Alex2ESP/readme.md

# Alex2ESP

Alex2ESP is an Arduino/PlatformIO library that makes an ESP8266 a set of Amazon Alexa smart home devices. The sketch declares devices and their capabilities (a lamp with `PowerController`, a blind with a `RangeController`); the library announces them to Alexa, hands every directive to a handler of the sketch and sends the answers and reports. It talks to Alexa through the [Alex2MQTT](https://alex2mqtt.stormysdream.club/) skill, over MQTT only.

One of the most popular libraries for controlling ESP devices with Alexa is [FauxmoESP](https://github.com/vintlabs/fauxmoESP). It works without an account by emulating a light bulb, so it is limited to the commands of one. Alex2ESP needs the Alex2MQTT account and announces the device as what it is.

- [What it needs](#what-it-needs) · [Supported boards](#supported-boards)
- [Quick start: PlatformIO](#quick-start-platformio) · [Quick start: Arduino IDE](#quick-start-arduino-ide) · [The Light example](#the-light-example)
- [Writing a sketch](#writing-a-sketch) · [Interface Types](#interface-types)
- [The MQTT contract](#the-mqtt-contract) · [Session and log lines](#session-and-log-lines)
- [Memory](#memory) · [Configuration](#configuration)
- [Migrating from 1.x](#migrating-from-1x) · [Limits](#limits) · [Tests](#tests) · [Changelog](CHANGELOG.md)

---

## What it needs

- **An Alex2MQTT account.** Log in with Amazon at https://alex2mqtt.stormysdream.club/. The page shows the three credentials a sketch passes to `begin()`: the MQTT user name, the MQTT password and the root topic.
- **An ESP8266 board** with the Arduino core 3.x.
- **Two libraries**: AsyncMqttClient 0.9.x (which needs ESPAsyncTCP) and ArduinoJson 7.
- **The network**: outbound TCP port 1883 to the broker, and DNS and outbound UDP port 123 for the time of day (`pool.ntp.org`, `time.nist.gov`).

## Supported boards

| Board | State |
|---|---|
| ESP8266 | Developed and measured on a Wemos D1 mini (PlatformIO, espressif8266 4.2.1, Arduino core 3.1.2). Every figure in this readme is from that board or that build. |
| ESP32 | Untested. The sources have `#ifdef` branches for it, but the library has never been compiled for an ESP32 or run on one. The examples include `ESP8266WiFi.h`, and `library.json` and `library.properties` name the ESP8266 only. |

---

## Quick start: PlatformIO

The library lives on Forgejo at https://git.stormysdream.club/platformio/Alex2ESP (public). It is not in the PlatformIO registry or the Arduino Library Manager: install it from git.

```ini
[env:d1_mini]
platform = espressif8266
board = d1_mini
framework = arduino
monitor_speed = 74880
lib_deps =
    https://git.stormysdream.club/platformio/Alex2ESP.git#v2.0.0
    marvinroger/AsyncMqttClient@^0.9.0
    bblanchon/ArduinoJson@^7
```

`#v2.0.0` pins the release; without it PlatformIO takes `main`. AsyncMqttClient pulls in ESPAsyncTCP by itself; if PlatformIO notes that more than one `ESPAsyncTCP` package matches, add `esp32async/ESPAsyncTCP@^2.0.0` to `lib_deps` to pick one explicitly. The examples print at 74880 baud, the ESP8266 boot-ROM rate, so the boot messages stay readable in the same monitor.

Copy [`examples/Light/Light.ino`](examples/Light/Light.ino) into `src/` of the project, fill in the five constants at its top and upload.

## Quick start: Arduino IDE

1. Download the repository as a ZIP (https://git.stormysdream.club/platformio/Alex2ESP/archive/v2.0.0.zip) and add it with *Sketch -> Include Library -> Add .ZIP Library*.
2. Install **AsyncMqttClient**, **ArduinoJson** (7.x) and **ESP Async TCP** (by ESP32Async) from the Library Manager.
3. Install the ESP8266 board package and select your board (for example *LOLIN(WEMOS) D1 R2 & mini*).
4. Open *File -> Examples -> Alex2ESP -> Light*, fill in the five constants at its top and upload.

The examples are built with PlatformIO. They are laid out as the Arduino IDE expects them, one folder per sketch, but they have not been built with the IDE yet.

