diff --git a/docs/mqtt-usage.md b/docs/mqtt-usage.md
index 5cb3555..faffdbd 100644
--- a/docs/mqtt-usage.md
+++ b/docs/mqtt-usage.md
@@ -114,7 +114,30 @@ myButton.setPayloadPress("PRESS");
 
 Defined in `ArduinoHADefines.h` or via build flags.
 
-- **`ARDUINOHA_DEBUG`** — enables library logging (useful when debugging MQTT). By default lines go to **`Serial`** unless you install a sink with **`arduinoHASetLogSink(ArduinoHALogSink*)`** (for example **DeviceFramework** forwards debug output into its main log stream).
+- **`ARDUINOHA_DEBUG`** — enables ArduinoHA logging by default and sets the initial maximum verbosity to `Debug`. Without this flag, structured logs are compiled in but remain disabled until you call `arduinoHASetLogEnabled(true)`.
+
+Structured logging is available through:
+
+```cpp
+arduinoHASetLogEnabled(true);
+arduinoHASetLogLevel(ArduinoHALogLevel::Trace);
+
+class MyLogSink : public ArduinoHALogSink {
+public:
+    void log(const ArduinoHALogMessage& msg) override {
+        Serial.print("[");
+        Serial.print(arduinoHALogLevelTag(msg.level));
+        Serial.print("][aha.");
+        Serial.print(msg.subsystem);
+        Serial.print("] ");
+        Serial.println(msg.text);
+    }
+};
+```
+
+`arduinoHALog(...)` and `arduinoHALogf(...)` support subsystem-tagged messages such as `mqtt`, `discovery`, `availability`, `serializer`, `entity`, and `device`. `DeviceFramework` installs a structured sink so ArduinoHA messages follow the same main log stream as the rest of the firmware.
+
+Optional transport context for diagnostics (registered automatically by **DeviceFramework**): **`arduinoHASetNetworkStatusFn`** (for example returning `WiFi.status()` so publish failure lines can include `wifi=`).
 
 **Exclude unused device types** (saves flash from vtables), e.g.:
 
diff --git a/src/ArduinoHA.h b/src/ArduinoHA.h
index 4b7e831..970786d 100644
--- a/src/ArduinoHA.h
+++ b/src/ArduinoHA.h
@@ -1,6 +1,8 @@
 #ifndef AHA_ARDUINOHA_H
 #define AHA_ARDUINOHA_H
 
+#include "ArduinoHALog.h"
+#include "ArduinoHALogTemplates.h"
 #include "HADevice.h"
 #include "HAMqtt.h"
 #include "device-types/HABinarySensor.h"
diff --git a/src/ArduinoHADefines.h b/src/ArduinoHADefines.h
index 686bc94..8f925f3 100644
--- a/src/ArduinoHADefines.h
+++ b/src/ArduinoHADefines.h
@@ -37,15 +37,17 @@
 #if defined(ARDUINOHA_DEBUG)
     #include <Arduino.h>
 
-    #define ARDUINOHA_DEBUG_INIT() do { if (!arduinoHAGetLogSink()) { Serial.begin(115200); } } while(0)
-    #define ARDUINOHA_DEBUG_PRINTLN(x) do { \
-        if (arduinoHAGetLogSink()) { arduinoHAGetLogSink()->println(String(x)); } \
-        else { Serial.println(x); } \
-    } while(0)
-    #define ARDUINOHA_DEBUG_PRINT(x) do { \
-        if (arduinoHAGetLogSink()) { arduinoHAGetLogSink()->print(String(x)); } \
-        else { Serial.print(x); } \
+    #define ARDUINOHA_DEBUG_INIT() do { \
+        if (!arduinoHAGetLogSink()) { Serial.begin(115200); } \
+        arduinoHASetLogEnabled(true); \
+        arduinoHASetLogLevel(ArduinoHALogLevel::Debug); \
     } while(0)
+    /** @deprecated Prefer arduinoHALog / arduinoHALogf; each macro emits one structured line. */
+    #define ARDUINOHA_DEBUG_PRINTLN(x) \
+        arduinoHALog(ArduinoHALogLevel::Debug, "aha", String(x))
+    /** @deprecated Prefer arduinoHALog / arduinoHALogf; each macro emits one structured line. */
+    #define ARDUINOHA_DEBUG_PRINT(x) \
+        arduinoHALog(ArduinoHALogLevel::Debug, "aha", String(x))
 #else
     #define ARDUINOHA_DEBUG_INIT()
     #define ARDUINOHA_DEBUG_PRINTLN(x)
diff --git a/src/ArduinoHALog.cpp b/src/ArduinoHALog.cpp
index ef7c253..5cb5340 100644
--- a/src/ArduinoHALog.cpp
+++ b/src/ArduinoHALog.cpp
@@ -1,6 +1,15 @@
 #include "ArduinoHALog.h"
 
