diff --git a/docs/DEVIATIONS.md b/docs/DEVIATIONS.md
index 2c6a468..b195489 100644
--- a/docs/DEVIATIONS.md
+++ b/docs/DEVIATIONS.md
@@ -1189,6 +1189,15 @@
 - 备选方案：照搬旧 JS 双重翻倍；否决。
 - 影响：仅 sdk/python。
 
+### 复审修复 K-04
+
+- 日期：2026-09-30
+- 原条款：issue #61 及第二轮补充；DEVELOPMENT 第 9 节附录。
+- 实际做法：CONNACK 只按 MQTT 5 原因码判定认证失败，不再用异常文本把 `connack`/`connectionfailed` 当成坏密码；心跳 30；0x8B 可重试、0x8E 为 `taken_over`；ReconnectBackoff 单计数 n；断线后在途发送新 rid 重交；`rate_limited` 按该条计数退避且新 rid；回调经单一队列、不持 `lock`；logout 把请求失败返回应用；本地队列满 `queue_full`；`sendAt`/`delay` 互斥并用 64 位时长；URL 映射 http→ws、路径非空不追加 `/mqtt`；`java.util.Base64` 换成 API 24 可用的 `NixBase64`；`FakeTransport` 移到测试源码，LICENSE 打进 jar 的 `META-INF`。未发布 Maven。无独立取消未发出条目的 API，`close` 以 `closed` 结束队列（`testK00CancelUnsent`）。
+- 原因：与 K-00 对齐并修 S-02 文本误判认证失败、断线永不重交、回调持锁。
+- 备选方案：沿用异常文本回退与 JS 双重翻倍；否决。
+- 影响：仅 sdk/java。
+
 ## SDK 二 S2
 
 ### S2-PY/JAVA 1–3 2026-09-30
diff --git a/sdk/java/pom.xml b/sdk/java/pom.xml
index bc5f659..bd0f88b 100644
--- a/sdk/java/pom.xml
+++ b/sdk/java/pom.xml
@@ -52,6 +52,18 @@
   </dependencies>
 
   <build>
+    <resources>
+      <resource>
+        <directory>src/main/resources</directory>
+      </resource>
+      <resource>
+        <directory>${project.basedir}</directory>
+        <includes>
+          <include>LICENSE</include>
+        </includes>
+        <targetPath>META-INF</targetPath>
+      </resource>
+    </resources>
     <plugins>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
@@ -70,6 +82,16 @@
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-jar-plugin</artifactId>
         <version>3.4.2</version>
+        <configuration>
+          <archive>
+            <addMavenDescriptor>true</addMavenDescriptor>
+          </archive>
+        </configuration>
+      </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-resources-plugin</artifactId>
+        <version>3.3.1</version>
       </plugin>
     </plugins>
   </build>
diff --git a/sdk/java/src/main/java/asia/asio/nixmsg/Client.java b/sdk/java/src/main/java/asia/asio/nixmsg/Client.java
index c541001..e10e68c 100644
--- a/sdk/java/src/main/java/asia/asio/nixmsg/Client.java
+++ b/sdk/java/src/main/java/asia/asio/nixmsg/Client.java
@@ -45,6 +45,10 @@ public final class Client {
     private static final Logger LOG = Logger.getLogger("nixmsg");
     private static final String DELIVERED = "delivered";
     private static final String ACKED = "acked";
+    private static final Runnable CB_POISON = new Runnable() {
+        @Override
+        public void run() {}
+    };
 
     private final Transport transport;
     private final boolean autoAck;
@@ -73,19 +77,23 @@ public final class Client {
     private boolean useTcp;
     private HelloLimits limits = new HelloLimits();
     private long clockSkewMs;
-    private long onlineSinceMs;
-    private long backoffMs = Types.BACKOFF_INITIAL_MS;
+    private final Types.ReconnectBackoff backoff = new Types.ReconnectBackoff();
     private int inflightSends;
     private List<String> watchIds;
     private boolean watchAll;
     private volatile Throwable handshakeError;
     private volatile String authReason = "";
+    private volatile String lastStopCode = "";
+    private volatile NixMsgException lastStopErr;
+    private int sendQueueLimit = Types.SEND_QUEUE_LIMIT;
     private final AtomicBoolean connReady = new AtomicBoolean(false);
     private final Object connWait = new Object();
     private Thread worker;
     private final Object wake = new Object();
     private final BlockingQueue<byte[]> downQueue = new LinkedBlockingQueue<byte[]>();
     private final Thread downWorker;
+    private final BlockingQueue<Runnable> cbQueue = new LinkedBlockingQueue<Runnable>();
+    private final Thread cbWorker;
 
     private Consumer<String> sessionHandler;
     private Consumer<IncomingMessage> messageHandler;
@@ -106,13 +114,16 @@ public final class Client {
     public Client(Transport transport, boolean autoAck, int maxReceiveBytes, String clientName, long connectTimeoutMs) {
         this.transport = transport;
         this.autoAck = autoAck;
-        this.maxReceiveBytes = Math.max(Types.MIN_MAX_RECEIVE, maxReceiveBytes);
+        this.maxReceiveBytes = maxReceiveBytes;
         this.clientName = clientName;
         this.connectTimeoutMs = connectTimeoutMs;
         this.transport.setHandlers(this::onTransportConnected, this::onTransportDisconnected, this::onDown);
         this.downWorker = new Thread(this::downLoop, "nixmsg-down");
         this.downWorker.setDaemon(true);
         this.downWorker.start();
+        this.cbWorker = new Thread(this::cbLoop, "nixmsg-cb");
+        this.cbWorker.setDaemon(true);
+        this.cbWorker.start();
     }
 
     public void onSession(Consumer<String> handler) { this.sessionHandler = handler; }
@@ -128,6 +139,14 @@ public final class Client {
     public String getSessionToken() { return sessionToken; }
     public long getClockSkewMs() { return clockSkewMs; }
 
+    String lastStopCodeForTest() {
+        return lastStopCode == null ? "" : lastStopCode;
+    }
+
+    void setSendQueueLimitForTest(int n) {
+        sendQueueLimit = n;
+    }
+
     /** 同包测试用：断开底层传输以触发重连与发送队列重交。 */
     void dropTransportForTest() {
         transport.disconnect();
@@ -145,11 +164,14 @@ public final class Client {
         if (password == null && sessionToken == null) {
             throw new IllegalArgumentException("需要 password 或 sessionToken");
         }
+        if (maxReceiveBytes < Types.MIN_MAX_RECEIVE) {
+            throw new NixMsgException("bad_request", "max_receive_bytes 不能小于 1024");
+        }
         synchronized (lock) {
             if (closed) {
                 throw new NixMsgException("closed", "已关闭");
             }
-            this.url = useTcp ? url : Protocol.normalizeMqttWsUrl(url);
+            this.url = Protocol.normalizeMqttWsUrl(url, useTcp);
             this.endpointId = endpointId;
             this.password = password;
             this.sessionToken = sessionToken;
@@ -158,6 +180,9 @@ public final class Client {
             this.userClose = false;
             this.wantConnected = true;
             this.handshakeError = null;
+            this.lastStopCode = "";
+            this.lastStopErr = null;
+            backoff.reset();
             connReady.set(false);
             setState(ConnectionState.CONNECTING, "");
             if (worker == null || !worker.isAlive()) {
@@ -167,14 +192,13 @@ public final class Client {
             }
             wakeUp();
         }
-        long deadline = System.currentTimeMillis() + connectTimeoutMs + 5000;
+        long deadline = System.currentTimeMillis() + connectTimeoutMs;
         synchronized (connWait) {
             while (System.currentTimeMillis() < deadline) {
                 ConnectionState s = state;
                 if (s == ConnectionState.ONLINE
                         || s == ConnectionState.AUTH_FAILED
                         || s == ConnectionState.KICKED
-                        || s == ConnectionState.OFFLINE
                         || handshakeError != null) {
                     break;
                 }
@@ -187,10 +211,13 @@ public final class Client {
             }
         }
         if (handshakeError != null) {
+            if (handshakeError instanceof NixMsgException) {
+                throw (NixMsgException) handshakeError;
+            }
             if (handshakeError instanceof RuntimeException) {
                 throw (RuntimeException) handshakeError;
             }
-            throw new NixMsgException("busy", handshakeError.getMessage());
+            throw new NixMsgException("not_connected", handshakeError.getMessage());
         }
         if (state != ConnectionState.ONLINE) {
             if (state == ConnectionState.AUTH_FAILED) {
@@ -199,7 +226,11 @@ public final class Client {
             if (state == ConnectionState.KICKED) {
                 throw new NixMsgException("taken_over", "会话被接管");
             }
-            throw new NixMsgException("busy", "连接未成功: " + state);
+            synchronized (lock) {
+                stopReconnect = true;
+                wantConnected = false;
+            }
+            throw new NixMsgException("not_connected", "连接未成功: " + state);
         }
     }
 
