Speech to text REST API for short audio

Use cases for the Speech to text REST API for short audio are limited. Use it only in cases where you can't use the Speech SDK.

Before you use the Speech to text REST API for short audio, consider the following limitations:

  • Requests that use the REST API for short audio and transmit audio directly can contain no more than 60 seconds of audio. The input audio formats are more limited compared to the Speech SDK.
  • The REST API for short audio returns only final results. It doesn't provide partial results.
  • Speech translation isn't supported via REST API for short audio. You need to use Speech SDK.
  • Batch transcription and custom speech aren't supported via REST API for short audio. You should always use the Speech to text REST API for batch transcription and custom speech.

Before you use the Speech to text REST API for short audio, understand that you need to complete a token exchange as part of authentication to access the service. For more information, see Authentication.

Regions and endpoints

The endpoint for the REST API for short audio has this format:


Replace <REGION_IDENTIFIER> with the identifier that matches the region of your Speech resource.


For Azure Government and Microsoft Azure operated by 21Vianet endpoints, see this article about sovereign clouds.

Audio formats

Audio is sent in the body of the HTTP POST request. It must be in one of the formats in this table:

Format Codec Bit rate Sample rate
WAV PCM 256 kbps 16 kHz, mono
OGG OPUS 256 kbps 16 kHz, mono


The preceding formats are supported through the REST API for short audio and WebSocket in the Speech service. The Speech SDK supports the WAV format with PCM codec as well as other formats.

Request headers

This table lists required and optional headers for speech to text requests:

Header Description Required or optional
Ocp-Apim-Subscription-Key Your resource key for the Speech service. Either this header or Authorization is required.
Authorization An authorization token preceded by the word Bearer. For more information, see Authentication. Either this header or Ocp-Apim-Subscription-Key is required.
Pronunciation-Assessment Specifies the parameters for showing pronunciation scores in recognition results. These scores assess the pronunciation quality of speech input, with indicators like accuracy, fluency, and completeness.

This parameter is a Base64-encoded JSON that contains multiple detailed parameters. To learn how to build this header, see Pronunciation assessment parameters.
Content-type Describes the format and codec of the provided audio data. Accepted values are audio/wav; codecs=audio/pcm; samplerate=16000 and audio/ogg; codecs=opus. Required
Transfer-Encoding Specifies that chunked audio data is being sent, rather than a single file. Use this header only if you're chunking audio data. Optional
Expect If you're using chunked transfer, send Expect: 100-continue. The Speech service acknowledges the initial request and awaits more data. Required if you're sending chunked audio data.
Accept If provided, it must be application/json. The Speech service provides results in JSON. Some request frameworks provide an incompatible default value. It's good practice to always include Accept. Optional, but recommended.

Query parameters

These parameters might be included in the query string of the REST request.


You must append the language parameter to the URL to avoid receiving a 4xx HTTP error. For example, the language set to US English via the West US endpoint is: https://westus.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1?language=en-US.

Parameter Description Required or optional
language Identifies the spoken language that's being recognized. See Supported languages. Required
format Specifies the result format. Accepted values are simple and detailed. Simple results include RecognitionStatus, DisplayText, Offset, and Duration. Detailed responses include four different representations of display text. The default setting is simple. Optional
profanity Specifies how to handle profanity in recognition results. Accepted values are:

masked, which replaces profanity with asterisks.
removed, which removes all profanity from the result.
raw, which includes profanity in the result.

The default setting is masked.
cid When you're using the Speech Studio to create custom models, you can take advantage of the Endpoint ID value from the Deployment page. Use the Endpoint ID value as the argument to the cid query string parameter. Optional

Pronunciation assessment parameters

This table lists required and optional parameters for pronunciation assessment:

Parameter Description Required or optional
ReferenceText The text that the pronunciation is evaluated against. Required
GradingSystem The point system for score calibration. The FivePoint system gives a 0-5 floating point score, and HundredMark gives a 0-100 floating point score. Default: FivePoint. Optional
Granularity The evaluation granularity. Accepted values are:

Phoneme, which shows the score on the full-text, word, and phoneme levels.
Word, which shows the score on the full-text and word levels.
FullText, which shows the score on the full-text level only.

The default setting is Phoneme.
Dimension Defines the output criteria. Accepted values are:

Basic, which shows the accuracy score only.
Comprehensive, which shows scores on more dimensions (for example, fluency score and completeness score on the full-text level, and error type on the word level).

