diff --git a/eng/vendored-files.json b/eng/vendored-files.json
index afd5faebec..e2598745b7 100644
--- a/eng/vendored-files.json
+++ b/eng/vendored-files.json
@@ -192,7 +192,7 @@
     {
       "id": "platform-ci-environment-detector",
       "local_path": "src/Platform/Microsoft.Testing.Platform/Helpers/CIEnvironmentDetector.cs",
-      "notes": "Adapted from dotnet CLI telemetry CI detection. Maintained per https://learn.microsoft.com/dotnet/core/tools/telemetry#continuous-integration-detection.",
+      "notes": "Adapted from dotnet CLI telemetry CI detection. Maintained per https://learn.microsoft.com/dotnet/core/tools/telemetry#continuous-integration-detection. This file is the single source of truth: it is linked into MSTest.TestFramework via src/TestFramework/TestFramework/TestFramework.csproj and toggled via the IS_CORE_MTP define.",
       "sources": [
         {
           "repo": "dotnet/sdk",
@@ -294,21 +294,6 @@
         }
       ]
     },
-    {
-      "id": "testframework-ci-environment-detector",
-      "local_path": "src/TestFramework/TestFramework/Internal/CIEnvironmentDetector.cs",
-      "notes": "Independent in-tree copy of the CI detector (test-framework variant exposing an IEnvironment-backed Instance) sharing the rules with the platform copy.",
-      "sources": [
-        {
-          "repo": "dotnet/sdk",
-          "ref": "main",
-          "path": "src/Cli/Microsoft.DotNet.Cli.Definitions/Telemetry/CIEnvironmentDetectorForTelemetry.cs",
-          "baseline_ref_sha": "1ab3ae8e0338cdd06b3974959c5802ccc5a85978",
-          "baseline_blob_sha": "e10e1fc2cd973c6123fd983b16a8f0c1e9191c31",
-          "scope": "CI detection rules and environment variable lists"
-        }
-      ]
-    },
     {
       "id": "source-gen-equatable-array",
       "local_path": "src/Analyzers/MSTest.SourceGeneration/Helpers/EquatableArray{T}.cs",
diff --git a/src/Platform/Microsoft.Testing.Platform/Helpers/CIEnvironmentDetector.cs b/src/Platform/Microsoft.Testing.Platform/Helpers/CIEnvironmentDetector.cs
index f8f91094c9..4b62cf6c16 100644
--- a/src/Platform/Microsoft.Testing.Platform/Helpers/CIEnvironmentDetector.cs
+++ b/src/Platform/Microsoft.Testing.Platform/Helpers/CIEnvironmentDetector.cs
@@ -1,14 +1,27 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+// NOTE: This file is the single source of truth for CI environment detection. It is the canonical
+// copy for Microsoft.Testing.Platform and is also linked into Microsoft.Testing.Extensions.Telemetry
+// (via Microsoft.Testing.Extensions.Telemetry.csproj) and MSTest.TestFramework (via
+// src/TestFramework/TestFramework/TestFramework.csproj). The TESTFRAMEWORK_CI_DETECTOR define
+// is set only in the MSTest.TestFramework project; the #if blocks below toggle namespace,
+// attributes, constructor accessibility, the static Instance helper, and the IsNullOrEmpty
+// implementation so the file fits both the Platform/Telemetry layer and the TestFramework layer.
+#if TESTFRAMEWORK_CI_DETECTOR
+namespace Microsoft.VisualStudio.TestTools.UnitTesting;
+#else
 using Microsoft.CodeAnalysis;
 
 namespace Microsoft.Testing.Platform.Helpers;
+#endif
 
 // Detection of CI: https://learn.microsoft.com/dotnet/core/tools/telemetry#continuous-integration-detection
 // Based on: https://github.com/dotnet/sdk/blob/main/src/Cli/Microsoft.DotNet.Cli.Definitions/Telemetry/CIEnvironmentDetectorForTelemetry.cs
+#if !TESTFRAMEWORK_CI_DETECTOR
 [Embedded]
 [ExcludeFromCodeCoverage]
+#endif
 internal sealed class CIEnvironmentDetector
 {
     // Systems that provide boolean values only, so we can simply parse and check for true
@@ -58,12 +71,23 @@ internal sealed class CIEnvironmentDetector
 
     private readonly IEnvironment _environment;
 
