Sending NICE CXone SMS Messages via REST API with Go

What You Will Build

You will build a production-grade Go service that constructs, validates, and dispatches SMS messages to NICE CXone using atomic POST operations. The service enforces messaging engine constraints, handles automatic segmentation, synchronizes delivery events via webhook callbacks, tracks latency, and generates audit logs for compliance governance.

Prerequisites

• OAuth2 client credentials with message:sms:send scope
• CXone account ID and base URL format https://{account}.my.cxone.com
• Go 1.21 or later
• Standard library only: net/http, encoding/json, context, fmt, log, net/url, regexp, sync, time
• Environment variables: CXONE_ACCOUNT, CXONE_CLIENT_ID, CXONE_CLIENT_SECRET, CXONE_FROM_NUMBER, CXONE_CAMPAIGN_ID, CXONE_WEBHOOK_URL

Authentication Setup

CXone uses standard OAuth2 client credentials flow. You must cache the access token and refresh it before expiration. The token endpoint returns a JSON payload containing access_token and expires_in.

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"sync"
	"time"
)

type OAuthToken struct {
	AccessToken string `json:"access_token"`
	ExpiresIn   int    `json:"expires_in"`
	ExpiresAt   time.Time
}

type CXoneAuth struct {
	account       string
	clientID      string
	clientSecret  string
	mu            sync.RWMutex
	token         *OAuthToken
	httpClient    *http.Client
}

func NewCXoneAuth(account, clientID, clientSecret string) *CXoneAuth {
	return &CXoneAuth{
		account:      account,
		clientID:     clientID,
		clientSecret: clientSecret,
		httpClient:   &http.Client{Timeout: 10 * time.Second},
	}
}

func (a *CXoneAuth) GetToken(ctx context.Context) (*OAuthToken, error) {
	a.mu.RLock()
	if a.token != nil && time.Until(a.token.ExpiresAt) > 2*time.Minute {
		token := a.token
		a.mu.RUnlock()
		return token, nil
	}
	a.mu.RUnlock()

	a.mu.Lock()
	defer a.mu.Unlock()
	if a.token != nil && time.Until(a.token.ExpiresAt) > 2*time.Minute {
		return a.token, nil
	}

	tokenURL := fmt.Sprintf("https://%s.my.cxone.com/api/v2/oauth/token", a.account)
	data := url.Values{}
	data.Set("grant_type", "client_credentials")
	data.Set("client_id", a.clientID)
	data.Set("client_secret", a.clientSecret)

	req, err := http.NewRequestWithContext(ctx, http.MethodPost, tokenURL, strings.NewReader(data.Encode()))
	if err != nil {
		return nil, fmt.Errorf("failed to create token request: %w", err)
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

	resp, err := a.httpClient.Do(req)
	if err != nil {
		return nil, fmt.Errorf("token request failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		body, _ := io.ReadAll(resp.Body)
		return nil, fmt.Errorf("token request returned %d: %s", resp.StatusCode, string(body))
	}

	var tokenResp OAuthToken
	if err := json.NewDecoder(resp.Body).Decode(&tokenResp); err != nil {
		return nil, fmt.Errorf("failed to decode token response: %w", err)
	}
	tokenResp.ExpiresAt = time.Now().Add(time.Duration(tokenResp.ExpiresIn) * time.Second)
	a.token = &tokenResp
	return a.token, nil
}

Implementation

Step 1: SMS Payload Construction and Schema Validation

CXone enforces strict SMS schema constraints. You must validate recipient numbers against E.164 format, verify opt-in status, and calculate character limits to trigger automatic segmentation safely. The messaging engine splits messages at 160 characters for GSM-7 encoding and 70 characters for UCS-2 encoding.

package main

import (
	"fmt"
	"regexp"
	"strings"
	"unicode/utf8"
)

type SMSMessage struct {
	To          []string `json:"to"`
	From        string   `json:"from"`
	Text        string   `json:"text"`
	CampaignID  string   `json:"campaignId"`
	WebhookURL  string   `json:"webhookUrl,omitempty"`
	Segmentation struct {
		Enabled bool `json:"enabled"`
	} `json:"segmentation"`
}

type SMSValidationResult struct {
	Valid        bool
	Segments     int
	EncodingType string
	Errors       []string
}

var e164Regex = regexp.MustCompile(`^\+?[1-9]\d{1,14}$`)

func ValidateSMS(msg SMSMessage, optInStore map[string]bool) SMSValidationResult {
	result := SMSValidationResult{Valid: true, Segments: 1, EncodingType: "GSM-7"}
	
	if msg.CampaignID == "" {
		result.Errors = append(result.Errors, "campaignId is required")
		result.Valid = false
	}

	for _, num := range msg.To {
		if !e164Regex.MatchString(num) {
			result.Errors = append(result.Errors, fmt.Sprintf("invalid E.164 format: %s", num))
			result.Valid = false
		}
		if !optInStore[num] {
			result.Errors = append(result.Errors, fmt.Sprintf("recipient not opted in: %s", num))
			result.Valid = false
		}
	}

