diff --git a/src/Build.UnitTests/BinaryLogger_Tests.cs b/src/Build.UnitTests/BinaryLogger_Tests.cs
index e47fb19b1e2..aa451d50564 100644
--- a/src/Build.UnitTests/BinaryLogger_Tests.cs
+++ b/src/Build.UnitTests/BinaryLogger_Tests.cs
@@ -74,10 +74,12 @@ public class BinaryLoggerTests : IDisposable
         </Project>";
 
         private readonly TestEnvironment _env;
+        private readonly ITestOutputHelper _output;
         private string _logFile;
 
         public BinaryLoggerTests(ITestOutputHelper output)
         {
+            _output = output;
             _env = TestEnvironment.Create(output);
 
             // this is needed to ensure the binary logger does not pollute the environment
@@ -437,30 +439,67 @@ private void AssemblyLoadsDuringTaskRun(string additionalEventText)
         [Fact]
         public void BinaryLoggerShouldEmbedFilesViaTaskOutput()
         {
+            // Pin the process current directory to a stable, isolated transient folder so
+            // both the WriteLinesToFile task (which resolves the relative file path against
+            // the process CWD) and the binary logger's file-existence/embed code observe
+            // the same directory. Historically this test has been flaky (#9412, #13762)
+            // because relative ItemSpecs on the EmbedInBinlog item were resolved against
+            // a different CWD on the binary logger's background embedding thread.
+            TransientTestFolder workingFolder = _env.CreateFolder(createFolder: true);
+            _env.SetCurrentDirectory(workingFolder.Path);
+
+            const string OutputFileName = "testtaskoutputfile.txt";
+            string expectedOutputFilePath = Path.Combine(workingFolder.Path, OutputFileName);
+
             using var buildManager = new BuildManager();
             var binaryLogger = new BinaryLogger()
             {
                 Parameters = $"LogFile={_logFile}",
                 CollectProjectImports = BinaryLogger.ProjectImportsCollectionMode.ZipFile,
             };
-            var testProject = @"
+
+            // Capture every event the binary logger sees so we can include it in the
+            // failure message if the assertion below fires intermittently.
+            var mockLogger = new MockLogger(_output);
+
+            var testProject = $@"
 <Project>
     <Target Name=""Build"">
-        <WriteLinesToFile File=""testtaskoutputfile.txt"" Lines=""abc;def;ghi""/>
-        <CreateItem Include=""testtaskoutputfile.txt"">
+        <WriteLinesToFile File=""{OutputFileName}"" Lines=""abc;def;ghi""/>
+        <CreateItem Include=""{OutputFileName}"">
             <Output TaskParameter=""Include"" ItemName=""EmbedInBinlog"" />
         </CreateItem>
     </Target>
 </Project>";
-            ObjectModelHelpers.BuildProjectExpectSuccess(testProject, binaryLogger);
+            ObjectModelHelpers.BuildProjectExpectSuccess(testProject, binaryLogger, mockLogger);
+
+            // Sanity-check the output file produced by WriteLinesToFile actually exists
+            // on disk after the build. If it does not, the diagnostic information will
+            // make the failure mode obvious instead of just reporting an empty zip.
+            bool outputFileExists = File.Exists(expectedOutputFilePath);
+
             var projectImportsZipPath = Path.ChangeExtension(_logFile, ".ProjectImports.zip");
             using var fileStream = new FileStream(projectImportsZipPath, FileMode.Open);
             using var zipArchive = new ZipArchive(fileStream, ZipArchiveMode.Read);
 