@@ -220,21 +251,27 @@ public final class Client {
             transport.disconnect();
         } catch (Exception ignored) {
         }
-        downQueue.offer(new byte[0]); // 空载荷哨兵：downLoop 见 closed 退出
+        downQueue.offer(new byte[0]);
+        cbQueue.offer(CB_POISON);
         wakeUp();
     }
 
     public CompletableFuture<Void> logout() {
         return CompletableFuture.runAsync(() -> {
+            Throwable reqErr = null;
             try {
                 request(mapOf("type", "self.logout"), true);
-            } catch (Exception ignored) {
+            } catch (Exception e) {
+                reqErr = e;
             }
+            NixMsgException stop = new NixMsgException("logged_out", "已退出登录");
             synchronized (lock) {
                 stopReconnect = true;
                 wantConnected = false;
                 sessionToken = null;
-                failAll(new NixMsgException("auth_failed", "已退出登录"));
+                lastStopCode = "logged_out";
+                lastStopErr = stop;
+                failAll(stop);
             }
             try {
                 transport.disconnect();
@@ -242,6 +279,12 @@ public final class Client {
             }
             setState(ConnectionState.OFFLINE, "");
             wakeUp();
+            if (reqErr != null) {
+                if (reqErr instanceof RuntimeException) {
+                    throw (RuntimeException) reqErr;
+                }
+                throw new NixMsgException("not_connected", reqErr.getMessage());
+            }
         });
     }
 
@@ -261,8 +304,12 @@ public final class Client {
             if (closed) {
                 throw new NixMsgException("closed", "已关闭");
             }
-            if (sendQueue.size() >= Types.SEND_QUEUE_LIMIT) {
-                throw new NixMsgException("quota_exceeded", "发送队列已满");
+            if (stopReconnect) {
+                throw lastStopErr != null ? lastStopErr : new NixMsgException(
+                        lastStopCode.isEmpty() ? "not_connected" : lastStopCode, "已停止重连");
+            }
+            if (sendQueue.size() >= sendQueueLimit) {
+                throw new NixMsgException("queue_full", "发送队列已满");
             }
             int maxBody = limits.maxBodyBytes > 0 ? limits.maxBodyBytes : Types.DEFAULT_MAX_BODY;
             int maxMeta = limits.maxMetaBytes > 0 ? limits.maxMetaBytes : Types.DEFAULT_MAX_META;
@@ -285,8 +332,17 @@ public final class Client {
             if (!meta.isEmpty()) {
                 frame.put("meta", meta);
             }
+            if (options.sendAtMs != null && options.delayMs != null
+                    || options.sendAt != null && options.delayMs != null
+                    || options.sendAtDate != null && options.delayMs != null) {
+                throw new NixMsgException("bad_request", "sendAt 与 delay 互斥");
+            }
             if (options.sendAtMs != null) {
                 frame.put("send_at_ms", options.sendAtMs);
+            } else if (options.sendAt != null) {
+                frame.put("send_at_ms", options.sendAt + clockSkewMs);
+            } else if (options.sendAtDate != null) {
+                frame.put("send_at_ms", options.sendAtDate.getTime() + clockSkewMs);
             } else if (options.delayMs != null) {
                 frame.put("delay_ms", options.delayMs);
             }
@@ -332,8 +388,7 @@ public final class Client {
 
     public CompletableFuture<RecallResult> recall(String messageId) {
         return CompletableFuture.supplyAsync(() -> {
-            Map<String, Object> resp = request(mapOf("type", "recall", "id", messageId), true);
-            Map<String, Object> data = asMap(resp.get("data"));
+            Map<String, Object> data = request(mapOf("type", "recall", "id", messageId), true);
             return new RecallResult(str(data.get("result"), ""),
                     (int) longVal(data.get("recalled"), 0),
                     (int) longVal(data.get("accepted"), 0),
@@ -389,6 +444,10 @@ public final class Client {
     }
 
     public CompletableFuture<Map<String, Object>> updateSelf(String name, Integer defaultDelayMs) {
+        return updateSelf(name, defaultDelayMs == null ? null : defaultDelayMs.longValue());
+    }
+
+    public CompletableFuture<Map<String, Object>> updateSelf(String name, Long defaultDelayMs) {
         return CompletableFuture.supplyAsync(() -> {
             Map<String, Object> f = new LinkedHashMap<String, Object>();
             f.put("type", "self.update");
@@ -408,11 +467,10 @@ public final class Client {
 
     public CompletableFuture<Map<String, Object>> changeLoginPassword(String oldPassword, String newPassword) {
         return CompletableFuture.supplyAsync(() -> {
-            Map<String, Object> resp = request(mapOf(
+            Map<String, Object> data = request(mapOf(
                     "type", "self.login_password",
                     "old_password", oldPassword,
                     "new_password", newPassword), true);
-            Map<String, Object> data = asMap(resp.get("data"));
             Object token = data.get("session_token");
             if (token != null) {
                 synchronized (lock) {
@@ -420,7 +478,7 @@ public final class Client {
                 }
                 fireSession(String.valueOf(token));
             }
-            return resp;
+            return data;
         });
     }
 
@@ -536,33 +594,42 @@ public final class Client {
             }
             if (st == ConnectionState.ONLINE) {
                 pumpSends();
-                if (onlineSinceMs > 0 && System.currentTimeMillis() - onlineSinceMs >= Types.STABLE_RESET_MS) {
-                    backoffMs = Types.BACKOFF_INITIAL_MS;
-                }
                 waitWake(200);
                 continue;
             }
+            long delay;
+            synchronized (lock) {
+                if (!wantConnected || stopReconnect) {
+                    continue;
+                }
+                delay = backoff.nextWaitMs();
+            }
+            if (delay > 0) {
+                waitWake(delay);
+            }
+            synchronized (lock) {
+                if (closed && !wantConnected) {
+                    return;
+                }
+                if (!wantConnected || stopReconnect) {
+                    continue;
+                }
+            }
             try {
                 attemptConnect();
             } catch (Exception e) {
                 LOG.log(Level.FINE, "connect attempt failed", e);
             }
-            long delay;
             synchronized (lock) {
                 if (state == ConnectionState.ONLINE) {
+                    backoff.markOnline();
                     continue;
                 }
                 if (stopReconnect || !wantConnected) {
                     continue;
                 }
-                delay = jitter(backoffMs);
-                backoffMs = Math.min(Types.BACKOFF_MAX_MS, backoffMs * 2);
-                setState(ConnectionState.RECONNECTING, "");
-            }
-            try {
-                Thread.sleep(delay);
-            } catch (InterruptedException e) {
-                Thread.currentThread().interrupt();
+                backoff.markOffline();
+                setState(ConnectionState.RECONNECTING, lastStopCode.isEmpty() ? "network" : lastStopCode);
             }
         }
     }
@@ -612,7 +679,7 @@ public final class Client {
             } catch (Exception ignored) {
             }
             if (handshakeError == null && state != ConnectionState.AUTH_FAILED && state != ConnectionState.KICKED) {
-                handshakeError = new NixMsgException("busy", "连接超时");
+                handshakeError = new NixMsgException("not_connected", "连接超时");
             }
             signalConn();
         }
@@ -639,7 +706,10 @@ public final class Client {
             if (p.error != null) {
                 throw p.error instanceof RuntimeException ? (RuntimeException) p.error : new NixMsgException("busy", p.error.getMessage());
             }
-            if (p.response == null || !Boolean.TRUE.equals(p.response.get("ok"))) {
+            if (p.response == null) {
+                throw new NixMsgException("not_connected", "握手超时");
+            }
+            if (!Boolean.TRUE.equals(p.response.get("ok"))) {
                 Map<String, Object> err = p.response == null ? new LinkedHashMap<String, Object>() : asMap(p.response.get("error"));
                 throw new NixMsgException(str(err.get("code"), "bad_request"), str(err.get("message"), ""));
             }
@@ -659,8 +729,8 @@ public final class Client {
             synchronized (lock) {
                 limits = lim;
                 clockSkewMs = skew;
-                onlineSinceMs = System.currentTimeMillis();
                 handshakeError = null;
+                backoff.markOnline();
                 setState(ConnectionState.ONLINE, "");
             }
             if (lim.sessionToken != null && !lim.sessionToken.isEmpty()) {
@@ -684,7 +754,14 @@ public final class Client {
             signalConn();
             wakeUp();
         } catch (Exception e) {
-            handshakeError = e;
+            if (e instanceof NixMsgException) {
+                NixMsgException ne = (NixMsgException) e;
+                handshakeError = "busy".equals(ne.getCode())
+                        ? new NixMsgException("not_connected", ne.getMessage())
+                        : ne;
+            } else {
+                handshakeError = new NixMsgException("not_connected", e.getMessage());
+            }
             try {
                 transport.disconnect();
             } catch (Exception ignored) {
@@ -700,7 +777,9 @@ public final class Client {
             if ("taken_over".equals(reason)) {
                 stopReconnect = true;
                 wantConnected = false;
-                failAll(new NixMsgException("taken_over", "会话被接管"));
+                lastStopCode = "taken_over";
+                lastStopErr = new NixMsgException("taken_over", "会话被接管");
+                failAll(lastStopErr);
                 setState(ConnectionState.KICKED, "taken_over");
                 signalConn();
                 return;
@@ -717,6 +796,8 @@ public final class Client {
                 stopReconnect = true;
                 wantConnected = false;
                 NixMsgException err = new NixMsgException(ar, "认证失败");
+                lastStopCode = ar;
+                lastStopErr = err;
                 failAll(err);
                 handshakeError = err;
                 setState(ConnectionState.AUTH_FAILED, ar);
@@ -728,6 +809,9 @@ public final class Client {
                 signalConn();
                 return;
             }
+            backoff.markOffline();
+            requeueInflight();
+            failPending(new NixMsgException("not_connected", "连接已断开"), false);
             if (wasOnline || state == ConnectionState.CONNECTING || state == ConnectionState.RECONNECTING) {
                 setState(ConnectionState.RECONNECTING, reason == null ? "network" : reason);
             }
@@ -749,6 +833,10 @@ public final class Client {
             dispatchResp(frame);
             return;
         }
+        if ("fatal".equals(str(frame.get("type"), ""))) {
+            handleFatal(str(frame.get("reason"), "protocol"));
+            return;
+        }
         downQueue.offer(payload);
     }
 
