Glossary
Instrumentation key
The GUID that identifies an Application Insights resource; it used to be the whole configuration and is now one part of the connection string.
Older SDKs and configuration files identify the resource by its instrumentation key alone. Telemetry sent that way goes to a global endpoint, https://dc.applicationinsights.azure.com, instead of the regional endpoints a connection string names.
Microsoft has ended support for that path: “On March 31, 2025, support for instrumentation key ingestion ended.” It still accepts data, but gets no updates or support. One practical consequence is documented on the daily cap page: with an instrumentation key alone, the Application Insights daily cap setting may not be effective across regions.
The key itself hasn’t gone away. It is the InstrumentationKey part of every connection string, so moving off the bare key means wrapping it, not replacing it. In a classic ApplicationInsights.config, which SDK 3.x still reads for ASP.NET on .NET Framework, the change looks like this:
<!-- Before -->
<InstrumentationKey>00000000-0000-0000-0000-000000000000</InstrumentationKey>
<!-- After: copy the full string from the resource's Overview section -->
<ConnectionString>InstrumentationKey=00000000-0000-0000-0000-000000000000;IngestionEndpoint=https://southcentralus.in.applicationinsights.azure.com/</ConnectionString>In code, the same move is from an InstrumentationKey property to ConnectionString, or better, to the APPLICATIONINSIGHTS_CONNECTION_STRING environment variable, which Microsoft recommends for production in every language.