Pre-Call diagnostic


This feature of Azure Communication Services is currently in preview.

Preview APIs and SDKs are provided without a service-level agreement. We recommend that you don't use them for production workloads. Some features might not be supported, or they might have constrained capabilities.

For more information, review Supplemental Terms of Use for Microsoft Azure Previews.

The Pre-Call API enables developers to programmatically validate a client’s readiness to join an Azure Communication Services Call. The Pre-Call APIs can be accessed through the Calling SDK. They provide multiple diagnostics including device, connection, and call quality. Pre-Call APIs are available only for Web (JavaScript). We'll be enabling these capabilities across platforms in the future, provide us with feedback on what platforms you would like to see Pre-Call APIs on.


Accessing Pre-Call APIs


Pre-Call diagnostics are available starting on the version 1.9.1-beta.1 of the Calling SDK. Make sure to use that version when trying the next instructions.

To Access the Pre-Call API, you need to initialize a callClient, and provision an Azure Communication Services access token. There you can access the PreCallDiagnostics feature and the startTest method.

import { CallClient, Features} from "@azure/communication-calling";
import { AzureCommunicationTokenCredential } from '@azure/communication-common';

const callClient = new CallClient(); 
const tokenCredential = new AzureCommunicationTokenCredential("INSERT ACCESS TOKEN");
const preCallDiagnosticsResult = await callClient.feature(Features.PreCallDiagnostics).startTest(tokenCredential);

Once it finishes running, developers can access the result object.

Diagnostic results

The Pre-Call API returns a full diagnostic of the device including details like device permissions, availability and compatibility, call quality stats and in-call diagnostics. The results are returned as a PreCallDiagnosticsResult object.

export declare type PreCallDiagnosticsResult  = {
    deviceAccess: Promise<DeviceAccess>;
    deviceEnumeration: Promise<DeviceEnumeration>;
    inCallDiagnostics: Promise<InCallDiagnostics>;
    browserSupport?: Promise<DeviceCompatibility>;
    id: string;
    callMediaStatistics?: Promise<MediaStatsCallFeature>;

Individual result objects can be accessed as such using the preCallDiagnosticsResult constant. Results for individual tests be returned as they're completed with many of the test results being available immediately. If you use the inCallDiagnostics test, the results might take up to 1 minute as the test validates quality of the video and audio.

Browser support

Browser compatibility check. Checks for Browser and OS compatibility and provides a Supported or NotSupported value back.

const browserSupport =  await preCallDiagnosticsResult.browserSupport;
  if(browserSupport) {

In the case that the test fails and the browser being used by the user is NotSupported, the easiest way to fix that is by asking the user to switch to a supported browser. Refer to the supported browsers in our documentation.


Known issue: browser support test returning Unknown in cases where it should be returning a correct value.

Device access

Permission check. Checks whether video and audio devices are available from a permissions perspective. Provides boolean value for audio and video devices.

  const deviceAccess =  await preCallDiagnosticsResult.deviceAccess;
  if(deviceAccess) {

In the case that the test fails and the permissions are false for audio and video, the user shouldn't continue into joining a call. Rather you need to prompt the user to enable the permissions. To do it, the best way is provided the specific instruction on how to access permission access based on the OS, version and browser they are on. For more information on permissions, check out our recommendations.

Device enumeration

Device availability. Checks whether microphone, camera and speaker devices are detected in the system and ready to use. Provides an Available or NotAvailable value back.

  const deviceEnumeration = await preCallDiagnosticsResult.deviceEnumeration;
  if(deviceEnumeration) {

In the case that devices aren't available, the user shouldn't continue into joining a call. Rather the user should be prompted to check device connections to ensure any headsets, cameras or speakers are properly connected. For more information on device management, check out our documentation

InCall diagnostics

Performs a quick call to check in-call metrics for audio and video and provides results back. Includes connectivity (connected, boolean), bandwidth quality (bandWidth, 'Bad' | 'Average' | 'Good') and call diagnostics for audio and video (diagnostics). Diagnostic are provided jitter, packetLoss and rtt and results are generated using a simple quality grade ('Bad' | 'Average' | 'Good').

InCall diagnostics uses media quality stats to calculate quality scores and diagnose issues. During the pre-call diagnostic, the full set of media quality stats are available for consumption. These stats include raw values across video and audio metrics that can be used programatically. The InCall diagnostic provides a convenience layer on top of media quality stats to consume the results without the need to process all the raw data. See section on media stats for instructions to access.

  const inCallDiagnostics =  await preCallDiagnosticsResult.inCallDiagnostics;
  if(inCallDiagnostics) {    

At this step, there are multiple failure points to watch out for. The values provided by the API are based on the threshold values required by the service. Those raw thresholds can be found in our media quality stats documentation.

  • If connection fails, the user should be prompted to recheck their network connectivity. Connection failures can also be attributed to network conditions like DNS, Proxies or Firewalls. For more information on recommended network setting, check out our documentation.
  • If bandwidth is Bad, the user should be prompted to try out a different network or verify the bandwidth availability on their current one. Ensure no other high bandwidth activities might be taking place.

Media stats

For granular stats on quality metrics like jitter, packet loss, rtt, etc. callMediaStatistics are provided as part of the preCallDiagnosticsResult feature. See the full list and description of the available metrics in the linked article. You can subscribe to the call media stats to get full collection of them. This stat is the raw metrics that are used to calculate InCall diagnostic results and which can be consumed granularly for further analysis.

const mediaStatsCollector = callMediaStatistics.startCollector(); 

mediaStatsCollector.on('mediaStatsEmitted', (mediaStats: SDK.MediaStats) => { 
    // process the stats for the call. 


When the Pre-Call diagnostic test runs, behind the scenes it uses calling minutes to run the diagnostic. The test lasts for roughly 30 seconds, using up 30 seconds of calling which is charged at the standard rate of $0.004 per participant per minute. For the case of Pre-Call diagnostic, the charge will be for 1 participant x 30 seconds = $0.002.

Next steps