diff --git a/apps/server/src/button-light.test.ts b/apps/server/src/button-light.test.ts
index 54ead90..1b4bdb4 100644
--- a/apps/server/src/button-light.test.ts
+++ b/apps/server/src/button-light.test.ts
@@ -1,6 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { randomUUID } from "node:crypto";
-import { devices, type Db } from "@parking/db";
+import { eq, devices, type Db } from "@parking/db";
 import { createTestDb } from "@parking/db/testing";
 import type { AuxOutputDevice } from "@parking/devices";
 import { ButtonLightController } from "./button-light.js";
@@ -72,107 +72,130 @@ function lane(entryBusy: boolean): void {
   deviceEvents.emitLaneStatus({ entry: entryBusy, exit: false });
 }
 
+/** Flush the microtask queue so serialized setAux promises (and their re-pump on
+ *  completion) settle. The lamp worker sends ONE UDP at a time and re-pumps on resolve;
+ *  a few turns drain a burst. Needed because sends are now async (was synchronous). */
+async function flush(): Promise<void> {
+  for (let i = 0; i < 6; i++) await Promise.resolve();
+}
+
 describe("ButtonLightController truth table", () => {
-  it("OFF at start (no radar, no car)", () => {
+  it("OFF at start (no radar, no car)", async () => {
     const calls: Array<{ ch: number; on: boolean }> = [];
     const ctl = new ButtonLightController(db, silentLogger(), () => fakeAux(calls));
     ctl.start();
+    await flush();
     expect(ctl.stateOf(CONTROLLER)).toBe("off");
-    // Initial apply drives the lamp off (false). It may de-dupe to no call since
-    // lastOn starts null -> false IS a change, so exactly one off write.
-    expect(calls).toEqual([{ ch: LAMP_RELAY, on: false }]);
+    // confirmedOn starts null; OFF de-dupes (null !== false → one off write), so the
+    // device is confirmed OFF and at most one call was made.
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(false);
     ctl.stop();
   });
 
-  it("radar present + lane busy -> SOLID on", () => {
+  it("radar present + lane busy -> SOLID on", async () => {
     const calls: Array<{ ch: number; on: boolean }> = [];
     const aux = fakeAux(calls);
     const ctl = new ButtonLightController(db, silentLogger(), () => aux);
     ctl.start();
-    calls.length = 0;
+    await flush();
     lane(true);
     radar(true);
+    await flush();
     expect(ctl.stateOf(CONTROLLER)).toBe("solid");
-    expect(calls.at(-1)).toEqual({ ch: LAMP_RELAY, on: true });
-    // Solid = no blinking: advancing time produces no further toggles.
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(true); // device latched ON
+    // Solid = no blinking: advancing time produces no further sends.
     const n = calls.length;
     vi.advanceTimersByTime(2000);
+    await flush();
     expect(calls.length).toBe(n);
     ctl.stop();
   });
 
-  it("radar present + lane free -> BLINK (toggles over time)", () => {
+  it("radar present + lane free -> BLINK (toggles the device over time)", async () => {
     const calls: Array<{ ch: number; on: boolean }> = [];
     const aux = fakeAux(calls);
     const ctl = new ButtonLightController(db, silentLogger(), () => aux);
     ctl.start();
-    calls.length = 0;
+    await flush();
     radar(true); // lane still free
+    await flush();
     expect(ctl.stateOf(CONTROLLER)).toBe("blink");
-    expect(calls.at(-1)).toEqual({ ch: LAMP_RELAY, on: true }); // on now
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(true); // on now
     vi.advanceTimersByTime(500);
-    expect(calls.at(-1)).toEqual({ ch: LAMP_RELAY, on: false }); // toggled off
+    await flush();
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(false); // toggled off
     vi.advanceTimersByTime(500);
-    expect(calls.at(-1)).toEqual({ ch: LAMP_RELAY, on: true }); // toggled on
+    await flush();
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(true); // toggled on
     ctl.stop();
   });
 
