[modbus] Command lifecycle: PDU-carrying callbacks, on_sent(), notified queue clearing (#17886)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

esphome/components/modbus/modbus.cpp

@@ -313,7 +313,6 @@ void ModbusClientHub::process_modbus_server_frame(uint8_t address, std::span<con
                  function_code, exception, address, this->last_modbus_byte_ - this->last_send_);
         if (device)
           device->on_error(request_pdu, static_cast<ExceptionCode>(exception));
-
       } else if (device) {  // Not an error response
         device->on_response(request_pdu, pdu);
       } else {  // Not an error response, but no device to respond to
@@ -507,16 +506,25 @@ void ModbusClientHub::send_next_frame_() {
     return;
   }
 
-  ModbusDeviceCommand &command = this->tx_buffer_.front();
+  // Move the command out and pop BEFORE attempting the send: no callback may run while the frame still
-
+  // sits in the queue (the same principle as the clear sweep). A failure callback that sends would
-  if (this->send_frame_(command.frame)) {
+  // otherwise queue a new frame and pop_front() could discard the wrong one - and the deque
-    this->waiting_for_response_ = std::move(command);
+  // reference / PDU span could be invalidated mid-callback.
-  } else {
+  ModbusDeviceCommand command = std::move(this->tx_buffer_.front());
-    if (command.device)
-      command.device->on_not_sent();
-  }
-
   this->tx_buffer_.pop_front();
+  ModbusClientDevice *device = command.device;
+  const bool sent = this->send_frame_(command.frame);
+
+  if (sent) {
+    // The frame now lives in the waiting slot; its PDU is the frame without the leading address and
+    // trailing CRC.
+    ModbusDeviceCommand &wfr = this->waiting_for_response_.emplace(std::move(command));
+    if (device != nullptr)
+      device->on_sent(wfr.frame.pdu());
+  } else {
+    if (device != nullptr)
+      device->trigger_not_sent(command.frame.pdu());
+  }
 
   if (!this->tx_buffer_.empty()) {
     ESP_LOGV(TAG, "Write queue contains %zu items.", this->tx_buffer_.size());
@@ -574,7 +582,7 @@ void ModbusServerHub::send_exception_(uint8_t address, uint8_t function_code, Ex
 void ModbusClientHub::notify_no_response_(ModbusDeviceCommand &wfr) {
   if (wfr.device == nullptr)
     return;
-  const bool retry = wfr.device->on_no_response();
+  const bool retry = wfr.device->on_no_response(wfr.frame.pdu());
   // The callback may have detached the device (e.g. clear_tx_queue_for_device()); honor the detach
   // over the retry request rather than re-queueing a frame that can no longer be routed.
   if (retry && wfr.device != nullptr)
@@ -588,7 +596,7 @@ void ModbusClientHub::requeue_waiting_frame_(ModbusDeviceCommand &wfr) {
   if (this->tx_buffer_.size() >= MODBUS_TX_BUFFER_SIZE) {
     ESP_LOGE(TAG, "Write buffer full, dropped retry for address %" PRIu8, frame.address());
     if (wfr.device != nullptr)
-      wfr.device->on_not_sent();
+      wfr.device->trigger_not_sent(frame.pdu());
     return;
   }
   // Re-queue a copy (not a move): the waiting entry may have to survive as an interrupted shell.
@@ -598,16 +606,16 @@ void ModbusClientHub::requeue_waiting_frame_(ModbusDeviceCommand &wfr) {
 // Raw send for client: pushes to tx queue. Everything except the CRC must be contained in payload.
 void ModbusClientHub::send_pdu(uint8_t address, std::span<const uint8_t> pdu, ModbusClientDevice *device) {
   if (pdu.empty()) {
-    if (device)
+    if (device != nullptr)
-      device->on_not_sent();
+      device->trigger_not_sent(pdu);
     return;
   }
 
   // Bound the PDU so the wire frame (address + pdu + CRC) stays within the Modbus RTU 256-byte limit.
   if (pdu.size() > MAX_PDU_SIZE) {
     ESP_LOGE(TAG, "Frame too large, dropped: %" PRIu8 ":%zu bytes", address, pdu.size());
-    if (device)
+    if (device != nullptr)
-      device->on_not_sent();
+      device->trigger_not_sent(pdu);
     return;
   }
 
@@ -624,17 +632,36 @@ void ModbusClientHub::send_pdu(uint8_t address, std::span<const uint8_t> pdu, Mo
 #endif
     ESP_LOGE(TAG, "Write buffer full, dropped: %" PRIu8 ":%s", address,
              format_hex_pretty_to(hex_buf, pdu.data(), pdu.size()));
-    if (device)
+    if (device != nullptr)
-      device->on_not_sent();
+      device->trigger_not_sent(pdu);
   }
 }
 