## The Light example

[`examples/Light/Light.ino`](examples/Light/Light.ino), complete:

```cpp
// Light: a lamp that Alexa switches on and off (Alexa.PowerController).
//
// The lamp is the on-board LED. One handler gets every directive of the device: it answers ReportState with the
// state of the lamp, carries out TurnOn and TurnOff, and refuses anything else.
#include <Arduino.h>
#include <ESP8266WiFi.h>
#include <Alex2ESP.h>

// Wi-Fi and the credentials of your Alex2MQTT account
const char *WIFI_SSID = "";
const char *WIFI_PASSWORD = "";

const char *ALEXA_USERNAME = "";
const char *ALEXA_PASSWORD = "";
const char *ALEXA_ROOT_TOPIC = "";

// The on-board LED: GPIO2 on a Wemos D1 mini, lit when the pin is low
#ifndef LED_BUILTIN
#define LED_BUILTIN 2
#endif

Alex2ESP alexa;
bool lampOn = false;

// Joins the Wi-Fi network and returns after 30 s at the latest. Without a connection the sketch carries on: the
// ESP8266 keeps trying, and alexa.loop() opens the MQTT session once Wi-Fi is up.
void connectWiFi()
{
  WiFi.mode(WIFI_STA);
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  Serial.printf("\n[WIFI] Connecting to \"%s\"\n", WIFI_SSID);

  unsigned long started = millis();
  while (WiFi.status() != WL_CONNECTED && millis() - started < 30000)
  {
    delay(100);
  }

  if (WiFi.status() == WL_CONNECTED)
  {
    Serial.printf("[WIFI] Connected, IP address %s\n", WiFi.localIP().toString().c_str());
  }
  else
  {
    Serial.printf("[WIFI] No connection after 30 s (status %d): check WIFI_SSID and WIFI_PASSWORD. Still trying.\n",
                  WiFi.status());
  }
}

// The state of the lamp: what the answer to a directive and the answer to ReportState carry
void sendState(AlexaStatusMessage message)
{
  message.addHealthProp(EndpointHealth::OK)
      .addPowerControllerProp(lampOn ? PowerController::ON : PowerController::OFF)
      .send();
}

void onDirective(AlexaDirective &directive)
{
  if (directive.isReportState())
  {
    sendState(directive.stateReport());
    return;
  }

  if (directive.type == AlexaInterfaceType::POWER_CONTROLLER && (directive.is("TurnOn") || directive.is("TurnOff")))
  {
    lampOn = directive.is("TurnOn");
    digitalWrite(LED_BUILTIN, lampOn ? LOW : HIGH);
    sendState(directive.response());
    return;
  }

  directive.error(AlexaErrorType::INVALID_DIRECTIVE, "This lamp switches on and off").send();
}

void setup()
{
  Serial.begin(74880);
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, HIGH);

  connectWiFi();

  // MQTT user name, MQTT password, root topic
  alexa.begin(ALEXA_USERNAME, ALEXA_PASSWORD, ALEXA_ROOT_TOPIC);

  // The name Alexa shows, and the id of the endpoint: every device of an account has its own
  AlexaDevice *lamp = alexa.getDevice("Desk Lamp", "esp-light");
  lamp->setDisplayCategory(DisplayCategory::LIGHT);
  lamp->addCapability(AlexaInterfaces::PowerController);
  lamp->addCapability(AlexaInterfaces::EndpointHealth);
  lamp->onDirective(onDirective);
}

void loop()
{
  alexa.loop();
}
```

After the upload, let Alexa discover devices. The serial monitor shows the session, the discovery and one line per directive.

The other examples, one folder per kind of device: `DimmableLight`, `ColorTemperatureLight`, `ColorLight`, `TemperatureSensor`, `ContactSensor`, `Blind`, `Thermostat`, `Lock`, `Scene`, `Doorbell` and `MultiDevice`. [`examples/README.md`](examples/README.md) says what each does. The examples of 1.x are in [`examples/legacy/`](examples/legacy/).

---

## Writing a sketch

A sketch has one `Alex2ESP`, calls `begin(username, password, rootTopic)` once and `loop()` from its own `loop()`. `getDevice(name, endpointId)` creates a device, `addCapability()` gives it an interface, `onDirective()` its handler. The code blocks of this section are excerpts of the examples; the file is named under each.

### Devices and capabilities

`getDevice()` is called after `begin()`. The endpoint id is what Alexa knows the device by: two boards with the same endpoint id on one account are one device to Alexa. The pointers returned by `getDevice()` and `addCapability()` stay valid for the lifetime of the client. `getDevice()` returns `nullptr`, with an error in the log, when the heap has no room for another device. The examples use the pointer unchecked, because they create their devices in `setup()`; a sketch that creates devices by the dozen, or later than `setup()`, checks it.

A device holds 8 capabilities. Every device of the examples has `EndpointHealth` and reports `connectivity` with its state.

### The handler

`onDirective()` registers a function that gets every directive of its device, `ReportState` included. `AlexaDirective` has:

| Member | What it is |
|---|---|
| `device` | the device the directive is for |
| `capability`, `type` | the capability of the device with the namespace and the instance of the directive; `nullptr` and `AlexaInterfaceType::UNKNOWN` when the device has none, and for `ReportState` |
| `ns`, `name`, `instance` | `"Alexa.PowerController"`, `"TurnOn"`, `"Blind.Lift"` (`""` for an interface without instances) |
| `correlationToken` | what the answer has to repeat; the library does that |
| `payload` | the payload of the directive, `directive.payload["rangeValue"]` |
| `is(name)`, `isReportState()` | compare the name of the directive |
| `response()`, `stateReport()`, `error(type, message)`, `deferred(seconds)`, `sceneStarted()`, `sceneStopped()` | build the answer; `send()` sends it |

The texts are those of the directive and valid until the handler returns. The backend waits 7 s for the answer to a directive, so the handler answers before it returns.

One function can serve several devices; the directive says which one it is for:

```cpp
void onDirective(AlexaDirective &directive)
{
  int lamp = directive.device == lamps[1] ? 1 : 0;

  if (directive.isReportState())
  {
    sendState(directive.stateReport(), lamp);
    return;
  }
```

[`examples/MultiDevice/MultiDevice.ino`](examples/MultiDevice/MultiDevice.ino)

What the library does when the handler sends nothing:

| Case | Answer | Log |
|---|---|---|
| the device has no handler | ErrorResponse `INVALID_DIRECTIVE` | `... refused as INVALID_DIRECTIVE: the device has no handler, give it one with onDirective()` |
| the directive names a capability the device does not have | ErrorResponse `INVALID_DIRECTIVE` | `... refused as INVALID_DIRECTIVE: the device has no such capability, and its handler sent no answer` |
| the directive is for a capability of the device | none; Alexa reports that the device does not respond | `... the handler sent no answer to ...` |