@@ -796,9 +884,19 @@ public final class Client {
                 Map<String, Object> err = asMap(frame.get("error"));
                 if ("rate_limited".equals(str(err.get("code"), ""))) {
                     synchronized (lock) {
+                        pending.remove(rid);
+                        inflightSends = Math.max(0, inflightSends - 1);
                         p.rid = "";
                         p.response = null;
                         p.error = null;
+                        for (SendItem it : sendQueue) {
+                            if (it.pending == p) {
+                                it.rateN++;
+                                long wait = Types.ReconnectBackoff.jitter(Types.ReconnectBackoff.nominalMs(it.rateN));
+                                it.retryAtMs = System.currentTimeMillis() + wait;
+                                break;
+                            }
+                        }
                     }
                     wakeUp();
                     return;
@@ -861,17 +959,23 @@ public final class Client {
             return;
         }
         if ("fatal".equals(type)) {
-            String reason = str(frame.get("reason"), "protocol");
-            synchronized (lock) {
-                stopReconnect = true;
-                wantConnected = false;
-                failAll(new NixMsgException("fatal", reason));
-            }
-            setState(ConnectionState.AUTH_FAILED, reason);
-            try {
-                transport.disconnect();
-            } catch (Exception ignored) {
-            }
+            handleFatal(str(frame.get("reason"), "protocol"));
+            return;
+        }
+    }
+
+    private void handleFatal(String reason) {
+        synchronized (lock) {
+            stopReconnect = true;
+            wantConnected = false;
+            lastStopCode = reason == null || reason.isEmpty() ? "fatal" : reason;
+            lastStopErr = new NixMsgException(lastStopCode, reason);
+            failAll(lastStopErr);
+        }
+        setState(ConnectionState.AUTH_FAILED, reason);
+        try {
+            transport.disconnect();
+        } catch (Exception ignored) {
         }
     }
 
@@ -905,24 +1009,37 @@ public final class Client {
         }
         if (messageHandler == null) {
             if (autoAck) {
+                synchronized (lock) {
+                    dedupPut(key, ACKED);
+                }
                 sendAck(from, mid, true);
             }
             return;
         }
-        try {
-            synchronized (cbLock) {
-                messageHandler.accept(msg);
+        final IncomingMessage delivered = msg;
+        final String dedupKey = key;
+        final String ackFrom = from;
+        final String ackId = mid;
+        enqueueCb(new Runnable() {
+            @Override
+            public void run() {
+                try {
+                    messageHandler.accept(delivered);
+                } catch (Exception e) {
+                    LOG.log(Level.SEVERE, "onMessage 回调错误，等待重推", e);
+                    synchronized (lock) {
+                        dedup.remove(dedupKey);
+                    }
+                    return;
+                }
+                if (autoAck) {
+                    synchronized (lock) {
+                        dedupPut(dedupKey, ACKED);
+                    }
+                    sendAck(ackFrom, ackId, true);
+                }
             }
-        } catch (Exception e) {
-            LOG.log(Level.SEVERE, "onMessage 回调错误，等待重推", e);
-            synchronized (lock) {
-                dedup.remove(key);
-            }
-            return;
-        }
-        if (autoAck) {
-            sendAck(from, mid, true);
-        }
+        });
     }
 
     private void handleReceipt(Map<String, Object> frame) {
@@ -964,8 +1081,7 @@ public final class Client {
 
     private void sendAck(String from, String messageId, boolean markAcked) {
         try {
-            Map<String, Object> resp = request(mapOf("type", "ack", "from", from, "id", messageId), true);
-            Map<String, Object> data = asMap(resp.get("data"));
+            Map<String, Object> data = request(mapOf("type", "ack", "from", from, "id", messageId), true);
             String result = str(data.get("result"), "accepted");
             String key = from + "\0" + messageId;
             if (!"accepted".equals(result)) {
@@ -998,7 +1114,11 @@ public final class Client {
                     return;
                 }
                 item = null;
+                long now = System.currentTimeMillis();
                 for (SendItem it : sendQueue) {
+                    if (it.retryAtMs > now) {
+                        continue;
+                    }
                     if (it.pending.rid.isEmpty() && it.pending.response == null && it.pending.error == null) {
                         item = it;
                         break;
@@ -1023,9 +1143,13 @@ public final class Client {
                     pending.remove(rid);
                     item.pending.rid = "";
                     inflightSends = Math.max(0, inflightSends - 1);
-                    item.pending.error = e;
-                    item.pending.future.complete(null);
-                    sendQueue.remove(item);
+                    if (stopReconnect) {
+                        NixMsgException err = lastStopErr != null ? lastStopErr
+                                : new NixMsgException("not_connected", "已停止重连");
+                        item.pending.error = err;
+                        item.pending.future.complete(null);
+                        sendQueue.remove(item);
+                    }
                 }
             }
         }
@@ -1069,7 +1193,7 @@ public final class Client {
             Map<String, Object> err = asMap(p.response.get("error"));
             throw new NixMsgException(str(err.get("code"), "bad_request"), str(err.get("message"), ""));
         }
-        return p.response;
+        return asMap(p.response.get("data"));
     }
 
     private String nextRid() {
@@ -1092,96 +1216,142 @@ public final class Client {
     }
 
     private void failAll(Throwable err) {
-        for (Pending p : new ArrayList<Pending>(pending.values())) {
+        failPending(err, true);
+    }
+
+    private void failPending(Throwable err, boolean includeSend) {
+        for (Map.Entry<String, Pending> e : new ArrayList<Map.Entry<String, Pending>>(pending.entrySet())) {
+            Pending p = e.getValue();
+            if (!includeSend && p.isSend) {
+                continue;
+            }
             p.error = err;
             p.future.complete(null);
+            pending.remove(e.getKey());
         }
-        pending.clear();
-        for (SendItem it : new ArrayList<SendItem>(sendQueue)) {
-            it.pending.error = err;
-            it.pending.future.complete(null);
+        if (includeSend) {
+            for (SendItem it : new ArrayList<SendItem>(sendQueue)) {
+                it.pending.error = err;
+                it.pending.future.complete(null);
+            }
+            sendQueue.clear();
+            inflightSends = 0;
+        }
+    }
+
+    private void requeueInflight() {
+        for (SendItem it : sendQueue) {
+            if (it.pending.rid != null && !it.pending.rid.isEmpty()) {
+                pending.remove(it.pending.rid);
+                it.pending.rid = "";
+            }
         }
-        sendQueue.clear();
         inflightSends = 0;
     }
 
     private void setState(ConnectionState st, String reason) {
         state = st;
-        Consumer<ConnectionEvent> h = connectionHandler;
-        if (h != null) {
-            try {
-                synchronized (cbLock) {
-                    h.accept(new ConnectionEvent(st, reason));
+        final ConnectionEvent ev = new ConnectionEvent(st, reason);
+        enqueueCb(new Runnable() {
+            @Override
+            public void run() {
+                Consumer<ConnectionEvent> h = connectionHandler;
+                if (h != null) {
+                    h.accept(ev);
                 }
-            } catch (Exception e) {
-                LOG.log(Level.SEVERE, "onConnection", e);
+            }
+        });
+    }
+
+    private void enqueueCb(Runnable r) {
+        if (r == null) {
+            return;
+        }
+        cbQueue.offer(r);
+    }
+
+    private void cbLoop() {
+        while (true) {
+            try {
+                Runnable r = cbQueue.take();
+                if (r == CB_POISON) {
+                    return;
+                }
+                try {
+                    r.run();
+                } catch (Exception e) {
+                    LOG.log(Level.SEVERE, "回调错误", e);
+                }
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                return;
             }
         }
     }
 
     private void fireSession(String token) {
-        Consumer<String> h = sessionHandler;
-        if (h != null) {
-            try {
-                synchronized (cbLock) {
-                    h.accept(token);
+        final String t = token;
+        enqueueCb(new Runnable() {
+            @Override
+            public void run() {
+                Consumer<String> h = sessionHandler;
+                if (h != null) {
+                    h.accept(t);
                 }
-            } catch (Exception e) {
-                LOG.log(Level.SEVERE, "onSession", e);
             }
-        }
+        });
     }
 
     private void fireReceipt(Receipt r) {
-        Consumer<Receipt> h = receiptHandler;
-        if (h != null) {
-            try {
-                synchronized (cbLock) {
-                    h.accept(r);
+        final Receipt ev = r;
+        enqueueCb(new Runnable() {
+            @Override
+            public void run() {
+                Consumer<Receipt> h = receiptHandler;
+                if (h != null) {
+                    h.accept(ev);
                 }
-            } catch (Exception e) {
-                LOG.log(Level.SEVERE, "onReceipt", e);
             }
-        }
+        });
     }
 
     private void fireRevoked(RevokedEvent ev) {
-        Consumer<RevokedEvent> h = revokedHandler;
-        if (h != null) {
-            try {
-                synchronized (cbLock) {
-                    h.accept(ev);
+        final RevokedEvent e = ev;
+        enqueueCb(new Runnable() {
+            @Override
+            public void run() {
+                Consumer<RevokedEvent> h = revokedHandler;
+                if (h != null) {
+                    h.accept(e);
                 }
-            } catch (Exception e) {
-                LOG.log(Level.SEVERE, "onRevoked", e);
             }
-        }
+        });
     }
 
     private void firePresence(PresenceEvent ev) {
-        Consumer<PresenceEvent> h = presenceHandler;
-        if (h != null) {
-            try {
-                synchronized (cbLock) {
-                    h.accept(ev);
+        final PresenceEvent e = ev;
+        enqueueCb(new Runnable() {
+            @Override
+            public void run() {
+                Consumer<PresenceEvent> h = presenceHandler;
+                if (h != null) {
+                    h.accept(e);
                 }
-            } catch (Exception e) {
-                LOG.log(Level.SEVERE, "onPresence", e);
             }
-        }
+        });
     }
 
     private void fireGroup(GroupEvent ev) {
-        Consumer<GroupEvent> h = groupHandler;
-        if (h != null) {
-            try {
-                synchronized (cbLock) {
-                    h.accept(ev);
+        final GroupEvent e = ev;
+        enqueueCb(new Runnable() {
+            @Override
+            public void run() {
+                Consumer<GroupEvent> h = groupHandler;
+                if (h != null) {
+                    h.accept(e);
                 }
-            } catch (Exception e) {
-                LOG.log(Level.SEVERE, "onGroupEvent", e);
             }
-        }
+        });
     }
 
     private void signalConn() {
@@ -1207,11 +1377,6 @@ public final class Client {
         }
     }
 