-  it("blink -> solid when the camera confirms a car (lane busy)", () => {
+  it("blink -> solid when the camera confirms a car (lane busy)", async () => {
     const calls: Array<{ ch: number; on: boolean }> = [];
     const aux = fakeAux(calls);
     const ctl = new ButtonLightController(db, silentLogger(), () => aux);
     ctl.start();
+    await flush();
     radar(true); // blink
+    await flush();
     expect(ctl.stateOf(CONTROLLER)).toBe("blink");
     lane(true); // camera confirms
+    await flush();
     expect(ctl.stateOf(CONTROLLER)).toBe("solid");
-    expect(calls.at(-1)).toEqual({ ch: LAMP_RELAY, on: true });
-    // No more toggles (blink torn down).
-    const n = calls.length;
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(true);
+    // No more toggles (blink torn down) — the device stays ON over time.
     vi.advanceTimersByTime(2000);
-    expect(calls.length).toBe(n);
+    await flush();
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(true);
     ctl.stop();
   });
 
-  it("radar clears -> OFF", () => {
+  it("radar clears -> OFF", async () => {
     const calls: Array<{ ch: number; on: boolean }> = [];
     const aux = fakeAux(calls);
     const ctl = new ButtonLightController(db, silentLogger(), () => aux);
     ctl.start();
+    await flush();
     lane(true);
     radar(true); // solid
-    calls.length = 0;
+    await flush();
     radar(false); // car gone
+    await flush();
     expect(ctl.stateOf(CONTROLLER)).toBe("off");
-    expect(calls.at(-1)).toEqual({ ch: LAMP_RELAY, on: false });
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(false); // device latched OFF
     ctl.stop();
   });
 
-  it("de-dupes redundant writes (no spam on repeat events)", () => {
+  it("de-dupes redundant writes (no spam on repeat events)", async () => {
     const calls: Array<{ ch: number; on: boolean }> = [];
     const aux = fakeAux(calls);
     const ctl = new ButtonLightController(db, silentLogger(), () => aux);
     ctl.start();
+    await flush();
     lane(true);
     radar(true); // solid, on
+    await flush();
     const n = calls.length;
     radar(true); // same state — no new edge (present unchanged)
     lane(true); // same lane — no change
+    await flush();
     expect(calls.length).toBe(n);
     ctl.stop();
   });
 
-  it("fails OFF: a setAux error does not throw or escalate", () => {
+  it("fails OFF: a setAux error does not throw or escalate", async () => {
     const calls: Array<{ ch: number; on: boolean }> = [];
     const throwOnce = { v: true };
     const aux = fakeAux(calls, throwOnce);
     const ctl = new ButtonLightController(db, silentLogger(), () => aux);
     // First write (initial off) throws — must be swallowed.
     expect(() => ctl.start()).not.toThrow();
-    // Subsequent writes work; driving to solid still succeeds.
+    await flush();
+    // Subsequent writes work; driving to solid still converges to ON.
     lane(true);
     radar(true);
-    expect(calls.at(-1)).toEqual({ ch: LAMP_RELAY, on: true });
+    await flush();
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(true);
     ctl.stop();
   });
 
