diff --git a/colaflow-api/src/ColaFlow.API/Handlers/GlobalExceptionHandler.cs b/colaflow-api/src/ColaFlow.API/Handlers/GlobalExceptionHandler.cs
index f3532f2..b437fc1 100644
--- a/colaflow-api/src/ColaFlow.API/Handlers/GlobalExceptionHandler.cs
+++ b/colaflow-api/src/ColaFlow.API/Handlers/GlobalExceptionHandler.cs
@@ -2,6 +2,7 @@ using System.Diagnostics;
 using FluentValidation;
 using Microsoft.AspNetCore.Diagnostics;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
 using ColaFlow.Modules.ProjectManagement.Domain.Exceptions;
 
 namespace ColaFlow.API.Handlers;
@@ -25,7 +26,7 @@ public sealed class GlobalExceptionHandler : IExceptionHandler
         CancellationToken cancellationToken)
     {
         // Log with appropriate level based on exception type
-        if (exception is ValidationException or DomainException or NotFoundException)
+        if (exception is ValidationException or DomainException or NotFoundException or UnauthorizedAccessException or ArgumentException)
         {
             _logger.LogWarning(exception, "Client error occurred: {Message}", exception.Message);
         }
@@ -39,6 +40,10 @@ public sealed class GlobalExceptionHandler : IExceptionHandler
             ValidationException validationEx => CreateValidationProblemDetails(httpContext, validationEx),
             DomainException domainEx => CreateDomainProblemDetails(httpContext, domainEx),
             NotFoundException notFoundEx => CreateNotFoundProblemDetails(httpContext, notFoundEx),
+            UnauthorizedAccessException unauthorizedEx => CreateUnauthorizedProblemDetails(httpContext, unauthorizedEx),
+            ArgumentException argumentEx => CreateBadRequestProblemDetails(httpContext, argumentEx),
+            InvalidOperationException invalidOpEx => CreateBadRequestProblemDetails(httpContext, invalidOpEx),
+            DbUpdateException dbUpdateEx when IsDuplicateKeyViolation(dbUpdateEx) => CreateConflictProblemDetails(httpContext, dbUpdateEx),
             _ => CreateInternalServerErrorProblemDetails(httpContext, exception)
         };
 
@@ -48,6 +53,15 @@ public sealed class GlobalExceptionHandler : IExceptionHandler
         return true; // Exception handled
     }
 
+    private static bool IsDuplicateKeyViolation(DbUpdateException exception)
+    {
+        // Check for duplicate key violation in SQL Server or PostgreSQL
+        var innerException = exception.InnerException?.Message ?? string.Empty;
+        return innerException.Contains("duplicate key", StringComparison.OrdinalIgnoreCase) ||
+               innerException.Contains("unique constraint", StringComparison.OrdinalIgnoreCase) ||
+               innerException.Contains("IX_", StringComparison.OrdinalIgnoreCase);
+    }
+
     private static ProblemDetails CreateValidationProblemDetails(
         HttpContext context,
         ValidationException exception)
@@ -110,6 +124,60 @@ public sealed class GlobalExceptionHandler : IExceptionHandler
         };
     }
 
+    private static ProblemDetails CreateUnauthorizedProblemDetails(
+        HttpContext context,
+        UnauthorizedAccessException exception)
+    {
+        return new ProblemDetails
+        {
+            Type = "https://tools.ietf.org/html/rfc7235#section-3.1",
+            Title = "Unauthorized",
+            Status = StatusCodes.Status401Unauthorized,
+            Detail = exception.Message,
+            Instance = context.Request.Path,
+            Extensions =
+            {
+                ["traceId"] = Activity.Current?.Id ?? context.TraceIdentifier
+            }
+        };
+    }
+
+    private static ProblemDetails CreateBadRequestProblemDetails(
+        HttpContext context,
+        Exception exception)
+    {
+        return new ProblemDetails
+        {
+            Type = "https://tools.ietf.org/html/rfc7231#section-6.5.1",
+            Title = "Bad Request",
+            Status = StatusCodes.Status400BadRequest,
+            Detail = exception.Message,
+            Instance = context.Request.Path,
+            Extensions =
+            {
+                ["traceId"] = Activity.Current?.Id ?? context.TraceIdentifier
+            }
+        };
+    }
+
+    private static ProblemDetails CreateConflictProblemDetails(
+        HttpContext context,
+        DbUpdateException exception)
+    {
+        return new ProblemDetails
+        {
+            Type = "https://tools.ietf.org/html/rfc7231#section-6.5.8",
+            Title = "Conflict",
+            Status = StatusCodes.Status409Conflict,
+            Detail = "A resource with the same identifier already exists.",
+            Instance = context.Request.Path,
+            Extensions =
+            {
+                ["traceId"] = Activity.Current?.Id ?? context.TraceIdentifier
+            }
+        };
+    }
+
     private static ProblemDetails CreateInternalServerErrorProblemDetails(
         HttpContext context,
         Exception exception)
diff --git a/colaflow-api/tests/Modules/Identity/ColaFlow.Modules.Identity.IntegrationTests/Infrastructure/DatabaseFixture.cs b/colaflow-api/tests/Modules/Identity/ColaFlow.Modules.Identity.IntegrationTests/Infrastructure/DatabaseFixture.cs
index 300d018..9a7986f 100644
--- a/colaflow-api/tests/Modules/Identity/ColaFlow.Modules.Identity.IntegrationTests/Infrastructure/DatabaseFixture.cs
+++ b/colaflow-api/tests/Modules/Identity/ColaFlow.Modules.Identity.IntegrationTests/Infrastructure/DatabaseFixture.cs
@@ -8,18 +8,28 @@ namespace ColaFlow.Modules.Identity.IntegrationTests.Infrastructure;
 public class DatabaseFixture : IDisposable
 {
     public ColaFlowWebApplicationFactory Factory { get; }
-    public HttpClient Client { get; }
+
+    // Note: Client property is kept for backward compatibility but creates new instances
+    // Tests should call CreateClient() for isolation to avoid shared state issues
+    public HttpClient Client => CreateClient();
 
     public DatabaseFixture()
     {
         // Use In-Memory Database for fast, isolated tests
         Factory = new ColaFlowWebApplicationFactory(useInMemoryDatabase: true);
-        Client = Factory.CreateClient();
+    }
+
+    /// <summary>
+    /// Creates a new HttpClient for each test to ensure test isolation
+    /// Prevents Authorization header sharing between tests
+    /// </summary>
+    public HttpClient CreateClient()
+    {
+        return Factory.CreateClient();
     }
 
     public void Dispose()
     {
-        Client?.Dispose();
         Factory?.Dispose();
         GC.SuppressFinalize(this);
     }