diff --git a/src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsDevelopmentManifest.cs b/src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsDevelopmentManifest.cs
index 00d4f39c69e1..c8483fca3427 100644
--- a/src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsDevelopmentManifest.cs
+++ b/src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsDevelopmentManifest.cs
@@ -13,10 +13,14 @@ namespace Microsoft.AspNetCore.StaticWebAssets.Tasks;
 // The manifest needs to always be case sensitive, since we don't know what the final runtime environment
 // will be. The runtime is responsible for merging the tree nodes in the manifest when the underlying OS
 // is case insensitive.
-public class GenerateStaticWebAssetsDevelopmentManifest : Task
+[MSBuildMultiThreadableTask]
+public class GenerateStaticWebAssetsDevelopmentManifest : Task, IMultiThreadableTask
 {
     private static readonly char[] _separator = ['/'];
 
+    /// <inheritdoc/>
+    public TaskEnvironment TaskEnvironment { get; set; } = TaskEnvironment.Fallback;
+
     [Required]
     public string Source { get; set; }
 
@@ -34,7 +38,8 @@ public class GenerateStaticWebAssetsDevelopmentManifest : Task
 
     public override bool Execute()
     {
-        if (File.Exists(ManifestPath) && File.GetLastWriteTimeUtc(ManifestPath) > File.GetLastWriteTimeUtc(CacheFilePath))
+        AbsolutePath manifestPath = TaskEnvironment.GetAbsolutePath(ManifestPath);
+        if (File.Exists(manifestPath) && File.GetLastWriteTimeUtc(manifestPath) > File.GetLastWriteTimeUtc(TaskEnvironment.GetAbsolutePath(CacheFilePath)))
         {
             Log.LogMessage(MessageImportance.Low, "Skipping manifest generation because manifest file '{0}' is up to date.", ManifestPath);
             return true;
@@ -49,10 +54,10 @@ public override bool Execute()
             }
 
             var manifest = ComputeDevelopmentManifest(
-                StaticWebAsset.FromTaskItemGroup(Assets),
+                StaticWebAsset.FromTaskItemGroup(Assets, TaskEnvironment),
                 DiscoveryPatterns.Select(StaticWebAssetsDiscoveryPattern.FromTaskItem));
 
-            PersistManifest(manifest);
+            PersistManifest(manifest, manifestPath);
         }
         catch (Exception ex)
         {
@@ -125,7 +130,7 @@ private IEnumerable<SegmentsAssetPair> ComputeManifestAssets(IEnumerable<StaticW
         }
     }
 
-    private void PersistManifest(StaticWebAssetsDevelopmentManifest manifest)
+    private void PersistManifest(StaticWebAssetsDevelopmentManifest manifest, AbsolutePath manifestPath)
     {
         var data = JsonSerializer.SerializeToUtf8Bytes(manifest, StaticWebAssetsJsonSerializerContext.RelaxedEscaping.StaticWebAssetsDevelopmentManifest);
 #if !NET9_0_OR_GREATER
@@ -134,19 +139,19 @@ private void PersistManifest(StaticWebAssetsDevelopmentManifest manifest)
 #else
         var currentHash = SHA256.HashData(data);
 #endif
-        var fileExists = File.Exists(ManifestPath);
+        var fileExists = File.Exists(manifestPath);
         var existingManifestHash = fileExists ?
 #if !NET9_0_OR_GREATER
-            sha256.ComputeHash(File.ReadAllBytes(ManifestPath)) :
+            sha256.ComputeHash(File.ReadAllBytes(manifestPath)) :
 #else
-            SHA256.HashData(File.ReadAllBytes(ManifestPath)) :
+            SHA256.HashData(File.ReadAllBytes(manifestPath)) :
 #endif
             [];
 
