diff --git a/src/ClaudeDo.Logging/BuildConfig.cs b/src/ClaudeDo.Logging/BuildConfig.cs
new file mode 100644
index 0000000..2e13542
--- /dev/null
+++ b/src/ClaudeDo.Logging/BuildConfig.cs
@@ -0,0 +1,14 @@
+using System.Diagnostics;
+using System.Reflection;
+
+namespace ClaudeDo.Logging;
+
+/// <summary>Runtime build-configuration detection — the replacement for #if DEBUG.
+/// Debug builds compile with the JIT optimizer disabled; Release builds enable it.</summary>
+public static class BuildConfig
+{
+    public static bool IsDebug { get; } =
+        Assembly.GetEntryAssembly()
+            ?.GetCustomAttribute<DebuggableAttribute>()
+            ?.IsJITOptimizerDisabled ?? false;
+}
diff --git a/tests/ClaudeDo.Worker.Tests/Logging/BuildConfigTests.cs b/tests/ClaudeDo.Worker.Tests/Logging/BuildConfigTests.cs
new file mode 100644
index 0000000..76d9293
--- /dev/null
+++ b/tests/ClaudeDo.Worker.Tests/Logging/BuildConfigTests.cs
@@ -0,0 +1,19 @@
+using System.Diagnostics;
+using System.Reflection;
+using ClaudeDo.Logging;
+
+namespace ClaudeDo.Worker.Tests.Logging;
+
+public sealed class BuildConfigTests
+{
+    [Fact]
+    public void IsDebug_MatchesEntryAssemblyDebuggableAttribute()
+    {
+        var entry = Assembly.GetEntryAssembly();
+        var expected = entry?
+            .GetCustomAttribute<DebuggableAttribute>()
+            ?.IsJITOptimizerDisabled ?? false;
+
+        Assert.Equal(expected, BuildConfig.IsDebug);
+    }
+}