 void ModbusClientHub::clear_tx_queue_for_address(uint8_t address, bool clear_sent) {
-  // Remove any pending commands for this address from the tx buffer
+  // Drop the queued frames for this address, delivering on_not_sent() to each frame's owner: other
-  auto &tx_buffer = this->tx_buffer_;
+  // devices talking to the same physical device (e.g. a modbus_client action alongside a controller that
-  tx_buffer.erase(std::remove_if(tx_buffer.begin(), tx_buffer.end(),
+  // just went offline) must observe the drop, or their command never resolves. Mark first, then sweep
-                                 [address](const ModbusDeviceCommand &cmd) { return cmd.frame.address() == address; }),
+  // only marked frames: anything a callback re-queues is unmarked, so it
-                  tx_buffer.end());
+  // is never swept - or re-notified - by the clear that triggered it. Each marked frame is moved out and erased BEFORE
+  // its callback runs, so handlers see a consistent queue; termination is guaranteed because only the initially-marked
+  // frames are ever swept.
+  for (auto &cmd : this->tx_buffer_) {
+    if (cmd.frame.address() == address)
+      cmd.marked_for_deletion = true;
+  }
+  for (;;) {
+    auto it = std::find_if(this->tx_buffer_.begin(), this->tx_buffer_.end(),
+                           [](const ModbusDeviceCommand &cmd) { return cmd.marked_for_deletion; });
+    if (it == this->tx_buffer_.end())
+      break;
+    ModbusDeviceCommand dropped = std::move(*it);
+    this->tx_buffer_.erase(it);
+    // The sweep delivers through the same per-device guard as refusals: a device clearing from inside
+    // its own on_not_sent() gets its remaining frames resolved silently (documented in the lifecycle
+    // contract), other owners are notified normally, and every nested clear stays bounded.
+    if (dropped.device != nullptr)
+      dropped.device->trigger_not_sent(dropped.frame.pdu());
+  }
 
   if (clear_sent && this->waiting_for_response_.has_value() && this->waiting_for_response_.value().device) {
     if (this->waiting_for_response_.value().frame.address() == address) {
@@ -662,8 +689,8 @@ void ModbusClientHub::clear_tx_queue_for_device(ModbusClientDevice *device) {
 
 void ModbusClientHub::send_raw(const std::vector<uint8_t> &payload, ModbusClientDevice *device) {
   if (payload.size() < 2) {
-    if (device)
+    if (device != nullptr)
-      device->on_not_sent();
+      device->trigger_not_sent({});  // too short to contain a PDU
     return;
   }
   this->send_pdu(payload[0], std::span<const uint8_t>(payload).subspan(1), device);

esphome/components/modbus/modbus.h

@@ -94,6 +94,9 @@ struct ModbusDeviceCommand {
   ModbusClientDevice *device;
   ModbusFrame frame;
   bool interrupted{false};
+  /// Marked by clear_tx_queue_for_address() before it starts notifying, so frames re-queued by an
+  /// on_not_sent() callback (which are unmarked) are never swept by the clear that triggered them.
+  bool marked_for_deletion{false};
 
   ModbusDeviceCommand(ModbusClientDevice *device, uint8_t address, const uint8_t *src, uint16_t len)
       : device(device), frame(address, src, len) {}
@@ -123,6 +126,10 @@ class ModbusClientHub : public Modbus {
   void send_pdu(uint8_t address, std::span<const uint8_t> pdu, ModbusClientDevice *device = nullptr);
   ESPDEPRECATED("Use send_pdu(payload[0], <pdu bytes>, device) instead. Removed in 2027.2.0", "2026.8.0")
   void send_raw(const std::vector<uint8_t> &payload, ModbusClientDevice *device = nullptr);
+  // Drop the queued commands for an address; every dropped frame resolves via its owner's on_not_sent(),
+  // so other devices sharing the address observe the drop. The in-flight frame is only detached (silently)
+  // when clear_sent is set. clear_tx_queue_for_device() SILENTLY discards the caller's own frames
+  // (supersede/teardown semantics); see the lifecycle note on ModbusClientDevice.
   void clear_tx_queue_for_address(uint8_t address, bool clear_sent = true);
   void clear_tx_queue_for_device(ModbusClientDevice *device);
 
@@ -177,6 +184,25 @@ class ModbusServerHub : public Modbus {
 // Transaction status: std::nullopt on success, otherwise a Modbus exception code
 using ResponseStatus = std::optional<ExceptionCode>;
 