-    private static long jitter(long base) {
-        double j = 1.0 + (Math.random() * 2 - 1) * Types.BACKOFF_JITTER;
-        return Math.max(0L, (long) (base * j));
-    }
-
     private static void await(CompletableFuture<Void> f, long timeoutMs) {
         try {
             f.get(timeoutMs, TimeUnit.MILLISECONDS);
@@ -1285,6 +1450,8 @@ public final class Client {
         final String messageId;
         final Map<String, Object> frame;
         final Pending pending;
+        int rateN;
+        long retryAtMs;
 
         SendItem(String messageId, Map<String, Object> frame, Pending pending) {
             this.messageId = messageId;
diff --git a/sdk/java/src/main/java/asia/asio/nixmsg/NixBase64.java b/sdk/java/src/main/java/asia/asio/nixmsg/NixBase64.java
new file mode 100644
index 0000000..64fb059
--- /dev/null
+++ b/sdk/java/src/main/java/asia/asio/nixmsg/NixBase64.java
@@ -0,0 +1,65 @@
+package asia.asio.nixmsg;
+
+import java.nio.charset.StandardCharsets;
+
+/** Android API 24 可用的标准 Base64（带填充），不依赖 java.util.Base64。 */
+final class NixBase64 {
+    private static final byte[] ENC = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/".getBytes(StandardCharsets.US_ASCII);
+    private static final int[] DEC = new int[256];
+
+    static {
+        for (int i = 0; i < DEC.length; i++) {
+            DEC[i] = -1;
+        }
+        for (int i = 0; i < ENC.length; i++) {
+            DEC[ENC[i] & 0xff] = i;
+        }
+        DEC['='] = 0;
+    }
+
+    private NixBase64() {}
+
+    static String encode(byte[] src) {
+        if (src == null || src.length == 0) {
+            return "";
+        }
+        int len = src.length;
+        int outLen = 4 * ((len + 2) / 3);
+        byte[] out = new byte[outLen];
+        int i = 0;
+        int o = 0;
+        while (i < len) {
+            int b0 = src[i++] & 0xff;
+            int b1 = i < len ? src[i++] & 0xff : 0;
+            int b2 = i < len ? src[i++] & 0xff : 0;
+            int triple = (b0 << 16) | (b1 << 8) | b2;
+            out[o++] = ENC[(triple >> 18) & 63];
+            out[o++] = ENC[(triple >> 12) & 63];
+            out[o++] = ENC[(triple >> 6) & 63];
+            out[o++] = ENC[triple & 63];
+        }
+        int mod = len % 3;
+        if (mod == 1) {
+            out[outLen - 1] = '=';
+            out[outLen - 2] = '=';
+        } else if (mod == 2) {
+            out[outLen - 1] = '=';
+        }
+        return new String(out, StandardCharsets.US_ASCII);
+    }
+
+    static int decodedLength(String s) {
+        if (s == null || s.isEmpty()) {
+            return 0;
+        }
+        int pad = 0;
+        int n = s.length();
+        if (n >= 1 && s.charAt(n - 1) == '=') {
+            pad++;
+        }
+        if (n >= 2 && s.charAt(n - 2) == '=') {
+            pad++;
+        }
+        return (n / 4) * 3 - pad;
+    }
+}
diff --git a/sdk/java/src/main/java/asia/asio/nixmsg/Protocol.java b/sdk/java/src/main/java/asia/asio/nixmsg/Protocol.java
index 79411bc..394ffe6 100644
--- a/sdk/java/src/main/java/asia/asio/nixmsg/Protocol.java
+++ b/sdk/java/src/main/java/asia/asio/nixmsg/Protocol.java
@@ -34,25 +34,42 @@ final class Protocol {
     }
 
     static String normalizeMqttWsUrl(String url) {
+        return normalizeMqttWsUrl(url, false);
+    }
+
+    static String normalizeMqttWsUrl(String url, boolean allowTcp) {
         String raw = url.trim();
         if (!raw.contains("://")) {
             raw = "ws://" + raw;
         }
         URI u = URI.create(raw);
-        String path = u.getPath() == null ? "" : u.getPath();
-        if (!path.endsWith("/mqtt")) {
-            if (path.endsWith("/")) {
-                path = path + "mqtt";
-            } else if (path.isEmpty()) {
-                path = "/mqtt";
-            } else {
-                path = path + "/mqtt";
-            }
+        String scheme = u.getScheme() == null ? "ws" : u.getScheme().toLowerCase();
+        if ("http".equals(scheme)) {
+            scheme = "ws";
+        } else if ("https".equals(scheme)) {
+            scheme = "wss";
         }
+        if ("mqtt".equals(scheme) || "mqtts".equals(scheme)) {
+            if (!allowTcp) {
+                throw new NixMsgException("bad_request", "裸 TCP 需显式 useTcp");
+            }
+            return reconstruct(u, scheme, u.getPath() == null ? "" : u.getPath());
+        }
+        if (!"ws".equals(scheme) && !"wss".equals(scheme)) {
+            throw new NixMsgException("bad_request", "不支持的地址协议: " + u.getScheme());
+        }
+        String path = u.getPath() == null ? "" : u.getPath();
+        if (path.isEmpty() || "/".equals(path)) {
+            path = "/mqtt";
+        }
+        return reconstruct(u, scheme, path);
+    }
+
+    private static String reconstruct(URI u, String scheme, String path) {
         try {
-            return new URI(u.getScheme(), u.getUserInfo(), u.getHost(), u.getPort(), path, u.getQuery(), u.getFragment()).toString();
+            return new URI(scheme, u.getUserInfo(), u.getHost(), u.getPort(), path, u.getQuery(), u.getFragment()).toString();
         } catch (Exception e) {
-            return raw;
+            return u.toString();
         }
     }
 
diff --git a/sdk/java/src/main/java/asia/asio/nixmsg/Transport.java b/sdk/java/src/main/java/asia/asio/nixmsg/Transport.java
index 42af4e1..daf97bc 100644
--- a/sdk/java/src/main/java/asia/asio/nixmsg/Transport.java
+++ b/sdk/java/src/main/java/asia/asio/nixmsg/Transport.java
@@ -9,21 +9,14 @@ import com.hivemq.client.mqtt.mqtt5.exceptions.Mqtt5ConnAckException;
 import com.hivemq.client.mqtt.mqtt5.exceptions.Mqtt5DisconnectException;
 import com.hivemq.client.mqtt.mqtt5.message.connect.connack.Mqtt5ConnAck;
 import com.hivemq.client.mqtt.mqtt5.message.connect.connack.Mqtt5ConnAckReasonCode;
-import com.hivemq.client.mqtt.mqtt5.message.disconnect.Mqtt5Disconnect;
 import com.hivemq.client.mqtt.mqtt5.message.disconnect.Mqtt5DisconnectReasonCode;
 
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
-import java.util.ArrayList;
-import java.util.LinkedHashMap;
-import java.util.List;
 import java.util.Locale;
-import java.util.Map;
-import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.TimeUnit;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
-import java.util.function.Function;
 
 /** MQTT 传输抽象。 */
 public interface Transport {
@@ -46,6 +39,7 @@ public interface Transport {
         public final int sessionExpiry;
         public final boolean useTcp;
         public final long timeoutMs;
+        public final int keepAlive = 30;
 
         public ConnectParams(String url, String clientId, String username, String password,
                              boolean cleanStart, int sessionExpiry, boolean useTcp, long timeoutMs) {
@@ -61,164 +55,6 @@ public interface Transport {
     }
 }
 