 static ArduinoHALogSink* g_arduinoHALogSink = nullptr;
+static ArduinoHANetworkStatusFn g_networkStatusFn = nullptr;
+
+#ifdef ARDUINOHA_DEBUG
+static bool g_logEnabled = true;
+static ArduinoHALogLevel g_logLevelMax = ArduinoHALogLevel::Debug;
+#else
+static bool g_logEnabled = false;
+static ArduinoHALogLevel g_logLevelMax = ArduinoHALogLevel::Info;
+#endif
 
 void arduinoHASetLogSink(ArduinoHALogSink* sink) {
     g_arduinoHALogSink = sink;
@@ -9,3 +18,80 @@ void arduinoHASetLogSink(ArduinoHALogSink* sink) {
 ArduinoHALogSink* arduinoHAGetLogSink() {
     return g_arduinoHALogSink;
 }
+
+void arduinoHASetLogEnabled(bool enabled) {
+    g_logEnabled = enabled;
+}
+
+bool arduinoHAIsLogEnabled() {
+    return g_logEnabled;
+}
+
+void arduinoHASetLogLevel(ArduinoHALogLevel level) {
+    g_logLevelMax = level;
+}
+
+ArduinoHALogLevel arduinoHAGetLogLevel() {
+    return g_logLevelMax;
+}
+
+void arduinoHASetNetworkStatusFn(ArduinoHANetworkStatusFn fn) {
+    g_networkStatusFn = fn;
+}
+
+int arduinoHANetworkStatusOptional(void) {
+    if (!g_networkStatusFn) {
+        return -1;
+    }
+    return g_networkStatusFn();
+}
+
+const char* arduinoHALogLevelTag(ArduinoHALogLevel level) {
+    switch (level) {
+        case ArduinoHALogLevel::Error:
+            return "ERROR";
+        case ArduinoHALogLevel::Warn:
+            return "WARN";
+        case ArduinoHALogLevel::Info:
+            return "INFO";
+        case ArduinoHALogLevel::Debug:
+            return "DEBUG";
+        case ArduinoHALogLevel::Trace:
+            return "TRACE";
+        default:
+            return "?";
+    }
+}
+
+void arduinoHALog(
+    ArduinoHALogLevel level,
+    const char* subsystem,
+    const String& text
+) {
+    if (!g_logEnabled) {
+        return;
+    }
+
+    if (static_cast<uint8_t>(level) > static_cast<uint8_t>(g_logLevelMax)) {
+        return;
+    }
+
+    const char* sub = (subsystem && subsystem[0] != '\0') ? subsystem : "aha";
+
+    ArduinoHALogMessage msg;
+    msg.level = level;
+    msg.subsystem = sub;
+    msg.text = text;
+
+    if (g_arduinoHALogSink) {
+        g_arduinoHALogSink->log(msg);
+        return;
+    }
+
+    Serial.print(F("["));
+    Serial.print(arduinoHALogLevelTag(level));
+    Serial.print(F("][aha."));
+    Serial.print(sub);
+    Serial.print(F("] "));
+    Serial.println(text);
+}
diff --git a/src/ArduinoHALog.h b/src/ArduinoHALog.h
index e96b90b..2364533 100644
--- a/src/ArduinoHALog.h
+++ b/src/ArduinoHALog.h
@@ -4,17 +4,62 @@
 #include <Arduino.h>
 
 /**
- * Optional sink for ArduinoHA debug output (ARDUINOHA_DEBUG).
- * When set, ARDUINOHA_DEBUG_* macros route here; otherwise Serial is used.
+ * Log severity for ArduinoHA. Numeric order is used for filtering: only messages with
+ * level <= the configured maximum (see arduinoHASetLogLevel) are emitted.
+ */
+enum class ArduinoHALogLevel : uint8_t {
+    Error = 1,
+    Warn,
+    Info,
+    Debug,
+    Trace,
+};
+
+struct ArduinoHALogMessage {
+    ArduinoHALogLevel level;
+    const char* subsystem;
+    String text;
+};
+
+/**
+ * Structured log sink (mirrors WiFiManager-style log callbacks).
+ * Install with arduinoHASetLogSink; when null, arduinoHALog writes to Serial if enabled.
  */
 class ArduinoHALogSink {
 public:
-    virtual void print(const String& s) = 0;
-    virtual void println(const String& s) = 0;
+    virtual void log(const ArduinoHALogMessage& msg) = 0;
     virtual ~ArduinoHALogSink() = default;
 };
 
 void arduinoHASetLogSink(ArduinoHALogSink* sink);
 ArduinoHALogSink* arduinoHAGetLogSink();
 