+/// Command lifecycle: each accepted command (a send_pdu()/typed-helper call, or a hub re-queue from
+/// a retry) ends in exactly ONE terminal callback: on_response() (valid response), on_error()
+/// (exception response), on_no_response() (timeout or interrupted transaction), or on_not_sent()
+/// (never transmitted: send failure or full queue). on_sent() is additional, not
+/// terminal: it fires once per wire transmission, before whichever of data/error/no_response follows,
+/// and never for a command that ends in on_not_sent().
+/// The exceptions to "exactly one terminal":
+///  - clear_tx_queue_for_device() drops the caller's OWN queued commands SILENTLY (supersede/teardown
+///    semantics), and both clear variants detach the in-flight frame silently.
+///    clear_tx_queue_for_address() DOES resolve every queued frame it drops via the owner's
+///    on_not_sent() (delivered one at a time, after that frame leaves the queue).
+///  - while a device's own on_not_sent() is on the stack, further on_not_sent() deliveries to THAT
+///    device are dropped (see trigger_not_sent()). In particular, a clear issued from inside your own
+///    on_not_sent() resolves your remaining frames silently - treat it like
+///    clear_tx_queue_for_device(): you cleared them, you know. Other owners are still notified.
+/// Sending from inside on_not_sent() is hazardous: the notification may itself mean the queue is full
+/// or refusing, and this device's retry that is refused again is dropped WITHOUT a callback (the
+/// guard above, which bounds what would otherwise be unbounded re-entry) - prefer re-sending from a
+/// later trigger or the component's update()/loop().
 class ModbusClientDevice {
  public:
   ModbusClientDevice() = default;
@@ -204,21 +230,34 @@ class ModbusClientDevice {
   virtual void on_error(std::span<const uint8_t> request_pdu, ExceptionCode exception_code) {
     this->dispatch_response_(request_pdu, {}, exception_code);
   }
-
+  /// Called when no request could be sent (e.g. queue full, transmission blocked).
-  /// Called when no request could be sent (e.g. queue full, transmission blocked)
   /// Do not attempt to queue a command in this callback.
-  /// (The on_modbus_* names are signature-identical renames, so the new defaults forward to the old
+  /// (The on_modbus_* names below are deprecated pre-rename spellings; the defaults forward so
-  /// virtuals: external devices overriding the old names keep working through the deprecation window.
+  /// external devices overriding them keep working through the deprecation window.)
-  /// Remove the forwards together with the deprecated names.)
+  virtual void on_not_sent(std::span<const uint8_t> request_pdu) {
-  virtual void on_not_sent() {
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
     this->on_modbus_not_sent();
 #pragma GCC diagnostic pop
   }
+  /// Non-virtual entry point the hub uses for EVERY on_not_sent() delivery (refusals and clear-queue
+  /// sweeps alike). While this device's on_not_sent() is on the stack, further deliveries to it are
+  /// dropped: this bounds every send->refuse and clear->sweep recursion, including cycles through
+  /// multiple devices (each device can appear on the stack at most once). The documented cost: a clear
+  /// issued from inside your own on_not_sent() resolves your remaining frames SILENTLY, while other
+  /// owners are still notified (their guards are not set) - see the lifecycle contract above.
+  void trigger_not_sent(std::span<const uint8_t> request_pdu) {
+    if (this->notifying_not_sent_)
+      return;
+    this->notifying_not_sent_ = true;
+    this->on_not_sent(request_pdu);
+    this->notifying_not_sent_ = false;
+  }
+  /// Called when this device's frame is actually written to the wire
+  virtual void on_sent(std::span<const uint8_t> request_pdu) {}
   /// Called when no matching, uninterrupted response arrived; return true to have the hub re-queue the frame for a
   /// retry. The hub does not bound retries: the device is responsible for limiting them.
-  virtual bool on_no_response() {
+  virtual bool on_no_response(std::span<const uint8_t> request_pdu) {
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
     return this->on_modbus_no_response();
@@ -287,7 +326,8 @@ class ModbusClientDevice {
   ESPDEPRECATED("Use send_pdu() instead (the device address is prepended for you). Removed in 2027.2.0", "2026.8.0")
   void send_raw(const std::vector<uint8_t> &payload) {
     if (payload.empty()) {
-      this->on_not_sent();  // match the hub-level send_raw(): a refused send is always signalled
+      // Through the guard like every other delivery, so a handler calling send_raw({}) cannot recurse.
+      this->trigger_not_sent({});
       return;
     }
     this->parent_->send_pdu(payload[0], std::span<const uint8_t>(payload).subspan(1), this);
@@ -345,6 +385,8 @@ class ModbusClientDevice {
                           ResponseStatus status);
 
   ModbusClientHub *parent_{nullptr};
+  /// True while this device's on_not_sent() is on the stack (see trigger_not_sent()).
+  bool notifying_not_sent_{false};
   uint8_t address_{0};
   bool custom_response_warned_{false};  // first unhandled custom response warns; repeats log at VERBOSE
 };

esphome/components/modbus_controller/modbus_controller.h

@@ -306,7 +306,8 @@ class ModbusController final : public PollingComponent, public modbus::ModbusCli
   /// controller-level raw sends stay supported until the command machinery is replaced.
   void send_raw(const std::vector<uint8_t> &payload) {
     if (payload.empty()) {
-      this->on_not_sent();  // match the hub-level send_raw(): a refused send is always signalled
+      // Through the guard like every other delivery, so a handler calling send_raw({}) cannot recurse.
+      this->trigger_not_sent({});
       return;
     }
     this->parent_->send_pdu(payload[0], std::span<const uint8_t>(payload).subspan(1), this);

tests/components/modbus/common.h

@@ -0,0 +1,22 @@
+#pragma once
+#include <cstdint>
+#include "esphome/components/uart/uart_component.h"
+
+namespace esphome::modbus::testing {
+
+// A UART that discards all writes, for tests that never inspect the wire.
+class NullUART : public uart::UARTComponent {
+ public:
+  NullUART() { this->set_baud_rate(115200); }
+  void write_array(const uint8_t *data, size_t len) override {}
+  bool peek_byte(uint8_t *data) override { return false; }
+  bool read_array(uint8_t *data, size_t len) override { return false; }
+  size_t available() override { return 0; }
+  uart::UARTFlushResult flush() override { return uart::UARTFlushResult::UART_FLUSH_RESULT_ASSUMED_SUCCESS; }
+#if defined(USE_ESP8266) || defined(USE_ESP32)
+  void load_settings(bool dump_config) override {}
+#endif
+  void check_logger_conflict() override {}
+};
+
+}  // namespace esphome::modbus::testing

tests/components/modbus/modbus_client_hub_test.cpp

@@ -1,9 +1,13 @@
 #include <gtest/gtest.h>
 
 #include <cstdint>
+#include <cstring>
 #include <span>
+#include <vector>
 
+#include "common.h"
 #include "esphome/components/modbus/modbus.h"
+#include "esphome/core/hal.h"
 
 namespace esphome::modbus::testing {
 
@@ -16,12 +20,14 @@ class NoResponseProbeHub : public ModbusClientHub {
  public:
   size_t queued_frames() const { return this->tx_buffer_.size(); }
   const ModbusDeviceCommand &front() const { return this->tx_buffer_.front(); }
+  const ModbusDeviceCommand &queued(size_t i) const { return this->tx_buffer_[i]; }
   bool waiting() const { return this->waiting_for_response_.has_value(); }
   const ModbusDeviceCommand &waiting_command() const {
     EXPECT_TRUE(this->waiting_for_response_.has_value());
     return *this->waiting_for_response_;  // NOLINT(bugprone-unchecked-optional-access)
   }
 
