diff --git a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/AspNetCore/AspNetCoreDiagnosticListener.cs b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/AspNetCore/AspNetCoreDiagnosticListener.cs
index f5731a3..ea3250f 100644
--- a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/AspNetCore/AspNetCoreDiagnosticListener.cs
+++ b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/AspNetCore/AspNetCoreDiagnosticListener.cs
@@ -40,7 +40,7 @@ protected override void OnEvent(string name, object? payload)
                          DiagnosticPayloadReader.GetInt32(payload, "http.response.status_code", "http.status_code");
         var errorType = DiagnosticPayloadReader.GetString(payload, "error.type", "exception.type");
 
-        using var activity = QylActivitySource.Source.StartActivity(QylActivityNames.HttpServer(method, route), ActivityKind.Server);
+        using var activity = QylActivitySource.StartAtAmbientStart(QylActivityNames.HttpServer(method, route), ActivityKind.Server);
 
         SemanticTagWriter.Set(activity, SemanticAttributes.QylInstrumentationDomain, QylInstrumentationDomains.HttpServer);
         SemanticTagWriter.Set(activity, SemanticAttributes.HttpRequestMethod, method);
diff --git a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/DiagnosticListenerSubscriber.cs b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/DiagnosticListenerSubscriber.cs
index ac9ff2f..23fa612 100644
--- a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/DiagnosticListenerSubscriber.cs
+++ b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/DiagnosticListenerSubscriber.cs
@@ -10,8 +10,11 @@ namespace Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners;
 /// Pre-swap: each library (HttpClient, EFCore, …) was instrumented by an IL-rewriting CallTarget
 /// integration injected by the substrate's CLR profiler. Post-swap: we subscribe to the same
 /// libraries' built-in <see cref="DiagnosticListener"/> events. DiagnosticSource is a managed BCL
-/// primitive that's been AOT-safe since .NET 8, so this layer publishes the same span shapes
-/// without any IL rewriting or runtime code generation.
+/// primitive that's been AOT-safe since .NET 8, so this layer emits spans without any IL rewriting
+/// or runtime code generation. Concrete subscribers react on the completion (<c>*.Stop</c>) event and
+/// stamp the span to the ambient framework activity's start (via
+/// <c>QylActivitySource.StartAtAmbientStart</c>) so the emitted duration reflects the real operation,
+/// not a ~0 span.
 /// </para>
 /// </summary>
 public abstract class DiagnosticListenerSubscriber : IObserver<KeyValuePair<string, object?>>, IDisposable
diff --git a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/GrpcClient/GrpcClientDiagnosticListener.cs b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/GrpcClient/GrpcClientDiagnosticListener.cs
index 5fac9dd..c2fc930 100644
--- a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/GrpcClient/GrpcClientDiagnosticListener.cs
+++ b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/GrpcClient/GrpcClientDiagnosticListener.cs
@@ -38,7 +38,7 @@ protected override void OnEvent(string name, object? payload)
                          DiagnosticPayloadReader.GetInt32(payload, "grpc.status_code");
         var errorType = DiagnosticPayloadReader.GetString(payload, "error.type", "exception.type");
 
-        using var activity = QylActivitySource.Source.StartActivity(QylActivityNames.GrpcClient(service, method), ActivityKind.Client);
+        using var activity = QylActivitySource.StartAtAmbientStart(QylActivityNames.GrpcClient(service, method), ActivityKind.Client);
 
         SemanticTagWriter.Set(activity, SemanticAttributes.QylInstrumentationDomain, QylInstrumentationDomains.RpcGrpc);
         SemanticTagWriter.Set(activity, SemanticAttributes.RpcSystem, QylSemanticAttributes.RpcSystemGrpc);
diff --git a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/HttpClient/HttpClientDiagnosticListener.cs b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/HttpClient/HttpClientDiagnosticListener.cs
index c937ad9..a92f1d7 100644
--- a/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/HttpClient/HttpClientDiagnosticListener.cs
+++ b/src/Qyl.OpenTelemetry.AutoInstrumentation.DiagnosticListeners/HttpClient/HttpClientDiagnosticListener.cs
@@ -39,7 +39,7 @@ protected override void OnEvent(string name, object? payload)
         var statusCode = DiagnosticPayloadReader.GetInt32(payload, "http.response.status_code", "http.status_code");
         var errorType = DiagnosticPayloadReader.GetString(payload, "error.type", "exception.type");
 