	// Character limit and segmentation logic
	runeCount := utf8.RuneCountInString(msg.Text)
	isGSM7 := true
	for _, r := range msg.Text {
		if r > 127 {
			isGSM7 = false
			break
		}
	}

	if isGSM7 {
		if runeCount > 160 {
			result.Segments = (runeCount + 159) / 160
			result.EncodingType = "GSM-7"
		}
	} else {
		if runeCount > 70 {
			result.Segments = (runeCount + 69) / 70
			result.EncodingType = "UCS-2"
		}
	}

	if len(result.Errors) > 0 {
		result.Valid = false
	}
	return result
}

Step 2: Atomic POST Dispatch with Retry and Format Verification

You dispatch the validated payload via an atomic POST operation. CXone returns a 202 Accepted with a message ID upon successful queueing. You must implement exponential backoff for 429 Too Many Requests responses to prevent rate-limit cascades. You also track dispatch latency and generate an audit log entry.

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"time"
)

type CXoneSMSClient struct {
	auth       *CXoneAuth
	baseURL    string
	httpClient *http.Client
}

type AuditLogEntry struct {
	Timestamp   time.Time `json:"timestamp"`
	CampaignID  string    `json:"campaignId"`
	Recipient   string    `json:"recipient"`
	MessageID   string    `json:"messageId,omitempty"`
	Status      string    `json:"status"`
	LatencyMs   float64   `json:"latencyMs"`
	ErrorCode   string    `json:"errorCode,omitempty"`
}

func NewCXoneSMSClient(auth *CXoneAuth) *CXoneSMSClient {
	return &CXoneSMSClient{
		auth: auth,
		baseURL: fmt.Sprintf("https://%s.my.cxone.com", auth.account),
		httpClient: &http.Client{Timeout: 30 * time.Second},
	}
}

func (c *CXoneSMSClient) SendSMS(ctx context.Context, msg SMSMessage) (string, AuditLogEntry, error) {
	start := time.Now()
	audit := AuditLogEntry{
		Timestamp:  start,
		CampaignID: msg.CampaignID,
		Recipient:  msg.To[0],
		Status:     "pending",
	}

	token, err := c.auth.GetToken(ctx)
	if err != nil {
		audit.Status = "auth_failed"
		audit.ErrorCode = err.Error()
		return "", audit, err
	}

	payload, err := json.Marshal(msg)
	if err != nil {
		audit.Status = "serialization_failed"
		audit.ErrorCode = err.Error()
		return "", audit, err
	}

	endpoint := fmt.Sprintf("%s/api/v2/message/sms/send", c.baseURL)
	req, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, bytes.NewReader(payload))
	if err != nil {
		audit.Status = "request_failed"
		audit.ErrorCode = err.Error()
		return "", audit, err
	}

	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", token.AccessToken))

	// Retry logic for 429
	var resp *http.Response
	maxRetries := 3
	for attempt := 0; attempt <= maxRetries; attempt++ {
		resp, err = c.httpClient.Do(req)
		if err != nil {
			audit.Status = "network_error"
			audit.ErrorCode = err.Error()
			return "", audit, err
		}

		if resp.StatusCode == http.StatusTooManyRequests {
			backoff := time.Duration(1<<uint(attempt)) * time.Second
			time.Sleep(backoff)
			continue
		}
		break
	}

	body, _ := io.ReadAll(resp.Body)
	resp.Body.Close()

	audit.LatencyMs = float64(time.Since(start).Microseconds()) / 1000.0

	if resp.StatusCode == http.StatusAccepted || resp.StatusCode == http.StatusOK {
		var result map[string]interface{}
		json.Unmarshal(body, &result)
		msgID, _ := result["id"].(string)
		audit.Status = "dispatched"
		audit.MessageID = msgID
		return msgID, audit, nil
	}

	audit.Status = "failed"
	audit.ErrorCode = fmt.Sprintf("HTTP %d: %s", resp.StatusCode, string(body))
	return "", audit, fmt.Errorf("SMS dispatch failed: %s", string(body))
}

Step 3: Webhook Synchronization and Delivery Tracking

CXone POSTs delivery status updates to the webhookUrl specified in the payload. You must expose an HTTP handler to parse these events, update latency metrics, and append final delivery states to the audit log. The webhook payload contains messageId, status, and timestamp.