+#if TESTFRAMEWORK_CI_DETECTOR
+    /// <summary>
+    /// Gets the default instance that uses the real environment.
+    /// </summary>
+    public static CIEnvironmentDetector Instance { get; } = new(EnvironmentWrapper.Instance);
+#endif
+
     /// <summary>
     /// Initializes a new instance of the <see cref="CIEnvironmentDetector"/> class.
     /// </summary>
     /// <param name="environment">The environment abstraction to use for reading environment variables.</param>
+#if TESTFRAMEWORK_CI_DETECTOR
+    internal /* for testing purposes */ CIEnvironmentDetector(IEnvironment environment) => _environment = environment;
+#else
     public CIEnvironmentDetector(IEnvironment environment)
         => _environment = environment;
+#endif
 
     /// <summary>
     /// Detects if the current environment is a CI environment.
@@ -84,7 +108,7 @@ public bool IsCIEnvironment()
             bool allVariablesPresent = true;
             foreach (string variable in variables)
             {
-                if (global::Microsoft.Testing.Platform.RoslynString.IsNullOrEmpty(_environment.GetEnvironmentVariable(variable)))
+                if (IsNullOrEmpty(_environment.GetEnvironmentVariable(variable)))
                 {
                     allVariablesPresent = false;
                     break;
@@ -99,7 +123,7 @@ public bool IsCIEnvironment()
 
         foreach (string variable in IfNonNullVariables)
         {
-            if (!global::Microsoft.Testing.Platform.RoslynString.IsNullOrEmpty(_environment.GetEnvironmentVariable(variable)))
+            if (!IsNullOrEmpty(_environment.GetEnvironmentVariable(variable)))
             {
                 return true;
             }
@@ -107,4 +131,12 @@ public bool IsCIEnvironment()
 