### Capabilities with an instance

`RangeController`, `ModeController` and `ToggleController` are generic controllers: a device may have several of one interface, each with an instance and at least one name. `addCapability()` takes the row and the instance; the setters of the capability return the capability, so they can be chained. What the discovery object needs beside the names (the range of a value, the modes) the sketch writes in the function it passes to `setConfiguration()`.

```cpp
// The "configuration" of the capability in discovery: the range of the value and its unit
void liftConfiguration(JsonObject configuration, void *)
{
  JsonObject range = configuration["supportedRange"].to<JsonObject>();
  range["minimumValue"] = 0;
  range["maximumValue"] = 100;
  range["precision"] = 1;
  configuration["unitOfMeasure"] = FPSTR(AlexaUnits::Percent);
}
```

```cpp
  blind->addCapability(AlexaInterfaces::RangeController, LIFT)
      ->addFriendlyAsset(AlexaAssets::Setting_Opening)
      .setConfiguration(liftConfiguration)
      .addActionMapping(ActionMapping({AlexaAction::Close}, "SetRangeValue", "{\"rangeValue\":0}"))
      .addActionMapping(ActionMapping({AlexaAction::Open}, "SetRangeValue", "{\"rangeValue\":100}"))
      .addActionMapping(ActionMapping({AlexaAction::Lower}, "AdjustRangeValue",
                                      "{\"rangeValueDelta\":-10,\"rangeValueDeltaDefault\":false}"))
      .addActionMapping(ActionMapping({AlexaAction::Raise}, "AdjustRangeValue",
                                      "{\"rangeValueDelta\":10,\"rangeValueDeltaDefault\":false}"))
      .addStateMapping({AlexaState::Closed}, 0)
      .addStateMapping({AlexaState::Open}, 1, 100);
```

[`examples/Blind/Blind.ino`](examples/Blind/Blind.ino)

- An `ActionMapping` maps words of Alexa ("open", "close", "raise", "lower") to a directive: the actions, the name of the directive and its payload as JSON text. The [Alexa documentation](https://developer.amazon.com/en-US/docs/alexa/device-apis/alexa-discovery-objects.html#action-mapping) lists the actions. Action and state mappings are taken by the three generic controllers only.
- `AlexaAssets` and `AlexaUnits` (`src/AlexaResources.h`) hold the names and units of the catalog of Alexa, which Alexa translates: `AlexaAssets::Setting_Opening` is `Alexa.Setting.Opening`. `addFriendlyName("Lift", "en-US")` adds a name as text. An id that the sketch does not name takes no flash.
- A capability holds 3 names, 4 action mappings and 2 state mappings. The instance and the text of a friendly name are copied. The locale, an asset id, the text value of a state mapping and the directive name and payload of an `ActionMapping` are kept as pointers: pass literals.
- A setter that refuses what it is given prints the reason and the chain goes on. `isValid()` of the capability is `false` from then on, and the device leaves the capability out of discovery with an error, so that Alexa does not learn half of it.
- `addCapability()` returns `nullptr` for a generic controller without an instance, for an interface the library has no row for, and for a ninth capability: check the pointer when the instance is not a literal.
- `setProactivelyReported(true)` announces that the sketch sends ChangeReports for the capability, `setNonControllable(true)` that it reports its state and takes no directives.

### Reports

`response()` answers a directive that changed something, `stateReport()` answers `ReportState`. Both take the properties of the device, one helper per property (see [Interface Types](#interface-types)), and are sent with `send()`. Every property carries the board's UTC time as `timeOfSample`; the last argument of a helper is the time since the value was read, in milliseconds:

```cpp
    directive.stateReport()
        .addHealthProp(EndpointHealth::OK)
        .addTemperatureSensorProp(temperature, TemperatureSensorScale::CELSIUS, millis() - lastRead)
        .send();
```

[`examples/TemperatureSensor/TemperatureSensor.ino`](examples/TemperatureSensor/TemperatureSensor.ino)

A value outside of the range Alexa gives for its property (brightness, percentage, power level and humidity 0 to 100, color temperature 1000 to 10000, hue 0 to 360, saturation and brightness of a color 0 to 1) is reported as the nearest value of the range, with an error in the log.

`send()` returns `false` when the message was not sent: no session with the broker, the MQTT client or the heap cannot take it, or it is over `ALEX2ESP_MAX_MESSAGE`. Each case prints its reason. Nothing is sent truncated.

### Change reports

A `ChangeReport` tells Alexa that a property changed without a directive: a door opened, somebody pressed the switch on the lamp. The capability is announced with `setProactivelyReported(true)`. The properties added before `context()` are those that changed, the ones after it the others. A `ChangeReport` without a property that changed is not sent and prints an error.

```cpp
    contact->changeReport(AlexaCause::PHYSICAL_INTERACTION)
        .addContactSensorProp(isOpen)
        .context()
        .addHealthProp(EndpointHealth::OK)
        .send();
```

[`examples/ContactSensor/ContactSensor.ino`](examples/ContactSensor/ContactSensor.ino)

A sketch that reports a value that drifts limits how often it does so, and keeps what it reported only when the report left:

```cpp
  bool sent = sensor->changeReport(AlexaCause::PERIODIC_POLL)
                  .addTemperatureSensorProp(temperature)
                  .context()
                  .addHealthProp(EndpointHealth::OK)
                  .send();
  if (sent)
  {
    reported = temperature;
    everReported = true;
    lastReport = millis();
  }
```

[`examples/TemperatureSensor/TemperatureSensor.ino`](examples/TemperatureSensor/TemperatureSensor.ino)