+void arduinoHASetLogEnabled(bool enabled);
+bool arduinoHAIsLogEnabled();
+
+/**
+ * Maximum verbosity to print: Error..Trace. Messages with level <= this value are shown.
+ */
+void arduinoHASetLogLevel(ArduinoHALogLevel level);
+ArduinoHALogLevel arduinoHAGetLogLevel();
+
+void arduinoHALog(
+    ArduinoHALogLevel level,
+    const char* subsystem,
+    const String& text
+);
+
+const char* arduinoHALogLevelTag(ArduinoHALogLevel level);
+
+/**
+ * Optional hook for transport diagnostics (e.g. return WiFi.status()).
+ * When unset, publish failure logs omit network fields.
+ */
+typedef int (*ArduinoHANetworkStatusFn)(void);
+
+void arduinoHASetNetworkStatusFn(ArduinoHANetworkStatusFn fn);
+
+/** Returns -1 when no hook is installed. */
+int arduinoHANetworkStatusOptional(void);
+
 #endif
diff --git a/src/ArduinoHALogTemplates.h b/src/ArduinoHALogTemplates.h
new file mode 100644
index 0000000..61e78b9
--- /dev/null
+++ b/src/ArduinoHALogTemplates.h
@@ -0,0 +1,58 @@
+#ifndef ARDUINOHA_LOG_TEMPLATES_H
+#define ARDUINOHA_LOG_TEMPLATES_H
+
+#include "ArduinoHALog.h"
+#include <IPAddress.h>
+
+inline String ahaLogStringifyArg(const __FlashStringHelper* v) {
+    return String(v);
+}
+
+inline String ahaLogStringifyArg(const IPAddress& v) {
+    return v.toString();
+}
+
+template<typename T>
+inline String ahaLogStringifyArg(const T& v) {
+    return String(v);
+}
+
+template<typename T>
+inline void arduinoHALogf(
+    ArduinoHALogLevel level,
+    const char* subsystem,
+    T&& text
+) {
+    arduinoHALog(level, subsystem, ahaLogStringifyArg(text));
+}
+
+template<typename T, typename U>
+inline void arduinoHALogf(
+    ArduinoHALogLevel level,
+    const char* subsystem,
+    T&& a,
+    U&& b
+) {
+    arduinoHALog(
+        level,
+        subsystem,
+        ahaLogStringifyArg(a) + ahaLogStringifyArg(b)
+    );
+}
+
+template<typename T, typename U, typename V>
+inline void arduinoHALogf(
+    ArduinoHALogLevel level,
+    const char* subsystem,
+    T&& a,
+    U&& b,
+    V&& c
+) {
+    arduinoHALog(
+        level,
+        subsystem,
+        ahaLogStringifyArg(a) + ahaLogStringifyArg(b) + ahaLogStringifyArg(c)
+    );
+}
+
+#endif
diff --git a/src/HAMqtt.cpp b/src/HAMqtt.cpp
index d05836b..7df1ef2 100644
--- a/src/HAMqtt.cpp
+++ b/src/HAMqtt.cpp
@@ -7,12 +7,19 @@
 #include <PubSubClient.h>
 #endif
 
+#include "ArduinoHALog.h"
+#include "ArduinoHALogTemplates.h"
 #include "HADevice.h"
 #include "device-types/HABaseDeviceType.h"
 #include "mocks/PubSubClientMock.h"
 #include "utils/HADictionary.h"
 #include "utils/HASerializer.h"
 
+namespace {
+constexpr char kMqtt[] = "mqtt";
+constexpr char kDiscovery[] = "discovery";
+} // namespace
+
 #define HAMQTT_INIT \
     _device(device), \
     _messageCallback(nullptr), \
@@ -35,6 +42,11 @@
     _lastWillMessage(nullptr), \
     _lastWillRetain(false), \
     _currentState(StateDisconnected), \
+    _lastLoopOkAt(0), \
+    _lastMessageAt(0), \
+    _lastPublishAt(0), \
+    _lastDisconnectAt(0), \
+    _lastDisconnectReason(DiagnosticDisconnectReason::None), \
     _messageDispatchDepth(0), \
     _deferredQueue{}, \
     _deferredHead(0), \
@@ -96,6 +108,49 @@ HAMqtt::~HAMqtt()
     _instance = nullptr;
 }
 