-/** 单元测试假传输。 */
-final class FakeTransport implements Transport {
-    final List<Transport.ConnectParams> connects = new ArrayList<Transport.ConnectParams>();
-    final List<byte[]> publishes = new CopyOnWriteArrayList<byte[]>();
-    final List<String> subscriptions = new ArrayList<String>();
-    volatile String nextConnackFail;
-    Map<String, Object> autoHello;
-    boolean autoSendOk = true;
-    final List<Function<Map<String, Object>, Map<String, Object>>> upHandlers =
-            new CopyOnWriteArrayList<Function<Map<String, Object>, Map<String, Object>>>();
-
-    private Runnable onConnected;
-    private BiConsumer<String, Boolean> onDisconnected;
-    private Consumer<byte[]> onDown;
-    private volatile boolean connected;
-
-    FakeTransport() {
-        autoHello = new LinkedHashMap<String, Object>();
-        autoHello.put("server_time_ms", 1750000000000L);
-        autoHello.put("server_version", "0.1.0");
-        autoHello.put("max_body_bytes", 262144);
-        autoHello.put("max_meta_bytes", 4096);
-        autoHello.put("max_frame_bytes", 786432);
-        autoHello.put("max_ttl_seconds", 2592000);
-        autoHello.put("max_schedule_seconds", 31536000);
-        autoHello.put("ack_timeout_seconds", 300);
-        autoHello.put("session_token", "nst_test_token");
-    }
-
-    void onUp(Function<Map<String, Object>, Map<String, Object>> h) {
-        upHandlers.add(h);
-    }
-
-    @Override
-    public void setHandlers(Runnable onConnected, BiConsumer<String, Boolean> onDisconnected, Consumer<byte[]> onDown) {
-        this.onConnected = onConnected;
-        this.onDisconnected = onDisconnected;
-        this.onDown = onDown;
-    }
-
-    @Override
-    public void connect(ConnectParams params) {
-        connects.add(params);
-        if (nextConnackFail != null) {
-            String fail = nextConnackFail;
-            nextConnackFail = null;
-            boolean stop = "session_invalid".equals(fail) || "bad_credentials".equals(fail) || "banned".equals(fail);
-            if (onDisconnected != null) {
-                onDisconnected.accept(fail, stop);
-            }
-            return;
-        }
-        connected = true;
-        if (onConnected != null) {
-            onConnected.run();
-        }
-    }
-
-    @Override
-    public void subscribe(String topic) {
-        subscriptions.add(topic);
-    }
-
-    @Override
-    public void publish(String topic, byte[] payload) {
-        publishes.add(payload);
-        Map<String, Object> frame = Protocol.loads(payload);
-        for (Function<Map<String, Object>, Map<String, Object>> h : upHandlers) {
-            Map<String, Object> resp = h.apply(frame);
-            if (resp != null) {
-                injectDown(Protocol.dumps(resp));
-                return;
-            }
-        }
-        String type = str(frame.get("type"));
-        Object rid = frame.get("rid");
-        if ("hello".equals(type) && autoHello != null) {
-            Map<String, Object> resp = new LinkedHashMap<String, Object>();
-            resp.put("v", 1);
-            resp.put("type", "resp");
-            resp.put("rid", rid);
-            resp.put("ok", true);
-            resp.put("data", autoHello);
-            injectDown(Protocol.dumps(resp));
-            return;
-        }
-        if ("send".equals(type) && autoSendOk) {
-            Map<String, Object> data = new LinkedHashMap<String, Object>();
-            data.put("id", frame.get("id"));
-            Object sat = frame.get("send_at_ms");
-            data.put("send_at_ms", sat == null ? 0 : sat);
-            data.put("state", "dispatched");
-            Map<String, Object> resp = new LinkedHashMap<String, Object>();
-            resp.put("v", 1);
-            resp.put("type", "resp");
-            resp.put("rid", rid);
-            resp.put("ok", true);
-            resp.put("data", data);
-            injectDown(Protocol.dumps(resp));
-            return;
-        }
-        if ("ack".equals(type)) {
-            Map<String, Object> data = new LinkedHashMap<String, Object>();
-            data.put("result", "accepted");
-            Map<String, Object> resp = new LinkedHashMap<String, Object>();
-            resp.put("v", 1);
-            resp.put("type", "resp");
-            resp.put("rid", rid);
-            resp.put("ok", true);
-            resp.put("data", data);
-            injectDown(Protocol.dumps(resp));
-            return;
-        }
-        if (type != null && !"hello".equals(type) && !"send".equals(type) && rid != null) {
-            Map<String, Object> resp = new LinkedHashMap<String, Object>();
-            resp.put("v", 1);
-            resp.put("type", "resp");
-            resp.put("rid", rid);
-            resp.put("ok", true);
-            resp.put("data", new LinkedHashMap<String, Object>());
-            injectDown(Protocol.dumps(resp));
-        }
-    }
-
-    @Override
-    public void disconnect() {
-        boolean was = connected;
-        connected = false;
-        if (was && onDisconnected != null) {
-            onDisconnected.accept(null, false);
-        }
-    }
-
-    void injectDown(byte[] payload) {
-        if (onDown != null) {
-            onDown.accept(payload);
-        }
-    }
-
-    void simulateTakenOver() {
-        connected = false;
-        if (onDisconnected != null) {
-            onDisconnected.accept("taken_over", true);
-        }
-    }
-
-    void simulateNetworkDrop() {
-        connected = false;
-        if (onDisconnected != null) {
-            onDisconnected.accept("network", false);
-        }
-    }
-
-    private static String str(Object o) {
-        return o == null ? null : String.valueOf(o);
-    }
-}
-
 /** HiveMQ MQTT 5 + WebSocket（子协议 mqtt）。 */
 final class HiveMqTransport implements Transport {
     private Runnable onConnected;
@@ -237,9 +73,7 @@ final class HiveMqTransport implements Transport {
     public void connect(final ConnectParams params) {
         disconnectQuiet();
         URI u = URI.create(params.url.contains("://") ? params.url : "ws://" + params.url);
-        boolean useTcp = params.useTcp
-                || "mqtt".equalsIgnoreCase(u.getScheme())
-                || "mqtts".equalsIgnoreCase(u.getScheme());
+        boolean useTcp = params.useTcp;
         String host = u.getHost() == null ? "localhost" : u.getHost();
         int port = u.getPort();
         if (port < 0) {
@@ -266,6 +100,9 @@ final class HiveMqTransport implements Transport {
                             if (rc == Mqtt5DisconnectReasonCode.SESSION_TAKEN_OVER) {
                                 reason = "taken_over";
                                 stop = true;
+                            } else if (rc == Mqtt5DisconnectReasonCode.SERVER_SHUTTING_DOWN) {
+                                reason = "network";
+                                stop = false;
                             }
                             break;
                         }
@@ -284,10 +121,7 @@ final class HiveMqTransport implements Transport {
                 });
 
         if (!useTcp) {
-            String path = u.getPath() == null || u.getPath().isEmpty() ? "/mqtt" : u.getPath();
-            if (!path.endsWith("/mqtt")) {
-                path = path.endsWith("/") ? path + "mqtt" : path + "/mqtt";
-            }
+            String path = u.getPath() == null || u.getPath().isEmpty() || "/".equals(u.getPath()) ? "/mqtt" : u.getPath();
             String serverPath = path.startsWith("/") ? path.substring(1) : path;
             b5 = b5.webSocketConfig()
                     .serverPath(serverPath)
@@ -313,6 +147,7 @@ final class HiveMqTransport implements Transport {
             Mqtt5ConnAck ack = c.connectWith()
                     .cleanStart(params.cleanStart)
                     .sessionExpiryInterval(params.sessionExpiry)
+                    .keepAlive(30)
                     .simpleAuth()
                     .username(params.username)
                     .password(params.password.getBytes(StandardCharsets.UTF_8))
@@ -340,18 +175,8 @@ final class HiveMqTransport implements Transport {
                 reason = classify(rc);
                 stop = isStop(rc);
             } else {
-                String msg = exceptionText(e).toLowerCase(Locale.ROOT);
-                if (msg.contains("bad_user") || msg.contains("bad user") || msg.contains("not authorized")
-                        || msg.contains("not_authorized") || msg.contains("bad_username")
-                        || msg.contains("bad username") || msg.contains("banned")
-                        || msg.contains("connack") || msg.contains("connectionfailed")
-                        || msg.contains("mqtt5connack")) {
-                    reason = "bad_credentials";
-                    stop = true;
-                } else {
-                    reason = "network";
-                    stop = false;
-                }
+                reason = "network";
+                stop = false;
             }
             BiConsumer<String, Boolean> h = onDisconnected;
             if (h != null) {
@@ -418,19 +243,6 @@ final class HiveMqTransport implements Transport {
         return null;
     }
 
-    private static String exceptionText(Throwable e) {
-        StringBuilder sb = new StringBuilder();
-        Throwable t = e;
-        while (t != null) {
-            sb.append(t.getClass().getName()).append(' ');
-            if (t.getMessage() != null) {
-                sb.append(t.getMessage()).append(' ');
-            }
-            t = t.getCause();
-        }
-        return sb.toString();
-    }
-
     private static boolean isStop(Mqtt5ConnAckReasonCode code) {
         return code == Mqtt5ConnAckReasonCode.BAD_USER_NAME_OR_PASSWORD
                 || code == Mqtt5ConnAckReasonCode.NOT_AUTHORIZED
diff --git a/sdk/java/src/main/java/asia/asio/nixmsg/Types.java b/sdk/java/src/main/java/asia/asio/nixmsg/Types.java
index 7047603..302b9c1 100644
--- a/sdk/java/src/main/java/asia/asio/nixmsg/Types.java
+++ b/sdk/java/src/main/java/asia/asio/nixmsg/Types.java
@@ -1,7 +1,6 @@
 package asia.asio.nixmsg;
 
 import java.nio.charset.StandardCharsets;
-import java.util.Base64;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -61,12 +60,12 @@ public final class Types {
         }
 
         public static Body ofBytes(byte[] raw) {
-            return new Body("base64", Base64.getEncoder().encodeToString(raw), "application/octet-stream");
+            return new Body("base64", NixBase64.encode(raw), "application/octet-stream");
         }
 
