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+<code C main.cpp>
+#include <Arduino.h>
+#include <WiFi.h>
+#include <WiFiUdp.h>
+#include <coap-simple.h>   // hirotakaster/CoAP simple library
+// ===== Wi-Fi creds =====
+const char* WIFI_SSID = "UPB-Guest";
+const char* WIFI_PASS = "";
+// ===== CoAP server (Californium demo) =====
+const char* COAP_HOST = "californium.eclipseprojects.io";
+const uint16_t COAP_PORT = 5683;   // RFC 7252 default port
+// UDP + CoAP objects
+WiFiUDP udp;
+Coap coap(udp);
+// ----- Response callback: prints payload as text -----
+void onCoapResponse(CoapPacket &packet, IPAddress ip, int port) {
+  // Copy payload into a printable buffer
+  const size_t n = packet.payloadlen;
+  String from = ip.toString() + ":" + String(port);
+  Serial.printf("[CoAP] Response from %s  code=%u (0x%02X)  len=%u\n",
+                from.c_str(), (packet.code >> 5), packet.code, (unsigned)n);
+  if (n > 0) {
+    // Make it printable; not all payloads are ASCII
+    size_t m = n;
+    if (m > 1023) m = 1023;     // clamp for serial
+    char buf[1024];
+    memcpy(buf, packet.payload, m);
+    buf[m] = '\0';
+    Serial.println(F("[CoAP Payload]"));
+    Serial.println(buf);
+  }
+}
+// Re-resolves hostname and sends GET to "/"
+bool coapGetRoot() {
+  IPAddress remote;
+  if (!WiFi.hostByName(COAP_HOST, remote)) {
+    Serial.println(F("[CoAP] DNS failed"));
+    return false;
+  }
+  Serial.printf("[CoAP] GET coap://%s:%u/\n", COAP_HOST, COAP_PORT);
+  // Path is "" for "/", per library examples
+  int msgid = coap.get(remote, COAP_PORT, "");
+  (void)msgid; // not used further in this demo
+  return true;
+}
+unsigned long lastGetMs = 0;
+const unsigned long GET_PERIOD_MS = 10000;
+void setup() {
+  Serial.begin(115200);
+  delay(200);
+  Serial.println(F("\n== CoAP client (Californium demo) =="));
+  // Wi-Fi
+  WiFi.mode(WIFI_STA);
+  WiFi.begin(WIFI_SSID, WIFI_PASS);
+  Serial.print(F("Connecting"));
+  while (WiFi.status() != WL_CONNECTED) { delay(300); Serial.print('.'); }
+  Serial.println();
+  Serial.print(F("Wi-Fi OK. IP: ")); Serial.println(WiFi.localIP());
+  // Bind UDP so we can receive responses (use standard CoAP port or 0 for ephemeral)
+  udp.begin(5683);
+  // Register response handler and start CoAP
+  coap.response(onCoapResponse);
+  coap.start();
+  // First GET right away
+  coapGetRoot();
+  lastGetMs = millis();
+}
+void loop() {
+  // Let the library process incoming UDP (and fire onCoapResponse)
+  coap.loop();
+  // Periodic GET
+  unsigned long now = millis();
+  if (now - lastGetMs >= GET_PERIOD_MS) {
+    lastGetMs = now;
+    coapGetRoot();
+  }
+}
+</code>