.NET: expose OpenTelemetryAgent.DefaultSourceName

docs/decisions/0003-agent-opentelemetry-instrumentation.md

@@ -124,21 +124,42 @@ The implementation is validated through:
 
 ### Usage Example
 
+The name passed to `TracerProviderBuilder.AddSource` must match the source name the agent emits under, otherwise the
+provider silently receives no agent spans. When no source name is supplied to `UseOpenTelemetry`, the agent emits under
+`OpenTelemetryAgent.DefaultSourceName`, so that is the value to register:
+
 ```csharp
 // Create TracerProvider
 using var tracerProvider = Sdk.CreateTracerProviderBuilder()
-    .AddSource(AgentOpenTelemetryConsts.DefaultSourceName)
+    .AddSource(OpenTelemetryAgent.DefaultSourceName)
     .AddConsoleExporter()
     .Build();
 
 // Create and wrap agent with telemetry
 var baseAgent = new ChatClientAgent(chatClient, options);
-using var telemetryAgent = baseAgent.WithOpenTelemetry();
+using var telemetryAgent = baseAgent.AsBuilder()
+    .UseOpenTelemetry()
+    .Build();
 
 // Use agent normally - telemetry is captured automatically
 var response = await telemetryAgent.RunAsync(messages);
 ```
 
+To emit under a custom source name, pass the same value to both calls:
+
+```csharp
+const string SourceName = "MyCompany.MyAgent";
+
+using var tracerProvider = Sdk.CreateTracerProviderBuilder()
+    .AddSource(SourceName)
+    .AddConsoleExporter()
+    .Build();
+
+using var telemetryAgent = baseAgent.AsBuilder()
+    .UseOpenTelemetry(sourceName: SourceName)
+    .Build();
+```
+
 ### Relationship to Microsoft.Extensions.AI
 
 This implementation follows the exact patterns established by Microsoft.Extensions.AI's OpenTelemetry instrumentation, ensuring consistency across the AI ecosystem and leveraging proven patterns for telemetry integration.

dotnet/src/Microsoft.Agents.AI/OpenTelemetryAgent.cs

@@ -41,6 +41,18 @@ public sealed class OpenTelemetryAgent : DelegatingAIAgent, IDisposable
     // inner agent not directly but rather via OpenTelemetryChatClient, which wraps a ForwardingChatClient that in turn
     // calls back into the inner agent.
 
+    /// <summary>
+    /// Gets the default <see cref="ActivitySource"/> name used by <see cref="OpenTelemetryAgent"/> when no source
+    /// name is supplied to the constructor.
+    /// </summary>
+    /// <remarks>
+    /// Pass this value to the tracing pipeline (for example, <c>TracerProviderBuilder.AddSource</c>) to subscribe to
+    /// the spans emitted by agents that use the default source name, instead of hardcoding the literal name. This is
+    /// a property rather than a constant so that the value is read at run time: a consumer that upgrades the package
+    /// picks up the current source name without recompiling.
+    /// </remarks>
+    public static string DefaultSourceName => OpenTelemetryConsts.DefaultSourceName;
+
     /// <summary>The <see cref="OpenTelemetryChatClient"/> providing the bulk of the telemetry.</summary>
     private readonly OpenTelemetryChatClient _otelClient;
     /// <summary>The provider name extracted from <see cref="AIAgentMetadata"/>.</summary>
@@ -62,8 +74,9 @@ public sealed class OpenTelemetryAgent : DelegatingAIAgent, IDisposable
     /// <summary>Initializes a new instance of the <see cref="OpenTelemetryAgent"/> class.</summary>
     /// <param name="innerAgent">The underlying <see cref="AIAgent"/> to be augmented with telemetry capabilities.</param>
     /// <param name="sourceName">
-    /// An optional source name that will be used to identify telemetry data from this agent.
-    /// If not provided, a default source name will be used for telemetry identification.
+    /// An optional source name used to identify telemetry data from this agent.
+    /// When specified, register the same value with <c>TracerProviderBuilder.AddSource</c> so the tracing pipeline
+    /// subscribes to these spans. When omitted, <see cref="DefaultSourceName"/> is used.
     /// </param>
     /// <exception cref="ArgumentNullException"><paramref name="innerAgent"/> is <see langword="null"/>.</exception>
     /// <remarks>
@@ -80,8 +93,9 @@ public OpenTelemetryAgent(AIAgent innerAgent, string? sourceName = null)
     /// <summary>Initializes a new instance of the <see cref="OpenTelemetryAgent"/> class.</summary>
     /// <param name="innerAgent">The underlying <see cref="AIAgent"/> to be augmented with telemetry capabilities.</param>
     /// <param name="sourceName">
-    /// An optional source name that will be used to identify telemetry data from this agent.
-    /// If not provided, a default source name will be used for telemetry identification.
+    /// An optional source name used to identify telemetry data from this agent.
+    /// When specified, register the same value with <c>TracerProviderBuilder.AddSource</c> so the tracing pipeline
+    /// subscribes to these spans. When omitted, <see cref="DefaultSourceName"/> is used.
     /// </param>
     /// <param name="autoWireChatClient">
     /// When <see langword="true"/> and the inner agent is a <see cref="ChatClientAgent"/>, the underlying

dotnet/src/Microsoft.Agents.AI/OpenTelemetryAgentBuilderExtensions.cs

@@ -15,8 +15,9 @@ public static class OpenTelemetryAgentBuilderExtensions
     /// </summary>
     /// <param name="builder">The <see cref="AIAgentBuilder"/> to which OpenTelemetry support will be added.</param>
     /// <param name="sourceName">
-    /// An optional source name that will be used to identify telemetry data from this agent.
-    /// If not specified, a default source name will be used.
+    /// An optional source name used to identify telemetry data from this agent.
+    /// When specified, register the same value with <c>TracerProviderBuilder.AddSource</c> so the tracing pipeline
+    /// subscribes to these spans. When omitted, <see cref="OpenTelemetryAgent.DefaultSourceName"/> is used.
     /// </param>
     /// <param name="configure">
     /// An optional callback that provides additional configuration of the <see cref="OpenTelemetryAgent"/> instance.

dotnet/tests/Microsoft.Agents.AI.UnitTests/OpenTelemetryAgentTests.cs

@@ -873,6 +873,17 @@ public async Task AutoWireChatClient_UserFactoryAddsOwnOTel_CoexistsWithBelowFic
         Assert.Equal(2, activities.Count(a => string.Equals(a.GetTagItem("gen_ai.operation.name") as string, "chat", StringComparison.Ordinal)));
     }
 
+    [Fact]
+    public void DefaultSourceName_ReturnsDocumentedSourceName()
+    {
+        // Callers pass this to TracerProviderBuilder.AddSource, so it must stay in sync with the source name the
+        // agent emits spans under, which Ctor_NullOrWhitespaceSourceName_AutoWiredChatClientUsesDefaultSource_Async
+        // pins to the same literal. Comparing against the literal here guards a rename of the internal constant.
+
+        // Arrange & Act & Assert
+        Assert.Equal("Experimental.Microsoft.Agents.AI", OpenTelemetryAgent.DefaultSourceName);
+    }
+
     [Theory]
     [InlineData(null)]
     [InlineData("")]