         return false;
     }
+
+#if TESTFRAMEWORK_CI_DETECTOR
+    private static bool IsNullOrEmpty(string? value)
+        => string.IsNullOrEmpty(value);
+#else
+    private static bool IsNullOrEmpty(string? value)
+        => global::Microsoft.Testing.Platform.RoslynString.IsNullOrEmpty(value);
+#endif
 }
diff --git a/src/TestFramework/TestFramework/Internal/CIEnvironmentDetector.cs b/src/TestFramework/TestFramework/Internal/CIEnvironmentDetector.cs
deleted file mode 100644
index 2a59282f82..0000000000
--- a/src/TestFramework/TestFramework/Internal/CIEnvironmentDetector.cs
+++ /dev/null
@@ -1,115 +0,0 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
-// Licensed under the MIT license. See LICENSE file in the project root for full license information.
-
-namespace Microsoft.VisualStudio.TestTools.UnitTesting;
-
-/// <summary>
-/// Provides CI environment detection capabilities.
-/// </summary>
-/// <remarks>
-/// Based on https://learn.microsoft.com/dotnet/core/tools/telemetry#continuous-integration-detection
-/// and https://github.com/dotnet/sdk/blob/main/src/Cli/dotnet/Telemetry/CIEnvironmentDetectorForTelemetry.cs.
-/// </remarks>
-internal sealed class CIEnvironmentDetector
-{
-    /// <summary>
-    /// Gets the default instance that uses the real environment.
-    /// </summary>
-    public static CIEnvironmentDetector Instance { get; } = new(EnvironmentWrapper.Instance);
-
-    private readonly IEnvironment _environment;
-
-    // Systems that provide boolean values only, so we can simply parse and check for true
-    private static readonly string[] BooleanVariables =
-    [
-        // Azure Pipelines - https://docs.microsoft.com/azure/devops/pipelines/build/variables#system-variables-devops-services
-        "TF_BUILD",
-
-        // GitHub Actions - https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables
-        "GITHUB_ACTIONS",
-
-        // AppVeyor - https://www.appveyor.com/docs/environment-variables/
-        "APPVEYOR",
-
-        // A general-use flag - Many of the major players support this: AzDo, GitHub, GitLab, AppVeyor, Travis CI, CircleCI.
-        "CI",
-
-        // Travis CI - https://docs.travis-ci.com/user/environment-variables/#default-environment-variables
-        "TRAVIS",
-
-        // CircleCI - https://circleci.com/docs/2.0/env-vars/#built-in-environment-variables
-        "CIRCLECI",
-    ];
-
-    // Systems where every variable must be present and not-null before returning true
-    private static readonly string[][] AllNotNullVariables =
-    [
-        // AWS CodeBuild - https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-env-vars.html
-        ["CODEBUILD_BUILD_ID", "AWS_REGION"],
-
-        // Jenkins - https://github.com/jenkinsci/jenkins/blob/master/core/src/main/resources/jenkins/model/CoreEnvironmentContributor/buildEnv.groovy
-        ["BUILD_ID", "BUILD_URL"],
-
-        // Google Cloud Build - https://cloud.google.com/build/docs/configuring-builds/substitute-variable-values#using_default_substitutions
-        ["BUILD_ID", "PROJECT_ID"],
-    ];
-
-    // Systems where the variable must be present and not-null
-    private static readonly string[] IfNonNullVariables =
-    [
-        // TeamCity - https://www.jetbrains.com/help/teamcity/predefined-build-parameters.html#Predefined+Server+Build+Parameters
-        "TEAMCITY_VERSION",
-
-        // JetBrains Space - https://www.jetbrains.com/help/space/automation-environment-variables.html#general
-        "JB_SPACE_API_URL",
-    ];
-
-    /// <summary>
-    /// Initializes a new instance of the <see cref="CIEnvironmentDetector"/> class.
-    /// </summary>
-    /// <param name="environment">The environment abstraction to use for reading environment variables.</param>
-    internal /* for testing purposes */ CIEnvironmentDetector(IEnvironment environment) => _environment = environment;
-
-    /// <summary>
-    /// Detects if the current environment is a CI environment.
-    /// </summary>
-    /// <returns><c>true</c> if running in a CI environment; otherwise, <c>false</c>.</returns>
-    public bool IsCIEnvironment()
-    {
-        foreach (string booleanVariable in BooleanVariables)
-        {
-            if (bool.TryParse(_environment.GetEnvironmentVariable(booleanVariable), out bool envVar) && envVar)
-            {
-                return true;
-            }
-        }
-
-        foreach (string[] variables in AllNotNullVariables)
-        {
-            bool allVariablesPresent = true;
-            foreach (string variable in variables)
-            {
-                if (string.IsNullOrEmpty(_environment.GetEnvironmentVariable(variable)))
-                {
-                    allVariablesPresent = false;
-                    break;
-                }
-            }
-
-            if (allVariablesPresent)
-            {
-                return true;
-            }
-        }
-
-        foreach (string variable in IfNonNullVariables)
-        {
-            if (!string.IsNullOrEmpty(_environment.GetEnvironmentVariable(variable)))
-            {
-                return true;
-            }
-        }
-
-        return false;
-    }
-}
diff --git a/src/TestFramework/TestFramework/TestFramework.csproj b/src/TestFramework/TestFramework/TestFramework.csproj
index 6afb0f4f90..a167fe2984 100644
--- a/src/TestFramework/TestFramework/TestFramework.csproj
+++ b/src/TestFramework/TestFramework/TestFramework.csproj
@@ -26,8 +26,16 @@
 
   <ItemGroup>
     <Compile Include="..\..\Platform\Microsoft.Testing.Platform\Helpers\RoslynHashCode.cs" Link="Internal\RoslynHashCode.cs" />
+    <!-- Single source of truth for CI environment detection lives in Microsoft.Testing.Platform. The
+         TESTFRAMEWORK_CI_DETECTOR define selects the TestFramework variant of the class (different
+         namespace, internal ctor, static Instance helper, string.IsNullOrEmpty). -->
+    <Compile Include="..\..\Platform\Microsoft.Testing.Platform\Helpers\CIEnvironmentDetector.cs" Link="Internal\CIEnvironmentDetector.cs" />
   </ItemGroup>
 
+  <PropertyGroup>
+    <DefineConstants>$(DefineConstants);TESTFRAMEWORK_CI_DETECTOR</DefineConstants>
+  </PropertyGroup>
+
   <ItemGroup>
     <Compile Include="$(RepoRoot)src/Polyfills/**/*.cs" Link="Polyfills\%(RecursiveDir)%(Filename)%(Extension)" />
   </ItemGroup>