+const char* HAMqtt::diagnosticDisconnectReasonText(DiagnosticDisconnectReason reason)
+{
+    switch (reason) {
+        case DiagnosticDisconnectReason::None:
+            return "none";
+        case DiagnosticDisconnectReason::LoopReturnedFalse:
+            return "loop_false";
+        case DiagnosticDisconnectReason::DeferredBeginPublishFailed:
+            return "deferred_begin_publish";
+        case DiagnosticDisconnectReason::DeferredEndPublishFailed:
+            return "deferred_end_publish";
+        case DiagnosticDisconnectReason::NotConnectedDuringDeferredFlush:
+            return "deferred_not_connected";
+        case DiagnosticDisconnectReason::UnderlyingPubSubStateChanged:
+            return "pubsub_state";
+        case DiagnosticDisconnectReason::ExplicitDisconnect:
+            return "explicit";
+        default:
+            return "?";
+    }
+}
+
+int HAMqtt::getPubSubState() const
+{
+    return _mqtt->state();
+}
+
+String HAMqtt::formatDirectPublishFailureDiagnostics(bool hamqttConnectedBefore, int pubsubStateBefore) const
+{
+    const bool hamqttConnectedAfter = isConnected();
+    const int pubsubStateAfter = getPubSubState();
+    String d = String(F(" hamqttConnBefore=")) + String(hamqttConnectedBefore ? 1 : 0) +
+        F(" hamqttConnAfter=") + String(hamqttConnectedAfter ? 1 : 0) +
+        F(" pubsubBefore=") + String(pubsubStateBefore) +
+        F(" pubsubAfter=") + String(pubsubStateAfter);
+    const int wifi = arduinoHANetworkStatusOptional();
+    if (wifi >= 0) {
+        d += F(" wifi=");
+        d += String(wifi);
+    }
+    return d;
+}
+
 bool HAMqtt::begin(
     const IPAddress serverIp,
     const uint16_t serverPort,
@@ -103,18 +158,16 @@ bool HAMqtt::begin(
     const char* password
 )
 {
-    ARDUINOHA_DEBUG_PRINT(F("AHA: init server "))
-    ARDUINOHA_DEBUG_PRINT(serverIp)
-    ARDUINOHA_DEBUG_PRINT(F(":"))
-    ARDUINOHA_DEBUG_PRINTLN(serverPort)
+    arduinoHALogf(ArduinoHALogLevel::Info, kMqtt, F("init server "), serverIp);
+    arduinoHALogf(ArduinoHALogLevel::Info, kMqtt, F("init port "), serverPort);
 
     if (_device.getUniqueId() == nullptr) {
-        ARDUINOHA_DEBUG_PRINTLN(F("AHA: init failed. Missing device unique ID"))
+        arduinoHALog(ArduinoHALogLevel::Error, kMqtt, F("init failed: missing device unique ID"));
         return false;
     }
 
     if (_initialized) {
-        ARDUINOHA_DEBUG_PRINTLN(F("AHA: already initialized"))
+        arduinoHALog(ArduinoHALogLevel::Warn, kMqtt, F("begin ignored: already initialized"));
         return false;
     }
 
@@ -144,18 +197,16 @@ bool HAMqtt::begin(
     const char* password
 )
 {
-    ARDUINOHA_DEBUG_PRINT(F("AHA: init server "))
-    ARDUINOHA_DEBUG_PRINT(serverHostname)
-    ARDUINOHA_DEBUG_PRINT(F(":"))
-    ARDUINOHA_DEBUG_PRINTLN(serverPort)
+    arduinoHALogf(ArduinoHALogLevel::Info, kMqtt, F("init server "), serverHostname);
+    arduinoHALogf(ArduinoHALogLevel::Info, kMqtt, F("init port "), serverPort);
 
     if (_device.getUniqueId() == nullptr) {
-        ARDUINOHA_DEBUG_PRINTLN(F("AHA: init failed. Missing device unique ID"))
+        arduinoHALog(ArduinoHALogLevel::Error, kMqtt, F("init failed: missing device unique ID"));
         return false;
     }
 
     if (_initialized) {
-        ARDUINOHA_DEBUG_PRINTLN(F("AHA: already initialized"))
+        arduinoHALog(ArduinoHALogLevel::Warn, kMqtt, F("begin ignored: already initialized"));
         return false;
     }
 
@@ -184,7 +235,9 @@ bool HAMqtt::disconnect()
         return false;
     }
 
-    ARDUINOHA_DEBUG_PRINTLN(F("AHA: disconnecting"))
+    _lastDisconnectReason = DiagnosticDisconnectReason::ExplicitDisconnect;
+    _lastDisconnectAt = millis();
+    arduinoHALog(ArduinoHALogLevel::Info, kMqtt, F("disconnect requested"));
 
     clearDeferredBuilder();
     _initialized = false;
@@ -201,16 +254,50 @@ void HAMqtt::loop()
     }
 
     bool result = _mqtt->loop();
-    if (_currentState != _mqtt->state()) {
-        setState(static_cast<ConnectionState>(_mqtt->state()));
+    const int rawState = _mqtt->state();
+
+    if (result) {
+        _lastLoopOkAt = millis();
+    }
+
+    if (_currentState != rawState) {
+        setState(static_cast<ConnectionState>(rawState));
     }
 