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"time"
)

type WebhookEvent struct {
	MessageID string    `json:"messageId"`
	Status    string    `json:"status"`
	To        string    `json:"to"`
	Timestamp time.Time `json:"timestamp"`
	Error     string    `json:"error,omitempty"`
}

type DeliveryTracker struct {
	mu      sync.Mutex
	events  map[string]WebhookEvent
	audit   []AuditLogEntry
}

func NewDeliveryTracker() *DeliveryTracker {
	return &DeliveryTracker{
		events: make(map[string]WebhookEvent),
		audit:  make([]AuditLogEntry, 0),
	}
}

func (t *DeliveryTracker) HandleWebhook(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodPost {
		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
		return
	}

	var event WebhookEvent
	if err := json.NewDecoder(r.Body).Decode(&event); err != nil {
		http.Error(w, "invalid payload", http.StatusBadRequest)
		return
	}

	t.mu.Lock()
	t.events[event.MessageID] = event
	t.audit = append(t.audit, AuditLogEntry{
		Timestamp:  time.Now(),
		CampaignID: "tracked",
		Recipient:  event.To,
		MessageID:  event.MessageID,
		Status:     event.Status,
		LatencyMs:  float64(time.Since(event.Timestamp).Seconds()) * 1000,
	})
	t.mu.Unlock()

	w.WriteHeader(http.StatusOK)
	fmt.Fprint(w, "accepted")
}

func (t *DeliveryTracker) GetAuditLog() []AuditLogEntry {
	t.mu.Lock()
	defer t.mu.Unlock()
	return t.audit
}

Complete Working Example

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"os"
	"strings"
	"time"
)

func main() {
	account := os.Getenv("CXONE_ACCOUNT")
	clientID := os.Getenv("CXONE_CLIENT_ID")
	clientSecret := os.Getenv("CXONE_CLIENT_SECRET")
	fromNumber := os.Getenv("CXONE_FROM_NUMBER")
	campaignID := os.Getenv("CXONE_CAMPAIGN_ID")
	webhookURL := os.Getenv("CXONE_WEBHOOK_URL")

	if account == "" || clientID == "" || clientSecret == "" {
		log.Fatal("missing required environment variables")
	}

	auth := NewCXoneAuth(account, clientID, clientSecret)
	client := NewCXoneSMSClient(auth)
	tracker := NewDeliveryTracker()

	// Expose webhook listener
	http.HandleFunc("/webhooks/cxone/sms", tracker.HandleWebhook)
	go func() {
		log.Println("listening for webhooks on :8080")
		if err := http.ListenAndServe(":8080", nil); err != nil {
			log.Fatalf("webhook server failed: %v", err)
		}
	}()

	// Simulate opt-in store
	optInStore := map[string]bool{
		"+15551234567": true,
	}

	msg := SMSMessage{
		To:         []string{"+15551234567"},
		From:       fromNumber,
		Text:       "Your account verification code is 849201. Valid for 10 minutes.",
		CampaignID: campaignID,
		WebhookURL: webhookURL,
		Segmentation: struct {
			Enabled bool `json:"enabled"`
		}{Enabled: true},
	}

	val := ValidateSMS(msg, optInStore)
	if !val.Valid {
		log.Fatalf("validation failed: %v", val.Errors)
	}

	fmt.Printf("validation passed: %s encoding, %d segments\n", val.EncodingType, val.Segments)

	ctx := context.Background()
	msgID, audit, err := client.SendSMS(ctx, msg)
	if err != nil {
		log.Fatalf("dispatch failed: %v", err)
	}

	fmt.Printf("message dispatched: %s\n", msgID)
	fmt.Printf("audit log: %+v\n", audit)

	// Keep main alive to receive webhooks
	time.Sleep(30 * time.Second)
}

Common Errors & Debugging

Error: HTTP 401 Unauthorized

• Cause: Expired OAuth token, invalid client credentials, or missing message:sms:send scope.
• Fix: Verify client credentials in CXone admin. Ensure the token cache refreshes before expires_in lapses. Check scope assignment on the OAuth client.
• Code Fix: The CXoneAuth.GetToken method already implements automatic refresh. If it persists, validate credentials against https://{account}.my.cxone.com/api/v2/oauth/token manually.

Error: HTTP 400 Bad Request

• Cause: Invalid E.164 format, missing campaignId, or payload schema mismatch.
• Fix: Run ValidateSMS before dispatch. Ensure campaignId matches an active campaign in CXone. Verify from number is registered and verified.
• Code Fix: The validation step catches E.164 violations and missing campaign IDs. Print val.Errors to identify exact field failures.

Error: HTTP 429 Too Many Requests

• Cause: Exceeding CXone SMS rate limits (typically 100 requests per second per account).
• Fix: Implement exponential backoff. The SendSMS method retries up to 3 times with doubling delays. For high volume, implement a worker pool with semaphore concurrency control.
• Code Fix: The retry loop in SendSMS handles this automatically. Monitor LatencyMs in audit logs to detect throttling patterns.

Error: HTTP 502/503 Bad Gateway

• Cause: CXone messaging engine temporarily unavailable or undergoing maintenance.
• Fix: Implement circuit breaker pattern. Retry after 5-10 seconds. Check CXone status page.
• Code Fix: Wrap SendSMS in a retry decorator with jitter. Log 5xx responses to your audit pipeline for SLA reporting.

Official References

• NICE CXone Message SMS API Documentation
• NICE CXone OAuth2 Authentication Guide
• NICE CXone Webhook Delivery Status Reference
• GSM 03.38 Character Encoding Standard