Speech-to-text REST API

As an alternative to the Speech SDK, Speech Services allow you to convert speech-to-text using a REST API. Each accessible endpoint is associated with a region. Your application requires a subscription key for the endpoint you plan to use.

Before using the speech-to-text REST API, understand:

  • Requests that use the REST API can only contain 10 seconds of recorded audio.
  • The speech-to-text REST API only returns final results. Partial results are not provided.

If sending longer audio is a requirement for your application, consider using the Speech SDK or batch transcription.


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

Supported authorization headers Speech-to-text Text-to-speech
Ocp-Apim-Subscription-Key Yes No
Authorization: Bearer Yes Yes

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

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

When using the Authorization: Bearer header, you're required to make a request to the issueToken endpoint. In this request, you exchange your subscription key for an access token that's valid for 10 minutes. In the next few sections you'll learn how to get a token, and use a token.

How to get an access token

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

These regions and endpoints are supported:

Region Token service endpoint
Australia East https://australiaeast.api.cognitive.microsoft.com/sts/v1.0/issueToken
Canada Central https://canadacentral.api.cognitive.microsoft.com/sts/v1.0/issueToken
Central US https://centralus.api.cognitive.microsoft.com/sts/v1.0/issueToken
East Asia https://eastasia.api.cognitive.microsoft.com/sts/v1.0/issueToken
East US https://eastus.api.cognitive.microsoft.com/sts/v1.0/issueToken
East US 2 https://eastus2.api.cognitive.microsoft.com/sts/v1.0/issueToken
France Central https://francecentral.api.cognitive.microsoft.com/sts/v1.0/issueToken
India Central https://centralindia.api.cognitive.microsoft.com/sts/v1.0/issueToken
Japan East https://japaneast.api.cognitive.microsoft.com/sts/v1.0/issueToken
Korea Central https://koreacentral.api.cognitive.microsoft.com/sts/v1.0/issueToken
North Central US https://northcentralus.api.cognitive.microsoft.com/sts/v1.0/issueToken
North Europe https://northeurope.api.cognitive.microsoft.com/sts/v1.0/issueToken
South Central US https://southcentralus.api.cognitive.microsoft.com/sts/v1.0/issueToken
Southeast Asia https://southeastasia.api.cognitive.microsoft.com/sts/v1.0/issueToken
UK South https://uksouth.api.cognitive.microsoft.com/sts/v1.0/issueToken
West Europe https://westeurope.api.cognitive.microsoft.com/sts/v1.0/issueToken
West US https://westus.api.cognitive.microsoft.com/sts/v1.0/issueToken
West US 2 https://westus2.api.cognitive.microsoft.com/sts/v1.0/issueToken

Use these 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 Speech Service subscription key. 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: westus.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 Speech Service subscription key. 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://westus.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 Speech Service subscription key. 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://westus.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 Speech Service subscription key 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://westus.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, however, to minimize network traffic and latency, we recommend using the same token for nine minutes.

Here's a sample HTTP request to the text-to-speech REST API:

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...

Regions and endpoints

These regions are supported for speech-to-text transcription using the REST API. Make sure that you select the endpoint that matches your subscription region.

Region Endpoint
Australia East https://australiaeast.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
Canada Central https://canadacentral.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
Central US https://centralus.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
East Asia https://eastasia.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
East US https://eastus.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
East US 2 https://eastus2.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
France Central https://francecentral.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
India Central https://centralindia.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
Japan East https://japaneast.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
Korea Central https://koreacentral.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
North Central US https://northcentralus.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
North Europe https://northeurope.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
South Central US https://southcentralus.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
Southeast Asia https://southeastasia.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
UK South https://uksouth.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
West Europe https://westeurope.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
West US https://westus.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1
West US 2 https://westus2.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1


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

Query parameters

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

Parameter Description Required / Optional
language Identifies the spoken language that is 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 multiple results with confidence values and four different representations. 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 remove all profanity from the result, or raw, which includes the profanity in the result. The default setting is masked. Optional

Request headers

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

Header Description Required / Optional
Ocp-Apim-Subscription-Key Your Speech Services subscription key. 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.
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. Only use this header if chunking audio data. Optional
Expect If using chunked transfer, send Expect: 100-continue. The Speech Services acknowledge the initial request and awaits additional data. Required if sending chunked audio data.
Accept If provided, it must be application/json. The Speech Services provide results in JSON. Some Web request frameworks provide an incompatible default value if you do not specify one, so it is good practice to always include Accept. Optional, but recommended.

