents/a2a/A2aTest.java
new file mode 100644
index 0000000..ad31770
--- /dev/null
+++ b/a2a/src/test/java/com/ankurm/agents/a2a/A2aTest.java
@@ -0,0 +1,203 @@
+package com.ankurm.agents.a2a;
+
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonParser;
+import io.a2a.spec.Task;
+import org.junit.jupiter.api.Test;
+
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class A2aTest {
+
+    private static String get(String url) throws Exception {
+        return HttpClient.newHttpClient().send(HttpRequest.newBuilder(URI.create(url)).build(),
+                HttpResponse.BodyHandlers.ofString()).body();
+    }
+
+    private static String post(String url, String json) throws Exception {
+        return HttpClient.newHttpClient().send(HttpRequest.newBuilder(URI.create(url))
+                        .header("Content-Type", "application/json").POST(HttpRequest.BodyPublishers.ofString(json)).build(),
+                HttpResponse.BodyHandlers.ofString()).body();
+    }
+
+    private static String text(Task t) {
+        StringBuilder sb = new StringBuilder();
+        t.artifacts().forEach(a -> a.parts().forEach(p -> sb.append(((io.a2a.spec.TextPart) p).text())));
+        return sb.toString();
+    }
+
+    private static String pretty(String json) {
+        return new GsonBuilder().setPrettyPrinting().create().toJson(JsonParser.parseString(json));
+    }
+
+    // ------------------------------------------------------------------ 1. the agent card
+
+    @Test
+    void theAgentCardIsPublishedAtAWellKnownPath() throws Exception {
+        try (A2aHttpServer glossary = A2aHttpServer.start(GlossaryAgent::card, new GlossaryAgent())) {
+            String card = pretty(get(glossary.url() + "/.well-known/agent-card.json"));
+            Transcript.write("01-agent-card.txt", "GET /.well-known/agent-card.json\n\n" + card + "\n");
+            assertThat(card).contains("\"name\": \"Glossary Agent\"").contains("\"streaming\": true").contains("\"id\": \"define\"");
+        }
+    }
+
+    // ------------------------------------------------------------------ 2. the raw exchange
+
+    @Test
+    void aMessageIsOneJsonRpcCallAndATaskIsTheAnswer() throws Exception {
+        try (A2aHttpServer glossary = A2aHttpServer.start(GlossaryAgent::card, new GlossaryAgent())) {
+            String request = "{\"jsonrpc\":\"2.0\",\"id\":\"1\",\"method\":\"SendMessage\",\"params\":{\"message\":"
+                    + "{\"messageId\":\"m-1\",\"role\":\"ROLE_USER\",\"parts\":[{\"text\":\"record\"}]},"
+                    + "\"configuration\":{\"blocking\":true}}}";
+            String response = post(glossary.url() + "/", request);
+            Transcript.write("02-raw-send-message.txt", "POST /\n" + pretty(request) + "\n\nResponse:\n" + pretty(response) + "\n");
+            assertThat(pretty(response)).contains("TASK_STATE_COMPLETED").contains("\"id\": 1,").contains("A record is a final class that is a transparent");
+        }
+    }
+
+    // ------------------------------------------------------------------ 3. blocking, streaming, and the task afterwards
+
+    @Test
+    void streamingShowsTheLifecycleAndTheTaskOutlivesTheCall() throws Exception {
+        try (A2aHttpServer glossary = A2aHttpServer.start(GlossaryAgent::card, new GlossaryAgent())) {
+            A2aCaller.Reply blocking = A2aCaller.send(glossary.url(), "record", null, false);
+            A2aCaller.Reply streamed = A2aCaller.send(glossary.url(), "record", null, true);
+            Task later = A2aCaller.getTask(glossary.url(), streamed.task().id());
+
+            StringBuilder sb = new StringBuilder("Same question, \"record\", asked twice.\n\n");
+            sb.append("Blocking (streaming off): ").append(blocking.events().size()).append(" event\n");