     if (!result) {
+        _lastDisconnectReason = DiagnosticDisconnectReason::LoopReturnedFalse;
+        const uint32_t now = millis();
+        arduinoHALog(
+            ArduinoHALogLevel::Warn,
+            kMqtt,
+            String(F("loop returned false pubsub=")) + String(rawState) +
+                F(" deferred=") + String(_deferredCount) +
+                F(" depth=") + String(_messageDispatchDepth) +
+                F(" sinceLastMsgMs=") +
+                String(_lastMessageAt ? (now - _lastMessageAt) : static_cast<uint32_t>(0)) +
+                F(" sinceLastPubMs=") +
+                String(_lastPublishAt ? (now - _lastPublishAt) : static_cast<uint32_t>(0))
+        );
         connectToServer();
     }
 
     if (_messageDispatchDepth == 0 && isConnected()) {
-        flushDeferredPublishes();
+        const uint8_t before = _deferredCount;
+        const bool ok = flushDeferredPublishes();
+        if (!ok) {
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("deferred flush failed queue=")) + String(_deferredCount) +
+                    F(" reason=") + diagnosticDisconnectReasonText(_lastDisconnectReason)
+            );
+        } else if (before > 0) {
+            arduinoHALog(
+                ArduinoHALogLevel::Debug,
+                kMqtt,
+                String(F("deferred flush ok count=")) + String(before)
+            );
+        }
     }
 }
 
@@ -258,12 +345,19 @@ bool HAMqtt::publish(const char* topic, const char* payload, bool retained)
 
     const uint16_t payloadLength = static_cast<uint16_t>(len);
 
-    ARDUINOHA_DEBUG_PRINT(F("AHA: publishing "))
-    ARDUINOHA_DEBUG_PRINT(topic)
-    ARDUINOHA_DEBUG_PRINT(F(", len: "))
-    ARDUINOHA_DEBUG_PRINTLN(payloadLength)
+    arduinoHALog(
+        ArduinoHALogLevel::Debug,
+        kMqtt,
+        String(F("publish topic=")) + topic + F(" len=") + String(payloadLength)
+    );
 
     if (isProcessingMessage()) {
+        arduinoHALog(
+            ArduinoHALogLevel::Trace,
+            kMqtt,
+            String(F("queue publish during callback topic=")) + topic +
+                F(" len=") + String(payloadLength)
+        );
         return enqueueDeferredPublish(
             topic,
             reinterpret_cast<const uint8_t*>(payload),
@@ -272,9 +366,32 @@ bool HAMqtt::publish(const char* topic, const char* payload, bool retained)
         );
     }
 
-    _mqtt->beginPublish(topic, payloadLength, retained);
+    const bool connBeforePub = isConnected();
+    const int psBeforePub = getPubSubState();
+    if (!_mqtt->beginPublish(topic, payloadLength, retained)) {
+        arduinoHALog(
+            ArduinoHALogLevel::Warn,
+            kMqtt,
+            String(F("beginPublish failed topic=")) + topic + F(" len=") + String(payloadLength) +
+                formatDirectPublishFailureDiagnostics(connBeforePub, psBeforePub)
+        );
+        return false;
+    }
     _mqtt->write(reinterpret_cast<const uint8_t*>(payload), payloadLength);
-    return _mqtt->endPublish();
+    const bool connBeforeEnd = isConnected();
+    const int psBeforeEnd = getPubSubState();
+    const bool ok = _mqtt->endPublish();
+    if (!ok) {
+        arduinoHALog(
+            ArduinoHALogLevel::Warn,
+            kMqtt,
+            String(F("endPublish failed topic=")) + topic +
+                formatDirectPublishFailureDiagnostics(connBeforeEnd, psBeforeEnd)
+        );
+    } else {
+        _lastPublishAt = millis();
+    }
+    return ok;
 }
 
 bool HAMqtt::beginPublish(
@@ -283,17 +400,30 @@ bool HAMqtt::beginPublish(
     bool retained
 )
 {
-    ARDUINOHA_DEBUG_PRINT(F("AHA: begin publish "))
-    ARDUINOHA_DEBUG_PRINT(topic)
-    ARDUINOHA_DEBUG_PRINT(F(", len: "))
-    ARDUINOHA_DEBUG_PRINTLN(payloadLength)
+    arduinoHALog(
+        ArduinoHALogLevel::Debug,
+        kMqtt,
+        String(F("beginPublish topic=")) + (topic ? topic : "") +
+            F(" len=") + String(payloadLength)
+    );
 
     if (!isConnected() || !topic) {
         return false;
     }
 
     if (!isProcessingMessage()) {
-        return _mqtt->beginPublish(topic, payloadLength, retained);
+        const bool connBefore = isConnected();
+        const int psBefore = getPubSubState();
+        const bool ok = _mqtt->beginPublish(topic, payloadLength, retained);
+        if (!ok) {
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("beginPublish failed topic=")) + topic + F(" len=") + String(payloadLength) +
+                    formatDirectPublishFailureDiagnostics(connBefore, psBefore)
+            );
+        }
+        return ok;
     }
 
