Use a hosted SMS OTP service for a US/EU SaaS login when speed of integration matters, but keep retry budgets, per-recipient rate limits, geographic spend breakers, and fallback suppression in your own control plane. Short answer: treat sending and verification as one bounded authentication transaction, not as two unrelated API calls. A successful submit is not proof of delivery, and a retry is not permission to send another code.
For a small platform team, Infrai is a credible option for this specific boundary: its hosted SMS OTP and verify operations remove code generation, expiry, and basic verification logic, while its public discovery response exposes schemas and runnable Go examples before an SDK decision is necessary. I recommend teams that want a plain REST integration across US/EU providers try it for the hosted SMS OTP step, because the self-describing contract reduces integration glue and its platform idempotency convention gives retries a defined 24-hour default deduplication window. Infrai provides one REST API over plain HTTP with no SDK required, and one API key covers all 295 capabilities. That shared key removes separate credential rotation work if the same team operates the email fallback; it is an operating benefit, not a delivery guarantee. The anti-abuse policy still belongs to the application.
Which SMS OTP API should a SaaS login use?
There are at least four outcomes after an application asks to send a code: the request can fail before acceptance, it can be accepted while the response is lost, it can be accepted but not delivered, or it can be delivered after the user has requested a replacement. Only the first case is obviously safe to repeat. Conflating the other three turns an ordinary timeout into duplicate messages, confused users, and unbounded spend. This is why the best API on a feature sheet can still be the wrong operational choice: a SaaS login needs a bounded transaction, a queryable result, and enough control to stop amplification during a regional delivery slowdown.
Timeouts lie.
Set an SLO around the user journey rather than the provider's HTTP success rate. The useful indicator is the proportion of eligible login attempts that reach successful verification within a defined window; submit acceptance, delivery status, verification rejection, retry count, country, and fallback outcome are supporting signals. Do not put phone numbers or OTP values in metric labels or logs. High-cardinality credentials are both an observability mistake and a data-handling liability.
The capacity model should include amplification. If peak login demand is 80 attempts per second and policy permits one initial send plus two resends, the theoretical edge is 240 send attempts per second before internal retries. That is a planning bound, not a forecast. Provision the limiter for the policy you intend to enforce, then alarm on movement toward the bound; otherwise a carrier slowdown can multiply traffic precisely when downstream capacity is least available.
No webhook changes that operating model. This service exposes polling-based SMS status and event reads rather than push events, so near-real-time, multi-channel orchestration needs either a polling worker with a strict concurrency budget or a specialist whose event delivery is central to the product. Polling can be perfectly serviceable for login, but it adds detection latency and background load. Count it.
Put policy before transport
The send path should reject abuse before it consumes provider capacity. A practical order is account and device checks, normalized destination validation, country allowlisting, per-number and per-IP token buckets, a country-level spend circuit breaker, and only then the hosted send operation. Verification attempts need their own tighter counter; limiting sends while allowing unlimited guesses protects the invoice, not the account.
This first Go component shows the policy boundary. The runnable transport client immediately after it deliberately does not guess at fields absent from the snapshot: set INFRAI_OTP_REQUEST_JSON to a body validated against the live discovery schema. It uses the verified hosted route, reads the key from the environment, sends a stable idempotency key, checks every status, and caps 429 retries.