         if (!fileExists)
         {
             Log.LogMessage(MessageImportance.Low, "Creating manifest because manifest file '{0}' does not exist.", ManifestPath);
-            File.WriteAllBytes(ManifestPath, data);
+            File.WriteAllBytes(manifestPath, data);
         }
         else if (!currentHash.SequenceEqual(existingManifestHash))
         {
@@ -155,7 +160,7 @@ private void PersistManifest(StaticWebAssetsDevelopmentManifest manifest)
                 "Updating manifest because manifest version '{0}' is different from existing manifest hash '{1}'.",
                 Convert.ToBase64String(currentHash),
                 Convert.ToBase64String(existingManifestHash));
-            File.WriteAllBytes(ManifestPath, data);
+            File.WriteAllBytes(manifestPath, data);
         }
         else
         {
diff --git a/test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsDevelopmentManifestMultiThreadingTest.cs b/test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsDevelopmentManifestMultiThreadingTest.cs
new file mode 100644
index 000000000000..a5656adda840
--- /dev/null
+++ b/test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsDevelopmentManifestMultiThreadingTest.cs
@@ -0,0 +1,146 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable disable
+
+using Microsoft.AspNetCore.StaticWebAssets.Tasks;
+using Microsoft.Build.Framework;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Moq;
+
+namespace Microsoft.NET.Sdk.StaticWebAssets.Tests;
+
+// These tests mutate the process current directory, so they must run sequentially.
+// The project disables parallelization (MSTestParallelizeScope=None) and
+// [DoNotParallelize] enforces that for this class.
+[DoNotParallelize]
+[TestClass]
+public class GenerateStaticWebAssetsDevelopmentManifestMultiThreadingTest
+{
+    [TestMethod]
+    public void WritesManifestRelativeToTaskEnvironmentProjectDirectory_NotProcessCurrentDirectory()
+    {
+        var testRoot = Path.Combine(AppContext.BaseDirectory, nameof(GenerateStaticWebAssetsDevelopmentManifestMultiThreadingTest), Guid.NewGuid().ToString("N"));
+        var projectDir = Path.Combine(testRoot, "project");
+        var spawnDir = Path.Combine(testRoot, "spawn");
+        var relativeManifestPath = Path.Combine("obj", "staticwebassets.development.json");
+        var relativeCacheFilePath = Path.Combine("obj", "staticwebassets.build.cache.json");
+        Directory.CreateDirectory(Path.Combine(projectDir, "obj"));
+        Directory.CreateDirectory(Path.Combine(spawnDir, "obj"));
+
+        var originalCurrentDirectory = Directory.GetCurrentDirectory();
+        try
+        {
+            Directory.SetCurrentDirectory(spawnDir);
+
+            var task = new GenerateStaticWebAssetsDevelopmentManifest
+            {
+                BuildEngine = new Mock<IBuildEngine>().Object,
+                TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment(projectDir),
+                Source = "CurrentProjectId",
+                Assets = [CreateAssetItem("css/site.css")],
+                DiscoveryPatterns = [],
+                ManifestPath = relativeManifestPath,
+                CacheFilePath = relativeCacheFilePath
+            };
+
+            task.Execute().Should().BeTrue();
+
+            var expectedPath = Path.Combine(projectDir, relativeManifestPath);
+            File.Exists(expectedPath).Should().BeTrue("the manifest should be written under the project dir, not the process CWD");
+
+            var incorrectPath = Path.Combine(spawnDir, relativeManifestPath);
+            File.Exists(incorrectPath).Should().BeFalse();
+        }
+        finally
+        {
+            Directory.SetCurrentDirectory(originalCurrentDirectory);
+            if (Directory.Exists(testRoot))
+            {
+                Directory.Delete(testRoot, recursive: true);
+            }
+        }
+    }
+
+    [TestMethod]
+    public void ResolvesExistingManifestProbeRelativeToProjectDirectory_NotProcessCurrentDirectory()
+    {
+        // Verifies that the File.Exists/File.ReadAllBytes change-detection probe in PersistManifest (and
+        // the up-to-date check in Execute) is rooted against TaskEnvironment.ProjectDirectory rather than
+        // the process CWD. A decoy manifest is planted in the process CWD at the same relative path. If the
+        // task read that decoy as the "existing" manifest it would overwrite the decoy or skip writing the
+        // project-dir file. The correct behavior is to ignore the decoy entirely: create the manifest under
+        // the project dir and leave the decoy untouched.
