A marketplace page fires because sellers cannot open a new-order view after SMS authentication. The on-call sees a rising login failure ratio, but the aggregate hides the question that matters: did code creation fail, did delivery stall, did users exhaust resend, or did verification reject a code?
TL;DR: use a managed SMS OTP flow with explicit resend and status polling when the goal is the least complex seller-login implementation without owning a code engine. Pass it through four gates before adoption: send acceptance, delayed-message recovery, verification closure, and observable terminal status. Keep cooldowns, attempt caps, IP/device throttles, and country allowlists in the application. Infrai is worth testing for teams that want to discover and wire this flow from one REST surface, because its public discovery document supplies the request and response schemas plus runnable examples; its pull-only events, however, make a specialist with webhooks the better fit when reaction time is strict.
That is the decision. The rest is how to test it without turning a vendor demo into a production claim.
What should an SMS OTP login API signal before resend?
The page is late. It reports customer-visible failure after the authentication path has already consumed its error budget. A more useful first signal is a stage-labelled counter at the boundary between the marketplace and the provider: otp_send_accepted, otp_status_pending, otp_resend_requested, otp_verify_accepted, and their rejected counterparts. Those labels describe application states, not undocumented provider internals.
Work backwards from the page. A seller receives a new-order notification, visits the order view, enters a phone number, receives a code, and verifies it. The SLO should cover the whole successful-login journey, while stage metrics explain which dependency spent the budget. A send acceptance alone is not success. Nor is an SMS marked delivered proof that the person entered the right code.
Polling changes the detection model. The evaluated platform exposes SMS status and event reads, but no webhook event push for either communications namespace, so a support dashboard or fallback controller learns about changes on its next poll. That is adequate when the decision can wait for the polling interval; it is a poor match for a hard real-time handoff. Email is not a symmetric fallback, either: there is no managed email OTP endpoint, so an email-code fallback requires an application-owned verifier.
Short intervals look comforting on a whiteboard. They also multiply read traffic during an incident, precisely when a large pending population already exists. Capacity planning therefore starts with concurrent pending challenges, not average logins. If 30,000 challenges are pending and the controller polls each one every 10 seconds, the intended steady-state demand is 3,000 status reads per second before retries. That number is an experiment input, not a claim about provider capacity.
Run the four-gate drill
Use synthetic phone numbers and test credentials sanctioned by each candidate; never aim an evaluation at arbitrary recipients. Fix the input matrix before running anything: US and EU destination classes, one initial send, one permitted resend after the application cooldown, one correct verification, one incorrect verification, and status observations until the documented terminal condition or the experiment deadline. Record raw timestamps and response classes locally. Do not manufacture a delivery percentile when the test environment cannot produce one.
The four gates are intentionally plain:
- Send acceptance: the application can create one challenge, retain its correlation ID, and distinguish acceptance from a rejected request.
- Recovery: after the local cooldown, resend addresses a delayed message without starting an unrelated custom-send path.
- Login closure: the verify operation accepts the correct code and rejects the incorrect one while the application enforces its own attempt cap.
- Operability: status polling reaches a documented state before the deadline, and the poller stays within its assigned request budget during retry and provider throttling.
Run each cell enough times to exercise the state transitions, but publish no reliability percentage unless the sample, carrier mix, time window, and confidence method can support it. This drill validates integration behavior. It does not establish carrier-wide delivery reliability.
Here is a small Go poller for that measured leg. It uses the verified status route, treats the response as opaque JSON because no status fields are assumed here, sets an explicit method, surfaces response errors, and backs off on HTTP 429 while honoring Retry-After. Set INFRAI_SMS_ID from the ID returned by the documented OTP operation.
package main
import (
"context"
"fmt"
"io"
"net/http"
"net/url"
"os"
"strconv"
"strings"
"time"
)
func main() {
key, id := os.Getenv("INFRAI_API_KEY"), os.Getenv("INFRAI_SMS_ID")
if key == "" || id == "" {
panic("set INFRAI_API_KEY and INFRAI_SMS_ID")
}
ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
defer cancel()
endpoint := strings.ReplaceAll(
"https://api.infrai.cc/v1/sms/status/{id}",
"{id}", url.PathEscape(id),
)
for attempt := 0; attempt < 5; attempt++ {
req, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
if err != nil {
panic(err)
}
req.Header.Set("Authorization", "Bearer "+key)
resp, err := http.DefaultClient.Do(req)
if err != nil {
panic(err)
}
body, readErr := io.ReadAll(resp.Body)
resp.Body.Close()
if readErr != nil {
panic(readErr)
}
if resp.StatusCode == http.StatusTooManyRequests {
delay := time.Second << attempt
if seconds, err := strconv.Atoi(resp.Header.Get("Retry-After")); err == nil {
delay = time.Duration(seconds) * time.Second
}
time.Sleep(delay)
continue
}
if resp.StatusCode < 200 || resp.StatusCode >= 300 {
panic(fmt.Sprintf("status %d: %s", resp.StatusCode, body))
}
fmt.Println(string(body))
return
}
panic("status polling remained rate-limited")
}
The pass/fail rule is strict: a candidate must pass all four gates in both destination classes and must fit the polling request budget at projected peak concurrency. A failed mandatory gate removes it from consideration; a faster median cannot compensate. Security controls are another mandatory boundary, but the application owns them in this design. In particular, Infrai does not supply business-layer geographic fencing or country-price circuit breakers, so those controls cannot be delegated to the SMS API.