+  void send_next_for_test() { this->send_next_frame_(); }
   void force_send_front() {
     this->waiting_for_response_ = std::move(this->tx_buffer_.front());
     this->tx_buffer_.pop_front();
@@ -41,7 +47,7 @@ class NoResponseProbeHub : public ModbusClientHub {
 class RetryingDevice : public ModbusClientDevice {
  public:
   RetryingDevice(ModbusClientHub *hub, uint8_t address, bool retry) : ModbusClientDevice(hub, address), retry_(retry) {}
-  bool on_no_response() override {
+  bool on_no_response(std::span<const uint8_t> request_pdu) override {
     this->no_response_count_++;
     return this->retry_;
   }
@@ -55,7 +61,7 @@ class RetryingDevice : public ModbusClientDevice {
 class ClearingRetryDevice : public ModbusClientDevice {
  public:
   ClearingRetryDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
-  bool on_no_response() override {
+  bool on_no_response(std::span<const uint8_t> request_pdu) override {
     this->no_response_count_++;
     this->clear_tx_queue_for_device();  // detaches this device from the waiting slot mid-callback
     return true;                        // and still requests a retry
@@ -176,6 +182,565 @@ TEST(ModbusClientHubNoResponse, MidCallbackClearCancelsRetry) {
   EXPECT_FALSE(hub.waiting());
 }
 