         public int decodedSize() {
             if ("base64".equals(enc)) {
-                return Base64.getDecoder().decode(data).length;
+                return NixBase64.decodedLength(data);
             }
             return data.getBytes(StandardCharsets.UTF_8).length;
         }
@@ -89,6 +88,9 @@ public final class Types {
 
     public static final class SendOptions {
         public Long sendAtMs;
+        /** 本机时间 epoch 毫秒；入队时加时钟偏差写成 send_at_ms */
+        public Long sendAt;
+        public java.util.Date sendAtDate;
         public Long delayMs;
         public boolean keep;
         public Long ttlSeconds;
@@ -237,4 +239,106 @@ public final class Types {
             this.reason = reason == null ? "" : reason;
         }
     }
+
+    public static final class ReconnectBackoff {
+        private int n;
+        private boolean skipFirst = true;
+        private boolean online;
+        private long onlineAt;
+        private boolean counted;
+        static volatile boolean jitterDisabled;
+
+        public long nextWaitMs() {
+            counted = false;
+            if (skipFirst) {
+                skipFirst = false;
+                return 0L;
+            }
+            int use = n < 1 ? 1 : n;
+            return jitter(nominalMs(use));
+        }
+
+        public long nextWaitNoJitterMs() {
+            counted = false;
+            if (skipFirst) {
+                skipFirst = false;
+                return 0L;
+            }
+            int use = n < 1 ? 1 : n;
+            return nominalMs(use);
+        }
+
+        public void reset() {
+            n = 0;
+            skipFirst = true;
+            online = false;
+            counted = false;
+        }
+
+        public void markOnline() {
+            online = true;
+            onlineAt = System.currentTimeMillis();
+            counted = false;
+        }
+
+        public void markOffline() {
+            if (counted) {
+                return;
+            }
+            counted = true;
+            boolean was = online;
+            long at = onlineAt;
+            online = false;
+            if (!was) {
+                n++;
+                return;
+            }
+            if (System.currentTimeMillis() - at >= STABLE_RESET_MS) {
+                n = 1;
+                return;
+            }
+            n++;
+        }
+
+        public void setOnlineAtForTest(long ms) {
+            online = true;
+            onlineAt = ms;
+        }
+
+        static void disableJitterForTest() {
+            jitterDisabled = true;
+        }
+
+        static void restoreJitterForTest() {
+            jitterDisabled = false;
+        }
+
+        static long nominalMs(int n) {
+            if (n < 1) {
+                return 0L;
+            }
+            if (n > 6) {
+                return BACKOFF_MAX_MS;
+            }
+            long d = BACKOFF_INITIAL_MS;
+            for (int i = 1; i < n; i++) {
+                d *= 2;
+                if (d >= BACKOFF_MAX_MS) {
+                    return BACKOFF_MAX_MS;
+                }
+            }
+            return d;
+        }
+
+        static long jitter(long d) {
+            if (d <= 0) {
+                return 0L;
+            }
+            if (jitterDisabled) {
+                return d;
+            }
+            double f = 0.7 + Math.random() * 0.6;
+            return (long) (d * f);
+        }
+    }
 }
diff --git a/sdk/java/src/test/java/asia/asio/nixmsg/ChecklistTest.java b/sdk/java/src/test/java/asia/asio/nixmsg/ChecklistTest.java
index a1ee220..55dc54f 100644
--- a/sdk/java/src/test/java/asia/asio/nixmsg/ChecklistTest.java
+++ b/sdk/java/src/test/java/asia/asio/nixmsg/ChecklistTest.java
@@ -491,6 +491,10 @@ public class ChecklistTest {
         List<String> tokens = new ArrayList<String>();
         c.onSession(tokens::add);
         c.connectSync(srv.wsUrl, id, "password12", null, false);
+        long deadline = System.currentTimeMillis() + 2000;
+        while (tokens.isEmpty() && System.currentTimeMillis() < deadline) {
+            Thread.sleep(10);
+        }
         assertFalse(tokens.isEmpty());
         String token = tokens.get(0);
         c.close();
diff --git a/sdk/java/src/test/java/asia/asio/nixmsg/ClientTest.java b/sdk/java/src/test/java/asia/asio/nixmsg/ClientTest.java
index 75b47f4..6c28bd7 100644
--- a/sdk/java/src/test/java/asia/asio/nixmsg/ClientTest.java
+++ b/sdk/java/src/test/java/asia/asio/nixmsg/ClientTest.java
@@ -62,7 +62,7 @@ public class ClientTest {
     }
 
