Python Speech SDK fails with "USP error: timeout waiting for the first audio chunk" when using managed identity authentication

[robotdad]

1. Environment Details:
   • Python SDK version: 1.42.0
   • Authentication: Using DefaultAzureCredential (falling back to Azure CLI)
   • Token acquisition is successful (token starts with "eyJ0eXAiOi...")
   • Identical configuration works successfully in C# but fails in Python
2. Tested Approaches:
   - Using the raw token directly: speech_config.authorization_token = token
   - Using a formatted token like C#: aad#{resourceId}#{token}
   - Setting token via property ID: set_property(SpeechServiceAuthorization_Token, token)
   - Setting auth type explicitly: set_property(SpeechServiceAuthorization_Type, "Bearer")
   - Using a single synthesizer for all segments (like C# does)
   - Using standard voices instead of HD voices
   - Using simplified SSML without custom parameters
3. Error Details:
   - Synthesis appears to start (synthesizer is created successfully)
   - Service times out after ~80 seconds
   - Error is always: "USP error: timeout waiting for the first audio chunk"
   - No audio data is received despite successful token acquisition
4. Working C# Implementation:

` private static async Task GetAiServicesAuthorizationTokenAsync(CancellationToken cancellationToken)
{
const string scope = "https://cognitiveservices.azure.com/.default";
var tokenCredential = new DefaultAzureCredential();
var token = await tokenCredential.GetTokenAsync(new Azure.Core.TokenRequestContext([scope]), cancellationToken);
return token.Token;
}

// Later used with:
var authorizationToken = await GetAiServicesAuthorizationTokenAsync(cancellationToken);
var speechConfig = SpeechConfig.FromAuthorizationToken($"aad#{speechResourceId}#{authorizationToken}", speechRegion);`

We've ruled out most common issues (token formatting, SSML structure, voice selection), this appears to be either:

1. A limitation in the Python SDK's implementation for managed identity
2. A possible network/connectivity issue specific to the Python SDK
3. An undocumented requirement for additional configuration when using managed identity with Python

Here is a repro script.

[github-actions]

Thank you for your feedback. Tagging and routing to the team member best able to assist.

[github-actions]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @oscholz @robch.

[robotdad]

I abandoned the SDK and switched to rest, working example code here.