### Deferred answers

A device that takes longer than the 7 s answers with a `DeferredResponse`, keeps a copy of the correlation token and sends the answer itself when it is done, with `sendAsync()`. Alexa accepts a deferred answer for `LockController` and `WakeOnLANController` only.

```cpp
  goal = directive.is("Lock") ? AlexaLockState::LOCKED : AlexaLockState::UNLOCKED;
  token = directive.correlationToken; // a copy: the text of the directive ends with the handler
  moving = true;
  movingSince = millis();
  digitalWrite(BOLT_PIN, goal == AlexaLockState::LOCKED ? HIGH : LOW);

  directive.deferred(5).send();
```

```cpp
  if (moving && millis() - movingSince >= BOLT_MS)
  {
    moving = false;
    state = digitalRead(BLOCKED_PIN) == LOW ? AlexaLockState::JAMMED : goal;
    doorLock->response(token).addHealthProp(EndpointHealth::OK).addLockControllerProp(state).sendAsync();
  }
```

[`examples/Lock/Lock.ino`](examples/Lock/Lock.ino): the handler, and `loop()`

### Errors

`error(type, message)` answers a directive that was not carried out. `AlexaErrorType` has the types of Alexa; the message is for the log of the skill. What a type carries beside it goes into `payload()`:

```cpp
    int position = directive.payload["rangeValue"] | -1;
    if (position < 0 || position > 100)
    {
      AlexaStatusMessage error = directive.error(AlexaErrorType::VALUE_OUT_OF_RANGE, "The blind takes 0 to 100");
      error.payload()["validRange"]["minimumValue"] = 0;
      error.payload()["validRange"]["maximumValue"] = 100;
      error.send();
      return;
    }
```

[`examples/Blind/Blind.ino`](examples/Blind/Blind.ino)

The types of a thermostat (`REQUESTED_SETPOINTS_TOO_CLOSE`, `THERMOSTAT_IS_OFF`, ...) are sent in the namespace `Alexa.ThermostatController`; [`examples/Thermostat/Thermostat.ino`](examples/Thermostat/Thermostat.ino) sends `THERMOSTAT_IS_OFF`, `DUAL_SETPOINTS_UNSUPPORTED`, `REQUESTED_SETPOINTS_TOO_CLOSE` and `UNSUPPORTED_THERMOSTAT_MODE`.

### Scenes

A scene has no state to report. `Activate` is answered with `sceneStarted()` in place of a Response, `Deactivate` with `sceneStopped()`:

```cpp
  if (directive.is("Activate"))
  {
    setScene(true);
    directive.sceneStarted().send();
  }
  else if (directive.is("Deactivate"))
  {
    setScene(false);
    directive.sceneStopped().send();
  }
```

[`examples/Scene/Scene.ino`](examples/Scene/Scene.ino)

### Doorbells

A doorbell sends an event and takes no directives:

```cpp
    if (pressed)
    {
      Serial.println("[DOORBELL] pressed");
      doorbell->doorbellPress().send();
    }
```

[`examples/Doorbell/Doorbell.ino`](examples/Doorbell/Doorbell.ino)

`device->event(row, name)` builds the event of another interface.

---

## Interface Types

The library describes 28 interfaces, each with a row in program memory (`src/AlexaInterfaces.cpp`). `addCapability()` takes the row, `AlexaInterfaces::PowerController`, or the type as 1.x sketches write it, `AlexaInterfaceType::POWER_CONTROLLER`. A sketch links the rows it names and no others, and the report helpers it calls and no others.

The last column says whether Alexa discovered and drove the interface **from this library**, on a Wemos D1 mini with a real Alexa account through the public Alex2MQTT service, on 2026-09-28. "no" means not tried, not that it failed.

| Row in `AlexaInterfaces` | Version | Properties | Report helper | Example | Driven through Alexa |
|---|---|---|---|---|---|
| `EndpointHealth` | 3.1 | connectivity | `addHealthProp` | all | not checked on its own |
| `PowerController` | 3 | powerState | `addPowerControllerProp` | Light | yes |
| `BrightnessController` | 3 | brightness | `addBrightnessControllerProp` | DimmableLight | yes |
| `ColorTemperatureController` | 3 | colorTemperatureInKelvin | `addColorTemperatureControllerProp` | ColorTemperatureLight | yes |
| `ColorController` | 3 | color | `addColorControllerProp(hue, saturation, brightness)` | ColorLight | no |
| `PowerLevelController` | 3 | powerLevel | `addPowerLevelControllerProp` | | no |
| `PercentageController` | 3 | percentage | `addPercentageControllerProp` | | no |
| `RangeController` | 3 | rangeValue | `addRangeControllerProp(instance, value)` | Blind | yes, one instance |
| `ModeController` | 3 | mode | `addModeControllerProp(instance, mode)` | | yes, one instance |
| `ToggleController` | 3 | toggleState | `addToggleControllerProp(instance, state)` | | yes, one instance |
| `ThermostatController` | 3.2 | targetSetpoint, lowerSetpoint, upperSetpoint, thermostatMode | `addThermostatSetpointProp`, `addThermostatLowerSetpointProp`, `addThermostatUpperSetpointProp` (both: `addThermostatDualSetpointProp(lower, upper)`), `addThermostatModeProp` | Thermostat | no |
| `TemperatureSensor` | 3 | temperature | `addTemperatureSensorProp(value, scale)` | TemperatureSensor | no |
| `HumiditySensor` | 3 | relativeHumidity | `addHumiditySensorProp` | | no |
| `LockController` | 3 | lockState | `addLockControllerProp` | Lock | no |
| `ContactSensor` | 3 | detectionState | `addContactSensorProp` | ContactSensor | no |
| `MotionSensor` | 3 | detectionState | `addMotionSensorProp` | | no |
| `TimeHoldController` | 3 | holdStartTime, holdEndTime | `addProperty` | | no |
| `Speaker` | 3 | volume, muted | `addProperty` | | no |
| `PlaybackStateReporter` | 3 | playbackState | `addProperty` | | no |
| `InputController` | 3 | input | `addProperty` | | no |
| `ChannelController` | 3 | channel | `addProperty` | | no |
| `InventoryLevelSensor` | 3 | level | `addProperty` | | no |
| `PlaybackController` | 3 | `"properties": {}` | | | no |
| `WakeOnLANController` | 3 | `"properties": {}` | | | no |
| `SceneController` | 3 | no `properties` object | | Scene | no |
| `DoorbellEventSource` | 3 | no `properties` object | | Doorbell | no |
| `StepSpeaker` | 3 | no `properties` object | | | no |
| `SimpleEventSource` | 1.0 | no `properties` object | | | no |

