diff --git a/changelog.d/unreleased/5121.fixed.md b/changelog.d/unreleased/5121.fixed.md
new file mode 100644
index 000000000..99a2fa041
--- /dev/null
+++ b/changelog.d/unreleased/5121.fixed.md
@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 5121
+affected:
+  - tests/CodeIndex.Tests/ExtractorPluginRegistryTests.cs
+  - tests/CodeIndex.PluginIsolationFixture/PluginIsolationFixture.cs
+---
+
+## English
+
+- **Plugin tests no longer depend on the size of `CodeIndex.Tests.dll` (#5121)** — executable extractor-plugin coverage now enforces use of the dedicated bounded `CodeIndex.PluginIsolationFixture` assembly, so growth in the main test assembly cannot exceed the production 4,096-type inspection limit and break the `net9.0` suite.
+
+## 日本語
+
+- **pluginテストが`CodeIndex.Tests.dll`のサイズに依存しなくなりました (#5121)** — 実行可能なextractor pluginのカバレッジで、専用かつ上限内の`CodeIndex.PluginIsolationFixture` assemblyを使うことを保証し、main test assemblyの増加によってproductionの4,096-type検査上限を超えて`net9.0` suiteが失敗しないようにしました。
diff --git a/tests/CodeIndex.Tests/ExtractorPluginRegistryTests.cs b/tests/CodeIndex.Tests/ExtractorPluginRegistryTests.cs
index 36696c811..e833cdb02 100644
--- a/tests/CodeIndex.Tests/ExtractorPluginRegistryTests.cs
+++ b/tests/CodeIndex.Tests/ExtractorPluginRegistryTests.cs
@@ -8,8 +8,6 @@
 using CodeIndex.Models;
 using CodeIndex.PluginIsolationFixture;
 
-[assembly: CdidxPlugin(ExtractorPluginRegistry.CurrentApiVersion, ExtractorPluginRegistry.CurrentApiVersion)]
-
 namespace CodeIndex.Tests;
 
 [Collection("Plugin registry sensitive")]
@@ -26,6 +24,18 @@ public ExtractorPluginRegistryTests(TrustedPluginAssemblyFixture trustedPluginAs
     internal const string SlowPluginConstructorEnvironmentVariable = PluginIsolationFixtureEnvironment.SlowConstructor;
     internal const string CrashingPluginConstructorEnvironmentVariable = PluginIsolationFixtureEnvironment.CrashingConstructor;
 
+    [Fact]
+    public void PluginFixture_IsDedicatedAndBounded_Issue5121()
+    {
+        var testAssembly = typeof(ExtractorPluginRegistryTests).Assembly;
+        var pluginAssembly = typeof(CollectiblePluginSymbolExtractor).Assembly;
+
+        Assert.NotSame(testAssembly, pluginAssembly);
+        Assert.Empty(testAssembly.GetCustomAttributes<CdidxPluginAttribute>());
+        Assert.Single(pluginAssembly.GetCustomAttributes<CdidxPluginAttribute>());
+        Assert.InRange(pluginAssembly.GetTypes().Length, 1, ExtractorPluginRegistry.MaxExtensionAssemblyTypes);
+    }
+
     [Fact]
     public void DoctorIntegrations_DoesNotStartConfiguredPluginWorker_Issue5102()
     {
@@ -337,7 +347,7 @@ public void LoadPluginAssemblies_RejectsUnsafeDirectoryModeAndAncestorSymlink_Is
             try
             {
                 Directory.CreateDirectory(pluginDirectory);
-                File.Copy(Assembly.GetExecutingAssembly().Location, Path.Combine(pluginDirectory, "plugin.dll"));
+                File.Copy(pluginAssemblyFixturePath, Path.Combine(pluginDirectory, "plugin.dll"));
 
                 ExtractorPluginRegistry.ResetForTests();
                 File.SetUnixFileMode(