+            blocking.events().forEach(e -> sb.append("  ").append(A2aCaller.describe(e)).append('\n'));
+            sb.append("  answer: ").append(blocking.text()).append("\n\n");
+            sb.append("Streaming: ").append(streamed.events().size()).append(" events\n");
+            streamed.events().forEach(e -> sb.append("  ").append(A2aCaller.describe(e)).append('\n'));
+            sb.append("  answer: ").append(streamed.text()).append("\n\n");
+            sb.append("GetTask for the streamed task afterwards: state ").append(later.status().state())
+                    .append(", artifacts ").append(later.artifacts().size()).append(", artifact text \"")
+                    .append(text(later)).append("\"\n");
+            Transcript.write("03-streaming.txt", sb.toString());
+
+            assertThat(blocking.events()).hasSize(1);
+            assertThat(streamed.events()).hasSize(5);
+            assertThat(blocking.text()).isEqualTo(streamed.text());
+            assertThat(later.status().state().name()).isEqualTo("TASK_STATE_COMPLETED");
+            assertThat(text(later)).isEqualTo(streamed.text());
+        }
+    }
+
+    // ------------------------------------------------------------------ 4. input-required and a follow-up
+
+    @Test
+    void anAgentCanAskForMoreAndTheSameTaskContinues() throws Exception {
+        try (A2aHttpServer glossary = A2aHttpServer.start(GlossaryAgent::card, new GlossaryAgent())) {
+            A2aCaller.Reply first = A2aCaller.send(glossary.url(), " ", null, false);
+            String taskId = first.task().id();
+            A2aCaller.Reply second = A2aCaller.send(glossary.url(), "sealed class", taskId, false);
+
+            String sb = "Turn 1: message with no term.\n  task " + taskId + " state " + A2aCaller.short_(first.task().status().state())
+                    + "\n  agent says: " + first.text()
+                    + "\n\nTurn 2: message \"sealed class\" sent with taskId " + taskId + ".\n  task " + second.task().id()
+                    + " state " + A2aCaller.short_(second.task().status().state()) + "\n  answer: " + second.text()
+                    + "\n\nSame task id both turns: " + taskId.equals(second.task().id()) + "\n";
+            Transcript.write("04-input-required.txt", sb);
+
+            assertThat(first.task().status().state().name()).isEqualTo("TASK_STATE_INPUT_REQUIRED");
+            assertThat(second.task().id()).isEqualTo(taskId);
+            assertThat(second.text()).contains("restricts which other classes may extend it");
+        }
+    }
+
+    // ------------------------------------------------------------------ 5. two Java agents
+
+    @Test
+    void oneAgentCallsAnotherOverA2a() throws Exception {
+        try (A2aHttpServer glossary = A2aHttpServer.start(GlossaryAgent::card, new GlossaryAgent())) {
+            BriefAgent briefAgent = new BriefAgent(glossary.url());
+            try (A2aHttpServer brief = A2aHttpServer.start(BriefAgent::card, briefAgent)) {
+                A2aCaller.Reply ok = A2aCaller.send(brief.url(), "record, sealed class", null, false);
+                int callsAfterOk = briefAgent.callLog.size();
+                A2aCaller.Reply bad = A2aCaller.send(brief.url(), "record, monad", null, false);
+
+                StringBuilder sb = new StringBuilder("Caller -> Brief Agent -> Glossary Agent.\n\n");
+                sb.append("Request: \"record, sealed class\"\n  Brief Agent task state: ").append(A2aCaller.short_(ok.task().status().state()))
+                        .append("\n  result:\n");
+                ok.text().lines().forEach(l -> sb.append("    ").append(l).append('\n'));
+                sb.append("  Calls the Brief Agent made to the Glossary Agent:\n");
+                briefAgent.callLog.subList(0, callsAfterOk).forEach(l -> sb.append("    ").append(l).append('\n'));