The six interfaces with "yes" were driven with another sketch than the examples, which have not run on a board yet. The directives were answered within 0.2 to 0.4 s, and a directive that was delivered twice was answered once. Voice commands have not been tried.

The Alex2MQTT service itself has carried more than that. With its Node.js client, alex2node 1.5.2, on the same day: Power, Brightness, Color, ColorTemperature, Percentage, PowerLevel, Thermostat, Range, Mode, Toggle, Lock and Scene, deferred responses, ErrorResponse, ReportState, and ChangeReports for contact, motion, lock, temperature and power. So the way from the broker to Alexa is tried for these; what this library sends for them is checked by the host tests only.

`addProperty(row, place, value)` adds a property that has no helper by its place in the row, 0 for the first: `addProperty(AlexaInterfaces::Speaker, 1, muted)` reports `muted`. `addContextProp(object)` adds a property that the sketch has written as JSON.

Every other value of `AlexaInterfaceType` (`COOKING`, `LAUNCHER`, `SECURITY_PANEL_CONTROLLER`, ...) names an interface the library has no row for. `addCapability()` adds nothing for it, prints `[Alex2ESP] error: <endpointId>: capability not added: ...` and returns `nullptr`.

---

## The MQTT contract

Everything goes over MQTT, to port 1883 of `alex2mqtt.stormysdream.club` or of the broker named with `setServer()`. The library makes no HTTP requests. It subscribes with QoS 1 and publishes with QoS 0.

| Topic | Direction | What |
|---|---|---|
| `<root>/discover` | in | the request to announce the devices |
| `<root>/discover_r` | out | one discovery object per device, one message each |
| `<root>/<endpointId>/alexaDirective` | in | the directive as JSON; subscribed as `<root>/+/alexaDirective` |
| `<root>/<endpointId>/alexaResponce` | out | Response, StateReport, ErrorResponse, DeferredResponse, ActivationStarted, DeactivationStarted |
| `<root>/<endpointId>/deferredResponse` | out | the answer after a DeferredResponse (`sendAsync()`) |
| `<root>/changeReport` | out | ChangeReport |
| `<root>/event` | out | DoorbellPress and the events of `event(row, name)` |

**Discovery.** The backend accepts one endpoint object per message and collects everything that arrives within 1 s for Alexa's discovery answer (up to 5 s for its proactive AddOrUpdate push), so all devices are published back to back from the next `loop()`. Each object sits on the heap (about 1 KB) until the MQTT client has sent it. When the client cannot take another one (free heap under 4 KB), the library prints `discovery deferred at <endpointId>` and sends the rest from `loop()` as the queue drains, for up to 5 s after the request.

**Directives.** A directive larger than one TCP segment arrives in fragments, which are put together in one heap block that exists until `loop()` has parsed it. Every call of `loop()` handles one directive, in the order of arrival.

**Sizes.**

| Limit | Default | Build flag |
|---|---|---|
| largest directive | 2047 bytes | `ALEX2ESP_MAX_DIRECTIVE` |
| largest message sent: a report, or the discovery object of one device | 3071 bytes, the largest directive plus 1024 | `ALEX2ESP_MAX_MESSAGE` |
| directives that wait for `loop()` | 8 | `ALEX2ESP_MAX_QUEUED_DIRECTIVES` |
| bytes they take together | 8188, four times the largest directive | `ALEX2ESP_MAX_QUEUED_BYTES` |

Alex2MQTT publishes directives of 600 to 900 bytes; most of that is the correlation token. An answer repeats the token and adds 140 to 170 bytes per property, so the largest directive can be answered with six properties. A directive over the limit, and one that finds no place among the waiting ones, is dropped with an error in the log.

**The queue.** Alexa sends a group command ("turn off the kitchen") as one directive per endpoint, and they arrive faster than a busy sketch calls `loop()`. Up to eight wait for it.

**Duplicates.** A directive that arrives twice is handled once. The Alex2MQTT broker currently delivers every message twice through a mirror. The repeat is recognised while it arrives, by a hash of its bytes, and takes no place in the queue; a directive that comes again with other bytes is recognised by its `messageId`. The last 16 directives are remembered.

**Other boards.** The subscription delivers the directives of every endpoint of the account. Those for endpoints of another board are recognised by their topic and neither buffered nor parsed.

**Messages.** Every message has a `messageId` of its own, a UUID of version 4 from the random source of the hardware.

**Boards with 1.x.** Boards that run 1.1.0 or older keep working: the backend still publishes the token on `<root>/<endpointId>/alexaDirective_e` and serves the HTTP routes they use.