@@ -191,4 +214,49 @@ describe("ButtonLightController truth table", () => {
     expect(ctl.stateOf("ctl-2")).toBeNull();
     ctl.stop();
   });
+
+  it("picks up a button light ADDED after start() (no restart needed)", async () => {
+    // Fresh controller with a radar input but NO buttonLight yet.
+    const calls: Array<{ ch: number; on: boolean }> = [];
+    const aux = fakeAux(calls);
+    const ctl = new ButtonLightController(db, silentLogger(), () => aux);
+    // Replace the seeded controller with one that has the radar but no lamp.
+    db.update(devices)
+      .set({
+        config: {
+          host: "10.0.0.5",
+          relays: [{ relay: 1, direction: "entry", presenceInput: RADAR_INPUT, presenceKind: "radar" }],
+        },
+      })
+      .where(eq(devices.id, CONTROLLER))
+      .run();
+    ctl.start();
+    await flush();
+    // No lamp configured → an input does nothing.
+    radar(true);
+    await flush();
+    expect(ctl.stateOf(CONTROLLER)).toBeNull();
+    expect(calls.length).toBe(0);
+    radar(false);
+    await flush();
+
+    // Admin saves a button light (relay 3) — without restarting the server.
+    db.update(devices)
+      .set({
+        config: {
+          host: "10.0.0.5",
+          relays: [{ relay: 1, direction: "entry", presenceInput: RADAR_INPUT, presenceKind: "radar" }],
+          buttonLight: { relay: LAMP_RELAY, blinkOnMs: 500, blinkOffMs: 500 },
+        },
+      })
+      .where(eq(devices.id, CONTROLLER))
+      .run();
+
+    // The very next radar edge reconciles + blinks (lane still free).
+    radar(true);
+    await flush();
+    expect(ctl.stateOf(CONTROLLER)).toBe("blink");
+    expect(ctl.confirmedOf(CONTROLLER)).toBe(true);
+    ctl.stop();
+  });
 });
diff --git a/apps/server/src/button-light.ts b/apps/server/src/button-light.ts
index 5ec7b47..62ecdaf 100644
--- a/apps/server/src/button-light.ts
+++ b/apps/server/src/button-light.ts
@@ -20,17 +20,25 @@ const DEFAULT_BLINK_MS = 500;
 
 /** Per-controller live state for the lamp rule. */
 interface LampState {
-  readonly spec: ButtonLightSpec;
+  /** Lamp config (relay #, blink ms). Mutable: #reconcile updates it in place when the
+   *  admin changes the button-light config without a restart. */
+  spec: ButtonLightSpec;
   /** Is the radar (presence input on an entry relay) currently active? */
   present: boolean;
-  /** The output we last commanded (de-dupe — avoid UDP spam at the 50ms input poll). */
-  lastOn: boolean | null;
   /** The high-level state we're rendering (to avoid restarting a running blink). */
   rendered: LightState | null;
   /** Active blink timer, if blinking. */
   blink: ReturnType<typeof setInterval> | null;
   /** Blink phase (true = currently on). */
   blinkOn: boolean;
+  /** The output we WANT the relay to be in. The serialized worker drives the device
+   *  toward this. The blink timer only flips this flag — it never sends directly. */
+  desiredOn: boolean;
+  /** The output we last CONFIRMED on the device (after a successful send). null = unknown. */
+  confirmedOn: boolean | null;
+  /** True while a send is in flight for this lamp — serializes UDP so on/off can't
+   *  overlap or reorder (UDP is unordered; concurrent toggles left the relay stuck). */
+  sending: boolean;
 }
 
 /** Resolves a controller's live aux-output adapter. The default goes through the
@@ -59,7 +67,7 @@ export class ButtonLightController {
 
   /** Subscribe to radar input edges + lane status, and initialise every lamp OFF. */
   start(): void {
-    this.#loadLamps();
+    this.#reconcile();
     // All lamps start OFF (known-safe baseline) regardless of prior device state.
     for (const [controllerId, lamp] of this.#lamps) this.#apply(controllerId, lamp);
 
@@ -67,23 +75,45 @@ export class ButtonLightController {
     this.#unsubLane = deviceEvents.onLaneStatus((s) => this.#onLane(s));
   }
 