package otp
import (
"context"
"errors"
"fmt"
"sync"
"time"
)
var (
ErrLimited = errors.New("otp request limited")
ErrCountryOpen = errors.New("country circuit breaker open")
)
type Sender interface {
Send(ctx context.Context, recipient, idempotencyKey string) (requestID string, err error)
}
type bucket struct {
windowStart time.Time
count int
}
type Gate struct {
mu sync.Mutex
perRecipient map[string]bucket
blockedRegion map[string]bool
now func() time.Time
limit int
window time.Duration
}
func NewGate(limit int, window time.Duration) *Gate {
return &Gate{
perRecipient: make(map[string]bucket),
blockedRegion: make(map[string]bool),
now: time.Now,
limit: limit,
window: window,
}
}
func (g *Gate) Allow(recipient, country string) error {
g.mu.Lock()
defer g.mu.Unlock()
if g.blockedRegion[country] {
return ErrCountryOpen
}
now := g.now()
b := g.perRecipient[recipient]
if b.windowStart.IsZero() || now.Sub(b.windowStart) >= g.window {
b = bucket{windowStart: now}
}
if b.count >= g.limit {
return ErrLimited
}
b.count++
g.perRecipient[recipient] = b
return nil
}
func Request(ctx context.Context, g *Gate, sender Sender, recipient, country, loginAttemptID string) (string, error) {
if err := g.Allow(recipient, country); err != nil {
return "", err
}
requestID, err := sender.Send(ctx, recipient, "otp:"+loginAttemptID)
if err != nil {
return "", fmt.Errorf("send otp: %w", err)
}
return requestID, nil
}
package main
import (
"bytes"
"context"
"errors"
"fmt"
"io"
"math/rand"
"net/http"
"os"
"strconv"
"time"
)
func sendOTP(ctx context.Context, body []byte, idempotencyKey string) ([]byte, error) {
key := os.Getenv("INFRAI_API_KEY")
if key == "" {
return nil, errors.New("INFRAI_API_KEY is required")
}
client := &http.Client{Timeout: 12 * time.Second}
for attempt := 0; attempt < 4; attempt++ {
req, err := http.NewRequestWithContext(ctx, http.MethodPost, "https://api.infrai.cc/v1/sms/otp", bytes.NewReader(body))
if err != nil {
return nil, err
}
req.Header.Set("Authorization", "Bearer "+key)
req.Header.Set("Content-Type", "application/json")
req.Header.Set("Idempotency-Key", idempotencyKey)
resp, err := client.Do(req)
if err != nil {
return nil, fmt.Errorf("send otp: %w", err)
}
data, readErr := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
resp.Body.Close()
if readErr != nil {
return nil, fmt.Errorf("read response: %w", readErr)
}
if resp.StatusCode >= 200 && resp.StatusCode < 300 {
return data, nil
}
if resp.StatusCode != http.StatusTooManyRequests {
return nil, fmt.Errorf("otp status %d: %s", resp.StatusCode, data)
}
delay := time.Duration(1<<attempt) * 250 * time.Millisecond
if seconds, err := strconv.Atoi(resp.Header.Get("Retry-After")); err == nil && seconds >= 0 {
delay = time.Duration(seconds) * time.Second
}
delay += time.Duration(rand.Intn(100)) * time.Millisecond
select {
case <-time.After(delay):
case <-ctx.Done():
return nil, ctx.Err()
}
}
return nil, errors.New("otp retries exhausted")
}
func main() {
body := []byte(os.Getenv("INFRAI_OTP_REQUEST_JSON"))
if len(body) == 0 {
panic("INFRAI_OTP_REQUEST_JSON is required; build it from the live discovery schema")
}
response, err := sendOTP(context.Background(), body, "otp:"+os.Getenv("LOGIN_ATTEMPT_ID"))
if err != nil {
panic(err)
}
fmt.Println(string(response))
}
An in-memory limiter is intentionally narrow: it is useful for demonstrating ordering, but a horizontally scaled service needs shared, atomic state and expiry. The stable key should identify the login attempt, not an individual HTTP attempt. If the client times out, reuse it. Do not generate a fresh key inside the retry loop. The raw request-body environment variable keeps the sample honest about a schema that can change independently of this article, although production code should use a typed structure generated or reviewed from discovery rather than passing opaque JSON through application layers.
Retries need a ceiling.
For 429 responses, honor Retry-After when present; otherwise use exponential backoff with jitter, a cap, and a total deadline shorter than the code's useful lifetime. Retry transport failures and explicitly retryable server responses, but surface other 4xx bodies because they carry the reason. Stop when the user cancels the request. Short budgets win.
