diff --git a/src/LiquidCode.Tester.Gateway/Services/WorkerClientService.cs b/src/LiquidCode.Tester.Gateway/Services/WorkerClientService.cs
index b37b2fe..18ee4a6 100644
--- a/src/LiquidCode.Tester.Gateway/Services/WorkerClientService.cs
+++ b/src/LiquidCode.Tester.Gateway/Services/WorkerClientService.cs
@@ -35,7 +35,7 @@ public class WorkerClientService : IWorkerClientService
             form.Add(new StringContent(submit.MissionId.ToString()), "MissionId");
             form.Add(new StringContent(submit.Language), "Language");
             form.Add(new StringContent(submit.LanguageVersion), "LanguageVersion");
-            form.Add(new StringContent(submit.SourceCode), "SourceCode");
+            form.Add(new StringContent(submit.SourceCode, System.Text.Encoding.UTF8, "text/plain"), "SourceCode");
             form.Add(new StringContent(submit.CallbackUrl), "CallbackUrl");
 
             // Add package file
diff --git a/src/LiquidCode.Tester.Worker/Services/CSharpCompilationServiceIsolate.cs b/src/LiquidCode.Tester.Worker/Services/CSharpCompilationServiceIsolate.cs
index 8f82480..e1343c3 100644
--- a/src/LiquidCode.Tester.Worker/Services/CSharpCompilationServiceIsolate.cs
+++ b/src/LiquidCode.Tester.Worker/Services/CSharpCompilationServiceIsolate.cs
@@ -103,7 +103,10 @@ public class CSharpCompilationServiceIsolate : ICompilationService
                 DirectoryBindings = new List<DirectoryBinding>
                 {
                     new DirectoryBinding { HostPath = "/usr/lib", SandboxPath = "/usr/lib", ReadOnly = true },
-                    new DirectoryBinding { HostPath = "/lib", SandboxPath = "/lib", ReadOnly = true }
+                    new DirectoryBinding { HostPath = "/lib", SandboxPath = "/lib", ReadOnly = true },
+                    new DirectoryBinding { HostPath = "/usr/bin", SandboxPath = "/usr/bin", ReadOnly = true },
+                    new DirectoryBinding { HostPath = "/usr/share", SandboxPath = "/usr/share", ReadOnly = true },
+                    new DirectoryBinding { HostPath = "/etc/mono", SandboxPath = "/etc/mono", ReadOnly = true }
                 }
             });
 
diff --git a/src/LiquidCode.Tester.Worker/Services/CSharpExecutionServiceIsolate.cs b/src/LiquidCode.Tester.Worker/Services/CSharpExecutionServiceIsolate.cs
index 3261ea9..47101ee 100644
--- a/src/LiquidCode.Tester.Worker/Services/CSharpExecutionServiceIsolate.cs
+++ b/src/LiquidCode.Tester.Worker/Services/CSharpExecutionServiceIsolate.cs
@@ -1,5 +1,6 @@
 using System.Diagnostics;
 using LiquidCode.Tester.Worker.Services.Isolate;
+using System.Collections.Generic;
 
 namespace LiquidCode.Tester.Worker.Services;
 
@@ -63,6 +64,7 @@ public class CSharpExecutionServiceIsolate : IExecutionService
 
             // Prepare input/output files inside the sandbox
             var outputFilePath = Path.Combine(boxDir, "output.txt");
+            var stderrFilePath = Path.Combine(boxDir, "stderr.txt");
             string? sandboxInputPath = null;
 
             if (!string.IsNullOrEmpty(inputFilePath) && File.Exists(inputFilePath))
@@ -71,20 +73,30 @@ public class CSharpExecutionServiceIsolate : IExecutionService
                 File.Copy(inputFilePath, sandboxInputPath, overwrite: true);
             }
 
-            // Run in Isolate
+            // Run in Isolate using mono runtime
             var isolateResult = await _isolateService.RunAsync(new IsolateRunOptions
             {
                 BoxId = boxId,
-                Executable = $"/box/{executableName}",
+                Executable = "/usr/bin/mono",
+                Arguments = new[] { $"/box/{executableName}" },
                 TimeLimitSeconds = timeLimitMs / 1000.0,
                 WallTimeLimitSeconds = (timeLimitMs / 1000.0) * 2,
                 MemoryLimitKb = memoryLimitMb * 1024,
                 StackLimitKb = 256 * 1024,
-                ProcessLimit = 1, // Single process for C#
+                ProcessLimit = 64, // Mono may create multiple threads/processes
                 EnableNetwork = false,
                 StdinFile = sandboxInputPath,
                 StdoutFile = outputFilePath,
-                WorkingDirectory = "/box"
+                StderrFile = stderrFilePath,
+                WorkingDirectory = "/box",
+                DirectoryBindings = new List<DirectoryBinding>
+                {
+                    new DirectoryBinding { HostPath = "/usr/lib", SandboxPath = "/usr/lib", ReadOnly = true },
+                    new DirectoryBinding { HostPath = "/lib", SandboxPath = "/lib", ReadOnly = true },
+                    new DirectoryBinding { HostPath = "/usr/share", SandboxPath = "/usr/share", ReadOnly = true },
+                    new DirectoryBinding { HostPath = "/etc/mono", SandboxPath = "/etc/mono", ReadOnly = true },
+                    new DirectoryBinding { HostPath = "/usr/bin", SandboxPath = "/usr/bin", ReadOnly = true }
+                }
             });
 
             stopwatch.Stop();
@@ -95,10 +107,19 @@ public class CSharpExecutionServiceIsolate : IExecutionService
                 result.Output = await File.ReadAllTextAsync(outputFilePath);
             }
 
+            // Read stderr
+            var stderrContent = string.Empty;
+            if (File.Exists(stderrFilePath))
+            {
+                stderrContent = await File.ReadAllTextAsync(stderrFilePath);
+            }
+
             // Map Isolate result to ExecutionResult
             result.ExecutionTimeMs = (long)(isolateResult.CpuTimeSeconds * 1000);
             result.MemoryUsedMb = isolateResult.MemoryUsedKb / 1024;
-            result.ErrorOutput = isolateResult.ErrorOutput;
+            result.ErrorOutput = string.IsNullOrEmpty(stderrContent)
+                ? isolateResult.ErrorOutput
+                : $"{stderrContent}\n{isolateResult.ErrorOutput}".Trim();
             result.ExitCode = isolateResult.ExitCode;
 
             if (isolateResult.TimeLimitExceeded)