+        var testRoot = Path.Combine(AppContext.BaseDirectory, nameof(GenerateStaticWebAssetsDevelopmentManifestMultiThreadingTest), Guid.NewGuid().ToString("N"));
+        var projectDir = Path.Combine(testRoot, "project");
+        var spawnDir = Path.Combine(testRoot, "spawn");
+        var relativeManifestPath = Path.Combine("obj", "staticwebassets.development.json");
+        var relativeCacheFilePath = Path.Combine("obj", "staticwebassets.build.cache.json");
+        Directory.CreateDirectory(Path.Combine(projectDir, "obj"));
+        Directory.CreateDirectory(Path.Combine(spawnDir, "obj"));
+
+        const string decoyContents = "DECOY - must not be read or overwritten";
+        var decoyPath = Path.Combine(spawnDir, relativeManifestPath);
+        File.WriteAllText(decoyPath, decoyContents);
+
+        var originalCurrentDirectory = Directory.GetCurrentDirectory();
+        try
+        {
+            Directory.SetCurrentDirectory(spawnDir);
+
+            var task = new GenerateStaticWebAssetsDevelopmentManifest
+            {
+                BuildEngine = new Mock<IBuildEngine>().Object,
+                TaskEnvironment = TaskEnvironment.CreateWithProjectDirectoryAndEnvironment(projectDir),
+                Source = "CurrentProjectId",
+                Assets = [CreateAssetItem("css/site.css")],
+                DiscoveryPatterns = [],
+                ManifestPath = relativeManifestPath,
+                CacheFilePath = relativeCacheFilePath
+            };
+
+            task.Execute().Should().BeTrue();
+
+            var expectedPath = Path.Combine(projectDir, relativeManifestPath);
+            File.Exists(expectedPath).Should().BeTrue("the existence probe must target the project dir, find nothing, and create the manifest there");
+
+            File.ReadAllText(decoyPath).Should().Be(decoyContents, "the decoy in the process CWD must be neither read nor overwritten");
+        }
+        finally
+        {
+            Directory.SetCurrentDirectory(originalCurrentDirectory);
+            if (Directory.Exists(testRoot))
+            {
+                Directory.Delete(testRoot, recursive: true);
+            }
+        }
+    }
+
+    private static ITaskItem CreateAssetItem(string relativePath)
+    {
+        var asset = new StaticWebAsset
+        {
+            Identity = relativePath,
+            SourceId = "OtherPackage",
+            SourceType = StaticWebAsset.SourceTypes.Discovered,
+            ContentRoot = "wwwroot",
+            BasePath = "_content/Base",
+            RelativePath = relativePath,
+            AssetKind = StaticWebAsset.AssetKinds.All,
+            AssetMode = StaticWebAsset.AssetModes.All,
+            AssetRole = StaticWebAsset.AssetRoles.Primary,
+            RelatedAsset = "",
+            AssetTraitName = "",
+            AssetTraitValue = "",
+            CopyToOutputDirectory = "",
+            CopyToPublishDirectory = "",
+            OriginalItemSpec = relativePath,
+            Integrity = "integrity",
+            Fingerprint = "fingerprint",
+            LastWriteTime = DateTime.UtcNow,
+            FileLength = 10,
+        };
+
+        return asset.ToTaskItem();
+    }
+}
diff --git a/test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsDevelopmentManifestTest.cs b/test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsDevelopmentManifestTest.cs
index e16a9d7c3203..a6e711c76888 100644
--- a/test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsDevelopmentManifestTest.cs
+++ b/test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsDevelopmentManifestTest.cs
@@ -28,11 +28,14 @@ public void SkipsManifestGenerationWhen_ThereAreNoAssetsNorDiscoveryPatterns()
             buildEngine.Setup(e => e.LogMessageEvent(It.IsAny<BuildMessageEventArgs>()))
                 .Callback<BuildMessageEventArgs>(args => messages.Add(args.Message));
 
+            var manifestDirectory = Path.Combine(AppContext.BaseDirectory, nameof(SkipsManifestGenerationWhen_ThereAreNoAssetsNorDiscoveryPatterns), Guid.NewGuid().ToString("N"));
             var task = new GenerateStaticWebAssetsDevelopmentManifest()
             {
                 BuildEngine = buildEngine.Object,
                 Assets = Array.Empty<ITaskItem>(),
-                DiscoveryPatterns = Array.Empty<ITaskItem>()
+                DiscoveryPatterns = Array.Empty<ITaskItem>(),
+                ManifestPath = Path.Combine(manifestDirectory, "staticwebassets.development.json"),
+                CacheFilePath = Path.Combine(manifestDirectory, "staticwebassets.build.cache.json")
             };
 
             // Act