@@ -194,6 +194,15 @@ public void DownloadFile(string address, string fileName)
             if (!DownloadFiles.Contains((address, fileName)))
                 throw new ArgumentException($"Missing DownloadFile, {address}, {fileName}");
         }
+
+        public IDiagnosticsWriter CreateDiagnosticsWriter(string filename) => new TestDiagnosticWriter();
+    }
+
+    internal class TestDiagnosticWriter : IDiagnosticsWriter
+    {
+        public IList<DiagnosticMessage> Diagnostics { get; } = new List<DiagnosticMessage>();
+
+        public void AddEntry(DiagnosticMessage message) => this.Diagnostics.Add(message);
     }
 
     /// <summary>
@@ -253,7 +262,7 @@ CppAutobuilder CreateAutoBuilder(bool isWindows,
             Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_TRAP_DIR"] = "";
             Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_SOURCE_ARCHIVE_DIR"] = "";
             Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_ROOT"] = $@"C:\codeql\{codeqlUpperLanguage.ToLowerInvariant()}";
-            Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_DIAGNOSTIC_DIR"] = Path.GetTempPath();
+            Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_DIAGNOSTIC_DIR"] = "";
             Actions.GetEnvironmentVariable["CODEQL_JAVA_HOME"] = @"C:\codeql\tools\java";
             Actions.GetEnvironmentVariable["CODEQL_PLATFORM"] = "win64";
             Actions.GetEnvironmentVariable["SEMMLE_DIST"] = @"C:\odasa";
 
@@ -210,6 +210,16 @@ public void DownloadFile(string address, string fileName)
             if (!DownloadFiles.Contains((address, fileName)))
                 throw new ArgumentException($"Missing DownloadFile, {address}, {fileName}");
         }
+
+
+        public IDiagnosticsWriter CreateDiagnosticsWriter(string filename) => new TestDiagnosticWriter();
+    }
+
+    internal class TestDiagnosticWriter : IDiagnosticsWriter
+    {
+        public IList<DiagnosticMessage> Diagnostics { get; } = new List<DiagnosticMessage>();
+
+        public void AddEntry(DiagnosticMessage message) => this.Diagnostics.Add(message);
     }
 
     /// <summary>
@@ -401,7 +411,7 @@ private CSharpAutobuilder CreateAutoBuilder(bool isWindows,
             actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_TRAP_DIR"] = "";
             actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_SOURCE_ARCHIVE_DIR"] = "";
             actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_ROOT"] = $@"C:\codeql\{codeqlUpperLanguage.ToLowerInvariant()}";
-            actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_DIAGNOSTIC_DIR"] = Path.GetTempPath();
+            actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_DIAGNOSTIC_DIR"] = "";
             actions.GetEnvironmentVariable["CODEQL_JAVA_HOME"] = @"C:\codeql\tools\java";
             actions.GetEnvironmentVariable["CODEQL_PLATFORM"] = isWindows ? "win64" : "linux64";
             actions.GetEnvironmentVariable["LGTM_INDEX_VSTOOLS_VERSION"] = vsToolsVersion;
 
@@ -241,7 +241,7 @@ protected Autobuilder(IBuildActions actions, TAutobuildOptions options, Diagnost
             SourceArchiveDir = RequireEnvironmentVariable(EnvVars.SourceArchiveDir(this.Options.Language));
             DiagnosticsDir = RequireEnvironmentVariable(EnvVars.DiagnosticDir(this.Options.Language));
 
-            this.diagnostics = new DiagnosticsStream(Path.Combine(DiagnosticsDir, $"autobuilder-{DateTime.UtcNow:yyyyMMddHHmm}.jsonc"));
+            this.diagnostics = actions.CreateDiagnosticsWriter(Path.Combine(DiagnosticsDir, $"autobuilder-{DateTime.UtcNow:yyyyMMddHHmm}.jsonc"));
         }
 
         /// <summary>
@@ -269,7 +269,7 @@ protected string RequireEnvironmentVariable(string name)
 
         private readonly ILogger logger = new ConsoleLogger(Verbosity.Info);
 
-        private readonly DiagnosticsStream diagnostics;
+        private readonly IDiagnosticsWriter diagnostics;
 
         /// <summary>
         /// Makes <see cref="path" /> relative to the root source directory.
 
@@ -167,6 +167,17 @@ public interface IBuildActions
         /// Downloads the resource with the specified URI to a local file.
         /// </summary>
         void DownloadFile(string address, string fileName);
+
+        /// <summary>
+        /// Creates an <see cref="IDiagnosticsWriter" /> for the given <paramref name="filename" />.
+        /// </summary>
+        /// <param name="filename">
+        /// The path suggesting where the diagnostics should be written to.
+        /// </param>
+        /// <returns>
+        /// A <see cref="IDiagnosticsWriter" /> to which diagnostic entries can be added.
+        /// </returns>
+        IDiagnosticsWriter CreateDiagnosticsWriter(string filename);
     }
 
     /// <summary>
@@ -288,6 +299,8 @@ private static async Task DownloadFileAsync(string address, string filename)
         public void DownloadFile(string address, string fileName) =>
             DownloadFileAsync(address, fileName).Wait();
 
+        public IDiagnosticsWriter CreateDiagnosticsWriter(string filename) => new DiagnosticsStream(filename);
+
         public static IBuildActions Instance { get; } = new SystemBuildActions();
     }
 }
@@ -174,11 +174,23 @@ public DiagnosticMessage(
         }
     }
 
+    /// <summary>
+    /// Provides the ability to write diagnostic messages to some output.
+    /// </summary>
+    public interface IDiagnosticsWriter
+    {
+        /// <summary>
+        /// Adds <paramref name="message" /> as a new diagnostics entry.
+        /// </summary>
+        /// <param name="message">The diagnostics entry to add.</param>
+        void AddEntry(DiagnosticMessage message);
+    }
+
     /// <summary>
     /// A wrapper around an underlying <see cref="StreamWriter" /> which allows
     /// <see cref="DiagnosticMessage" /> objects to be serialized to it.
     /// </summary>
-    public sealed class DiagnosticsStream : IDisposable
+    public sealed class DiagnosticsStream : IDiagnosticsWriter, IDisposable
     {
         private readonly JsonSerializer serializer;
         private readonly StreamWriter writer;