-  /** (Re)build the lamp map from the current device config. Each enabled access
-   *  controller with a `buttonLight` gets a lamp; others are skipped. */
-  #loadLamps(): void {
-    this.#lamps.clear();
+  /** Reconcile the lamp map with the CURRENT device config (the booth can add/change a
+   *  button light without a server restart). Mirrors DeviceMonitor, which re-reads the
+   *  device set each tick. Adds lamps for newly-configured controllers, updates the spec
+   *  (relay #, blink ms) in place — preserving live `present`/blink state — and drops
+   *  lamps whose controller lost its buttonLight or was disabled. Called at start() and
+   *  before handling each event, so a just-saved lamp takes effect immediately. */
+  #reconcile(): void {
     const rows = this.#db.select().from(devices).where(eq(devices.category, "access")).all();
+    const seen = new Set<string>();
     for (const row of rows) {
       if (!row.enabled) continue;
       const spec = buttonLightOf(row);
       if (!spec) continue;
-      this.#lamps.set(row.id, {
-        spec,
-        present: false,
-        lastOn: null,
-        rendered: null,
-        blink: null,
-        blinkOn: false,
-      });
+      seen.add(row.id);
+      const existing = this.#lamps.get(row.id);
+      if (existing) {
+        existing.spec = spec; // pick up a changed relay # / blink cadence
+      } else {
+        this.#lamps.set(row.id, {
+          spec,
+          present: false,
+          rendered: null,
+          blink: null,
+          blinkOn: false,
+          desiredOn: false,
+          confirmedOn: null,
+          sending: false,
+        });
+      }
+    }
+    // Drop lamps whose controller no longer declares one (or was disabled/removed).
+    for (const [id, lamp] of this.#lamps) {
+      if (seen.has(id)) continue;
+      if (lamp.blink) {
+        clearInterval(lamp.blink);
+        lamp.blink = null;
+      }
+      this.#finalOff(id, lamp); // best-effort fail-OFF before forgetting it
+      this.#lamps.delete(id);
     }
   }
 
@@ -91,6 +121,8 @@ export class ButtonLightController {
    *  edge the SAME way the entry flow does (relayForPresence on an entry/both relay),
    *  so the lamp and the one-car-one-ticket gate always agree on "a car is here". */
   #onInput(e: DeviceInputEvent): void {
+    // Reconcile first so a lamp added/changed since boot (no restart) is picked up.
+    this.#reconcile();
     const lamp = this.#lamps.get(e.deviceId);
     if (!lamp) return; // no lamp on this controller
     const presence = relayForPresence(this.#db, e.deviceId, e.input);
@@ -123,19 +155,24 @@ export class ButtonLightController {
     lamp.rendered = target;
 
     if (target === "off") {
-      void this.#drive(controllerId, lamp, false);
+      lamp.desiredOn = false;
+      this.#pump(controllerId, lamp);
     } else if (target === "solid") {
-      void this.#drive(controllerId, lamp, true);
+      lamp.desiredOn = true;
+      this.#pump(controllerId, lamp);
     } else {
-      // BLINK: arm the toggle timer SYNCHRONOUSLY (it must not wait on a UDP write), then
-      // drive the first "on". A symmetric cadence uses one interval; an asymmetric one
-      // re-arms each phase with its own duration.
+      // BLINK: a wall-clock timer flips ONLY the desired flag; #pump does the actual
+      // (serialized) UDP send. A symmetric cadence uses one interval; an asymmetric one
+      // re-arms each phase with its own duration. Sends never overlap or reorder, so the
+      // relay can't get stuck on a stale packet.
       const onMs = lamp.spec.blinkOnMs && lamp.spec.blinkOnMs > 0 ? lamp.spec.blinkOnMs : DEFAULT_BLINK_MS;
       const offMs = lamp.spec.blinkOffMs && lamp.spec.blinkOffMs > 0 ? lamp.spec.blinkOffMs : DEFAULT_BLINK_MS;
       lamp.blinkOn = true;
+      lamp.desiredOn = true;
       const tick = () => {
         lamp.blinkOn = !lamp.blinkOn;
-        void this.#drive(controllerId, lamp, lamp.blinkOn);
+        lamp.desiredOn = lamp.blinkOn;
+        this.#pump(controllerId, lamp);
         if (onMs !== offMs && lamp.blink) {
           clearInterval(lamp.blink);
           lamp.blink = setInterval(tick, lamp.blinkOn ? onMs : offMs);
@@ -144,23 +181,37 @@ export class ButtonLightController {
       };
       lamp.blink = setInterval(tick, onMs);
       lamp.blink.unref?.();
-      void this.#drive(controllerId, lamp, true);
+      this.#pump(controllerId, lamp);
     }
   }
 