+                sb.append("\nRequest: \"record, monad\"\n  Brief Agent task state: ").append(A2aCaller.short_(bad.task().status().state()))
+                        .append("\n  message: ").append(bad.text()).append("\n  Calls the Brief Agent made to the Glossary Agent:\n");
+                briefAgent.callLog.subList(callsAfterOk, briefAgent.callLog.size()).forEach(l -> sb.append("    ").append(l).append('\n'));
+                Transcript.write("05-agent-to-agent.txt", sb.toString());
+
+                assertThat(ok.task().status().state().name()).isEqualTo("TASK_STATE_COMPLETED");
+                assertThat(ok.text()).contains("- record: A record is a final class").contains("- sealed class: A sealed class restricts");
+                assertThat(bad.task().status().state().name()).isEqualTo("TASK_STATE_FAILED");
+                assertThat(bad.text()).contains("monad");
+            }
+        }
+    }
+
+    // ------------------------------------------------------------------ 2b. without blocking the call returns early
+
+    @Test
+    void withoutBlockingTheCallReturnsBeforeTheAgentIsDone() throws Exception {
+        io.a2a.server.agentexecution.AgentExecutor slow = new io.a2a.server.agentexecution.AgentExecutor() {
+            private final GlossaryAgent inner = new GlossaryAgent();
+
+            @Override
+            public void execute(io.a2a.server.agentexecution.RequestContext c, io.a2a.server.tasks.AgentEmitter e) throws io.a2a.spec.A2AError {
+                e.submit();
+                e.startWork();
+                try {
+                    Thread.sleep(400);
+                } catch (InterruptedException ex) {
+                    Thread.currentThread().interrupt();
+                }
+                e.addArtifact(java.util.List.<io.a2a.spec.Part<?>>of(new io.a2a.spec.TextPart("slow answer")), "answer", "answer", java.util.Map.of(), false, true);
+                e.complete();
+            }
+
+            @Override
+            public void cancel(io.a2a.server.agentexecution.RequestContext c, io.a2a.server.tasks.AgentEmitter e) throws io.a2a.spec.A2AError {
+                inner.cancel(c, e);
+            }
+        };
+        try (A2aHttpServer glossary = A2aHttpServer.start(GlossaryAgent::card, slow)) {
+            String body = "{\"jsonrpc\":\"2.0\",\"id\":\"1\",\"method\":\"SendMessage\",\"params\":{\"message\":"
+                    + "{\"messageId\":\"m-%s\",\"role\":\"ROLE_USER\",\"parts\":[{\"text\":\"record\"}]}%s}}";
+            long t0 = System.nanoTime();
+            String early = post(glossary.url() + "/", body.formatted("a", ",\"configuration\":{\"blocking\":false}"));
+            long earlyMs = (System.nanoTime() - t0) / 1_000_000;
+            t0 = System.nanoTime();
+            String blocking = post(glossary.url() + "/", body.formatted("b", ",\"configuration\":{\"blocking\":true}"));
+            long blockingMs = (System.nanoTime() - t0) / 1_000_000;
+            t0 = System.nanoTime();
+            String omitted = post(glossary.url() + "/", body.formatted("c", ""));
+            long omittedMs = (System.nanoTime() - t0) / 1_000_000;
+            String omittedState = com.google.gson.JsonParser.parseString(omitted).getAsJsonObject().getAsJsonObject("result")
+                    .getAsJsonObject("task").getAsJsonObject("status").get("state").getAsString();
+            String earlyState = com.google.gson.JsonParser.parseString(early).getAsJsonObject().getAsJsonObject("result")
+                    .getAsJsonObject("task").getAsJsonObject("status").get("state").getAsString();
+            String blockingState = com.google.gson.JsonParser.parseString(blocking).getAsJsonObject().getAsJsonObject("result")
