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To keep speech recognition on the device, your app has to request local processing explicitly and then confirm that the platform service can honor the request. On Apple platforms, set requiresOnDeviceRecognition to true only after checking that supportsOnDeviceRecognition is also true. On Android, call isOnDeviceRecognitionAvailable(context) and create the recognizer with createOnDeviceSpeechRecognizer(context), both of which were added in API level 31. If either check fails, show an unavailable state and stop. Do not quietly send the audio to a server instead.
The default paths do send audio off the device. Apple’s standard SFSpeechRecognizer processing captures speech and sends it to Apple servers, and Android’s generic SpeechRecognizer is documented as likely to stream audio to remote servers. A privacy requirement therefore has to be enforced in code, not assumed from the platform.
Why the default recognition paths are not local
Neither platform’s standard speech API should be treated as on-device by default.
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SFSpeechRecognizerflow as capturing speech and sending audio to Apple servers. The app must supply anNSSpeechRecognitionUsageDescriptionvalue and request authorization before recognition starts. - Android: The SpeechRecognizer API reference states: “The implementation of this API is likely to stream audio to remote servers to perform speech recognition.” The wording is “likely,” so your code should not assume a particular device or service behaves otherwise without checking.
Apple platforms: the SFSpeechRecognizer path
1. Declare the usage description and request authorization
- Add
NSSpeechRecognitionUsageDescriptionto the app’s Info.plist. The string should name the specific purpose, for example “Transcribes your dictation on this device.” Only write this if the app really does what the string says.<key>NSSpeechRecognitionUsageDescription</key> <string>Transcribes your dictation on this device.</string> - Call
SFSpeechRecognizer.requestAuthorizationand handle the result before creating any request. - Create the recognizer for the locale the user actually speaks. Check that it is available before you begin.
2. Check capability, then set the request flag
The request property is necessary but not sufficient. Apple’s requiresOnDeviceRecognition documentation says: “Set this property to true to prevent an SFSpeechRecognitionRequest from sending audio over the network.” It is honored only when the recognizer supports on-device recognition, which the supportsOnDeviceRecognition property reports. That property is documented as “A Boolean value that indicates whether the speech recognizer can operate without network access.”
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import Speech
func makeLocalRequest(locale: Locale) -> (SFSpeechRecognizer, SFSpeechAudioBufferRecognitionRequest)? {
guard let recognizer = SFSpeechRecognizer(locale: locale),
recognizer.isAvailable,
recognizer.supportsOnDeviceRecognition else {
return nil
}
let request = SFSpeechAudioBufferRecognitionRequest()
request.requiresOnDeviceRecognition = true
return (recognizer, request)
}The function returns nothing when local recognition cannot be guaranteed, so the calling code must handle the missing case. It should not fall back to a request without the flag.
3. Account for the accuracy trade-off and the deployment target
Apple notes that on-device requests may be less accurate than server-based ones. The reviewed Apple pages do not give a numeric accuracy comparison, so test your own languages and vocabulary before you rely on local results for a given feature.
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The pages reviewed for this article do not state a minimum iOS, iPadOS, or macOS version for these properties. Check the availability annotations on each symbol in the current Apple documentation and set your deployment target to match.
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Apple’s authorization guide also distinguishes the SFSpeechRecognizer flow from SpeechAnalyzer transcriber modules, which it says do not send users’ voice audio to Apple servers. The sources reviewed here do not establish SpeechAnalyzer’s OS version or device requirements, so confirm them in Apple’s current documentation before you choose it.
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Android: the dedicated on-device recognizer
1. Check availability, then create the recognizer
The on-device methods were added in API level 31 (Android 12). On a lower minSdkVersion, the call to these methods must be guarded, or the app can fail at runtime on older devices. Creation throws UnsupportedOperationException when the on-device service is not available, so catch it and treat the result as unavailable.
import android.content.Context
import android.os.Build
import android.speech.SpeechRecognizer
fun createLocalRecognizer(context: Context): SpeechRecognizer? {
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.S) return null
if (!SpeechRecognizer.isOnDeviceRecognitionAvailable(context)) return null
return try {
SpeechRecognizer.createOnDeviceSpeechRecognizer(context)
} catch (e: UnsupportedOperationException) {
null
}
}Do not fall back to SpeechRecognizer.createSpeechRecognizer(context) when this function returns null. That call goes to the generic implementation that the reference describes as likely to stream audio to remote servers.
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2. Permissions, threading and lifecycle
- Declare
android.permission.RECORD_AUDIOin the manifest and request it at runtime before you start listening. - Register a
RecognitionListeneron the recognizer before you send it any commands. - Call the API from the main thread, as the reference requires.
- Call
destroy()when recognition is finished so the service is released.
3. Package visibility on Android 11 and later
The reference documents a manifest <queries> declaration for interacting with a recognition service in apps that target Android 11 (API level 30). Confirm that requirement against your target SDK and your integration before adding it to every project as a standard step.
Comparing the two platforms
| Area | Apple (SFSpeechRecognizer) | Android (SpeechRecognizer) |
|---|---|---|
| Enforcement mechanism | Set requiresOnDeviceRecognition to true on the request; honored only if the recognizer supports it |
Create the recognizer with createOnDeviceSpeechRecognizer(context) instead of the generic constructor |
| Availability check | supportsOnDeviceRecognition on the recognizer, plus recognizer availability for the chosen locale |
isOnDeviceRecognitionAvailable(context); API level 31 and later |
| Permission and setup | NSSpeechRecognitionUsageDescription and speech authorization |
RECORD_AUDIO at runtime; <queries> declaration for API 30 targets where applicable |
| Failure behavior | The flag is ignored unless the recognizer supports on-device recognition, so the app must check the property first | Creation throws UnsupportedOperationException when the service is unavailable |
| Documented trade-off | On-device requests may be less accurate (Apple documentation) | No accuracy trade-off stated in the reviewed Android reference |
Availability in both cases depends on the OS version, the device, the language, and any installed recognition service. The platform references establish API behavior, not whether a particular language works offline on a particular phone, so the runtime check is the authority.
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When on-device recognition is unavailable
Use this sequence whenever either platform reports that local recognition is not possible:
Quick Recap
- Identify the reason. The cause may be the OS version, the device, the selected locale, or the installed recognition service. Record the reason in your logs without logging the audio or transcript.
- Show an unavailable state. Tell the user that speech input cannot run on the device for this language or device, and leave the voice feature disabled until the user acts.
- Offer a network-backed option only with consent. If you want to offer server-based recognition, ask first and say that audio will leave the device. This is a design recommendation based on the explicit support checks and the documented remote behavior, not a platform requirement.
- Never switch silently. Do not start a request without the on-device flag or the on-device recognizer because the local path failed.
Making the privacy claim match the code
- Describe what the code enforces. For example, say that speech is processed on the device when the platform reports local support, and that the app does not start recognition otherwise.
- Avoid saying that the phone never sends voice data. That claim is broader than the checks guarantee.
- Do not imply that asking for ordinary system recognition makes processing local. On Apple platforms the standard path sends audio to Apple servers, and on Android the generic implementation is likely to stream audio remotely.
- Apple’s SFSpeechRecognizer reference advises against using speech recognition on private or sensitive information, including passwords, health data, or financial data. On-device processing does not change that advice, so explain the specific data and purpose to users.
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