CoAP Response Code Meaning Misunderstood

In this tutorial, you'll learn about CoAP Response Code Meaning Misunderstood. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

CoAP response code does not match the expected semantics (e.g., using 2.05 for empty body).

Quick Fix

Wrong

res.code = '2.05';  // Content — but no payload sent
res.end();```

Client confused: 2.05 Content with no payload. Should use 2.03 Valid.

### Right

```cpp
#include <coap.h>

void get_handler(coap_context_t *ctx, coap_resource_t *res,
                 coap_session_t *session, coap_pdu_t *request,
                 coap_binary_t *token, coap_string_t *query,
                 coap_pdu_t *response) {
  // 2.05 Content — has payload
  unsigned char buf[] = "temperature data";
  coap_pdu_set_code(response, COAP_RESPONSE_CODE_CONTENT);
  coap_add_data(response, sizeof(buf) - 1, buf);
}

void check_handler(coap_context_t *ctx, coap_resource_t *res,
                   coap_session_t *session, coap_pdu_t *request,
                   coap_binary_t *token, coap_string_t *query,
                   coap_pdu_t *response) {
  // 2.03 Valid — no payload, just validation
  coap_pdu_set_code(response, COAP_RESPONSE_CODE_VALID);
}```

/gets: 2.05 Content with data. /check: 2.03 Valid with no body.

## Prevention

CoAP response codes are three-digit: 2.XX (Success), 4.XX (Client Error), 5.XX (Server Error). Key codes: 2.01=Created, 2.02=Deleted, 2.03=Valid (no body), 2.04=Changed, 2.05=Content (has body). 4.00=Bad Request, 4.01=Unauthorized, 4.04=Not Found, 4.05=Method Not Allowed. 5.00=Internal Server Error, 5.01=Not Implemented, 5.02=Bad Gateway, 5.03=Service Unavailable.

DodaTech engineers apply these same patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro for production IoT reliability.

## FAQ

<details style="margin-bottom:12px;border:1px solid #e2e8f0;border-radius:10px;overflow:hidden"><summary style="cursor:pointer;padding:14px 18px;font-weight:600;font-size:1.05rem;background:#f8fafc;border-bottom:1px solid #e2e8f0;color:#1e293b">### 2.03 Valid vs 2.05 Content?</summary><div style="padding:14px 18px;color:#475569;line-height:1.7;background:#fff"><p>2.03 Valid = success, no payload (e.g., ETag validation). 2.05 Content = success with response payload.</p>
<h3 id="404-vs-405">4.04 vs 4.05?</h3><p>4.04 Not Found = resource doesn't exist. 4.05 Method Not Allowed = resource exists but method not supported.</p>
<h3 id="what-code-for-server-error">What code for server error?</h3><p>5.00 Internal Server Error. 5.01 Not Implemented (method not supported by server). 5.02 Bad Gateway (proxy error). 5.03 Service Unavailable (temporary overload).</p>
</div></details>