     if (_deferredBuilder.active) {
@@ -367,7 +497,20 @@ void HAMqtt::writePayload(const __FlashStringHelper* src)
 bool HAMqtt::endPublish()
 {
     if (!isProcessingMessage()) {
-        return _mqtt->endPublish();
+        const bool connBefore = isConnected();
+        const int psBefore = getPubSubState();
+        const bool ok = _mqtt->endPublish();
+        if (ok) {
+            _lastPublishAt = millis();
+        } else {
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("endPublish failed (direct)")) +
+                    formatDirectPublishFailureDiagnostics(connBefore, psBefore)
+            );
+        }
+        return ok;
     }
 
     if (!_deferredBuilder.active ||
@@ -383,24 +526,32 @@ bool HAMqtt::endPublish()
         _deferredBuilder.expectedLength,
         _deferredBuilder.retained
     );
+    if (ok) {
+        arduinoHALog(
+            ArduinoHALogLevel::Trace,
+            kMqtt,
+            String(F("queued deferred built publish len=")) + String(_deferredBuilder.expectedLength)
+        );
+    }
     clearDeferredBuilder();
     return ok;
 }
 
 bool HAMqtt::subscribe(const char* topic)
 {
-    ARDUINOHA_DEBUG_PRINT(F("AHA: subscribing "))
-    ARDUINOHA_DEBUG_PRINTLN(topic)
-
+    arduinoHALog(ArduinoHALogLevel::Debug, kMqtt, String(F("subscribe ")) + (topic ? topic : ""));
     return _mqtt->subscribe(topic);
 }
 
 void HAMqtt::processMessage(const char* topic, const uint8_t* payload, uint16_t length)
 {
-    ARDUINOHA_DEBUG_PRINT(F("AHA: received call "))
-    ARDUINOHA_DEBUG_PRINT(topic)
-    ARDUINOHA_DEBUG_PRINT(F(", len: "))
-    ARDUINOHA_DEBUG_PRINTLN(length)
+    _lastMessageAt = millis();
+    arduinoHALog(
+        ArduinoHALogLevel::Debug,
+        kMqtt,
+        String(F("rx topic=")) + (topic ? topic : "") + F(" len=") + String(length) +
+            F(" depth=") + String(_messageDispatchDepth) + F(" deferred=") + String(_deferredCount)
+    );
 
     _messageDispatchDepth++;
 
@@ -415,7 +566,22 @@ void HAMqtt::processMessage(const char* topic, const uint8_t* payload, uint16_t
     _messageDispatchDepth--;
 
     if (_messageDispatchDepth == 0) {
-        flushDeferredPublishes();
+        const uint8_t before = _deferredCount;
+        const bool ok = flushDeferredPublishes();
+        if (!ok) {
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("deferred flush failed after rx queue=")) + String(_deferredCount) +
+                    F(" reason=") + diagnosticDisconnectReasonText(_lastDisconnectReason)
+            );
+        } else if (before > 0) {
+            arduinoHALog(
+                ArduinoHALogLevel::Debug,
+                kMqtt,
+                String(F("deferred flush ok after rx count=")) + String(before)
+            );
+        }
     }
 }
 
@@ -426,11 +592,18 @@ void HAMqtt::connectToServer()
         return;
     }
 
-    _lastConnectionAttemptAt = millis();
+    const uint32_t now = millis();
+    const uint32_t sinceLastAttempt = (_lastConnectionAttemptAt > 0) ? (now - _lastConnectionAttemptAt) : 0;
+    _lastConnectionAttemptAt = now;
     setState(StateConnecting);
 
-    ARDUINOHA_DEBUG_PRINT(F("AHA: MQTT connecting, client ID: "))
-    ARDUINOHA_DEBUG_PRINTLN(_device.getUniqueId())
+    arduinoHALog(
+        ArduinoHALogLevel::Info,
+        kMqtt,
+        String(F("connect attempt clientId=")) + (_device.getUniqueId() ? _device.getUniqueId() : "") +
+            F(" sincePrevAttemptMs=") + String(sinceLastAttempt) +
+            F(" reconnectIntervalMs=") + String(_reconnectInterval)
+    );
 
     _mqtt->connect(
         _device.getUniqueId(),
@@ -446,7 +619,11 @@ void HAMqtt::connectToServer()
     if (isConnected()) {
         setState(StateConnected);
     } else {
-        ARDUINOHA_DEBUG_PRINTLN(F("AHA: failed to connect"))
+        arduinoHALog(
+            ArduinoHALogLevel::Warn,
+            kMqtt,
+            String(F("connect failed pubsub=")) + String(_mqtt->state())
+        );
     }
 }
 
@@ -574,7 +751,16 @@ bool HAMqtt::publishDeviceDiscovery()
     strcat_P(topic, HASerializerSlash);
     strcat_P(topic, HAConfigTopic);
 