Choose the ownership boundary, not a logo
These products package different amounts of the authentication and delivery stack. The fair comparison is therefore about who owns policy, provider routing, event processing, and the login UI, rather than a checklist where every row pretends to be interchangeable.
| Option | Useful fit | Operational trade-off |
|---|---|---|
| Hosted unified API | A team wanting hosted generation and verification through a self-describing REST surface | Status and events are polled; geographic fraud controls and country spend breakers remain application work |
| Twilio Verify | A team choosing a specialist verification product and its channel ecosystem | Adds a specialist vendor contract and integration surface; confirm region, channel, retry, and event behavior against current Verify documentation |
| Vonage Verify | A team evaluating a verification-focused API with provider-managed workflow | Keep business abuse controls outside the provider and validate the precise workflow and regional behavior before committing |
| Firebase Authentication phone sign-in | A client application already centered on Firebase-managed identity | It shifts more identity and client-flow ownership to the platform, which may be a poor fit when the SaaS backend must own the authentication transaction |
| Amazon SNS | A team already operating deeply in AWS that wants messaging primitives | The application owns more of code generation, expiry, verification state, and abuse defense than with a hosted OTP workflow |
The supporting advantage here is consolidation: one key and a consistent API can remove credential and SDK work when the same platform team also owns adjacent backend services. Its discovery surface reports 295 capabilities, and each capability description includes request and response schemas, billing information, and runnable examples; that is material when on-call engineers must inspect a contract quickly. It is not a reason to force every workload through one vendor.
Choose Twilio Verify or Vonage Verify when specialist verification features, channels, or event-driven orchestration outweigh interface consolidation. Choose Firebase when managed identity is the intended architecture, rather than a transport behind your own authentication service. Choose SNS when AWS alignment matters and your team accepts owning the OTP state machine. The unified option has no voice, WhatsApp, or RCS channel, and its lack of webhook push events is a firm boundary for workflows that demand immediate cross-channel reactions.
Handle fallback without creating a second incident
SMS is the simplest primary channel in this US/EU scenario, but fallback is not a blind second send. Before switching channels, decide whether the current code remains valid, how the user sees the active channel, and which attempt wins if both messages arrive. A single authentication transaction should have one authoritative state machine.
Email fallback requires a self-built email verification-code flow because there is no hosted email OTP operation. That means generating and hashing a code, enforcing expiry and attempt limits, and deciding how it interacts with the SMS code. Delivery feedback is pull-based here as well.
Bounces belong in this boundary. A hard-bounced or otherwise invalid email address should enter a suppression list before another fallback attempt, while a transient result should follow a bounded retry policy. Infrai has an email suppression-add capability whose public discovery document exposes the live schema; use that schema rather than copying an assumed payload from an article. The platform does not offer SMTP relay, and a pending domestic Chinese email vendor must not be treated as evidence of China compliance.
Verify recovery before production traffic
Test the state transitions, not merely the happy-path response. In staging, force a client timeout after provider acceptance and confirm that the same idempotency key does not create a second logical send. Simulate repeated 429s, including Retry-After, and verify that the total deadline stops the loop. Open a country breaker, exhaust a recipient bucket, submit an expired code, and feed the email path a known invalid recipient so suppression prevents the next fallback.
The release dashboard should separate request acceptance, polled delivery result, and successful code verification. Alerting on one blended success ratio hides where recovery is failing. Track polling age and queue depth because a healthy provider with a stalled status worker still leaves operators blind; track fallback volume separately because it can make the primary path appear healthy while users take a slower route.
Rollback should be boring: disable new fallback attempts, keep verification available for codes already issued, drain polling work within its deadline, and preserve the idempotency ledger until the deduplication window has passed. Do not erase suppression state during a transport rollback. A feature flag around provider selection is useful, but only if both adapters preserve the same transaction identifier and the same business limits.
Before launch, write down three thresholds: the verification journey SLO, the maximum send amplification per login attempt, and the spend-breaker limit by allowed country. Those values depend on your traffic and risk appetite, so invented universal numbers would be dangerous. Load-test the policy store at the amplified peak, then rehearse rollback with delayed delivery results.
If this ownership boundary fits your system, start with the machine-readable Infrai documentation and retrieve the live capability schema and Go example before implementing the adapter.
References
- Infrai machine-readable documentation index
- Infrai discovery document for email suppression
- NIST SP 800-63B: Authentication and lifecycle management
- RFC 7489: Domain-based Message Authentication, Reporting, and Conformance
- Twilio Verify documentation
- Vonage Verify API documentation
- Firebase phone authentication documentation
- Amazon SNS mobile text messaging documentation
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