To see definitions of different score dimensions and word error types, see Response properties. The default setting is Basic.
EnableMiscue Enables miscue calculation. With this parameter enabled, the pronounced words are compared to the reference text. They are marked with omission or insertion based on the comparison. Accepted values are False and True. The default setting is False. Optional
ScenarioId A GUID that indicates a customized point system. Optional

Here's example JSON that contains the pronunciation assessment parameters:

  "ReferenceText": "Good morning.",
  "GradingSystem": "HundredMark",
  "Granularity": "FullText",
  "Dimension": "Comprehensive"

The following sample code shows how to build the pronunciation assessment parameters into the Pronunciation-Assessment header:

var pronAssessmentParamsJson = $"{{\"ReferenceText\":\"Good morning.\",\"GradingSystem\":\"HundredMark\",\"Granularity\":\"FullText\",\"Dimension\":\"Comprehensive\"}}";
var pronAssessmentParamsBytes = Encoding.UTF8.GetBytes(pronAssessmentParamsJson);
var pronAssessmentHeader = Convert.ToBase64String(pronAssessmentParamsBytes);

We strongly recommend streaming (chunked transfer) uploading while you're posting the audio data, which can significantly reduce the latency. To learn how to enable streaming, see the sample code in various programming languages.


For more For more information, see pronunciation assessment.

Sample request

The following sample includes the host name and required headers. It's important to note that the service also expects audio data, which isn't included in this sample. As mentioned earlier, chunking is recommended but not required.

POST speech/recognition/conversation/cognitiveservices/v1?language=en-US&format=detailed HTTP/1.1
Accept: application/json;text/xml
Content-Type: audio/wav; codecs=audio/pcm; samplerate=16000
Ocp-Apim-Subscription-Key: YOUR_RESOURCE_KEY
Host: westus.stt.speech.microsoft.com
Transfer-Encoding: chunked
Expect: 100-continue

To enable pronunciation assessment, you can add the following header. To learn how to build this header, see Pronunciation assessment parameters.

Pronunciation-Assessment: eyJSZWZlcm...

HTTP status codes

The HTTP status code for each response indicates success or common errors.

HTTP status code Description Possible reasons
100 Continue The initial request is accepted. Proceed with sending the rest of the data. (This code is used with chunked transfer.)
200 OK The request was successful. The response body is a JSON object.
400 Bad request The language code wasn't provided, the language isn't supported, or the audio file is invalid (for example).
401 Unauthorized A resource key or an authorization token is invalid in the specified region, or an endpoint is invalid.
403 Forbidden A resource key or authorization token is missing.

Sample responses

Here's a typical response for simple recognition:

  "RecognitionStatus": "Success",
  "DisplayText": "Remind me to buy 5 pencils.",
  "Offset": "1236645672289",
  "Duration": "1236645672289"

Here's a typical response for detailed recognition:

  "RecognitionStatus": "Success",
  "Offset": "1236645672289",
  "Duration": "1236645672289",
  "NBest": [
      "Confidence": 0.9052885,
      "Display": "What's the weather like?",
      "ITN": "what's the weather like",
      "Lexical": "what's the weather like",
      "MaskedITN": "what's the weather like"
      "Confidence": 0.92459863,
      "Display": "what is the weather like",
      "ITN": "what is the weather like",
      "Lexical": "what is the weather like",
      "MaskedITN": "what is the weather like"

Here's a typical response for recognition with pronunciation assessment:

  "RecognitionStatus": "Success",
  "Offset": "400000",
  "Duration": "11000000",
  "NBest": [
        "Confidence" : "0.87",
        "Lexical" : "good morning",
        "ITN" : "good morning",
        "MaskedITN" : "good morning",
        "Display" : "Good morning.",
        "PronScore" : 84.4,
        "AccuracyScore" : 100.0,
        "FluencyScore" : 74.0,
        "CompletenessScore" : 100.0,
        "Words": [
              "Word" : "Good",
              "AccuracyScore" : 100.0,
              "ErrorType" : "None",
              "Offset" : 500000,
              "Duration" : 2700000
              "Word" : "morning",
              "AccuracyScore" : 100.0,
              "ErrorType" : "None",
              "Offset" : 5300000,
              "Duration" : 900000

Response properties

Results are provided as JSON. The simple format includes the following top-level fields:

Property Description
RecognitionStatus Status, such as Success for successful recognition. See the next table.
DisplayText The recognized text after capitalization, punctuation, inverse text normalization, and profanity masking. Present only on success. Inverse text normalization is conversion of spoken text to shorter forms, such as 200 for "two hundred" or "Dr. Smith" for "doctor smith."
Offset The time (in 100-nanosecond units) at which the recognized speech begins in the audio stream.
Duration The duration (in 100-nanosecond units) of the recognized speech in the audio stream.