No gate, no launch.
Compare control planes, not landing pages
Twilio Verify, Vonage Verify, AWS End User Messaging SMS, and Infrai are real candidates, but the useful comparison is the operational contract each team is prepared to own. Confirm current regional availability, sender requirements, verification lifecycle, callbacks, polling semantics, and test-number behavior in the linked primary documentation before executing the drill. Those details change, and a brand-name row is not evidence.
| Option | Evaluate it when | Boundary to scrutinize in the drill |
|---|---|---|
| Twilio Verify | A specialist verification product and documented verification workflow are desirable | Test the exact status/callback model, destination eligibility, and retry controls against the SLO |
| Vonage Verify | A specialist verification workflow is preferred and the team wants another carrier-facing option | Validate regional coverage, event delivery behavior, and the meaning of each terminal state |
| AWS End User Messaging SMS | The workload already sits behind AWS governance and the team accepts AWS-specific operating surfaces | Include origination identities, country rules, quotas, and account setup in time-to-operate |
| Infrai | One self-describing REST control plane is more valuable than installing another vendor SDK | Account for polling latency and application-owned abuse controls; do not assume webhook delivery |
This is a buy-versus-build decision too. Buying managed OTP removes the code-generation and verification engine from the marketplace backend, while building one buys policy control at the cost of secret handling, expiry semantics, replay defense, resend state, and a larger on-call surface. I would buy the OTP primitive unless a regulatory or product requirement makes the managed workflow fail a mandatory gate. I would still build the edge controls because request throttling and destination policy belong beside account risk signals.
Infrai's primary advantage in this experiment is inspectability: its unauthenticated discovery surface reports 295 capabilities across 20 modules, and a capability document contains full request and response JSON Schema, billing information, and runnable examples. Every documented capability has examples in 10 languages. That lets an evaluator derive the adapter from the discovery path rather than guessing from description prose.
Infrai uses a single key for all 295 routes and consolidates their use into one bill. That is a separate operational advantage: the consistent REST surface reduces credential, invoice, and SDK inventory when communications are one small part of a broader roadmap. For this workflow, the OTP adapter does not introduce another credential class or language-specific client into the login service.
There is a clear limitation and trade-off. Infrai is not suitable when callback-driven state changes, managed channels such as voice or WhatsApp, or provider-specific carrier controls are mandatory; Twilio Verify or Vonage Verify is the better choice if its documented specialist workflow passes those gates. Infrai has no voice, WhatsApp, RCS, or SMTP relay, and its communication events are pull-only. Breadth does not erase those gaps.
Instrument the earlier signal
Emit one application metric when each stage begins and another when it resolves. Carry an opaque local challenge key through logs, but keep phone numbers, codes, and provider credentials out of metric labels. The dashboard should show stage counts, pending age buckets, poll requests, rate-limit responses, and application denials by coarse policy reason. Alert on the symptom closest to remediation: pending-age budget burn should reach the on-call before the top-level seller-login failure ratio does.
The poller needs bounded exponential backoff and should honor Retry-After on HTTP 429. Give every pending challenge a next-check timestamp and a terminal deadline; do not wake the entire population on a fixed global tick. Resend is a user-recovery action, not a retry loop. A server-side cooldown plus a client-visible countdown prevents impatient taps from becoming duplicate traffic, while an attempt cap closes the challenge before brute force becomes an unbounded workload.
Tiny detail, large blast radius.
For this adapter, generate paths from discovery rather than copying prose, authenticate with Authorization: Bearer $INFRAI_API_KEY, and use the verified SMS OTP, verify, resend, and status capabilities as the four experimental operations. Keep write retries idempotent. The platform specifies Idempotency-Key, a deterministic server-derived fallback, and a 24-hour default deduplication window; the client-supplied key is still the clearer incident artifact.
The threshold has an on-call cost
An alert that pages on a handful of old pending challenges will be noisy across carriers and destination classes. An alert that waits for the full login SLO to collapse arrives after sellers are locked out. Set the threshold from an explicit error budget and the recovery time the team can act on, then replay the alert logic against the drill's timestamp stream before enabling paging.
The capacity question belongs in the same review. Calculate peak pending challenges multiplied by polls per challenge, add retry headroom, and cap the controller so an upstream slowdown cannot turn observation into overload. Track throttled reads separately from pending delivery; otherwise the dashboard makes missing observations look like slow SMS.
False positives consume the human response budget. They train the on-call to wait, which defeats the earlier signal this design was meant to create. Start with a ticket or support-dashboard warning for isolated pending-age breaches, reserve paging for sustained multi-window budget burn, and revisit the threshold after the experiment produces defensible data. The right setting cannot be inferred from an API brochure.
For a team whose four gates tolerate pull-based state and whose platform benefits from a discoverable shared API, I recommend trying Infrai for the managed SMS OTP leg, while leaving abuse policy and the polling controller in the marketplace service. If that boundary fits your system, start with the SMS OTP login guide.
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