In Better Auth’s Phone Number plugin, the sendOTP callback is the delivery hook. It receives the destination phone number and the generated code, and your application passes them to an SMS provider. Verification stays inside the plugin unless you also supply verifyOTP, which replaces that internal check. A five-line adapter therefore handles sending only, and it does not verify anything.
How the plugin is wired
The Phone Number plugin is imported as phoneNumber from better-auth/plugins and added to the plugins array in your Better Auth configuration, as described in the current Phone Number plugin documentation. The plugin owns the request logic, and your callbacks own message delivery and, if you choose, verification.
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The five-line sendOTP adapter
The snippet below shows the shape of the integration. It is illustrative only. It has not been run or tested, sms is a placeholder for your provider client, and it omits credentials, error handling, and background execution.
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phoneNumber({
sendOTP: ({ phoneNumber, code }) => sms.send({ to: phoneNumber, body: `Code: ${code}` }),
})The callback destructures two values. phoneNumber is the destination, and code is the one-time code the plugin generated. The documented callback also receives a second context argument, which this snippet does not use. Confirm the exact callback types against the Better Auth version installed in your project.
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What your application must supply
- A real provider call. Replace the placeholder with your SMS provider’s SDK or HTTP call, and write the message template yourself.
- Credentials from secure configuration. Keep provider keys in environment variables or secret storage, not in source files.
- Error handling. The snippet discards the provider’s result. Decide whether a failed send is logged, retried, or surfaced in some other way.
- Background execution where needed. On serverless platforms, the send may need the runtime’s background-task mechanism, covered below.
Why the callback should not be awaited
The documentation is direct on this point: “We highly recommend not awaiting the sendOTP function. If you await it, it’ll slow down the request and could cause timing attacks.” (Better Auth documentation, Phone Number plugin page; the page does not name an individual author.)
Not awaiting the send keeps the authentication response from waiting on the provider, and it keeps response timing from reflecting whether a send happened. The cost is that a failed send no longer blocks the request. If you do not handle the promise, a rejection can go unnoticed, so add your own logging or failure handling inside the callback.
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On serverless platforms, the docs mention waitUntil as an option to make sure delivery finishes after the response has been returned. Use the background-task API your platform provides for that purpose.
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verifyOTP replaces verification; it does not add delivery
The verifyOTP callback receives phoneNumber and code and returns a boolean or a promise that resolves to one. When you configure it, Better Auth uses it instead of its internal verification logic. It is a custom verifier, not a second sending path. Sending an SMS and checking a code are separate responsibilities, and configuring one does not configure the other.
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The documentation names Twilio Verify and AWS SNS as examples of external integrations. Its example code for these is illustrative only, so treat it as a pattern rather than a verified integration.
| Question | Built-in verification (sendOTP only) | Custom verifyOTP |
|---|---|---|
| Who generates the code | The plugin generates it and passes it to sendOTP |
Not stated in the documentation reviewed; the plugin still passes its generated code to sendOTP |
| Where verification happens | Inside Better Auth | In your verifyOTP function, which replaces the internal logic |
| Expiry and attempt limits | Documented defaults of 300 seconds and 3 attempts (see below) | Not stated in the documentation reviewed; check the expiry and attempt rules of your verifier |
| Single-use under parallel redemption | Not stated in the documentation reviewed | For strict single-use acceptance, the docs say to use a provider that atomically consumes accepted codes |
| Effect on delivery | Set by your sendOTP callback |
None; verifyOTP sends nothing |
Documented defaults and attempt limits
The current Phone Number documentation and the v1.6 Phone Number page both list the following defaults. Treat them as documented defaults and confirm them against the version installed in your project.
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| Option | Documented default | Meaning |
|---|---|---|
otpLength |
6 | Number of digits in the code |
expiresIn |
300 seconds | How long a code remains valid after it is issued |
allowedAttempts |
3 | Verification attempts allowed before the code is discarded |
After the allowed attempts are exceeded, the OTP is deleted and the user must request a new one. Build your interface to handle that case, because a user who mistypes the code three times will need to start over.
Schema changes
The plugin schema requires two user fields, phoneNumber and phoneNumberVerified. Add them before testing any sign-in flow.
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- Enable the Phone Number plugin in your Better Auth configuration.
- Run the migration or schema generation step described in the Phone Number documentation.
- If you manage your schema by hand, add the
phoneNumberandphoneNumberVerifiedfields to the user table yourself, matching the plugin’s schema exactly.
Flows the plugin covers
- Send and verify an OTP, the core flow that both callbacks serve.
- Optional sign-up on verification, where a successful code can create the account.
- Phone-number password sign-in, where a phone number and password authenticate the user.
- Phone-number change after authentication, which verifies a new number for a signed-in user.
- Password reset through a phone-number OTP.
- Server-only
consumePhoneNumberOTP, for custom sign-up or account-linking flows. It does not return a session or a reusable proof of verification, and the docs do not describe it as adding stronger concurrency guarantees. Do not treat its result as a token you can hand to the browser.
Check the version before you copy the page
The current Phone Number page and the v1.6 page both describe sendOTP, custom verifyOTP, and the same defaults. The current page adds detail on server-only consumption and concurrency. For a given project, the documentation matching your deployed Better Auth release is the authoritative source, so check which version your project installs before relying on any single page.
Choosing a delivery option
Better Auth’s documentation identifies a managed SMS service for OTP delivery, described on the v1.6 SMS service page. It also names Twilio Verify and AWS SNS as external integration examples. Choosing among them is a question of delivery, not verification. If you pick an external verifier, you also take on the job of confirming single-use behavior, as described above.
The Better Auth documentation does not compare providers on price, latency, deliverability, or reliability. Evaluate those points directly with each provider before committing.
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