+            // Snapshot the zip entries up-front so the assertion message can describe
+            // every entry (FullName + Length), not just whichever one tripped Shouldly.
+            var entries = zipArchive.Entries
+                .Select(e => $"  Name='{e.Name}' FullName='{e.FullName}' Length={e.Length}")
+                .ToList();
+
+            string diagnostics =
+                $"Working directory: {workingFolder.Path}{Environment.NewLine}" +
+                $"Process CWD at assert time: {Directory.GetCurrentDirectory()}{Environment.NewLine}" +
+                $"Expected output file: {expectedOutputFilePath} (Exists={outputFileExists}){Environment.NewLine}" +
+                $"Binlog path: {_logFile}{Environment.NewLine}" +
+                $"ProjectImports.zip path: {projectImportsZipPath}{Environment.NewLine}" +
+                $"Embedded files ({entries.Count}):{Environment.NewLine}{string.Join(Environment.NewLine, entries)}{Environment.NewLine}" +
+                $"=== Build log ==={Environment.NewLine}{mockLogger.FullLog}";
+
             // Can't just compare `Name` because `ZipArchive` does not handle unix directory separators well
             // thus producing garbled fully qualified paths in the actual .ProjectImports.zip entries
-            zipArchive.Entries.ShouldContain(zE => zE.Name.EndsWith("testtaskoutputfile.txt"),
-                $"Embedded files: {string.Join(",", zipArchive.Entries)}");
+            zipArchive.Entries.ShouldContain(zE => zE.Name.EndsWith(OutputFileName), diagnostics);
         }
 
         [RequiresSymbolicLinksFact]
diff --git a/src/Build/BackEnd/Components/Logging/LoggingContext.cs b/src/Build/BackEnd/Components/Logging/LoggingContext.cs
index f08d8845008..b7a372c973d 100644
--- a/src/Build/BackEnd/Components/Logging/LoggingContext.cs
+++ b/src/Build/BackEnd/Components/Logging/LoggingContext.cs
@@ -70,6 +70,20 @@ public LoggingContext(LoggingContext baseContext, BuildEventContext newEventCont
         /// </summary>
         internal IBuildEngineDataConsumer BuildEngineDataConsumer => this;
 
+        /// <summary>
+        /// The full path of the project this context is associated with, if any. Returns
+        /// <see langword="null"/> for contexts that are not project-scoped (e.g.
+        /// <see cref="NodeLoggingContext"/>).
+        /// </summary>
+        /// <remarks>
+        /// Used for resolving relative paths to absolute on the engine side before they
+        /// flow out to loggers, where neither the process current directory nor the
+        /// background thread CWD is a reliable base directory (especially in
+        /// multithreaded MSBuild execution; see
+        /// <c>documentation/specs/multithreading/multithreaded-msbuild.md</c>).
+        /// </remarks>
+        internal virtual string? ProjectFullPath => null;
+
         /// <summary>
         /// Retrieves the logging service
         /// </summary>
diff --git a/src/Build/BackEnd/Components/Logging/ProjectLoggingContext.cs b/src/Build/BackEnd/Components/Logging/ProjectLoggingContext.cs
index 6f467e25d9b..589e4e6a576 100644
--- a/src/Build/BackEnd/Components/Logging/ProjectLoggingContext.cs
+++ b/src/Build/BackEnd/Components/Logging/ProjectLoggingContext.cs
@@ -153,6 +153,11 @@ private ProjectLoggingContext(
             this.IsValid = true;
         }
 
+        /// <summary>
+        /// The full path of the project this context was created for.
+        /// </summary>
+        internal override string ProjectFullPath => _projectFullPath;
+
         private static BuildEventContext CreateInitialContext(
             NodeLoggingContext nodeLoggingContext,
             int submissionId,
diff --git a/src/Build/BackEnd/Components/Logging/TargetLoggingContext.cs b/src/Build/BackEnd/Components/Logging/TargetLoggingContext.cs
index 1f44ab275de..120352aa197 100644
--- a/src/Build/BackEnd/Components/Logging/TargetLoggingContext.cs
+++ b/src/Build/BackEnd/Components/Logging/TargetLoggingContext.cs
@@ -69,6 +69,9 @@ internal ProjectLoggingContext ProjectLoggingContext
             }
         }
 
+        /// <inheritdoc />
+        internal override string ProjectFullPath => _projectLoggingContext?.ProjectFullPath;
+
         /// <summary>
         /// Retrieves the target.
         /// </summary>
diff --git a/src/Build/BackEnd/Components/Logging/TaskLoggingContext.cs b/src/Build/BackEnd/Components/Logging/TaskLoggingContext.cs
index 43d668805e4..a2b09eb4566 100644
--- a/src/Build/BackEnd/Components/Logging/TaskLoggingContext.cs
+++ b/src/Build/BackEnd/Components/Logging/TaskLoggingContext.cs
@@ -101,6 +101,9 @@ internal TargetLoggingContext TargetLoggingContext
             }
         }
 
