Detect which version of yarn is being used

src/Aspire.Hosting.NodeJs/NodeExtensions.cs

@@ -475,11 +475,29 @@ public static IResourceBuilder<TResource> WithYarn<TResource>(this IResourceBuil
         return resource;
     }
 
-    private static string[] GetDefaultYarnInstallArgs(IResourceBuilder<JavaScriptAppResource> resource) =>
-        resource.ApplicationBuilder.ExecutionContext.IsPublishMode &&
-            File.Exists(Path.Combine(resource.Resource.WorkingDirectory, "yarn.lock"))
-            ? ["--immutable"]
-            : [];
+    private static string[] GetDefaultYarnInstallArgs(IResourceBuilder<JavaScriptAppResource> resource)
+    {
+        var workingDirectory = resource.Resource.WorkingDirectory;
+        if (!resource.ApplicationBuilder.ExecutionContext.IsPublishMode ||
+            !File.Exists(Path.Combine(workingDirectory, "yarn.lock")))
+        {
+            // Not publish mode or no yarn.lock, use default install args
+            return [];
+        }
+
+        var yarnRcYml = Path.Combine(workingDirectory, ".yarnrc.yml");
+        var yarnBerryReleaseDir = Path.Combine(workingDirectory, ".yarn", "releases");
+        var hasYarnBerry = File.Exists(yarnRcYml) || Directory.Exists(yarnBerryReleaseDir);
+
+        if (hasYarnBerry)
+        {
+            // Yarn 2+ detected
+            return ["--immutable"];
+        }
+
+        // Fallback: default to Yarn v1.x behavior
+        return ["--frozen-lockfile"];
+    }
 
     /// <summary>
     /// Configures the Node.js resource to use pnmp as the package manager and optionally installs packages before the application starts.

tests/Aspire.Hosting.NodeJs.Tests/PackageInstallationTests.cs

@@ -476,7 +476,7 @@ public void WithYarn_DefaultsArgsInPublishMode()
             .WithYarn();
 
         Assert.True(app.Resource.TryGetLastAnnotation<JavaScriptInstallCommandAnnotation>(out var installCommand));
-        Assert.Equal(["install", "--immutable"], installCommand.Args);
+        Assert.Equal(["install", "--frozen-lockfile"], installCommand.Args);
 
         var app2 = builder.AddViteApp("test-app2", tempDir.Path)
             .WithYarn(installArgs: ["--immutable-cache"]);
@@ -485,6 +485,38 @@ public void WithYarn_DefaultsArgsInPublishMode()
         Assert.Equal(["install", "--immutable-cache"], installCommand.Args);
     }
 
+    [Fact]
+    public void WithYarn_ReturnsImmutable_WhenYarnRcYmlExists()
+    {
+        using var tempDir = new TempDirectory();
+        File.WriteAllText(Path.Combine(tempDir.Path, "yarn.lock"), "empty");
+        File.WriteAllText(Path.Combine(tempDir.Path, ".yarnrc.yml"), "empty"); // .yarnrc.yml present
+
+        using var builder = TestDistributedApplicationBuilder.Create(DistributedApplicationOperation.Publish);
+
+        var app = builder.AddViteApp("test-app", tempDir.Path)
+            .WithYarn();
+
+        Assert.True(app.Resource.TryGetLastAnnotation<JavaScriptInstallCommandAnnotation>(out var installCommand));
+        Assert.Equal(["install", "--immutable"], installCommand.Args);
+    }
+
+    [Fact]
+    public void WithYarn_ReturnsImmutable_WhenYarnReleasesDirExists()
+    {
+        using var tempDir = new TempDirectory();
+        File.WriteAllText(Path.Combine(tempDir.Path, "yarn.lock"), "empty");
+        Directory.CreateDirectory(Path.Combine(tempDir.Path, ".yarn", "releases"));
+
+        using var builder = TestDistributedApplicationBuilder.Create(DistributedApplicationOperation.Publish);
+
+        var app = builder.AddViteApp("test-app", tempDir.Path)
+            .WithYarn();
+
+        Assert.True(app.Resource.TryGetLastAnnotation<JavaScriptInstallCommandAnnotation>(out var installCommand));
+        Assert.Equal(["install", "--immutable"], installCommand.Args);
+    }
+
     [Fact]
     public void WithPnpm_DefaultsArgsInPublishMode()
     {