The RecognitionStatus field might contain these values:

Status Description
Success The recognition was successful, and the DisplayText field is present.
NoMatch Speech was detected in the audio stream, but no words from the target language were matched. This status usually means that the recognition language is different from the language that the user is speaking.
InitialSilenceTimeout The start of the audio stream contained only silence, and the service timed out while waiting for speech.
BabbleTimeout The start of the audio stream contained only noise, and the service timed out while waiting for speech.
Error The recognition service encountered an internal error and couldn't continue. Try again if possible.


If the audio consists only of profanity, and the profanity query parameter is set to remove, the service does not return a speech result.

The detailed format includes more forms of recognized results. When you're using the detailed format, DisplayText is provided as Display for each result in the NBest list.

The object in the NBest list can include:

Property Description
Confidence The confidence score of the entry, from 0.0 (no confidence) to 1.0 (full confidence).
Lexical The lexical form of the recognized text: the actual words recognized.
ITN The inverse-text-normalized (ITN) or canonical form of the recognized text, with phone numbers, numbers, abbreviations ("doctor smith" to "dr smith"), and other transformations applied.
MaskedITN The ITN form with profanity masking applied, if requested.
Display The display form of the recognized text, with punctuation and capitalization added. This parameter is the same as what DisplayText provides when the format is set to simple.
AccuracyScore Pronunciation accuracy of the speech. Accuracy indicates how closely the phonemes match a native speaker's pronunciation. The accuracy score at the word and full-text levels is aggregated from the accuracy score at the phoneme level.
FluencyScore Fluency of the provided speech. Fluency indicates how closely the speech matches a native speaker's use of silent breaks between words.
CompletenessScore Completeness of the speech, determined by calculating the ratio of pronounced words to reference text input.
PronScore Overall score that indicates the pronunciation quality of the provided speech. This score is aggregated from AccuracyScore, FluencyScore, and CompletenessScore with weight.
ErrorType Value that indicates whether a word is omitted, inserted, or badly pronounced, compared to ReferenceText. Possible values are None (meaning no error on this word), Omission, Insertion, and Mispronunciation.

Chunked transfer

Chunked transfer (Transfer-Encoding: chunked) can help reduce recognition latency. It allows the Speech service to begin processing the audio file while it's transmitted. The REST API for short audio doesn't provide partial or interim results.

The following code sample shows how to send audio in chunks. Only the first chunk should contain the audio file's header. request is an HttpWebRequest object that's connected to the appropriate REST endpoint. audioFile is the path to an audio file on disk.

var request = (HttpWebRequest)HttpWebRequest.Create(requestUri);
request.SendChunked = true;
request.Accept = @"application/json;text/xml";
request.Method = "POST";
request.ProtocolVersion = HttpVersion.Version11;
request.Host = host;
request.ContentType = @"audio/wav; codecs=audio/pcm; samplerate=16000";
request.Headers["Ocp-Apim-Subscription-Key"] = "YOUR_RESOURCE_KEY";
request.AllowWriteStreamBuffering = false;

using (var fs = new FileStream(audioFile, FileMode.Open, FileAccess.Read))
    // Open a request stream and write 1,024-byte chunks in the stream one at a time.
    byte[] buffer = null;
    int bytesRead = 0;
    using (var requestStream = request.GetRequestStream())
        // Read 1,024 raw bytes from the input audio file.
        buffer = new Byte[checked((uint)Math.Min(1024, (int)fs.Length))];
        while ((bytesRead = fs.Read(buffer, 0, buffer.Length)) != 0)
            requestStream.Write(buffer, 0, bytesRead);



Each request requires an authorization header. This table illustrates which headers are supported for each feature:

Supported authorization header Speech to text Text to speech
Ocp-Apim-Subscription-Key Yes Yes
Authorization: Bearer Yes Yes

When you're using the Ocp-Apim-Subscription-Key header, you're only required to provide your resource key. For example:

'Ocp-Apim-Subscription-Key': 'YOUR_SUBSCRIPTION_KEY'

When you're using the Authorization: Bearer header, you're required to make a request to the issueToken endpoint. In this request, you exchange your resource key for an access token that's valid for 10 minutes.