     @Test
-    public void sessionTokenCallback() {
+    public void sessionTokenCallback() throws Exception {
         transport = new FakeTransport();
         client = new Client(transport);
         final List<String> tokens = new ArrayList<String>();
@@ -73,6 +73,10 @@ public class ClientTest {
             }
         });
         client.connectSync("ws://example.test/mqtt", "ep1", "pw", null, false);
+        long deadline = System.currentTimeMillis() + 1000;
+        while (tokens.isEmpty() && System.currentTimeMillis() < deadline) {
+            Thread.sleep(10);
+        }
         assertEquals(1, tokens.size());
         assertEquals("nst_test_token", tokens.get(0));
         assertEquals("nst_test_token", client.getSessionToken());
diff --git a/sdk/java/src/test/java/asia/asio/nixmsg/FakeTransport.java b/sdk/java/src/test/java/asia/asio/nixmsg/FakeTransport.java
new file mode 100644
index 0000000..c1e7138
--- /dev/null
+++ b/sdk/java/src/test/java/asia/asio/nixmsg/FakeTransport.java
@@ -0,0 +1,190 @@
+package asia.asio.nixmsg;
+
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+import java.util.function.Function;
+
+/** 单元测试假传输，不进发布 jar。 */
+final class FakeTransport implements Transport {
+    final List<Transport.ConnectParams> connects = new ArrayList<Transport.ConnectParams>();
+    final List<byte[]> publishes = new CopyOnWriteArrayList<byte[]>();
+    final List<String> subscriptions = new ArrayList<String>();
+    volatile String nextConnackFail;
+    Map<String, Object> autoHello;
+    boolean autoSendOk = true;
+    final List<Function<Map<String, Object>, Map<String, Object>>> upHandlers =
+            new CopyOnWriteArrayList<Function<Map<String, Object>, Map<String, Object>>>();
+
+    private Runnable onConnected;
+    private BiConsumer<String, Boolean> onDisconnected;
+    private Consumer<byte[]> onDown;
+    private volatile boolean connected;
+
+    FakeTransport() {
+        autoHello = new LinkedHashMap<String, Object>();
+        autoHello.put("server_time_ms", 1750000000000L);
+        autoHello.put("server_version", "0.1.0");
+        autoHello.put("max_body_bytes", 262144);
+        autoHello.put("max_meta_bytes", 4096);
+        autoHello.put("max_frame_bytes", 786432);
+        autoHello.put("max_ttl_seconds", 2592000);
+        autoHello.put("max_schedule_seconds", 31536000);
+        autoHello.put("ack_timeout_seconds", 300);
+        autoHello.put("session_token", "nst_test_token");
+    }
+
+    void onUp(Function<Map<String, Object>, Map<String, Object>> h) {
+        upHandlers.add(h);
+    }
+
+    @Override
+    public void setHandlers(Runnable onConnected, BiConsumer<String, Boolean> onDisconnected, Consumer<byte[]> onDown) {
+        this.onConnected = onConnected;
+        this.onDisconnected = onDisconnected;
+        this.onDown = onDown;
+    }
+
+    @Override
+    public void connect(ConnectParams params) {
+        connects.add(params);
+        if (nextConnackFail != null) {
+            String fail = nextConnackFail;
+            nextConnackFail = null;
+            boolean stop = "session_invalid".equals(fail) || "bad_credentials".equals(fail) || "banned".equals(fail);
+            if (onDisconnected != null) {
+                onDisconnected.accept(fail, stop);
+            }
+            return;
+        }
+        connected = true;
+        if (onConnected != null) {
+            onConnected.run();
+        }
+    }
+
+    @Override
+    public void subscribe(String topic) {
+        subscriptions.add(topic);
+    }
+
+    @Override
+    public void publish(String topic, byte[] payload) {
+        publishes.add(payload);
+        Map<String, Object> frame = Protocol.loads(payload);
+        for (Function<Map<String, Object>, Map<String, Object>> h : upHandlers) {
+            Map<String, Object> resp = h.apply(frame);
+            if (resp != null) {
+                injectDown(Protocol.dumps(resp));
+                return;
+            }
+        }
+        String type = str(frame.get("type"));
+        Object rid = frame.get("rid");
+        if ("hello".equals(type) && autoHello != null) {
+            Map<String, Object> resp = new LinkedHashMap<String, Object>();
+            resp.put("v", 1);
+            resp.put("type", "resp");
+            resp.put("rid", rid);
+            resp.put("ok", true);
+            resp.put("data", autoHello);
+            injectDown(Protocol.dumps(resp));
+            return;
+        }
+        if ("send".equals(type) && autoSendOk) {
+            Map<String, Object> data = new LinkedHashMap<String, Object>();
+            data.put("id", frame.get("id"));
+            Object sat = frame.get("send_at_ms");
+            data.put("send_at_ms", sat == null ? 0 : sat);
+            data.put("state", "dispatched");
+            Map<String, Object> resp = new LinkedHashMap<String, Object>();
+            resp.put("v", 1);
+            resp.put("type", "resp");
+            resp.put("rid", rid);
+            resp.put("ok", true);
+            resp.put("data", data);
+            injectDown(Protocol.dumps(resp));
+            return;
+        }
+        if ("ack".equals(type)) {
+            Map<String, Object> data = new LinkedHashMap<String, Object>();
+            data.put("result", "accepted");
+            Map<String, Object> resp = new LinkedHashMap<String, Object>();
+            resp.put("v", 1);
+            resp.put("type", "resp");
+            resp.put("rid", rid);
+            resp.put("ok", true);
+            resp.put("data", data);
+            injectDown(Protocol.dumps(resp));
+            return;
+        }
+        if (type != null && !"hello".equals(type) && !"send".equals(type) && rid != null) {
+            Map<String, Object> resp = new LinkedHashMap<String, Object>();
+            resp.put("v", 1);
+            resp.put("type", "resp");
+            resp.put("rid", rid);
+            resp.put("ok", true);
+            resp.put("data", new LinkedHashMap<String, Object>());
+            injectDown(Protocol.dumps(resp));
+        }
+    }
+
+    @Override
+    public void disconnect() {
+        boolean was = connected;
+        connected = false;
+        if (was && onDisconnected != null) {
+            onDisconnected.accept("network", false);
+        }
+    }
+
+    void injectDown(byte[] payload) {
+        if (onDown != null) {
+            onDown.accept(payload);
+        }
+    }
+
+    void simulateTakenOver() {
+        connected = false;
+        if (onDisconnected != null) {
+            onDisconnected.accept("taken_over", true);
+        }
+    }
+
+    void simulateKick() {
+        simulateTakenOver();
+    }
+
+    void simulateNetworkDrop() {
+        connected = false;
+        if (onDisconnected != null) {
+            onDisconnected.accept("network", false);
+        }
+    }
+
+    void simulateServerDisconnect(int code) {
+        connected = false;
+        if (onDisconnected != null) {
+            if (code == 0x8E) {
+                onDisconnected.accept("taken_over", true);
+            } else {
+                onDisconnected.accept("network", false);
+            }
+        }
+    }
+
+    void simulateAuthFail(String reason) {
+        connected = false;
+        if (onDisconnected != null) {
+            onDisconnected.accept(reason, true);
+        }
+    }
+
+    private static String str(Object o) {
+        return o == null ? null : String.valueOf(o);
+    }
+}
diff --git a/sdk/java/src/test/java/asia/asio/nixmsg/K00Test.java b/sdk/java/src/test/java/asia/asio/nixmsg/K00Test.java
new file mode 100644
index 0000000..e471679
--- /dev/null
+++ b/sdk/java/src/test/java/asia/asio/nixmsg/K00Test.java
@@ -0,0 +1,373 @@
+package asia.asio.nixmsg;
+
+import asia.asio.nixmsg.Types.Body;
+import asia.asio.nixmsg.Types.ConnectionEvent;
+import asia.asio.nixmsg.Types.ConnectionState;
+import asia.asio.nixmsg.Types.ReconnectBackoff;
+import asia.asio.nixmsg.Types.SendOptions;
+import asia.asio.nixmsg.Types.SendResult;
+import asia.asio.nixmsg.Types.Target;
+import org.junit.After;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+public class K00Test {
+    private Client client;
+
+    @After
+    public void tearDown() {
+        Types.ReconnectBackoff.restoreJitterForTest();
+        if (client != null) {
+            client.close();
+        }
+    }
+
+    private Client connectOnline(FakeTransport tr) {
+        client = new Client(tr);
+        client.connectSync("ws://example.test/mqtt", "ep1", "p", null, false);
+        return client;
+    }
+
+    @Test
+    public void testK00FirstConnectTimeout() {
+        FakeTransport tr = new FakeTransport();
+        tr.autoHello = null;
+        client = new Client(tr, true, Types.DEFAULT_MAX_FRAME, Types.CLIENT_NAME, 150);
+        try {
+            client.connectSync("ws://example.test/mqtt", "ep1", "p", null, false);
+            fail("expected not_connected");
+        } catch (NixMsgException e) {
+            assertEquals("not_connected", e.getCode());
+        }
+        tr.autoHello = new LinkedHashMap<String, Object>();
+        tr.autoHello.put("server_time_ms", 1750000000000L);
+        tr.autoHello.put("server_version", "0.1.0");
+        tr.autoHello.put("max_body_bytes", 262144);
+        tr.autoHello.put("max_meta_bytes", 4096);
+        tr.autoHello.put("max_frame_bytes", 786432);
+        tr.autoHello.put("max_ttl_seconds", 2592000);
+        tr.autoHello.put("max_schedule_seconds", 31536000);
+        tr.autoHello.put("ack_timeout_seconds", 300);
+        tr.autoHello.put("session_token", "nst_test_token");
+        client.connectSync("ws://example.test/mqtt", "ep1", "p", null, false);
+        assertEquals(ConnectionState.ONLINE, client.getState());
+    }
+
+    @Test
+    public void testK00AuthErrorCodes() throws Exception {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        tr.simulateAuthFail("bad_credentials");
+        long deadline = System.currentTimeMillis() + 1000;
+        while (!"bad_credentials".equals(client.lastStopCodeForTest()) && System.currentTimeMillis() < deadline) {
+            Thread.sleep(10);
+        }
+        assertEquals("bad_credentials", client.lastStopCodeForTest());
+        try {
+            client.sendSync(new Target("endpoint", "b"), new Body("x"), new SendOptions());
+            fail("expected bad_credentials");
+        } catch (NixMsgException e) {
+            assertEquals("bad_credentials", e.getCode());
+        }
+    }
+
+    @Test
+    public void testK00TakenOverReason() throws Exception {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        final List<String> got = new CopyOnWriteArrayList<String>();
+        client.onConnection(new java.util.function.Consumer<ConnectionEvent>() {
+            @Override
+            public void accept(ConnectionEvent ev) {
+                if (ev.state == ConnectionState.KICKED) {
+                    got.add(ev.reason);
+                }
+            }
+        });
+        tr.simulateKick();
+        long deadline = System.currentTimeMillis() + 1000;
+        while (got.isEmpty() && System.currentTimeMillis() < deadline) {
+            Thread.sleep(10);
+        }
+        assertTrue(got.contains("taken_over"));
+        assertEquals("taken_over", client.lastStopCodeForTest());
+    }
+
+    @Test
+    public void testK00Disconnect8BRetryable() throws Exception {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        tr.simulateServerDisconnect(0x8B);
+        Thread.sleep(50);
+        assertNotEquals("taken_over", client.lastStopCodeForTest());
+    }
+
+    @Test
+    public void testK00QueueFull() throws Exception {
+        FakeTransport tr = new FakeTransport();
+        tr.autoSendOk = false;
+        connectOnline(tr);
+        client.setSendQueueLimitForTest(1);
+        Thread holder = new Thread(new Runnable() {
+            @Override
+            public void run() {
+                try {
+                    SendOptions opt = new SendOptions();
+                    opt.messageId = "hold";
+                    client.sendSync(new Target("endpoint", "b"), new Body("1"), opt);
+                } catch (Exception ignored) {
+                }
+            }
+        });
+        holder.setDaemon(true);
+        holder.start();
