diff --git a/Directory.Packages.props b/Directory.Packages.props
index ded4255..3d5dabf 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -4,6 +4,8 @@
     <CentralPackageTransitivePinningEnabled>false</CentralPackageTransitivePinningEnabled>
   </PropertyGroup>
   <ItemGroup>
+    <PackageVersion Include="Microsoft.Build" Version="18.3.3" />
+    <PackageVersion Include="Microsoft.Build.Locator" Version="1.11.2" />
     <PackageVersion Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="10.0.103" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="10.0.103" />
diff --git a/IntelliTect.Multitool.Tests/CIDetectionTests.cs b/IntelliTect.Multitool.Tests/CIDetectionTests.cs
new file mode 100644
index 0000000..42b1e09
--- /dev/null
+++ b/IntelliTect.Multitool.Tests/CIDetectionTests.cs
@@ -0,0 +1,101 @@
+using System.IO;
+using System.Text.RegularExpressions;
+using System.Xml.Linq;
+using System.Xml;
+using Microsoft.Build.Construction;
+using Microsoft.Build.Evaluation;
+using Xunit;
+
+namespace IntelliTect.Multitool.Tests;
+
+[Collection(MSBuildCollection.CollectionName)]
+public class CIDetectionTests
+{
+    private static readonly string TargetsPath = Path.Combine(
+        RepositoryPaths.GetDefaultRepoRoot(),
+        "IntelliTect.Multitool", "Build", "IntelliTect.Multitool.targets");
+
+    // Derived from the targets file itself — automatically stays in sync when new CI variables are added.
+    private static readonly Lazy<IReadOnlyCollection<string>> _ciVarNames = new(() =>
+    {
+        var doc = XDocument.Load(TargetsPath);
+        var conditions = doc.Descendants()
+            .Where(e => e.Name.LocalName == "CI" && e.Attribute("Condition") != null)
+            .Select(e => e.Attribute("Condition")!.Value);
+        var names = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        foreach (string condition in conditions)
+            foreach (Match m in Regex.Matches(condition, @"\$\((\w+)\)"))
+                names.Add(m.Groups[1].Value);
+        return names;
+    });
+
+    [Theory]
+    [InlineData("GITHUB_ACTIONS", "true")]
+    [InlineData("GITLAB_CI", "true")]
+    [InlineData("CIRCLECI", "true")]
+    [InlineData("CONTINUOUS_INTEGRATION", "true")]
+    [InlineData("TF_BUILD", "true")]
+    [InlineData("TEAMCITY_VERSION", "1.0")]
+    [InlineData("APPVEYOR", "True")]
+    [InlineData("BuildRunner", "MyGet")]
+    [InlineData("JENKINS_URL", "http://jenkins")]
+    [InlineData("TRAVIS", "true")]
+    [InlineData("BUDDY", "true")]
+    [InlineData("CODEBUILD_CI", "true")]
+    public void CiEnvVar_SetsCIPropertyToTrue(string envVar, string value)
+    {
+        string ci = EvaluateCIProperty(new Dictionary<string, string> { [envVar] = value });
+        Assert.Equal("true", ci);
+    }
+
+    [Fact]
+    public void NoCiEnvVars_SetsCIPropertyToFalse()
+    {
+        string ci = EvaluateCIProperty([]);
+        Assert.Equal("false", ci);
+    }
+
+    [Fact]
+    public void CiAlreadyTrue_IsNotOverridden()
+    {
+        string ci = EvaluateCIProperty(new Dictionary<string, string> { ["CI"] = "true" });
+        Assert.Equal("true", ci);
+    }
+
+    private static string EvaluateCIProperty(Dictionary<string, string> overrides)
+    {
+        // Clear all CI-related variables parsed from the targets file so that any env vars
+        // set on the host runner (e.g. GITHUB_ACTIONS=true on GitHub Actions) don't leak in.
+        // Global properties override process env vars in MSBuild evaluation.
+        var globalProperties = _ciVarNames.Value
+            .ToDictionary(v => v, _ => "", StringComparer.OrdinalIgnoreCase);
+        foreach (var (key, value) in overrides)
+            globalProperties[key] = value;
+
+        // CI itself is not in _ciVarNames (it's the output property, not an input condition
+        // variable), but GitHub Actions sets CI=true in the OS environment. If it leaks in,
+        // the outer <PropertyGroup Condition="'$(CI)' == ''"> guard short-circuits and the
+        // entire detection block is skipped. Temporarily remove it from the process environment
+        // so MSBuild doesn't see it, unless the caller is explicitly testing the CI=true case.
+        string? savedCI = Environment.GetEnvironmentVariable("CI");
+        if (!overrides.ContainsKey("CI"))
+            Environment.SetEnvironmentVariable("CI", null);
+        try
+        {
+            using var collection = new ProjectCollection(globalProperties);
+            string xml = $"""
+                <Project>
+                  <Import Project="{TargetsPath.Replace(@"\", "/")}" />
+                </Project>
+                """;
+            using var reader = XmlReader.Create(new StringReader(xml));
+            ProjectRootElement rootElement = ProjectRootElement.Create(reader, collection);
+            var project = new Project(rootElement, null, null, collection);
+            return project.GetPropertyValue("CI");
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable("CI", savedCI);
+        }
+    }
+}
diff --git a/IntelliTect.Multitool.Tests/IntelliTect.Multitool.Tests.csproj b/IntelliTect.Multitool.Tests/IntelliTect.Multitool.Tests.csproj
index a734b6f..6474654 100644
--- a/IntelliTect.Multitool.Tests/IntelliTect.Multitool.Tests.csproj
+++ b/IntelliTect.Multitool.Tests/IntelliTect.Multitool.Tests.csproj
@@ -13,6 +13,8 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.Build" ExcludeAssets="runtime" />
+    <PackageReference Include="Microsoft.Build.Locator" />
     <PackageReference Include="Moq" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
     <PackageReference Include="xunit" />
diff --git a/IntelliTect.Multitool.Tests/MSBuildFixture.cs b/IntelliTect.Multitool.Tests/MSBuildFixture.cs
new file mode 100644
index 0000000..08140a6
--- /dev/null
+++ b/IntelliTect.Multitool.Tests/MSBuildFixture.cs
@@ -0,0 +1,21 @@
+using Microsoft.Build.Locator;
+using Xunit;
+
+namespace IntelliTect.Multitool.Tests;
+
+[CollectionDefinition(CollectionName)]
+public class MSBuildCollection : ICollectionFixture<MSBuildFixture>
+{
+    public const string CollectionName = "MSBuild";
+}
+
+public class MSBuildFixture : IDisposable
+{
+    public MSBuildFixture()
+    {
+        if (!MSBuildLocator.IsRegistered)
+            MSBuildLocator.RegisterDefaults();
+    }
+
+    public void Dispose() { }
+}