+// A device whose sent/not-sent callbacks are counted.
+namespace {
+class SentCountingDevice : public ModbusClientDevice {
+ public:
+  SentCountingDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_sent(std::span<const uint8_t> request_pdu) override {
+    this->sent_count_++;
+    this->last_sent_pdu_.assign(request_pdu.begin(), request_pdu.end());
+  }
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    this->last_not_sent_pdu_.assign(request_pdu.begin(), request_pdu.end());
+  }
+  int sent_count_{0};
+  int not_sent_count_{0};
+  std::vector<uint8_t> last_sent_pdu_;
+  std::vector<uint8_t> last_not_sent_pdu_;
+};
+}  // namespace
+
+// on_sent() fires when the frame goes onto the wire, not when it is queued.
+TEST(ModbusClientHubSent, FiresOnWireNotOnQueue) {
+  NullUART uart;
+  NoResponseProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();  // frame timing derives from the baud rate
+  SentCountingDevice device(&hub, 0x02);
+
+  device.send_pdu(read_pdu());
+  EXPECT_EQ(device.sent_count_, 0);  // queued only - nothing on the wire yet
+
+  hub.send_next_for_test();
+  EXPECT_EQ(device.sent_count_, 1);
+  EXPECT_EQ(device.not_sent_count_, 0);
+  // The callback identifies which command transmitted: it carries the request PDU.
+  EXPECT_EQ(device.last_sent_pdu_, (std::vector<uint8_t>(READ_PDU, READ_PDU + sizeof(READ_PDU))));
+  EXPECT_TRUE(hub.waiting());
+}
+
+// Counts response deliveries so requeue semantics can be pinned end to end.
+namespace {
+class DataCountingDevice : public ModbusClientDevice {
+ public:
+  DataCountingDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_response(std::span<const uint8_t> request_pdu, std::span<const uint8_t> response_pdu) override {
+    this->data_count_++;
+  }
+  void on_error(std::span<const uint8_t> request_pdu, ExceptionCode exception_code) override { this->error_count_++; }
+  bool on_no_response(std::span<const uint8_t> request_pdu) override {
+    this->no_response_count_++;
+    this->last_no_response_pdu_.assign(request_pdu.begin(), request_pdu.end());
+    if (this->retries_ == 0)
+      return false;
+    this->retries_--;
+    return true;
+  }
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    this->last_not_sent_pdu_.assign(request_pdu.begin(), request_pdu.end());
+  }
+  void on_sent(std::span<const uint8_t> request_pdu) override { this->sent_count_++; }
+  int terminals() const {
+    return this->data_count_ + this->error_count_ + this->no_response_count_ + this->not_sent_count_;
+  }
+  int data_count_{0};
+  int error_count_{0};
+  int no_response_count_{0};
+  int not_sent_count_{0};
+  int sent_count_{0};
+  int retries_{0};
+  std::vector<uint8_t> last_not_sent_pdu_;
+  std::vector<uint8_t> last_no_response_pdu_;
+};
+
+// Runs full send/respond cycles until the queue drains; returns the number of cycles executed.
+int drain_with_responses(NoResponseProbeHub &hub, std::span<const uint8_t> response_pdu, int max_cycles = 10) {
+  int cycles = 0;
+  while (hub.queued_frames() != 0 && cycles < max_cycles) {
+    hub.force_send_front();
+    hub.receive_frame_for_test(0x02, response_pdu);
+    cycles++;
+  }
+  return cycles;
+}
+}  // namespace
+
+constexpr uint8_t OK_RESPONSE[] = {0x03, 0x04, 0x00, 0x2A, 0x01, 0x00};
+
+// One request produces exactly one data callback.
+TEST(ModbusClientHubCallbackCount, SingleReadSingleCallback) {
+  NoResponseProbeHub hub;
+  DataCountingDevice device(&hub, 0x02);
+
+  device.send_pdu(read_pdu());
+  drain_with_responses(hub, OK_RESPONSE);
+
+  EXPECT_EQ(device.data_count_, 1);
+  EXPECT_EQ(device.not_sent_count_, 0);
+  EXPECT_EQ(hub.queued_frames(), 0u);
+  EXPECT_FALSE(hub.waiting());
+}
+
+// An exception response is a terminal on its own: exactly one on_error(), no others,
+// preceded by exactly one on_sent().
+TEST(ModbusClientHubCallbackCount, ErrorResponseIsSoleTerminal) {
+  NullUART uart;
+  NoResponseProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+  DataCountingDevice device(&hub, 0x02);
+
+  device.send_pdu(read_pdu());
+  hub.send_next_for_test();
+  const uint8_t exception_response[] = {0x83, 0x02};
+  hub.receive_frame_for_test(0x02, exception_response);
+
+  EXPECT_EQ(device.error_count_, 1);
+  EXPECT_EQ(device.terminals(), 1);
+  EXPECT_EQ(device.sent_count_, 1);
+}
+
+// A timeout is a terminal on its own: exactly one on_no_response(), preceded by one
+// on_sent(); a refused duplicate ends in on_not_sent() with NO on_sent().
+TEST(ModbusClientHubCallbackCount, NoResponseIsSoleTerminalAndNotSentHasNoSent) {
+  NullUART uart;
+  NoResponseProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+  DataCountingDevice device(&hub, 0x02);
+
+  device.send_pdu(read_pdu());
+  hub.send_next_for_test();
+  hub.timeout_waiting();
+  EXPECT_EQ(device.no_response_count_, 1);
+  EXPECT_EQ(device.terminals(), 1);
+  EXPECT_EQ(device.sent_count_, 1);
+
+  // A refused send (empty PDU) is a not_sent terminal, never sent.
+  const uint8_t write_pdu[] = {0x06, 0x00, 0x10, 0xBE, 0xEF};
+  device.send_pdu(write_pdu);
+  device.send_pdu(std::span<const uint8_t>{});
+  EXPECT_EQ(device.not_sent_count_, 1);
+  EXPECT_EQ(device.terminals(), 2);  // the accepted write is still queued - no terminal for it yet
+  EXPECT_EQ(device.sent_count_, 1);  // and it has not transmitted yet
+
+  // Drain it: the write echo response is its data terminal, and the books balance.
+  hub.send_next_for_test();
+  hub.receive_frame_for_test(0x02, write_pdu);
+  EXPECT_EQ(device.data_count_, 1);
+  EXPECT_EQ(device.terminals(), 3);  // 3 accepted lifecycles, 3 terminals
+  EXPECT_EQ(device.sent_count_, 2);  // 2 transmissions (read + write); the refused send never sent
