diff --git a/src/helpers/CommonCLI_Observer.cpp b/src/helpers/CommonCLI_Observer.cpp
index 37982da4..12c403dc 100644
--- a/src/helpers/CommonCLI_Observer.cpp
+++ b/src/helpers/CommonCLI_Observer.cpp
@@ -405,6 +405,34 @@ bool CommonCLI::handleObserverSetCmd(uint32_t sender_timestamp, const char* conf
           } else {
             sprintf(reply, "OK - slot %d preset: %s", slot + 1, preset_name);
           }
+          // Warn when this slot won't actually connect on this hardware. The set
+          // is never blocked — prefs persist so the config carries over if the
+          // device is moved to a board with more slots — but flag it, or the
+          // operator waits for a connection that never comes. Mirrors the bridge
+          // setup loop: slots past the runtime array (RUNTIME_MQTT_SLOTS) are
+          // never iterated; within it, only the first getMaxActiveSlots()
+          // *enabled* slots connect (each WSS/TLS link costs ~40 KB heap).
+          if (strcmp(preset_name, MQTT_PRESET_NONE) != 0) {
+            size_t used = strlen(reply);
+            if (slot >= RUNTIME_MQTT_SLOTS) {
+              if (used < 158) {
+                snprintf(reply + used, 160 - used, " (slot inactive on this hardware)");
+              }
+            } else {
+              const int max_active = MQTTBridge::getMaxActiveSlots();
+              int rank = 0;  // this slot's position among enabled slots, by index
+              for (int s = 0; s <= slot; s++) {
+                if (_mqtt_prefs.mqtt_slot_preset[s][0] != '\0' &&
+                    strcmp(_mqtt_prefs.mqtt_slot_preset[s], MQTT_PRESET_NONE) != 0) {
+                  rank++;
+                }
+              }
+              if (rank > max_active && used < 158) {
+                snprintf(reply + used, 160 - used,
+                         " (won't connect: %d-slot limit on this hardware)", max_active);
+              }
+            }
+          }
         }
       } else {
         strcpy(reply, "Error: unknown preset. Use 'get mqtt.presets'");
diff --git a/src/helpers/bridges/MQTTBridge.cpp b/src/helpers/bridges/MQTTBridge.cpp
index 2066f9c6..39cd494d 100644
--- a/src/helpers/bridges/MQTTBridge.cpp
+++ b/src/helpers/bridges/MQTTBridge.cpp
@@ -337,6 +337,18 @@ void MQTTBridge::formatMqttStatsReply(char* buf, size_t bufsize) {
 uint8_t MQTTBridge::getLastWifiDisconnectReason() { return s_wifi_disconnect_reason; }
 unsigned long MQTTBridge::getLastWifiDisconnectTime() { return s_wifi_disconnect_time; }
 
+// Each WSS/TLS connection needs ~40KB for mbedTLS buffers. Without PSRAM even 3
+// concurrent connections would exhaust internal heap, so cap at 2; with PSRAM
+// cap at 5 (6 configurable but 5 active max). Static so the CLI can report, at
+// config time, that a slot beyond this cap won't connect.
+int MQTTBridge::getMaxActiveSlots() {
+#if defined(ESP_PLATFORM) && defined(BOARD_HAS_PSRAM)
+  return psramFound() ? 5 : 2;
+#else
+  return 2;
+#endif
+}
+
 unsigned long MQTTBridge::getSlotCurrentOutageStartMs(int slot_index) const {
   if (slot_index < 0 || slot_index >= RUNTIME_MQTT_SLOTS) return 0;
   return _slots[slot_index].current_outage_started_ms;
@@ -627,15 +639,9 @@ void MQTTBridge::begin() {
   MQTT_DEBUG_PRINTLN("PSRAM: not configured for this board (no BOARD_HAS_PSRAM)");
   #endif
 
-  // Limit active slots based on available memory.
-  // Each WSS/TLS connection needs ~40KB for mbedTLS buffers.
-  // Without PSRAM, even 3 concurrent connections would exhaust internal heap.
-  // With PSRAM, cap at 5 for safety (6 configurable but 5 active max).
-  #if defined(ESP_PLATFORM) && defined(BOARD_HAS_PSRAM)
-  _max_active_slots = psramFound() ? 5 : 2;
-  #else
-  _max_active_slots = 2;
-  #endif
+  // Limit active slots based on available memory (single source of truth in
+  // getMaxActiveSlots(), which the CLI also uses to warn about inactive slots).
+  _max_active_slots = getMaxActiveSlots();
   MQTT_DEBUG_PRINTLN("Max active slots: %d", _max_active_slots);
 
   // Check if WiFi credentials are configured first
diff --git a/src/helpers/bridges/MQTTBridge.h b/src/helpers/bridges/MQTTBridge.h
index 5e7e2d01..17346c43 100644
--- a/src/helpers/bridges/MQTTBridge.h
+++ b/src/helpers/bridges/MQTTBridge.h
@@ -498,6 +498,10 @@ public:
   static void formatSlotDiagReply(char* buf, size_t bufsize, int slot_index);
   static uint8_t getLastWifiDisconnectReason();
   static unsigned long getLastWifiDisconnectTime();
+  /** Max slots that can be connected at once on this hardware (each WSS/TLS link
+   *  needs ~40 KB internal heap): 5 with PSRAM, 2 without. Note this is below
+   *  RUNTIME_MQTT_SLOTS, so more slots can be configured than will connect. */
+  static int getMaxActiveSlots();
 
   #if defined(WITH_MQTT_NEIGHBORS)
   void requestPublishNeighbors(const char* json, size_t len);