---

## Session and log lines

`begin()` starts SNTP and returns. `loop()` opens the MQTT session once Wi-Fi is up and the clock is set, or after 5 s of Wi-Fi without an answer from a time server. `getState()` is `Alex2ESPState::CONNECTED` when the broker has acknowledged both subscriptions.

A session that ends or cannot be opened is opened again, whatever ended it, for as long as Wi-Fi is up:

| | |
|---|---|
| wait before the next attempt | 1 s, then 2 s, 4 s, ... up to 60 s |
| the wait starts at 1 s again | after a session that lasted a minute |
| an attempt without an answer | given up after 30 s |
| MQTT keep-alive | 30 s |
| while Wi-Fi is down | nothing is tried; the session is opened when Wi-Fi is back |

Tried on a D1 mini against a scripted broker (2026-09-28): the waits of 1 to 60 s, a refused password, a broker that does not answer, and loss of Wi-Fi.

Every line of the library starts with `[Alex2ESP]`, a problem with `[Alex2ESP] error:`. The user name, the password, the root topic and correlation tokens are never printed, at any level, so a log can be posted as it is: a topic appears without its root (`esp-light/alexaResponce`), a directive under its endpoint id.

| Line | What it means | What to do |
|---|---|---|
| `connected to <host>, subscribing` | the broker accepted the session | |
| `ready: N device(s)` | both subscriptions are acknowledged | |
| `discovery: N of M device(s) announced` | the answer to a discovery request | |
| `esp-light <- Alexa.PowerController.TurnOn` | a directive was handed to the handler | |
| `waiting for Wi-Fi: the sketch has to join a network with WiFi.begin()` | `loop()` runs and Wi-Fi is not up; printed once | check the SSID and the password of the sketch |
| `error: Wi-Fi is down, waiting for it` | the link was lost; printed once per loss | nothing: the session is opened again when Wi-Fi is back |
| `error: disconnected: the broker refused the username or the password, ...; next attempt in N s` | the first two arguments of `begin()` are not those of the account | copy them again from the Alex2MQTT page. This is the reason to look for when a board never shows up in Alexa |
| `error: disconnected: the broker cannot be reached or the connection was lost; ...` | no TCP connection, or it ended | check the network and port 1883; the library tries again |
| `error: disconnected: the broker did not answer within 30 s; ...` | the attempt was given up | as above |
| `error: the broker refused the subscription to <root>/discover: check the root topic passed to begin()` | the root topic is not the one of the account; the state stays `SUBSCRIBING` | correct the third argument of `begin()` |
| `error: the clock is not set, reports carry a time in 1970: ...` | no answer from a time server; printed when the session opens and at most once a minute while reports are sent | allow DNS and outbound UDP port 123, or set the clock in the sketch and call `setTimeSource(false)` |
| `discovery deferred at <endpointId>` | the heap is short, the rest is sent from `loop()` | nothing |
| `error: discovery gave up: N device(s) not announced` | those devices missed this answer; on the backend's proactive discovery that can remove them from Alexa until the next one | fewer devices on the board, or more free heap |
| `error: device <endpointId> not announced: its discovery object has N bytes, ...` | the object is over `ALEX2ESP_MAX_MESSAGE` | fewer capabilities or mappings on the device, or a higher limit |
| `error: <endpointId>: directive of N bytes dropped: ...` | over `ALEX2ESP_MAX_DIRECTIVE`, no place in the queue, or no memory | call `loop()` more often, or raise the limit |
| `<endpointId>: repeated directive ignored` | the directive arrived a second time | nothing |
| `error: <endpointId>: the handler sent no answer to ...` | the handler returned without `send()` | answer with `response()` or `error()` |
| `error: <topic>: N bytes not sent, ...` | `send()` returned `false`: no session, the MQTT client or the heap is full, or the message is over `ALEX2ESP_MAX_MESSAGE` | |

---

## Memory

Static RAM and flash as PlatformIO reports them for `d1_mini` (espressif8266 4.2.1, Arduino core 3.1.2, AsyncMqttClient 0.9.0, ArduinoJson 7.4.3), built from the examples as committed, with empty credentials and the default log level. A D1 mini has 81,920 bytes of RAM; what is not static is the heap.

| Sketch | Static RAM (bytes) | Flash (bytes) |
|---|---:|---:|
| Light | 30,616 | 332,565 |
| DimmableLight | 30,692 | 336,765 |
| ColorTemperatureLight | 30,808 | 337,489 |
| ColorLight | 30,748 | 338,657 |
| TemperatureSensor | 30,572 | 333,213 |
| ContactSensor | 30,584 | 332,861 |
| Blind | 30,860 | 336,525 |
| Thermostat | 31,044 | 337,517 |
| Lock | 30,648 | 333,441 |
| Scene | 30,640 | 333,277 |
| Doorbell | 30,584 | 333,013 |
| MultiDevice | 30,672 | 332,725 |
| legacy/basicLight | 30,520 | 333,989 |
| legacy/lightWithBrightness | 30,648 | 337,929 |
| legacy/lightWithColorTemp | 30,828 | 338,653 |
| legacy/tempSensor | 30,412 | 332,781 |
| legacy/blindControl | 30,568 | 335,261 |

For comparison: `basicLight` with 1.1.0 took 52,768 bytes of static RAM and 350,885 of flash. A sketch with Wi-Fi and Serial and nothing else takes 28,152 and 269,771 with the same toolchain, one that also uses the two dependencies 29,000 and 299,401.

Devices and capabilities are on the heap, a capability with 116 bytes. `MultiDevice`, with two devices, has 56 bytes of static RAM more than `Light`: the arrays of the sketch.