-  /** Latch the lamp's relay via the device's aux-output capability. De-duped + fail-OFF:
-   *  an error logs and leaves `lastOn` unchanged so the next compute retries. */
-  async #drive(controllerId: string, lamp: LampState, on: boolean): Promise<void> {
-    if (lamp.lastOn === on) return; // no redundant UDP writes
+  /** Serialized per-lamp worker: drive the relay toward `desiredOn`, one UDP send at a
+   *  time. Because UDP is unordered, concurrent on/off sends previously raced and left
+   *  the relay stuck on a stale packet. Here a single in-flight send is guaranteed
+   *  (`sending` guard); when it resolves, if the desired state moved on we send again —
+   *  so the LAST desired state is always the one finally asserted on the device. */
+  #pump(controllerId: string, lamp: LampState): void {
+    if (lamp.sending) return; // a send is already in flight; it'll re-check on completion
+    if (lamp.confirmedOn === lamp.desiredOn) return; // already there — no redundant UDP
     const aux = this.#resolveAux(controllerId);
     if (!aux) return;
-    try {
-      await aux.setAux(lamp.spec.relay, on);
-      lamp.lastOn = on;
-    } catch (err) {
-      this.#logger.error(`button-light setAux failed (${controllerId} R${lamp.spec.relay}): ${(err as Error).message}`);
-      // Leave lastOn unchanged → retried on the next state compute. Never escalates.
-    }
+    const target = lamp.desiredOn;
+    lamp.sending = true;
+    void aux
+      .setAux(lamp.spec.relay, target)
+      .then(() => {
+        lamp.confirmedOn = target;
+      })
+      .catch((err: unknown) => {
+        // Leave confirmedOn unchanged so the next pump retries this state. Never escalates.
+        this.#logger.error(`button-light setAux failed (${controllerId} R${lamp.spec.relay}): ${(err as Error).message}`);
+      })
+      .finally(() => {
+        lamp.sending = false;
+        // Desired state may have changed (or the send failed) while we were busy —
+        // re-pump to converge. This is what makes the final state authoritative.
+        if (lamp.confirmedOn !== lamp.desiredOn) this.#pump(controllerId, lamp);
+      });
   }
 
   /** Build the live aux-output adapter for a controller, or null (logged once). */
@@ -197,14 +248,28 @@ export class ButtonLightController {
         lamp.blink = null;
       }
       // Best-effort fail-OFF on shutdown.
-      void this.#drive(controllerId, lamp, false);
+      this.#finalOff(controllerId, lamp);
     }
   }
 
+  /** Drive a lamp OFF as a one-shot (used when dropping/stopping a lamp): set desired
+   *  OFF and pump. The serialized worker still applies, so this can't collide with an
+   *  in-flight send — it converges to OFF. */
+  #finalOff(controllerId: string, lamp: LampState): void {
+    lamp.desiredOn = false;
+    this.#pump(controllerId, lamp);
+  }
+
   /** Test seam: current high-level state being rendered for a controller. */
   stateOf(controllerId: string): LightState | null {
     return this.#lamps.get(controllerId)?.rendered ?? null;
   }
+
+  /** Test seam: the state last CONFIRMED on the device for a controller (after a
+   *  successful send). null = unknown / nothing sent yet. */
+  confirmedOf(controllerId: string): boolean | null {
+    return this.#lamps.get(controllerId)?.confirmedOn ?? null;
+  }
 }
 
 /** Build a controller row's live aux device (exported for reuse/tests). */