+}
+
+// A device-requested retry starts a new lifecycle: each transmission gets its own sent + terminal.
+TEST(ModbusClientHubCallbackCount, RetryLifecyclesEachGetSentAndTerminal) {
+  NullUART uart;
+  NoResponseProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+  DataCountingDevice device(&hub, 0x02);
+  device.retries_ = 1;  // ask for exactly one retry
+
+  device.send_pdu(read_pdu());
+  hub.send_next_for_test();
+  hub.timeout_waiting();  // lifecycle 1: sent + no_response (retry requested -> re-queued)
+  ASSERT_EQ(hub.queued_frames(), 1u);
+  hub.send_next_for_test();
+  hub.timeout_waiting();  // lifecycle 2: sent + no_response (retry declined -> done)
+
+  EXPECT_EQ(device.no_response_count_, 2);
+  EXPECT_EQ(device.terminals(), 2);
+  EXPECT_EQ(device.sent_count_, 2);
+  EXPECT_EQ(hub.queued_frames(), 0u);
+  // The retried lifecycle's timeout carries the SAME request PDU as the first attempt.
+  EXPECT_EQ(device.last_no_response_pdu_, std::vector<uint8_t>(READ_PDU, READ_PDU + sizeof(READ_PDU)));
+}
+
+// A retry re-queue that finds the buffer full is refused like any other send: the device gets
+// on_not_sent() carrying the request PDU (the previously uncovered requeue_waiting_frame_ branch).
+TEST(ModbusClientHubCallbackCount, FullQueueRetryRefusalDeliversNotSentWithPdu) {
+  NullUART uart;
+  NoResponseProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+  DataCountingDevice device(&hub, 0x02);
+  device.retries_ = 1;
+  SentCountingDevice filler(&hub, 0x05);
+
+  device.send_pdu(read_pdu());
+  hub.force_send_front();  // in flight
+  // Fill the queue with distinct frames.
+  for (uint16_t i = 0; i < MODBUS_TX_BUFFER_SIZE; i++) {
+    const uint8_t fill[] = {0x03, static_cast<uint8_t>(i >> 8), static_cast<uint8_t>(i & 0xFF), 0x00, 0x01};
+    filler.send_pdu(fill);
+  }
+  ASSERT_EQ(hub.queued_frames(), MODBUS_TX_BUFFER_SIZE);
+
+  hub.timeout_waiting();  // retry requested, but the re-queue is refused: not_sent terminal instead
+
+  EXPECT_EQ(device.no_response_count_, 1);
+  EXPECT_EQ(device.not_sent_count_, 1);
+  EXPECT_EQ(device.last_not_sent_pdu_, std::vector<uint8_t>(READ_PDU, READ_PDU + sizeof(READ_PDU)));
+  EXPECT_EQ(hub.queued_frames(), MODBUS_TX_BUFFER_SIZE);
+}
+
+// The deprecated device-side send_raw() refusal delivers through the same guard as every other
+// path: a handler that reacts to its own refusal with another empty send_raw() stays bounded.
+namespace {
+class SendRawOnNotSentDevice : public ModbusClientDevice {
+ public:
+  SendRawOnNotSentDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+    this->send_raw({});  // refused again; the guard must suppress the nested delivery
+#pragma GCC diagnostic pop
+  }
+  int not_sent_count_{0};
+};
+}  // namespace
+
+TEST(ModbusClientHubQueue, SendRawRefusalIsGuardedAgainstRecursion) {
+  NoResponseProbeHub hub;
+  SendRawOnNotSentDevice device(&hub, 0x02);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+  device.send_raw({});  // empty payload refused -> on_not_sent -> nested send_raw({}) suppressed
+#pragma GCC diagnostic pop
+  EXPECT_EQ(device.not_sent_count_, 1);
+}
+
+namespace {
+// A device that chains a follow-up send from inside on_sent().
+class ChainOnSentDevice : public ModbusClientDevice {
+ public:
+  ChainOnSentDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_sent(std::span<const uint8_t> request_pdu) override {
+    if (!this->chained_) {
+      this->chained_ = true;
+      const uint8_t follow[] = {0x03, 0x00, 0x09, 0x00, 0x01};  // read holding 0x0009 x1
+      this->send_pdu(follow);
+    }
+  }
+  bool chained_{false};
+};
+}  // namespace
+
+// clear_tx_queue_for_address() resolves every dropped frame via its owner's on_not_sent(), so a device
+// sharing the address with the clearer (e.g. a modbus_client action alongside an offline controller)
+// observes the drop; frames for other addresses are untouched.
+TEST(ModbusClientHubQueue, ClearAddressQueueNotifiesEveryOwner) {
+  NoResponseProbeHub hub;
+  SentCountingDevice controller_like(&hub, 0x02);
+  SentCountingDevice bystander_same(&hub, 0x02);
+  SentCountingDevice bystander_other(&hub, 0x03);
+
+  const uint8_t read_a[] = {0x03, 0x01, 0x00, 0x00, 0x02};
+  const uint8_t read_b[] = {0x03, 0x02, 0x00, 0x00, 0x02};
+  const uint8_t read_c[] = {0x03, 0x03, 0x00, 0x00, 0x02};
+  controller_like.send_pdu(read_a);
+  bystander_same.send_pdu(read_b);
+  bystander_other.send_pdu(read_c);
+  ASSERT_EQ(hub.queued_frames(), 3u);
+
+  controller_like.clear_tx_queue_for_address(false);
+
+  ASSERT_EQ(hub.queued_frames(), 1u);  // only the other-address frame remains
+  EXPECT_EQ(hub.front().frame.address(), 0x03);
+  EXPECT_EQ(controller_like.not_sent_count_, 1);
+  EXPECT_EQ(bystander_same.not_sent_count_, 1);
+  EXPECT_EQ(bystander_other.not_sent_count_, 0);
+  // each owner saw its own request PDU
+  EXPECT_EQ(bystander_same.last_not_sent_pdu_, std::vector<uint8_t>(std::begin(read_b), std::end(read_b)));
+}
+
+namespace {
+// Re-sends its frame once from inside on_not_sent - the re-queued frame must survive the sweep.
+class ResendOnNotSentDevice : public ModbusClientDevice {
+ public:
+  ResendOnNotSentDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    if (this->not_sent_count_ == 1) {
+      const uint8_t again[] = {0x06, 0x00, 0x40, 0x00, 0x01};
+      this->send_pdu(again);
+    }
+  }
+  int not_sent_count_{0};
+};
+}  // namespace
+
+// A handler that re-sends to the same address from inside on_not_sent() neither corrupts the sweep nor
+// loops it: only initially-marked frames are swept, so the re-queued frame stays queued.