+    const bool discConnBefore = isConnected();
+    const int discPsBefore = getPubSubState();
     if (!_mqtt->beginPublish(topic, payloadLength, true)) {
+        arduinoHALog(
+            ArduinoHALogLevel::Warn,
+            kDiscovery,
+            String(F("device discovery beginPublish failed topic=")) + topic +
+                F(" len=") + String(payloadLength) +
+                formatDirectPublishFailureDiagnostics(discConnBefore, discPsBefore)
+        );
         for (uint8_t i = 0; i < componentSerializerCount; i++) {
             delete componentSerializers[i];
         }
@@ -615,7 +801,11 @@ bool HAMqtt::publishDeviceDiscovery()
 
     writePayload(AHATOFSTR(HASerializerJsonDataSuffix));
     writePayload(AHATOFSTR(HASerializerJsonDataSuffix));
-    return endPublish();
+    const bool published = endPublish();
+    if (!published) {
+        arduinoHALog(ArduinoHALogLevel::Warn, kDiscovery, F("device discovery endPublish failed"));
+    }
+    return published;
 }
 
 void HAMqtt::setState(ConnectionState state)
@@ -623,16 +813,40 @@ void HAMqtt::setState(ConnectionState state)
     ConnectionState previousState = _currentState;
     _currentState = state;
 
-    ARDUINOHA_DEBUG_PRINT(F("AHA: MQTT state changed to "))
-    ARDUINOHA_DEBUG_PRINT(_currentState)
-    ARDUINOHA_DEBUG_PRINT(F(", previous state: "))
-    ARDUINOHA_DEBUG_PRINTLN(previousState)
+    arduinoHALog(
+        ArduinoHALogLevel::Info,
+        kMqtt,
+        String(F("mqtt state ")) + String(static_cast<int>(previousState)) +
+            F(" -> ") + String(static_cast<int>(_currentState)) +
+            F(" pubsub=") + String(_mqtt->state())
+    );
 
     if (_currentState == StateConnected) {
-        ARDUINOHA_DEBUG_PRINTLN(F("AHA: MQTT connected"))
+        _lastDisconnectReason = DiagnosticDisconnectReason::None;
+        arduinoHALog(ArduinoHALogLevel::Info, kMqtt, F("connected"));
         onConnectedLogic();
     } else if (previousState == StateConnected && _currentState != StateConnected) {
-        ARDUINOHA_DEBUG_PRINTLN(F("AHA: MQTT disconnected"))
+        const uint32_t now = millis();
+        _lastDisconnectAt = now;
+        if (_lastDisconnectReason == DiagnosticDisconnectReason::None) {
+            _lastDisconnectReason = DiagnosticDisconnectReason::UnderlyingPubSubStateChanged;
+        }
+
+        arduinoHALog(
+            ArduinoHALogLevel::Warn,
+            kMqtt,
+            String(F("disconnect ahaState=")) + String(static_cast<int>(_currentState)) +
+                F(" pubsub=") + String(_mqtt->state()) +
+                F(" reason=") + diagnosticDisconnectReasonText(_lastDisconnectReason) +
+                F(" deferred=") + String(_deferredCount) +
+                F(" depth=") + String(_messageDispatchDepth) +
+                F(" sinceLastMsgMs=") +
+                String(_lastMessageAt ? (now - _lastMessageAt) : static_cast<uint32_t>(0)) +
+                F(" sinceLastPubMs=") +
+                String(_lastPublishAt ? (now - _lastPublishAt) : static_cast<uint32_t>(0)) +
+                F(" sinceLastLoopOkMs=") +
+                String(_lastLoopOkAt ? (now - _lastLoopOkAt) : static_cast<uint32_t>(0))
+        );
 
         if (_disconnectedCallback) {
             _disconnectedCallback();
@@ -652,6 +866,13 @@ bool HAMqtt::enqueueDeferredPublish(
 )
 {
     if (!topic || _deferredCount >= DeferredQueueCapacity) {
+        if (topic && _deferredCount >= DeferredQueueCapacity) {
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("deferred queue full; drop topic=")) + topic + F(" len=") + String(length)
+            );
+        }
         return false;
     }
 
@@ -724,7 +945,17 @@ bool HAMqtt::flushDeferredPublishes()
     while (_deferredCount > 0) {
         DeferredPublishMessage& msg = _deferredQueue[_deferredHead];
 
+        const bool defConnSnap = isConnected();
+        const int defPsSnap = getPubSubState();
         if (!isConnected()) {
+            _lastDisconnectReason = DiagnosticDisconnectReason::NotConnectedDuringDeferredFlush;
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("deferred flush: not connected topic=")) +
+                    (msg.topic ? msg.topic : "") + F(" queue=") + String(_deferredCount) +
+                    formatDirectPublishFailureDiagnostics(defConnSnap, defPsSnap)
+            );
 #ifdef ARDUINOHA_TEST
             _deferredFlushFailedForTest = true;
             _lastDeferredFlushErrorForTest = DeferredFlushErrorNotConnected;
@@ -732,7 +963,17 @@ bool HAMqtt::flushDeferredPublishes()
             return false;
         }
 