### What a capability costs

Measured on the prototype of the 2.0 design, not on the release: a lamp with `PowerController` and `EndpointHealth` (29,412 bytes of static RAM, 329,293 of flash) plus one capability as a sketch uses it, with its `addCapability()`, the handling of its directives and its report helper in the Response and the StateReport. The figures of the release differ by some bytes; the table shows what is cheap and what is not.

| + capability | Static RAM | Flash |
|---|---:|---:|
| TemperatureSensor, report only | +0 | +356 |
| ContactSensor with its ChangeReport | +0 | +368 |
| ToggleController, instance and name | +32 | +620 |
| LockController with the deferred answer | +24 | +984 |
| SceneController | +48 | +1,000 |
| ColorTemperatureController | +88 | +1,280 |
| ColorController | +48 | +1,316 |
| BrightnessController, PowerLevelController, PercentageController, each | +56 | +1,376 |
| ModeController, two modes with their names | +116 | +2,640 |
| RangeController, range, one action and one state mapping | +136 | +2,872 |
| ThermostatController with TemperatureSensor | +204 | +3,112 |

An interface that the sketch does not name costs nothing: the ESP8266 core links with `--gc-sections`, in PlatformIO and in the Arduino IDE, and every row is an object of its own. In the examples the step from `Light` to `DimmableLight` is larger than the table says (+4,200 bytes of flash) because the sketch also begins to use `analogWrite()`.

### Build flags that change the figures

| Flag | Effect, measured on `legacy/basicLight` (30,520 / 333,989) |
|---|---|
| `-DALEX2ESP_LOG_MAX=0` | no log lines compiled in: 30,496 / 327,953 |
| `-DALEX2ESP_LOG_MAX=3` | the `DEBUG` lines compiled in: 30,520 / 334,477 |
| `-DALEX2ESP_MAX_CAPABILITIES=<n>` | 4 bytes of heap per place in every device |
| `-DALEX2ESP_MAX_FRIENDLY_NAMES=<n>`, `-DALEX2ESP_MAX_ACTION_MAPPINGS=<n>`, `-DALEX2ESP_MAX_STATE_MAPPINGS=<n>` | 8 or 12 bytes of heap per place in every capability |

---

## Configuration

```cpp
  // Another broker than alex2mqtt.stormysdream.club:1883. The text is not copied: a literal, or a buffer that stays
  alexa.setServer("broker.example.org", 1883);

  // The sketch sets the clock itself; the library then leaves SNTP alone
  alexa.setTimeSource(false);

  // NONE, ERROR, INFO (the default) or DEBUG; DEBUG needs -DALEX2ESP_LOG_MAX=3
  alexa.setLogLevel(AlexaLogLevel::DEBUG);

  // Where the lines go: any Print, Serial unless changed
  AlexaLog::setOutput(&Serial);

  alexa.begin(ALEXA_USERNAME, ALEXA_PASSWORD, ALEXA_ROOT_TOPIC);
```

[`test/readme/Configuration/Configuration.ino`](test/readme/Configuration/Configuration.ino)

| Call | When | What |
|---|---|---|
| `setServer(host, port)` | before `begin()` | another broker, for example your own instance of Alex2MQTT. After `begin()` it is ignored with an error |
| `setTimeSource(false)` | before `begin()` | for a sketch with its own `configTime()` or an RTC |
| `setLogLevel(level)` | any time | `ERROR` leaves only the problems, `NONE` nothing; `INFO` adds the session, discovery and one line per directive; `DEBUG` adds sizes and free heap per message |
| `AlexaLog::setOutput(print)` | any time | where the lines go; `Serial` unless changed, `nullptr` for nowhere |

The user name and the password passed to `begin()` are not copied either.

The limits are build flags, set for the whole project in `platformio.ini`:

```ini
build_flags =
    -DALEX2ESP_LOG_MAX=3
    -DALEX2ESP_MAX_CAPABILITIES=12
    -DALEX2ESP_MAX_DIRECTIVE=3071
```

| Flag | Default | Range |
|---|---|---|
| `ALEX2ESP_LOG_MAX` | 2 (`INFO`) | 0 to 3: the highest level that is compiled in |
| `ALEX2ESP_MAX_DIRECTIVE` | 2047 | |
| `ALEX2ESP_MAX_MESSAGE` | `ALEX2ESP_MAX_DIRECTIVE` + 1024 | under `ALEX2ESP_MAX_DIRECTIVE` + 300 the answer of a lamp to the largest directive is refused |
| `ALEX2ESP_MAX_QUEUED_DIRECTIVES` | 8 | 1 to 64 |
| `ALEX2ESP_MAX_QUEUED_BYTES` | 4 x `ALEX2ESP_MAX_DIRECTIVE` | `ALEX2ESP_MAX_DIRECTIVE` or more |
| `ALEX2ESP_MAX_CAPABILITIES` | 8 | 1 to 100 |
| `ALEX2ESP_MAX_FRIENDLY_NAMES` | 3 | 1 to 255 |
| `ALEX2ESP_MAX_ACTION_MAPPINGS` | 4 | 1 to 255 |
| `ALEX2ESP_MAX_STATE_MAPPINGS` | 2 | 1 to 255 |

The Arduino IDE has no flags per sketch and builds with the defaults. `setLogLevel()` with a level that is not compiled in prints an error that names the flag.

A sketch that defines a macro named `DEBUG`, `ERROR` or `INFO` cannot write the level of that name; it passes the number instead, `alexa.setLogLevel(static_cast<AlexaLogLevel>(3))` for `DEBUG`.