How to get an access token

To get an access token, you need to make a request to the issueToken endpoint by using Ocp-Apim-Subscription-Key and your resource key.

The issueToken endpoint has this format:


Replace <REGION_IDENTIFIER> with the identifier that matches the region of your subscription.

Use the following samples to create your access token request.

HTTP sample

This example is a simple HTTP request to get a token. Replace YOUR_SUBSCRIPTION_KEY with your resource key for the Speech service. If your subscription isn't in the West US region, replace the Host header with your region's host name.

POST /sts/v1.0/issueToken HTTP/1.1
Ocp-Apim-Subscription-Key: YOUR_SUBSCRIPTION_KEY
Host: eastus.api.cognitive.microsoft.com
Content-type: application/x-www-form-urlencoded
Content-Length: 0

The body of the response contains the access token in JSON Web Token (JWT) format.

PowerShell sample

This example is a simple PowerShell script to get an access token. Replace YOUR_SUBSCRIPTION_KEY with your resource key for the Speech service. Make sure to use the correct endpoint for the region that matches your subscription. This example is currently set to West US.

$FetchTokenHeader = @{
  'Content-Length'= '0';
  'Ocp-Apim-Subscription-Key' = 'YOUR_SUBSCRIPTION_KEY'

$OAuthToken = Invoke-RestMethod -Method POST -Uri https://eastus.api.cognitive.microsoft.com/sts/v1.0/issueToken
 -Headers $FetchTokenHeader

# show the token received

cURL sample

cURL is a command-line tool available in Linux (and in the Windows Subsystem for Linux). This cURL command illustrates how to get an access token. Replace YOUR_SUBSCRIPTION_KEY with your resource key for the Speech service. Make sure to use the correct endpoint for the region that matches your subscription. This example is currently set to West US.

curl -v -X POST \
 "https://eastus.api.cognitive.microsoft.com/sts/v1.0/issueToken" \
 -H "Content-type: application/x-www-form-urlencoded" \
 -H "Content-Length: 0" \
 -H "Ocp-Apim-Subscription-Key: YOUR_SUBSCRIPTION_KEY"

C# sample

This C# class illustrates how to get an access token. Pass your resource key for the Speech service when you instantiate the class. If your subscription isn't in the West US region, change the value of FetchTokenUri to match the region for your subscription.

public class Authentication
    public static readonly string FetchTokenUri =
    private string subscriptionKey;
    private string token;

    public Authentication(string subscriptionKey)
        this.subscriptionKey = subscriptionKey;
        this.token = FetchTokenAsync(FetchTokenUri, subscriptionKey).Result;

    public string GetAccessToken()
        return this.token;

    private async Task<string> FetchTokenAsync(string fetchUri, string subscriptionKey)
        using (var client = new HttpClient())
            client.DefaultRequestHeaders.Add("Ocp-Apim-Subscription-Key", subscriptionKey);
            UriBuilder uriBuilder = new UriBuilder(fetchUri);

            var result = await client.PostAsync(uriBuilder.Uri.AbsoluteUri, null);
            Console.WriteLine("Token Uri: {0}", uriBuilder.Uri.AbsoluteUri);
            return await result.Content.ReadAsStringAsync();

Python sample

# Request module must be installed.
# Run pip install requests if necessary.
import requests

subscription_key = 'REPLACE_WITH_YOUR_KEY'

def get_token(subscription_key):
    fetch_token_url = 'https://eastus.api.cognitive.microsoft.com/sts/v1.0/issueToken'
    headers = {
        'Ocp-Apim-Subscription-Key': subscription_key
    response = requests.post(fetch_token_url, headers=headers)
    access_token = str(response.text)

How to use an access token

The access token should be sent to the service as the Authorization: Bearer <TOKEN> header. Each access token is valid for 10 minutes. You can get a new token at any time, but to minimize network traffic and latency, we recommend using the same token for nine minutes.

Here's a sample HTTP request to the Speech to text REST API for short audio:

POST /cognitiveservices/v1 HTTP/1.1
Authorization: Bearer YOUR_ACCESS_TOKEN
Host: westus.stt.speech.microsoft.com
Content-type: application/ssml+xml
Content-Length: 199
Connection: Keep-Alive

// Message body here...

Next steps