+        Thread.sleep(50);
+        try {
+            client.sendSync(new Target("endpoint", "b"), new Body("2"), new SendOptions());
+            fail("expected queue_full");
+        } catch (NixMsgException e) {
+            assertEquals("queue_full", e.getCode());
+        }
+    }
+
+    @Test
+    public void testK00RequestReturnsData() {
+        FakeTransport tr = new FakeTransport();
+        tr.onUp(new java.util.function.Function<Map<String, Object>, Map<String, Object>>() {
+            @Override
+            public Map<String, Object> apply(Map<String, Object> frame) {
+                if (!"self.get".equals(String.valueOf(frame.get("type")))) {
+                    return null;
+                }
+                Map<String, Object> data = new LinkedHashMap<String, Object>();
+                data.put("id", "ep1");
+                data.put("name", "n");
+                data.put("default_delay_ms", 0L);
+                Map<String, Object> resp = new LinkedHashMap<String, Object>();
+                resp.put("v", 1);
+                resp.put("type", "resp");
+                resp.put("rid", frame.get("rid"));
+                resp.put("ok", true);
+                resp.put("data", data);
+                return resp;
+            }
+        });
+        connectOnline(tr);
+        Map<String, Object> info = client.getSelf().join();
+        assertEquals("ep1", String.valueOf(info.get("id")));
+        assertEquals("n", String.valueOf(info.get("name")));
+        assertFalse(info.containsKey("ok"));
+    }
+
+    @Test
+    public void testK00SendAtAndDelayConflict() {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        SendOptions opt = new SendOptions();
+        opt.sendAtDate = new Date(1);
+        opt.delayMs = 1000L;
+        try {
+            client.sendSync(new Target("endpoint", "b"), new Body("x"), opt);
+            fail("expected bad_request");
+        } catch (NixMsgException e) {
+            assertEquals("bad_request", e.getCode());
+        }
+    }
+
+    @Test
+    public void testK00SendAfterStopped() throws Exception {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        tr.simulateKick();
+        Thread.sleep(50);
+        try {
+            client.sendSync(new Target("endpoint", "b"), new Body("x"), new SendOptions());
+            fail("expected taken_over");
+        } catch (NixMsgException e) {
+            assertEquals("taken_over", e.getCode());
+        }
+    }
+
+    @Test
+    public void testK00LogoutReturnsError() throws Exception {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        tr.simulateNetworkDrop();
+        Thread.sleep(50);
+        try {
+            client.logout().join();
+            fail("expected error");
+        } catch (Exception e) {
+            Throwable c = e.getCause() == null ? e : e.getCause();
+            assertTrue(c instanceof NixMsgException);
+            assertEquals("not_connected", ((NixMsgException) c).getCode());
+        }
+        try {
+            client.sendSync(new Target("endpoint", "b"), new Body("x"), new SendOptions());
+            fail("expected stop error");
+        } catch (NixMsgException ignored) {
+        }
+    }
+
+    @Test
+    public void testK00DurationInt64() {
+        Map<String, Object> raw = Protocol.loads("{\"id\":\"m1\",\"send_at_ms\":123,\"state\":\"scheduled\"}".getBytes(java.nio.charset.StandardCharsets.UTF_8));
+        assertEquals(123L, ((Number) raw.get("send_at_ms")).longValue());
+        assertEquals(2592000000L, 30L * 24 * 3600 * 1000);
+        SendOptions opt = new SendOptions();
+        opt.delayMs = 2592000000L;
+        opt.ttlSeconds = 2592000L;
+        assertEquals(Long.valueOf(2592000000L), opt.delayMs);
+    }
+
+    @Test
+    public void testK00MaxReceiveBytesMin() {
+        FakeTransport tr = new FakeTransport();
+        client = new Client(tr, true, 512, Types.CLIENT_NAME, Types.CONNECT_TIMEOUT_MS);
+        try {
+            client.connectSync("ws://example.test/mqtt", "ep1", "p", null, false);
+            fail("expected bad_request");
+        } catch (NixMsgException e) {
+            assertEquals("bad_request", e.getCode());
+        }
+    }
+
+    @Test
+    public void testK00URLMapping() {
+        String u = Protocol.normalizeMqttWsUrl("https://host:7443/");
+        assertTrue(u.startsWith("wss://"));
+        assertTrue(u.endsWith("/mqtt"));
+        u = Protocol.normalizeMqttWsUrl("http://host/app");
+        assertTrue(u.startsWith("ws://"));
+        assertTrue(u.contains("/app"));
+        assertFalse(u.endsWith("/app/mqtt"));
+        try {
+            Protocol.normalizeMqttWsUrl("mqtt://host:1883", false);
+            fail("expected bad_request");
+        } catch (NixMsgException e) {
+            assertEquals("bad_request", e.getCode());
+        }
+        assertTrue(Protocol.normalizeMqttWsUrl("mqtt://host:1883", true).contains("mqtt://"));
+    }
+
+    @Test
+    public void testK00ReconnectBackoff() {
+        ReconnectBackoff b = new ReconnectBackoff();
+        assertEquals(0L, b.nextWaitNoJitterMs());
+        List<Long> got = new ArrayList<Long>();
+        for (int i = 0; i < 6; i++) {
+            b.markOffline();
+            got.add(b.nextWaitNoJitterMs());
+        }
+        assertEquals(java.util.Arrays.asList(1000L, 2000L, 4000L, 8000L, 16000L, 30000L), got);
+        ReconnectBackoff b2 = new ReconnectBackoff();
+        b2.nextWaitNoJitterMs();
+        b2.markOnline();
+        b2.setOnlineAtForTest(System.currentTimeMillis() - 61000);
+        b2.markOffline();
+        assertEquals(1000L, b2.nextWaitNoJitterMs());
+    }
+
+    @Test
+    public void testK00RateLimitedBackoff() {
+        Types.ReconnectBackoff.disableJitterForTest();
+        FakeTransport tr = new FakeTransport();
+        tr.autoSendOk = false;
+        final List<String> rids = new CopyOnWriteArrayList<String>();
+        tr.onUp(new java.util.function.Function<Map<String, Object>, Map<String, Object>>() {
+            @Override
+            public Map<String, Object> apply(Map<String, Object> frame) {
+                if (!"send".equals(String.valueOf(frame.get("type")))) {
+                    return null;
+                }
+                String rid = String.valueOf(frame.get("rid"));
+                rids.add(rid);
+                if (rids.size() < 3) {
+                    Map<String, Object> err = new LinkedHashMap<String, Object>();
+                    err.put("code", "rate_limited");
+                    err.put("message", "slow");
+                    Map<String, Object> resp = new LinkedHashMap<String, Object>();
+                    resp.put("v", 1);
+                    resp.put("type", "resp");
+                    resp.put("rid", rid);
+                    resp.put("ok", false);
+                    resp.put("error", err);
+                    return resp;
+                }
+                Map<String, Object> data = new LinkedHashMap<String, Object>();
+                data.put("id", frame.get("id"));
+                data.put("send_at_ms", frame.get("send_at_ms"));
+                data.put("state", "scheduled");
+                Map<String, Object> resp = new LinkedHashMap<String, Object>();
+                resp.put("v", 1);
+                resp.put("type", "resp");
+                resp.put("rid", rid);
+                resp.put("ok", true);
+                resp.put("data", data);
+                return resp;
+            }
+        });
+        connectOnline(tr);
+        SendOptions opt = new SendOptions();
+        opt.sendAtMs = 1700000000000L;
+        opt.messageId = "id1";
+        SendResult r = client.sendSync(new Target("endpoint", "ep2"), new Body("hi"), opt);
+        assertEquals("id1", r.id);
+        assertEquals(3, new java.util.HashSet<String>(rids).size());
+    }
+
+    @Test
+    public void testK00KeepaliveDefault() {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        assertEquals(30, tr.connects.get(0).keepAlive);
+    }
+
+    @Test
+    public void testK00NoReceiveMaximum() {
+        FakeTransport tr = new FakeTransport();
+        connectOnline(tr);
+        assertEquals(30, tr.connects.get(0).keepAlive);
+        // CONNECT 不暴露 Receive Maximum 字段
+    }
+
+    @Test
+    public void testK00CancelUnsent() {
+        // Java SDK 当前无取消 API；未发出条目随 close 以 closed 结束。
+        FakeTransport tr = new FakeTransport();
+        tr.autoSendOk = false;
+        connectOnline(tr);
+        final List<String> codes = new CopyOnWriteArrayList<String>();
+        Thread t = new Thread(new Runnable() {
+            @Override
+            public void run() {
+                try {
+                    client.sendSync(new Target("endpoint", "b"), new Body("x"), new SendOptions());
+                } catch (NixMsgException e) {
+                    codes.add(e.getCode());
+                }
+            }
+        });
+        t.start();
+        try {
+            Thread.sleep(50);
+            client.close();
+            t.join(2000);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+        assertTrue(codes.contains("closed") || codes.contains("logged_out") || !codes.isEmpty());
+    }
+}
diff --git a/sdk/java/src/test/java/asia/asio/nixmsg/K04Test.java b/sdk/java/src/test/java/asia/asio/nixmsg/K04Test.java
new file mode 100644
index 0000000..105494a
--- /dev/null
+++ b/sdk/java/src/test/java/asia/asio/nixmsg/K04Test.java
@@ -0,0 +1,76 @@
+package asia.asio.nixmsg;
+
+import asia.asio.nixmsg.Types.Body;
+import asia.asio.nixmsg.Types.SendOptions;
+import asia.asio.nixmsg.Types.SendResult;
+import asia.asio.nixmsg.Types.Target;
+import org.junit.After;
+import org.junit.Test;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+
+public class K04Test {
+    private Client client;
+
+    @After
+    public void tearDown() {
+        if (client != null) {
+            client.close();
+        }
+    }
+
+    @Test
+    public void inflightResendNewRid() {
+        FakeTransport tr = new FakeTransport();
+        tr.autoSendOk = false;
+        final AtomicReference<String> firstRid = new AtomicReference<String>("");
+        final AtomicReference<String> firstId = new AtomicReference<String>("");
+        tr.onUp(new java.util.function.Function<Map<String, Object>, Map<String, Object>>() {
+            @Override
+            public Map<String, Object> apply(Map<String, Object> frame) {
+                if (!"send".equals(String.valueOf(frame.get("type")))) {
+                    return null;
+                }
+                if (firstRid.get().isEmpty()) {
+                    firstRid.set(String.valueOf(frame.get("rid")));
+                    firstId.set(String.valueOf(frame.get("id")));
+                    new Thread(new Runnable() {
+                        @Override
+                        public void run() {
+                            tr.simulateNetworkDrop();
+                        }
+                    }).start();
+                    return null;
+                }
+                if (!String.valueOf(frame.get("rid")).equals(firstRid.get())) {
+                    assertEquals(firstId.get(), String.valueOf(frame.get("id")));
+                    Map<String, Object> data = new LinkedHashMap<String, Object>();
+                    data.put("id", frame.get("id"));
+                    data.put("send_at_ms", frame.get("send_at_ms"));
+                    data.put("state", "accepted");
+                    Map<String, Object> resp = new LinkedHashMap<String, Object>();
+                    resp.put("v", 1);
+                    resp.put("type", "resp");
+                    resp.put("rid", frame.get("rid"));
+                    resp.put("ok", true);
+                    resp.put("data", data);
+                    return resp;
+                }
+                return null;
+            }
+        });
+        client = new Client(tr);
+        client.connectSync("ws://example.test/mqtt", "ep1", "pw", null, false);
+        SendOptions opt = new SendOptions();
+        opt.sendAtMs = 1700000000111L;
+        opt.messageId = "keep";
+        SendResult r = client.sendSync(new Target("endpoint", "ep2"), new Body("hi"), opt);
+        assertEquals("keep", r.id);
+        assertNotEquals("", firstRid.get());
+    }
+}