+TEST(ModbusClientHubQueue, ClearAddressReentrantResendSurvives) {
+  NoResponseProbeHub hub;
+  ResendOnNotSentDevice device(&hub, 0x02);
+
+  const uint8_t read[] = {0x03, 0x00, 0x10, 0x00, 0x01};
+  device.send_pdu(read);
+  ASSERT_EQ(hub.queued_frames(), 1u);
+
+  hub.clear_tx_queue_for_address(0x02, false);
+
+  // The original frame resolved via on_not_sent; the re-send from inside that callback remains queued.
+  EXPECT_EQ(device.not_sent_count_, 1);
+  ASSERT_EQ(hub.queued_frames(), 1u);
+  EXPECT_EQ(hub.front().frame.address(), 0x02);
+}
+
+namespace {
+// Retries from EVERY on_not_sent - against a full queue this recursed without bound before the guard.
+class AlwaysRetryDevice : public ModbusClientDevice {
+ public:
+  AlwaysRetryDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    const uint8_t again[] = {0x03, 0x00, 0x50, 0x00, 0x01};
+    this->send_pdu(again);
+  }
+  int not_sent_count_{0};
+};
+
+// From inside on_not_sent, clears ANOTHER address - those victims must still be notified (the per-device
+// guard suppresses deliveries only to a device already inside its own on_not_sent()).
+class ClearOtherOnNotSentDevice : public ModbusClientDevice {
+ public:
+  ClearOtherOnNotSentDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    this->parent_->clear_tx_queue_for_address(0x03, false);
+  }
+  int not_sent_count_{0};
+};
+}  // namespace
+
+// A handler that retries from every on_not_sent() against a FULL queue must not recurse: the first
+// refusal notifies once, the nested refusal is dropped without a callback (the documented guard).
+TEST(ModbusClientHubQueue, FullQueueRetryFromNotSentDoesNotRecurse) {
+  NoResponseProbeHub hub;
+  SentCountingDevice filler(&hub, 0x05);
+  AlwaysRetryDevice retrier(&hub, 0x02);
+
+  // Fill the queue with distinct frames.
+  for (uint16_t i = 0; i < MODBUS_TX_BUFFER_SIZE; i++) {
+    const uint8_t fill[] = {0x03, static_cast<uint8_t>(i >> 8), static_cast<uint8_t>(i & 0xFF), 0x00, 0x01};
+    filler.send_pdu(fill);
+  }
+  ASSERT_EQ(hub.queued_frames(), MODBUS_TX_BUFFER_SIZE);
+
+  const uint8_t read[] = {0x03, 0x00, 0x10, 0x00, 0x01};
+  retrier.send_pdu(read);  // refused (full) -> on_not_sent -> retry -> refused under the guard, silently
+
+  EXPECT_EQ(retrier.not_sent_count_, 1);
+  EXPECT_EQ(hub.queued_frames(), MODBUS_TX_BUFFER_SIZE);
+}
+
+namespace {
+// From inside on_not_sent, triggers ANOTHER device's send (which will be refused too).
+class SendOtherOnNotSentDevice : public ModbusClientDevice {
+ public:
+  SendOtherOnNotSentDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    if (this->other_ != nullptr) {
+      const uint8_t read[] = {0x03, 0x00, 0x60, 0x00, 0x01};
+      this->other_->send_pdu(read);
+    }
+  }
+  ModbusClientDevice *other_{nullptr};
+  int not_sent_count_{0};
+};
+}  // namespace
+
+// The refusal recursion guard is per-device: a refusal that lands on a DIFFERENT device while one
+// device's notification is on the stack must still deliver - that device did not cause the recursion
+// and would otherwise silently lose its terminal callback.
+TEST(ModbusClientHubQueue, RefusalForOtherDeviceDeliversDuringNotification) {
+  NoResponseProbeHub hub;
+  SentCountingDevice filler(&hub, 0x05);
+  SendOtherOnNotSentDevice first(&hub, 0x02);
+  SentCountingDevice second(&hub, 0x03);
+  first.other_ = &second;
+
+  // Fill the queue with distinct frames.
+  for (uint16_t i = 0; i < MODBUS_TX_BUFFER_SIZE; i++) {
+    const uint8_t fill[] = {0x03, static_cast<uint8_t>(i >> 8), static_cast<uint8_t>(i & 0xFF), 0x00, 0x01};
+    filler.send_pdu(fill);
+  }
+  ASSERT_EQ(hub.queued_frames(), MODBUS_TX_BUFFER_SIZE);
+
+  const uint8_t read[] = {0x03, 0x00, 0x10, 0x00, 0x01};
+  first.send_pdu(read);  // refused -> first.on_not_sent -> second's send refused -> second notified
+
+  EXPECT_EQ(first.not_sent_count_, 1);
+  EXPECT_EQ(second.not_sent_count_, 1);
+}
+
+// Two devices whose handlers each trigger the other's send cannot recurse without bound: each device
+// can be on the notification stack at most once, so the cycle dies as soon as it returns to a device
+// whose own on_not_sent() is still running.
+TEST(ModbusClientHubQueue, TwoDeviceRefusalCycleTerminates) {
+  NoResponseProbeHub hub;
+  SentCountingDevice filler(&hub, 0x05);
+  SendOtherOnNotSentDevice first(&hub, 0x02);
+  SendOtherOnNotSentDevice second(&hub, 0x03);
+  first.other_ = &second;
+  second.other_ = &first;
+
+  // Fill the queue with distinct frames.
+  for (uint16_t i = 0; i < MODBUS_TX_BUFFER_SIZE; i++) {
+    const uint8_t fill[] = {0x03, static_cast<uint8_t>(i >> 8), static_cast<uint8_t>(i & 0xFF), 0x00, 0x01};
+    filler.send_pdu(fill);
+  }
+  ASSERT_EQ(hub.queued_frames(), MODBUS_TX_BUFFER_SIZE);
+
+  const uint8_t read[] = {0x03, 0x00, 0x10, 0x00, 0x01};
+  first.send_pdu(read);  // refuse -> first -> second refused -> second -> first suppressed -> unwind
+
+  EXPECT_EQ(first.not_sent_count_, 1);
+  EXPECT_EQ(second.not_sent_count_, 1);
+}
+
+namespace {
+// From inside on_not_sent, clears its OWN address - its remaining queued frames resolve silently
+// (the guard suppresses self-deliveries), while other owners on the address are still notified.
+class ClearOwnAddressOnNotSentDevice : public ModbusClientDevice {
+ public:
+  ClearOwnAddressOnNotSentDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    this->clear_tx_queue_for_address(/*clear_sent=*/false);
+  }
+  int not_sent_count_{0};
+};
+}  // namespace
+
+// The documented cost of the per-device guard: a clear issued from inside your own on_not_sent()