+        /// <inheritdoc />
+        internal override string ProjectFullPath => _targetLoggingContext?.ProjectFullPath;
+
         /// <summary>
         /// Retrieves the task node.
         /// </summary>
diff --git a/src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/ItemGroupLoggingHelper.cs b/src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/ItemGroupLoggingHelper.cs
index 23f3d96c1f6..00308dd50de 100644
--- a/src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/ItemGroupLoggingHelper.cs
+++ b/src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/ItemGroupLoggingHelper.cs
@@ -5,6 +5,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Globalization;
+using System.IO;
 #if FEATURE_APPDOMAIN
 using System.Runtime.Remoting;
 #endif
@@ -263,6 +264,20 @@ internal static void LogTaskParameter(
             bool logItemMetadata,
             IElementLocation location = null)
         {
+            // For items destined for the binary logger via the EmbedInBinlog hint, resolve
+            // any relative item specs against the project's directory before the event
+            // leaves the engine. The logger consumes events on background threads where
+            // the process current directory is not a reliable base directory; this is
+            // especially true under multithreaded MSBuild execution where multiple
+            // projects may be running concurrently in the same process and the
+            // process-level CWD is not a per-project concept at all (see
+            // documentation/specs/multithreading/multithreaded-msbuild.md).
+            if (items != null
+                && string.Equals(itemType, ItemTypeNames.EmbedInBinlog, StringComparison.OrdinalIgnoreCase))
+            {
+                items = MakeEmbedInBinlogItemSpecsAbsolute(items, loggingContext.ProjectFullPath);
+            }
+
             var args = CreateTaskParameterEventArgs(
                 loggingContext.BuildEventContext,
                 messageKind,
@@ -278,6 +293,81 @@ internal static void LogTaskParameter(
             loggingContext.LogBuildEvent(args);
         }
 
+        /// <summary>
+        /// Returns an item list equivalent to <paramref name="items"/> in which every relative
+        /// <see cref="ITaskItem.ItemSpec"/> (or relative bare-string spec) has been resolved to
+        /// an absolute path against <paramref name="projectFullPath"/>'s directory. The original
+        /// items are not mutated; a snapshot is allocated only when at least one rewrite is
+        /// required.
+        /// </summary>
+        private static IList MakeEmbedInBinlogItemSpecsAbsolute(IList items, string projectFullPath)
+        {
+            if (string.IsNullOrEmpty(projectFullPath))
+            {
+                return items;
+            }
+
+            string projectDirectory = Path.GetDirectoryName(projectFullPath);
+            if (string.IsNullOrEmpty(projectDirectory))
+            {
+                return items;
+            }
+
+            int count = items.Count;
+            object[] cloned = null;
+
+            for (int i = 0; i < count; i++)
+            {
+                object item = items[i];
+                string originalSpec = item switch
+                {
+                    ITaskItem taskItem => taskItem.ItemSpec,
+                    string s => s,
+                    _ => null,
+                };
+
+                if (string.IsNullOrEmpty(originalSpec) || Path.IsPathRooted(originalSpec))
+                {
+                    if (cloned != null)
+                    {
+                        cloned[i] = item;
+                    }
+
+                    continue;
+                }
+
+                // Path.GetFullPath of an already-rooted path does not consult the process
+                // current directory, so this is safe in any execution mode.
+                string absoluteSpec = Path.GetFullPath(Path.Combine(projectDirectory, originalSpec));
+
+                if (cloned == null)
+                {
+                    cloned = new object[count];
+                    for (int j = 0; j < i; j++)
+                    {
+                        cloned[j] = items[j];
+                    }
+                }
+
+                cloned[i] = item is ITaskItem origTaskItem
+                    ? new TaskItemData(absoluteSpec, CopyMetadata(origTaskItem))
+                    : (object)absoluteSpec;
+            }
+
+            return cloned ?? items;
+        }
+
+        private static IDictionary<string, string> CopyMetadata(ITaskItem item)
+        {
+            var dictionary = new Dictionary<string, string>();
+            foreach (KeyValuePair<string, string> kvp in item.EnumerateMetadata())
+            {
+                dictionary[kvp.Key] = kvp.Value;
+            }
+
+            return dictionary;
+        }
+
         internal static TaskParameterEventArgs CreateTaskParameterEventArgs(
             BuildEventContext buildEventContext,
             TaskParameterMessageKind messageKind,