-        using var activity = QylActivitySource.Source.StartActivity(QylActivityNames.HttpClient(method), ActivityKind.Client);
+        using var activity = QylActivitySource.StartAtAmbientStart(QylActivityNames.HttpClient(method), ActivityKind.Client);
 
         SemanticTagWriter.Set(activity, SemanticAttributes.QylInstrumentationDomain, QylInstrumentationDomains.HttpClient);
         SemanticTagWriter.Set(activity, SemanticAttributes.HttpRequestMethod, method);
diff --git a/src/Qyl.OpenTelemetry.AutoInstrumentation/QylActivitySource.cs b/src/Qyl.OpenTelemetry.AutoInstrumentation/QylActivitySource.cs
index d986310..4749073 100644
--- a/src/Qyl.OpenTelemetry.AutoInstrumentation/QylActivitySource.cs
+++ b/src/Qyl.OpenTelemetry.AutoInstrumentation/QylActivitySource.cs
@@ -25,4 +25,21 @@ internal static bool IsRecordingEnabled
         => Source.HasListeners()
             ? Source.StartActivity(operationName, activityKind)
             : null;
+
+    /// <summary>
+    /// Starts a qyl span stamped to the ambient (framework) <see cref="Activity"/>'s real start time,
+    /// so DiagnosticListener bridges that only observe the completion (<c>*.Stop</c>) event emit the
+    /// operation's TRUE duration instead of a ~0 span. Parents to the current activity to preserve
+    /// trace correlation; falls back to a now-stamped span when there is no ambient activity.
+    /// </summary>
+    internal static Activity? StartAtAmbientStart(string operationName, ActivityKind activityKind)
+    {
+        if (!Source.HasListeners())
+            return null;
+
+        var ambient = Activity.Current;
+        return ambient is null
+            ? Source.StartActivity(operationName, activityKind)
+            : Source.StartActivity(operationName, activityKind, ambient.Context, tags: null, links: null, startTime: ambient.StartTimeUtc);
+    }
 }
diff --git a/src/Qyl.OpenTelemetry.AutoInstrumentation/QylRuntimeProcessMetrics.cs b/src/Qyl.OpenTelemetry.AutoInstrumentation/QylRuntimeProcessMetrics.cs
index 371e11d..78a723c 100644
--- a/src/Qyl.OpenTelemetry.AutoInstrumentation/QylRuntimeProcessMetrics.cs
+++ b/src/Qyl.OpenTelemetry.AutoInstrumentation/QylRuntimeProcessMetrics.cs
@@ -47,9 +47,12 @@ private static class NetRuntimeMetrics
             QylMetricNames.ProcessRuntimeDotnetGcObjectsSize,
             static () => GC.GetTotalMemory(false),
             "By");
-        private static readonly ObservableGauge<int> ThreadPoolThreads = Meter.CreateObservableGauge(
+        // OTel semconv: dotnet.thread_pool.thread.count is an (Observable)UpDownCounter with unit {thread},
+        // not a unitless gauge — the pool size goes up and down, and UCUM units are required.
+        private static readonly ObservableUpDownCounter<int> ThreadPoolThreads = Meter.CreateObservableUpDownCounter(
             QylMetricNames.ProcessRuntimeDotnetThreadPoolThreadsCount,
-            static () => ThreadPool.ThreadCount);
+            static () => ThreadPool.ThreadCount,
+            unit: "{thread}");
         private static readonly ObservableGauge<long> ThreadPoolQueueLength = Meter.CreateObservableGauge(
             QylMetricNames.ProcessRuntimeDotnetThreadPoolQueueLength,
             static () => ThreadPool.PendingWorkItemCount);