diff --git a/fan-controller/TODO.md b/fan-controller/TODO.md
index a06d94e..b89dab0 100644
--- a/fan-controller/TODO.md
+++ b/fan-controller/TODO.md
@@ -383,7 +383,7 @@ urgent if subscriptions ever become dynamic.
 |---|---|
 | `src/mqtt/packet/publish.rs:158` | Validate there is enough space left in the buffer |
 | `src/mqtt/packet/publish.rs:146` | Validate the topic name contains no MQTT wildcard characters |
-| `src/mqtt/packet/publish.rs:66`, `src/mqtt/mod.rs:54` | Set flags |
+| `src/mqtt/packet/publish.rs:66`, `src/mqtt/mod.rs:54` | Set the duplicate and QoS flags. Retain is done — the reset cause is published with it |
 | `src/mqtt/packet/connect.rs:40` | Check the fixed header can even be written |
 | `src/mqtt/packet/subscribe.rs:127` | Support packet identifiers greater than `u8::MAX` |
 | `src/mqtt/packet/subscribe_acknowledgement.rs:18` | Convert to the decode trait |
diff --git a/fan-controller/src/main.rs b/fan-controller/src/main.rs
index d420721..04d7e93 100644
--- a/fan-controller/src/main.rs
+++ b/fan-controller/src/main.rs
@@ -40,6 +40,7 @@ mod debounce;
 mod fan;
 mod modbus;
 mod mqtt;
+mod reset_cause;
 mod task;
 
 bind_interrupts!(struct Irqs {
@@ -540,6 +541,9 @@ const _: () = core::assert!(
 
 enum OutgoingPublish {
     Discovery,
+    /// Why the controller last reset. Published once per boot, retained, because the resets worth
+    /// diagnosing happen when nobody is watching
+    ResetCause(reset_cause::ResetCause),
     UpdateSpeed {
         fan: Fan,
         payload: UpdateSpeedPayload,
@@ -561,6 +565,7 @@ impl Publish for OutgoingPublish {
     fn topic(&self) -> &str {
         match self {
             OutgoingPublish::Discovery => topic::fan_controller::DISCOVERY,
+            OutgoingPublish::ResetCause(_) => topic::fan_controller::RESET_CAUSE,
             OutgoingPublish::UpdateSpeed {
                 fan: Fan::One,
                 payload: _,
@@ -589,6 +594,7 @@ impl Publish for OutgoingPublish {
     fn payload(&self) -> &[u8] {
         match self {
             OutgoingPublish::Discovery => DISCOVERY_PAYLOAD,
+            OutgoingPublish::ResetCause(cause) => cause.as_str().as_bytes(),
             OutgoingPublish::UpdateSpeed { fan: _, payload } => {
                 // set_point.0.to_be_bytes()
                 payload.0.as_bytes()
@@ -600,6 +606,10 @@ impl Publish for OutgoingPublish {
             OutgoingPublish::UpdateSensors { fan: _, payload } => payload.as_bytes(),
         }
     }
+
+    fn is_retained(&self) -> bool {
+        matches!(self, OutgoingPublish::ResetCause(_))
+    }
 }
 
 /// Sets up and manages the MQTT connection like keeping it alive
@@ -1235,6 +1245,10 @@ const CHANNEL_SIZE: usize = 8;
 
 #[embassy_executor::main]
 async fn main(spawner: Spawner) {
+    // Read before anything else touches the chip, so nothing here can be mistaken for the reason
+    // the last boot ended
+    let boot_reset_cause = reset_cause::read();
+
     let Peripherals {
         PIN_23: pin_23,
         PIN_25: pin_25,
@@ -1329,6 +1343,11 @@ async fn main(spawner: Spawner) {
     info!("[Main] Seinding discovery");
     sender_out.send(OutgoingPublish::Discovery).await;
     info!("[Main] Sent out discocery");
+
+    info!("[Main] Reset cause: {}", boot_reset_cause);
+    sender_out
+        .send(OutgoingPublish::ResetCause(boot_reset_cause))
+        .await;
     unwrap!(spawner.spawn(display_routine(display_receivers, &LED_STATE, sender_out)));
 
     static FAN_ONE_STATE: SetPointSignal = Signal::new();
diff --git a/fan-controller/src/mqtt/mod.rs b/fan-controller/src/mqtt/mod.rs
index b430ac6..d773b1d 100644
--- a/fan-controller/src/mqtt/mod.rs
+++ b/fan-controller/src/mqtt/mod.rs
@@ -51,8 +51,8 @@ impl<T: crate::task::Publish> TryEncode for T {
         }
 