---

## Migrating from 1.x

The five examples of 1.x compile against 2.0 as they are, without a warning; they are in [`examples/legacy/`](examples/legacy/). `registerEvent()`, `buildStatusMessage()`, the helpers with a capital letter (`AddHealthProp`, `AddPowerControllerProp`, ...), `addCapability(AlexaInterfaceType::...)`, `AlexaInterface` and `AlexaActions` stay:

```cpp
  device1->registerEvent("ReportState", [](const JsonDocument& directive, const AlexaInterfaceType& type) {
    // Build and send status report
    device1->buildStatusMessage(directive["header"]["correlationToken"])
      .AddHealthProp(EndpointHealth::OK)
      .AddPowerControllerProp(outputState)
      .send();
  });
```

[`examples/legacy/basicLight/basicLight.ino`](examples/legacy/basicLight/basicLight.ino)

A device with a handler of `onDirective()` does not call the handlers of `registerEvent()`.

What changes for a 1.x sketch that is compiled against 2.0:

| 1.x | 2.0 | What to do |
|---|---|---|
| directives fetched and reports posted over HTTP | both over MQTT; the HTTP fallback is removed | nothing. The library no longer includes `ESP8266HTTPClient`: a sketch that uses it includes it itself |
| `timeOfSample` filled in by the backend | the board's own time | the board has to reach a time server (DNS, UDP port 123), or the sketch sets the clock and calls `setTimeSource(false)` |
| the session opens in `begin()` | `loop()` opens it when the clock is set, at most 5 s later | nothing |
| reconnect every 5 s after a lost TCP connection only, without a line | after every end of a session, with the reason and a growing wait | nothing |
| `EndpointHealth` announced as 3.3, `ThermostatController` as 3 without properties, `Speaker`, `StepSpeaker`, `PlaybackStateReporter`, `InventoryLevelSensor` and `WakeOnLANController` as 1 | 3.1, 3.2 with four properties, and 3, as the interface pages of Alexa have them; the interfaces that were announced with an empty list of properties have their properties | let Alexa discover the devices again |
| `AlexaInterfaceType::AUTHORIZATION_CONTROLLER`, `AUTOMOTIVE_VEHICLE_DATA` | removed; Alexa has withdrawn both | remove the capability |
| `addCapability()` with a type the library has no description of announced it as version 1 without properties | adds nothing, prints an error and returns `nullptr` | check the pointer; announce the interface when the library has a row for it |
| `AddTemperatureSensorProp(TemperatureSensorScale::FAHRENHEIT, 69)` reported 20.56 `CELSIUS` | reports 69 `FAHRENHEIT` | nothing |
| a value outside of its range was sent as it was | the nearest value of the range, with an error | nothing |
| the handler of `Event` got the type of the namespace | the type of the capability of its device, `UNKNOWN` when the device has none | add the capability |
| a second `registerEvent()` for a name was ignored | replaces the handler | |
| the locale and the directive name and payload of an `ActionMapping` were copied | kept as pointers | pass literals |
| any number of capabilities, names and mappings | 8 capabilities per device, 3 names, 4 action and 2 state mappings per capability | build flags, see [Configuration](#configuration) |
| a directive nobody answered ran into the timeout | the library answers `INVALID_DIRECTIVE` when the device has no handler for it | |
| an `AlexaDevice` or `AlexaStatusMessage` the sketch constructed itself could send | `send()` returns `false`: it has no client | use `getDevice()` and `buildStatusMessage()` |
| `#define Alex2ESP_DEBUG` in `AlexaUtils.cpp` | `setLogLevel()` and `-DALEX2ESP_LOG_MAX` | |
| `AlexaInterfaceUtils`, the queue functions of `AlexaUtils`, `AlexaInterface::getJSON()` and `getProps()`, `ActionMapping::getJSON()`, `FriendlyName`, `MAX_STATUS_REPORT_SIZE` | removed | `alexaInterfaceRow(type)` gives the row of a type, with its namespace, version and properties; the limit is `ALEX2ESP_MAX_MESSAGE` |

[`CHANGELOG.md`](CHANGELOG.md) has every change. A sketch written for 1.0.0 also meets the change of 1.1.0: `begin()` takes the user name, the password and the root topic, in that order.

---

## Limits

- ESP8266 only. The library has never been compiled for an ESP32.
- The session is MQTT over TCP on port 1883, without TLS.
- Reports are published with QoS 0 and sent once. An event or a change while there is no session with the broker is not sent later; `send()` returns `false` and the sketch decides.
- Tried with Alexa from this library: the six interfaces marked in [Interface Types](#interface-types). Not tried from this library: `ColorController`, `ThermostatController`, `LockController` and the deferred answer, `SceneController`, the ErrorResponse, the ChangeReports, `DoorbellPress` and every interface without an example. Voice commands have not been tried.
- The examples are compiled, with 0 warnings, and what each announces in discovery is checked by the host tests. They have not run on a board and have not been built with the Arduino IDE.
- The library keeps no state of the devices. What a device is set to is a variable of the sketch and starts from its initial value after a reset.
- 28 of the interfaces of Alexa have a row. The others cannot be announced.
- The Arduino IDE builds with the default limits.

---

## Tests

`pio test -e native` in the repository runs 147 host tests: 52 of the receive and publish logic, 35 of discovery, 16 of dispatch, 32 of the messages and 12 of the examples (what each announces in discovery). No board and no broker is needed. The `platformio.ini` of the repository is for these tests; a sketch does not need it.

## Contributing

Feel free to submit pull requests or issues for feature requests and bug fixes. The host tests have to pass, and the examples have to build without a warning.

## License

This project is licensed under the MIT License. See the LICENSE file for details.