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 Bitrate Sample Rate
WAV PCM 16-bit 16 kHz, mono
OGG OPUS 16-bit 16 kHz, mono


The above formats are supported through REST API and WebSocket in the Speech Services. The Speech SDK currently only supports the WAV format with PCM codec.

Sample request

This is a typical HTTP request. The sample below includes the hostname and required headers. It's important to note that the service also expects audio data, which is not included in this sample. As mentioned earlier, chunking is recommended, however, 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_SUBSCRIPTION_KEY
Host: westus.stt.speech.microsoft.com
Transfer-Encoding: chunked
Expect: 100-continue

HTTP status codes

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

HTTP status code Description Possible reason
100 Continue The initial request has been accepted. Proceed with sending the rest of the data. (Used with chunked transfer.)
200 OK The request was successful; the response body is a JSON object.
400 Bad request Language code not provided or is not a supported language; invalid audio file.
401 Unauthorized Subscription key or authorization token is invalid in the specified region, or invalid endpoint.
403 Forbidden Missing subscription key or authorization token.

Chunked transfer

Chunked transfer (Transfer-Encoding: chunked) can help reduce recognition latency because it allows the Speech Services to begin processing the audio file while it's being transmitted. The REST API does not provide partial or interim results. This option is intended solely to improve responsiveness.

This 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 connected to the appropriate REST endpoint. audioFile is the path to an audio file on disk.

    HttpWebRequest request = null;
    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"] = args[1];
    request.AllowWriteStreamBuffering = false;

using (fs = new FileStream(audioFile, FileMode.Open, FileAccess.Read))
    * Open a request stream and write 1024 byte chunks in the stream one at a time.
    byte[] buffer = null;
    int bytesRead = 0;
    using (Stream requestStream = request.GetRequestStream())
        * Read 1024 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);

        // Flush

Response parameters

Results are provided as JSON. The simple format includes these top-level fields.

Parameter Description
RecognitionStatus Status, such as Success for successful recognition. See next table.
DisplayText The recognized text after capitalization, punctuation, inverse text normalization (conversion of spoken text to shorter forms, such as 200 for "two hundred" or "Dr. Smith" for "doctor smith"), and profanity masking. Present only on success.
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 may 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. Usually means the recognition language is a different language from the one the user is speaking.
InitialSilenceTimeout The start of the audio stream contained only silence, and the service timed out waiting for speech.
BabbleTimeout The start of the audio stream contained only noise, and the service timed out waiting for speech.
Error The recognition service encountered an internal error and could not 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 the same data as the simple format, along with NBest, a list of alternative interpretations of the same recognition result. These results are ranked from most likely to least likely. The first entry is the same as the main recognition result. When using the detailed format, DisplayText is provided as Display for each result in the NBest list.

Each object in the NBest list includes:

Parameter 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 ("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 DisplayText provided when format is set to simple.

Sample responses

This is a typical response for simple recognition.

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

This is a typical response for detailed recognition.

  "RecognitionStatus": "Success",
  "Offset": "1236645672289",
  "Duration": "1236645672289",
  "NBest": [
        "Confidence" : "0.87",
        "Lexical" : "remind me to buy five pencils",
        "ITN" : "remind me to buy 5 pencils",
        "MaskedITN" : "remind me to buy 5 pencils",
        "Display" : "Remind me to buy 5 pencils.",
        "Confidence" : "0.54",
        "Lexical" : "rewind me to buy five pencils",
        "ITN" : "rewind me to buy 5 pencils",
        "MaskedITN" : "rewind me to buy 5 pencils",
        "Display" : "Rewind me to buy 5 pencils.",

Next steps