         // Fixed header
-        //TODO set flags
-        buffer[*offset] = Self::TYPE << 4;
+        //TODO set the duplicate and QoS flags
+        buffer[*offset] = (Self::TYPE << 4) | u8::from(self.is_retained());
         *offset += 1;
 
         let topic_name = self.topic();
diff --git a/fan-controller/src/reset_cause.rs b/fan-controller/src/reset_cause.rs
new file mode 100644
index 0000000..196f01a
--- /dev/null
+++ b/fan-controller/src/reset_cause.rs
@@ -0,0 +1,65 @@
+//! Why the RP2040 last came up.
+//!
+//! A fan controller that reboots on its own looks identical from the outside to one that was
+//! power cycled, and the two want completely different fixes. The chip records the answer across
+//! the reset, so read it before anything else has a chance to overwrite it.
+
+use embassy_rp::pac;
+
+#[derive(Clone, Copy, defmt::Format)]
+pub enum ResetCause {
+    /// The supply dropped below the brown-out threshold, or power was applied. A controller that
+    /// reports this without anyone touching the plug browned out.
+    PowerOnOrBrownOut,
+    /// The RUN pin was pulled low, which is the reset button and the probe's reset line.
+    RunPin,
+    /// The watchdog was not fed in time.
+    WatchdogTimeout,
+    /// Something asked for the reset deliberately — a debugger, or `probe-rs run`.
+    Forced,
+    /// The power-on state machine restarted without any of the above, which is what a debugger
+    /// attaching usually looks like.
+    DebuggerRestart,
+    /// None of the bits were set. Documented as the plain hardware reset case.
+    Unknown,
+}
+
+impl ResetCause {
+    /// The string published to MQTT. Kept short and stable so it can be grepped and compared
+    /// across boots.
+    pub fn as_str(self) -> &'static str {
+        match self {
+            ResetCause::PowerOnOrBrownOut => "power-on-or-brown-out",
+            ResetCause::RunPin => "run-pin",
+            ResetCause::WatchdogTimeout => "watchdog-timeout",
+            ResetCause::Forced => "forced",
+            ResetCause::DebuggerRestart => "debugger-restart",
+            ResetCause::Unknown => "unknown",
+        }
+    }
+}
+
+/// Read why the chip last reset.
+///
+/// The watchdog reason is checked first: a watchdog reset runs through the same power-on state
+/// machine, so `CHIP_RESET` alone would report it as a restart and hide the real cause.
+pub fn read() -> ResetCause {
+    let watchdog_reason = pac::WATCHDOG.reason().read();
+    if watchdog_reason.timer() {
+        return ResetCause::WatchdogTimeout;
+    }
+    if watchdog_reason.force() {
+        return ResetCause::Forced;
+    }
+
+    let chip_reset = pac::VREG_AND_CHIP_RESET.chip_reset().read();
+    if chip_reset.had_por() {
+        ResetCause::PowerOnOrBrownOut
+    } else if chip_reset.had_run() {
+        ResetCause::RunPin
+    } else if chip_reset.had_psm_restart() {
+        ResetCause::DebuggerRestart
+    } else {
+        ResetCause::Unknown
+    }
+}
diff --git a/fan-controller/src/task.rs b/fan-controller/src/task.rs
index 5847a8a..360ba2c 100644
--- a/fan-controller/src/task.rs
+++ b/fan-controller/src/task.rs
@@ -470,6 +470,12 @@ pub(super) trait Publish {
     /// For example, they can be defined as an enum and match internally to provide the appropriate string for the enum variant.
     fn topic(&self) -> &str;
     fn payload(&self) -> &[u8];
+    /// Whether the broker should keep this message and hand it to whoever subscribes next.
+    /// Almost nothing wants this: state that is published on every change is better re-read than
+    /// remembered. A message about an event nobody was watching for is the exception.
+    fn is_retained(&self) -> bool {
+        false
+    }
 }
 
 pub(super) async fn set_up_network_stack(
diff --git a/topic/src/lib.rs b/topic/src/lib.rs
index e14524f..534c109 100644
--- a/topic/src/lib.rs
+++ b/topic/src/lib.rs
@@ -12,6 +12,11 @@ pub mod fan_controller {
 
     pub const DISCOVERY: &str = formatcp!("{DISCOVERY_PREFIX}/{COMPONENT}/{OBJECT_ID}/config");
 
+    /// Why the controller last reset, published retained on boot. Retained because the resets
+    /// worth diagnosing are rare and nobody is subscribed when they happen — the answer has to
+    /// still be there whenever someone next looks.
+    pub const RESET_CAUSE: &str = formatcp!("{OBJECT_ID}/reset-cause");
+
     /// The topic to publish the on/off state of the fan controller.
     pub const STATE: &str = formatcp!("{OBJECT_ID}/on/state");
     /// The topic to subscribe to for setting the on/off state of the fan controller.