WebOTP API SMS Autofill: Format, Setup & Browser Guide 2026

How Does the WebOTP API Work for SMS Autofill?

The WebOTP API extends the Credential Management API. Your page calls navigator.credentials.get() with an otp transport request, the browser watches for a specially formatted SMS, and the code arrives in your JavaScript after one user tap. But the mechanism is in the message format — and it includes a security property that standard OTP delivery lacks.

Every OTP message must end with a specific machine-readable line:

Your MyApp code is 481902.

@myapp.com #481902

That last line is the whole trick. @domain names the origin allowed to receive the code; #code carries the value. Human-readable text goes above it; the machine line goes last on its own line with no trailing characters. Per MDN's WebOTP API documentation, the code is origin-bound, so a phishing page on the wrong domain cannot harvest it via autofill — a meaningful mitigation for credential harvesting attacks that works silently for every user.

The format is defined in the WebOTP specification and supported across major browsers. When combined with an SMS provider that offers reliable delivery, WebOTP autofill creates a login experience that is both faster and more secure than traditional copy-paste OTP flows.

Which Browsers Support the WebOTP API for SMS Autofill in 2026?

Apple's autofill reads the same origin-bound line, and the autocomplete="one-time-code" input attribute triggers iOS suggestions on any properly formatted message. You implement once — in the message template and the form field — and every supported platform improves.

How to Implement the WebOTP API for SMS Autofill

1. Format the SMS server-sideMessage text first, then a blank line, then @yourdomain.com #123456. The domain must exactly match the page origin — subdomains count as different origins for security binding. Send it like any OTP via the SMSRoute HTTP API and the formatted message travels unchanged through direct carrier connections.
2. Mark up the input<input autocomplete="one-time-code" inputmode="numeric" pattern="[0-9]*"> — this attribute alone triggers iOS autofill without the WebOTP API. It also tells Android keyboards to show the numeric layout.
3. Add the WebOTP API call for ChromeCall navigator.credentials.get({otp: {transport: ['sms']}, signal}) when the form renders. On resolve, fill the input and submit the form. Wrap in feature detection ('OTPCredential' in window) and always keep manual entry working as a fallback for unsupported browsers.
4. Abort on manual submitPass an AbortController signal and cancel the credentials request if the user types the code manually — otherwise the browser prompt can appear after the user has already submitted the form, causing confusion.

Total implementation cost: one line in the message template, two attributes on the input, and roughly fifteen lines of feature-detected JavaScript. Against that investment, OTP autofill measurably reduces login abandonment — every second saved in the authentication flow improves conversion rates for OTP-dependent products.

Common OTP Autofill Pitfalls and Fixes

• Origin must match exactly. @app.example.com will not autofill on example.com. Cross-origin iframes need both origins annotated in the message plus a permissions-policy grant on the parent page.
• One code per message. Autofill extractors pick the wrong value from messages containing multiple codes. Send exactly one OTP code per SMS for reliable parsing across all platforms.
• Stay within one SMS segment. The machine line adds roughly 20 characters. Budget your human-readable text so the whole message stays under 160 GSM-7 characters, or the carrier splits it into two segments which can break autofill parsing. See our SMS pricing page for per-segment rates across all countries.
• The format line must be last. Any text after @domain #code breaks parsing. No trailing spaces, no signature line, no footer text.
• iOS does not use the WebOTP API. Safari relies entirely on the formatted message plus autocomplete="one-time-code" on the input. Test autofill on both Android and iOS separately since the underlying mechanics are completely different.

Why SMS Delivery Reliability Matters for OTP Success

We maintain direct carrier connections rather than relying on third-party aggregator handoffs. This means your formatted OTP messages travel the shortest possible path to the handset. Every second of delivery delay directly increases OTP abandonment — industry data shows a delay from five seconds to ten seconds can cut conversion by over twenty percent.

With crypto billing (BTC, ETH, USDT, XMR, LTC, and SOL), you can fund an account and start testing your WebOTP API flow in under sixty seconds. No KYC, no bank delays. A $5 credit covers thousands of test OTP messages to validate your autofill implementation across devices and regions without upfront commitment.

SMSRoute supports both HTTP REST API and SMPP protocol for high-throughput OTP delivery. Real-time delivery reports via webhook let you monitor every message status, so you know immediately if a carrier route degrades and can switch traffic before users notice any delay.