+        const bool defBpConnBefore = isConnected();
+        const int defBpPsBefore = getPubSubState();
         if (!_mqtt->beginPublish(msg.topic, msg.length, msg.retained)) {
+            _lastDisconnectReason = DiagnosticDisconnectReason::DeferredBeginPublishFailed;
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("deferred beginPublish failed topic=")) +
+                    (msg.topic ? msg.topic : "") + F(" len=") + String(msg.length) +
+                    formatDirectPublishFailureDiagnostics(defBpConnBefore, defBpPsBefore)
+            );
 #ifdef ARDUINOHA_TEST
             _deferredFlushFailedForTest = true;
             _lastDeferredFlushErrorForTest = DeferredFlushErrorBeginPublish;
@@ -744,7 +985,16 @@ bool HAMqtt::flushDeferredPublishes()
             _mqtt->write(msg.payload, msg.length);
         }
 
+        const bool defEpConnBefore = isConnected();
+        const int defEpPsBefore = getPubSubState();
         if (!_mqtt->endPublish()) {
+            _lastDisconnectReason = DiagnosticDisconnectReason::DeferredEndPublishFailed;
+            arduinoHALog(
+                ArduinoHALogLevel::Warn,
+                kMqtt,
+                String(F("deferred endPublish failed topic=")) + (msg.topic ? msg.topic : "") +
+                    formatDirectPublishFailureDiagnostics(defEpConnBefore, defEpPsBefore)
+            );
 #ifdef ARDUINOHA_TEST
             _deferredFlushFailedForTest = true;
             _lastDeferredFlushErrorForTest = DeferredFlushErrorEndPublish;
@@ -752,6 +1002,7 @@ bool HAMqtt::flushDeferredPublishes()
             return false;
         }
 
+        _lastPublishAt = millis();
         clearDeferredMessage(msg);
         _deferredHead = static_cast<uint8_t>((_deferredHead + 1) % DeferredQueueCapacity);
         _deferredCount--;
diff --git a/src/HAMqtt.h b/src/HAMqtt.h
index 6c27171..a317512 100644
--- a/src/HAMqtt.h
+++ b/src/HAMqtt.h
@@ -37,6 +37,22 @@ using namespace arduino;
 class HAMqtt
 {
 public:
+    /**
+     * Best-effort classification of the last disconnect or transport failure
+     * (for diagnostics; not an MQTT DISCONNECT reason code).
+     */
+    enum class DiagnosticDisconnectReason : uint8_t {
+        None = 0,
+        LoopReturnedFalse,
+        DeferredBeginPublishFailed,
+        DeferredEndPublishFailed,
+        NotConnectedDuringDeferredFlush,
+        UnderlyingPubSubStateChanged,
+        ExplicitDisconnect,
+    };
+
+    static const char* diagnosticDisconnectReasonText(DiagnosticDisconnectReason reason);
+
     enum ConnectionState {
         StateConnecting = -5,
         StateConnectionTimeout = -4,
@@ -194,6 +210,26 @@ public:
     inline ConnectionState getState() const
         { return _currentState; }
 
+    inline uint8_t getDeferredQueueCount() const
+        { return _deferredCount; }
+
+    inline uint32_t getLastLoopOkAt() const
+        { return _lastLoopOkAt; }
+
+    inline uint32_t getLastMessageAt() const
+        { return _lastMessageAt; }
+
+    inline uint32_t getLastPublishAt() const
+        { return _lastPublishAt; }
+
+    inline uint32_t getLastDisconnectAt() const
+        { return _lastDisconnectAt; }
+
+    inline DiagnosticDisconnectReason getLastDisconnectReason() const
+        { return _lastDisconnectReason; }
+
+    int getPubSubState() const;
+
     /**
      * Sets parameters of the MQTT connection using the IP address and port.
      * The library will try to connect to the broker in first loop cycle.
@@ -526,6 +562,11 @@ private:
      */
     bool flushDeferredPublishes();
 
+    /**
+     * PubSubClient snapshot when a direct publish step fails (header or endPublish).
+     */
+    String formatDirectPublishFailureDiagnostics(bool hamqttConnectedBefore, int pubsubStateBefore) const;
+
 #ifdef ARDUINOHA_TEST
     PubSubClientMock* _mqtt;
 #else
@@ -598,6 +639,12 @@ private:
     /// The last known state of the MQTT connection.
     ConnectionState _currentState;
 
+    uint32_t _lastLoopOkAt;
+    uint32_t _lastMessageAt;
+    uint32_t _lastPublishAt;
+    uint32_t _lastDisconnectAt;
+    DiagnosticDisconnectReason _lastDisconnectReason;
+
     /// Nesting depth for inbound message dispatch (processMessage).
     uint8_t _messageDispatchDepth;