+                    .getAsJsonObject("task").getAsJsonObject("status").get("state").getAsString();
+            String sb = "The agent accepts the task at once, then works for 400 ms before finishing.\n\n"
+                    + "SendMessage with configuration.blocking = false: task state " + earlyState + ", returned in under 400 ms: "
+                    + (earlyMs < 400) + "\n"
+                    + "SendMessage with configuration.blocking = true: task state " + blockingState + ", returned in at least 400 ms: "
+                    + (blockingMs >= 400) + "\n"
+                    + "SendMessage with no configuration at all: task state " + omittedState + ", returned in under 400 ms: "
+                    + (omittedMs < 400) + "\n";
+            Transcript.write("07-blocking.txt", sb);
+            assertThat(earlyState).isIn("TASK_STATE_SUBMITTED", "TASK_STATE_WORKING");
+            assertThat(blockingState).isEqualTo("TASK_STATE_COMPLETED");
+            assertThat(omittedState).isIn("TASK_STATE_SUBMITTED", "TASK_STATE_WORKING");
+            assertThat(omittedMs).isLessThan(400);
+            assertThat(earlyMs).isLessThan(400);
+            assertThat(blockingMs).isGreaterThanOrEqualTo(400);
+        }
+    }
+}
diff --git a/a2a/src/test/java/com/ankurm/agents/a2a/Transcript.java b/a2a/src/test/java/com/ankurm/agents/a2a/Transcript.java
new file mode 100644
index 0000000..8b66a8b
--- /dev/null
+++ b/a2a/src/test/java/com/ankurm/agents/a2a/Transcript.java
@@ -0,0 +1,39 @@
+package com.ankurm.agents.a2a;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.regex.Pattern;
+
+/** Writes what a test observed to output/NN-name.txt so every figure in the post comes from a file. */
+final class Transcript {
+
+    private static final Path DIR = Path.of("output");
+    private static final Pattern UUID = Pattern.compile("[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}");
+    private static final Pattern PORT = Pattern.compile("127\\.0\\.0\\.1:\\d+");
+    private static final Pattern TS = Pattern.compile("\\d{4}-\\d\\d-\\d\\dT[\\d:.]+Z");
+
+    private Transcript() {
+    }
+
+    /** Ports, ids and timestamps change on every run, so they are replaced by stable placeholders. */
+    static String scrub(String s) {
+        var ids = new java.util.LinkedHashMap<String, String>();
+        var m = UUID.matcher(s);
+        StringBuilder out = new StringBuilder();
+        while (m.find()) {
+            m.appendReplacement(out, ids.computeIfAbsent(m.group(), k -> "<id-" + (ids.size() + 1) + ">"));
+        }
+        m.appendTail(out);
+        return TS.matcher(PORT.matcher(out.toString()).replaceAll("127.0.0.1:PORT")).replaceAll("<time>");
+    }
+
+    static void write(String name, String content) {
+        try {
+            Files.createDirectories(DIR);
+            Files.writeString(DIR.resolve(name), scrub(content));
+        } catch (IOException e) {
+            throw new IllegalStateException(e);
+        }
+    }
+}
diff --git a/a2a/src/test/resources/simplelogger.properties b/a2a/src/test/resources/simplelogger.properties
new file mode 100644
index 0000000..cd90c2a
--- /dev/null
+++ b/a2a/src/test/resources/simplelogger.properties
@@ -0,0 +1 @@
+org.slf4j.simpleLogger.defaultLogLevel=warn
diff --git a/pom.xml b/pom.xml
index e85e54b..8b7ac69 100644
--- a/pom.xml
+++ b/pom.xml
@@ -11,6 +11,7 @@
 
   <modules>
     <module>embabel</module>
+    <module>a2a</module>
   </modules>
 
   <properties>
@@ -18,6 +19,7 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <!-- Verified on Maven Central 2026-10-11 -->
     <embabel.version>1.5.3</embabel.version>
+    <a2a.sdk.version>1.0.0.Alpha3</a2a.sdk.version>
     <junit.version>6.1.3</junit.version>
     <assertj.version>3.27.6</assertj.version>
     <slf4j.version>2.0.17</slf4j.version>