+// resolves your remaining frames silently (like clear_tx_queue_for_device() - you cleared them, you
+// know), while other owners sharing the address are still notified.
+TEST(ModbusClientHubQueue, SelfClearFromNotSentSilentForClearerNotifiesOthers) {
+  NoResponseProbeHub hub;
+  ClearOwnAddressOnNotSentDevice clearer(&hub, 0x02);
+  SentCountingDevice bystander(&hub, 0x02);
+
+  const uint8_t read_a[] = {0x03, 0x00, 0x10, 0x00, 0x01};
+  const uint8_t read_b[] = {0x03, 0x00, 0x20, 0x00, 0x01};
+  const uint8_t read_c[] = {0x03, 0x00, 0x30, 0x00, 0x01};
+  clearer.send_pdu(read_a);
+  clearer.send_pdu(read_b);
+  bystander.send_pdu(read_c);
+  ASSERT_EQ(hub.queued_frames(), 3u);
+
+  clearer.send_pdu(std::span<const uint8_t>{});  // refused (empty) -> the handler clears the shared address
+
+  EXPECT_EQ(clearer.not_sent_count_, 1);    // only the refusal; the two swept frames resolve silently
+  EXPECT_EQ(bystander.not_sent_count_, 1);  // the bystander's swept frame is still notified
+  EXPECT_EQ(hub.queued_frames(), 0u);
+}
+
+// The guard must not over-suppress: a sweep started from inside on_not_sent() still delivers its
+// victims' notifications (only nested refusals are silenced).
+TEST(ModbusClientHubQueue, NestedClearFromNotSentStillNotifiesVictims) {
+  NoResponseProbeHub hub;
+  ClearOtherOnNotSentDevice clearer(&hub, 0x02);
+  SentCountingDevice victim(&hub, 0x03);
+
+  const uint8_t read_a[] = {0x03, 0x00, 0x10, 0x00, 0x01};
+  const uint8_t read_b[] = {0x03, 0x00, 0x20, 0x00, 0x01};
+  clearer.send_pdu(read_a);
+  victim.send_pdu(read_b);
+  ASSERT_EQ(hub.queued_frames(), 2u);
+
+  hub.clear_tx_queue_for_address(0x02, false);  // clearer's on_not_sent clears address 0x03 in turn
+
+  EXPECT_EQ(clearer.not_sent_count_, 1);
+  EXPECT_EQ(victim.not_sent_count_, 1);  // delivered despite arriving from a nested sweep
+  EXPECT_EQ(hub.queued_frames(), 0u);
+}
+
+namespace {
+// tx_blocked() flips to blocked after the first check, so send_next_frame_() passes its own gate but
+// send_frame_() refuses - a deterministic transmit failure.
+class FlakyBlockHub : public NoResponseProbeHub {
+ public:
+  bool tx_blocked() override {
+    this->tx_blocked_calls_++;
+    return this->tx_blocked_calls_ > 1;
+  }
+  int tx_blocked_calls_{0};
+};
+
+// Reacts to a transmit failure by sending another frame from inside the failure callback.
+class WriteOnNotSentDevice : public ModbusClientDevice {
+ public:
+  WriteOnNotSentDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
+  void on_not_sent(std::span<const uint8_t> request_pdu) override {
+    this->not_sent_count_++;
+    const uint8_t write[] = {0x06, 0x00, 0x40, 0x01, 0x02};
+    this->send_pdu(write);
+  }
+  int not_sent_count_{0};
+};
+
+}  // namespace
+
+// A transmit failure must resolve with the failed frame OUT of the queue before its on_not_sent runs: a
+// handler that reacts by sending a new frame must not have that frame discarded by the pop that
+// follows - the failed frame is popped first, the new frame survives.
+TEST(ModbusClientHubQueue, TransmitFailurePopsBeforeNotify) {
+  FlakyBlockHub hub;
+  WriteOnNotSentDevice device(&hub, 0x02);
+
+  const uint8_t read[] = {0x03, 0x00, 0x10, 0x00, 0x01};
+  device.send_pdu(read);
+  ASSERT_EQ(hub.queued_frames(), 1u);
+
+  hub.send_next_for_test();  // tx_blocked gate passes, send_frame_ refuses -> failure path
+
+  EXPECT_EQ(device.not_sent_count_, 1);
+  ASSERT_EQ(hub.queued_frames(), 1u);           // the handler's write survives...
+  EXPECT_EQ(hub.front().frame.pdu()[0], 0x06);  // ...and it is the write, not the failed read
+}
+
+// clear_tx_queue_for_device() drops queued frames SILENTLY - no terminal callback (the documented
+// exception to the exactly-one-terminal contract; used during teardown/offline handling).
+TEST(ModbusClientHubQueue, ClearDeviceQueueDropsSilently) {
+  NoResponseProbeHub hub;
+  SentCountingDevice device(&hub, 0x02);
+
+  const uint8_t read_a[] = {0x03, 0x01, 0x00, 0x00, 0x02};
+  const uint8_t read_b[] = {0x03, 0x02, 0x00, 0x00, 0x02};
+  device.send_pdu(read_a);
+  device.send_pdu(read_b);
+  ASSERT_EQ(hub.queued_frames(), 2u);
+
+  device.clear_tx_queue_for_device();
+
+  EXPECT_EQ(hub.queued_frames(), 0u);
+  EXPECT_EQ(device.not_sent_count_, 0);  // silent drop: no terminal callback
+}
+
+// A send_pdu() from inside on_sent() enqueues behind the in-flight frame rather than sending
+// immediately or corrupting the in-flight transaction.
+TEST(ModbusClientHubSent, ReentrantSendFromOnSentQueues) {
+  NullUART uart;
+  NoResponseProbeHub hub;
+  hub.set_uart_parent(&uart);
+  hub.setup();
+  ChainOnSentDevice device(&hub, 0x02);
+
+  device.send_pdu(read_pdu());
+  hub.send_next_for_test();  // first frame goes on the wire -> on_sent chains a follow-up
+
+  EXPECT_TRUE(hub.waiting());                     // first frame is in flight
+  ASSERT_EQ(hub.queued_frames(), 1u);             // the follow-up queued behind it, not sent
+  EXPECT_EQ(hub.queued(0).frame.pdu()[2], 0x09);  // it is the chained read (start address 0x0009)
+}
+
 namespace {
 // Overrides only the DEPRECATED on_modbus_* names: the new-name default implementations must forward, so
 // external devices written against the old names keep working through the deprecation window.
@@ -336,7 +901,7 @@ namespace {
 class NotSentCountingDevice : public ModbusClientDevice {
  public:
   NotSentCountingDevice(ModbusClientHub *hub, uint8_t address) : ModbusClientDevice(hub, address) {}
-  void on_not_sent() override { this->not_sent_++; }
+  void on_not_sent(std::span<const uint8_t> request_pdu) override { this->not_sent_++; }
   int not_sent_{0};
 };
 }  // namespace