diff --git a/readme.md b/readme.md
index 3204f2c9..404cbc1f 100644
--- a/readme.md
+++ b/readme.md
@@ -46,6 +46,7 @@ DiffEngine manages launching and cleanup of diff tools. It is designed to be use
   * [Closing a tool](#closing-a-tool)
   * [File type detection](#file-type-detection)
   * [BuildServerDetector](#buildserverdetector)
+    * [Override in tests](#override-in-tests)
   * [AiCliDetector](#aiclidetector)
   * [Disable for a machine/process](#disable-for-a-machineprocess)
   * [Disable in code](#disable-in-code)
@@ -162,6 +163,50 @@ var isAppVeyor = BuildServerDetector.IsAppVeyor;
 <!-- endSnippet -->
 
 
+### Override in tests
+
+`BuildServerDetector.Detected` can be set at test time. The value is stored in an `AsyncLocal`, so it is scoped to the current async context and does not leak to other threads or tests running in parallel.
+
+<!-- snippet: BuildServerDetectorDetectedOverride -->
+<a id='snippet-BuildServerDetectorDetectedOverride'></a>
+```cs
+[Fact]
+public async Task SetDetectedPersistsInAsyncContext()
+{
+    var original = BuildServerDetector.Detected;
+    try
+    {
+        BuildServerDetector.Detected = true;
+        Assert.True(BuildServerDetector.Detected);
+
+        await Task.Delay(1);
+
+        Assert.True(BuildServerDetector.Detected);
+    }
+    finally
+    {
+        BuildServerDetector.Detected = original;
+    }
+}
+
+[Fact]
+public async Task SetDetectedDoesNotLeakToOtherContexts()
+{
+    var parentValue = BuildServerDetector.Detected;
+
+    await Task.Run(() =>
+    {
+        BuildServerDetector.Detected = true;
+        Assert.True(BuildServerDetector.Detected);
+    });
+
+    Assert.Equal(parentValue, BuildServerDetector.Detected);
+}
+```
+<sup><a href='/src/DiffEngine.Tests/BuildServerDetectorTest.cs#L28-L63' title='Snippet source file'>snippet source</a> | <a href='#snippet-BuildServerDetectorDetectedOverride' title='Start of snippet'>anchor</a></sup>
+<!-- endSnippet -->
+
+
 ## AiCliDetector
 
 `AiCliDetector.Detected` returns true if the current code is running in an AI-powered CLI environment.
diff --git a/readme.source.md b/readme.source.md
index 0f8979a6..fd77687d 100644
--- a/readme.source.md
+++ b/readme.source.md
@@ -84,6 +84,13 @@ There are also individual properties to check for each specific build system
 snippet: BuildServerDetectorProps
 
 
+### Override in tests
+
+`BuildServerDetector.Detected` can be set at test time. The value is stored in an `AsyncLocal`, so it is scoped to the current async context and does not leak to other threads or tests running in parallel.
+
+snippet: BuildServerDetectorDetectedOverride
+
+
 ## AiCliDetector
 
 `AiCliDetector.Detected` returns true if the current code is running in an AI-powered CLI environment.
diff --git a/src/DiffEngine.Tests/BuildServerDetectorTest.cs b/src/DiffEngine.Tests/BuildServerDetectorTest.cs
index 64f83eb3..3e2f96e5 100644
--- a/src/DiffEngine.Tests/BuildServerDetectorTest.cs
+++ b/src/DiffEngine.Tests/BuildServerDetectorTest.cs
@@ -24,4 +24,41 @@ public void Props()
 
         // ReSharper restore UnusedVariable
     }
+
+    #region BuildServerDetectorDetectedOverride
+
+    [Fact]
+    public async Task SetDetectedPersistsInAsyncContext()
+    {
+        var original = BuildServerDetector.Detected;
+        try
+        {
+            BuildServerDetector.Detected = true;
+            Assert.True(BuildServerDetector.Detected);
+
+            await Task.Delay(1);
+
+            Assert.True(BuildServerDetector.Detected);
+        }
+        finally
+        {
+            BuildServerDetector.Detected = original;
+        }
+    }
+
+    [Fact]
+    public async Task SetDetectedDoesNotLeakToOtherContexts()
+    {
+        var parentValue = BuildServerDetector.Detected;
+
+        await Task.Run(() =>
+        {
+            BuildServerDetector.Detected = true;
+            Assert.True(BuildServerDetector.Detected);
+        });
+
+        Assert.Equal(parentValue, BuildServerDetector.Detected);
+    }
+
+    #endregion
 }
\ No newline at end of file
diff --git a/src/DiffEngine/BuildServerDetector.cs b/src/DiffEngine/BuildServerDetector.cs
index 1e5bcf34..980deae1 100644
--- a/src/DiffEngine/BuildServerDetector.cs
+++ b/src/DiffEngine/BuildServerDetector.cs
@@ -52,7 +52,7 @@ static BuildServerDetector()
         // https://docs.microsoft.com/en-us/azure/devops/pipelines/process/variables?view=azure-devops&tabs=yaml%2Cbatch#access-variables-through-the-environment
         IsAzureDevops = ValueEquals(variables, "TF_BUILD", "True");
 
-        Detected = IsTravis ||
+        detected = IsTravis ||
                    IsJenkins ||
                    IsGithubAction ||
                    IsAzureDevops ||
@@ -65,6 +65,9 @@ static BuildServerDetector()
                    IsAppVeyor;
     }
 
+    static bool detected;
+    static AsyncLocal<bool?> overrideDetected = new();
+
     static bool ValueEquals(IDictionary variables, string key, string value)
     {
         var variable = variables[key];
@@ -98,5 +101,9 @@ static bool ValueEquals(IDictionary variables, string key, string value)
 
     public static bool IsJenkins { get; }
 
-    public static bool Detected { get; set; }
+    public static bool Detected
+    {
+        get => overrideDetected.Value ?? detected;
+        set => overrideDetected.Value = value;
+    }
 }
\ No newline at end of file
diff --git a/src/Directory.Build.props b/src/Directory.Build.props
index 958f0b6d..6e729e13 100644
--- a/src/Directory.Build.props
+++ b/src/Directory.Build.props
@@ -2,7 +2,7 @@
 <Project>
   <PropertyGroup>
     <NoWarn>CS1591;CS0649;NU1608;NU1109</NoWarn>
-    <Version>18.3.0</Version>
+    <Version>18.4.0</Version>
     <AssemblyVersion>1.0.0</AssemblyVersion>
     <PackageTags>Testing, Snapshot, Diff, Compare</PackageTags>
     <Description>Launches diff tools based on file extensions. Designed to be consumed by snapshot testing libraries.</Description>