diff --git a/server/src/services/homeAssistantService/runtimeEngine.js b/server/src/services/homeAssistantService/runtimeEngine.js
index e389e0ef..66acc316 100644
--- a/server/src/services/homeAssistantService/runtimeEngine.js
+++ b/server/src/services/homeAssistantService/runtimeEngine.js
@@ -80,6 +80,27 @@ function normalizeRgbColor(color) {
 function createRuntimeEngine(deps) {
   const { logger, enabled, haConfig, callHomeAssistantService } = deps;
 
+  async function turnOnLightAtFullBrightness(entityId, serviceData = {}) {
+    /*
+      Every server-owned interaction that turns on or changes a light must
+      also restore it to full brightness. Home Assistant remembers a bulb's
+      previous brightness, so sending only a color or color temperature can
+      otherwise make a light appear unexpectedly dim even though this service
+      requested an on-state.
+
+      Keeping this rule in one helper makes it apply consistently to ordinary
+      on commands, RGB changes, white-temperature changes, bulk operations,
+      random scenes, and lock-on behavior. brightness_pct is deliberately
+      written after the caller's service data so future call sites cannot
+      accidentally override the service-wide 100 percent requirement.
+    */
+    await callHomeAssistantService('light', 'turn_on', {
+      entity_id: entityId,
+      ...serviceData,
+      brightness_pct: 100,
+    });
+  }
+
   function emitUpdate(getState) {
     events.emit('update', getState());
   }
@@ -143,7 +164,14 @@ function createRuntimeEngine(deps) {
     const domain = String(meta.domain || (meta.type === 'light' ? 'light' : 'switch')).toLowerCase();
     const service = nextState === 'on' ? 'turn_on' : 'turn_off';
     const source = String(options?.source || 'unknown');
-    await callHomeAssistantService(domain, service, { entity_id: entityId });
+    if (domain === 'light' && service === 'turn_on') {
+      await turnOnLightAtFullBrightness(entityId);
+    } else {
+      // Off commands and non-light domains do not accept a meaningful light
+      // brightness value, so their existing Home Assistant payload stays
+      // intentionally unchanged.
+      await callHomeAssistantService(domain, service, { entity_id: entityId });
+    }
     logger.info('Issued Home Assistant command', { entityId, domain, service, source });
   }
 
@@ -467,7 +495,7 @@ function createRuntimeEngine(deps) {
     if (!runtime.connection) throw new Error('Home Assistant not connected');
 
     const normalized = normalizeRgbColor(color);
-    await callHomeAssistantService('light', 'turn_on', { entity_id: entityId, rgb_color: normalized });
+    await turnOnLightAtFullBrightness(entityId, { rgb_color: normalized });
     logger.info('Issued Home Assistant color command', { entityId, rgbColor: normalized });
   }
 
@@ -481,7 +509,7 @@ function createRuntimeEngine(deps) {
     const normalizedKelvin = Number.isFinite(nextKelvin)
       ? Math.max(2000, Math.min(6500, Math.round(nextKelvin)))
       : DEFAULT_WHITE_KELVIN;
-    await callHomeAssistantService('light', 'turn_on', { entity_id: entityId, color_temp_kelvin: normalizedKelvin });
+    await turnOnLightAtFullBrightness(entityId, { color_temp_kelvin: normalizedKelvin });
     logger.info('Issued Home Assistant white command', { entityId, colorTempKelvin: normalizedKelvin });
   }
 
diff --git a/server/src/services/homeAssistantService/runtimeEngine.test.js b/server/src/services/homeAssistantService/runtimeEngine.test.js
new file mode 100644
index 00000000..e56af405
--- /dev/null
+++ b/server/src/services/homeAssistantService/runtimeEngine.test.js
@@ -0,0 +1,83 @@
+// Home Assistant Runtime Engine Tests
+// Purpose: Verifies the service-wide full-brightness rule for light commands.
+// Scope: Exercises injected Home Assistant calls without opening a real connection or starting the server.
+
+const assert = require('node:assert/strict');
+const test = require('node:test');
+const { createRuntimeEngine } = require('./runtimeEngine');
+const { entityConfig, entityState, runtime } = require('./state');
+
+function createHarness() {
+  const calls = [];
+  const engine = createRuntimeEngine({
+    enabled: true,
+    haConfig: { whiteKelvin: 4000 },
+    callHomeAssistantService: async (domain, service, serviceData) => {
+      calls.push({ domain, service, serviceData });
+    },
+    // These tests only verify outbound service payloads. A no-op logger keeps
+    // the harness faithful to the runtime dependency contract without adding
+    // unrelated output to the test run.
+    logger: {
+      info() {},
+      warn() {},
+    },
+  });
+
+  return { calls, engine };
+}
+
+test('light interactions force full brightness without changing switches or off commands', async (t) => {
+  const { calls, engine } = createHarness();
+
+  /*
+    runtimeEngine uses the shared entity registry populated from configuration
+    in production. Seed the smallest representative registry here and restore
+    the shared state afterward so this focused unit test cannot leak state into
+    other Home Assistant tests added later.
+  */
+  entityConfig.clear();
+  entityState.clear();
+  entityConfig.set('light.room', { id: 'light.room', type: 'light', domain: 'light' });
+  entityConfig.set('switch.lamp', { id: 'switch.lamp', type: 'switch', domain: 'switch' });
+  runtime.connection = {};
+  t.after(() => {
+    entityConfig.clear();
+    entityState.clear();
+    runtime.connection = null;
+  });
+
+  await engine.setEntityState('light.room', 'on');
+  await engine.setLightColor('light.room', [12, 34, 56]);
+  await engine.setLightWhite('light.room', 4500);
+  await engine.setEntityState('light.room', 'off');
+  await engine.setEntityState('switch.lamp', 'on');
+
+  assert.deepEqual(calls, [
+    {
+      domain: 'light',
+      service: 'turn_on',
+      serviceData: { entity_id: 'light.room', brightness_pct: 100 },
+    },
+    {
+      domain: 'light',
+      service: 'turn_on',
+      serviceData: { entity_id: 'light.room', rgb_color: [12, 34, 56], brightness_pct: 100 },
+    },
+    {
+      domain: 'light',
+      service: 'turn_on',
+      serviceData: { entity_id: 'light.room', color_temp_kelvin: 4500, brightness_pct: 100 },
+    },
+    {
+      domain: 'light',
+      service: 'turn_off',
+      serviceData: { entity_id: 'light.room' },
+    },
+    {
+      domain: 'switch',
+      service: 'turn_on',
+      serviceData: { entity_id: 'switch.lamp' },
+    },
+  ]);
+});