diff --git a/Api.SeaHavenIndustries.Tests/VendorCompanyRosterControllerTests.cs b/Api.SeaHavenIndustries.Tests/VendorCompanyRosterControllerTests.cs
index 5542289..b10a37d 100644
--- a/Api.SeaHavenIndustries.Tests/VendorCompanyRosterControllerTests.cs
+++ b/Api.SeaHavenIndustries.Tests/VendorCompanyRosterControllerTests.cs
@@ -133,6 +133,28 @@ public class VendorCompanyRosterControllerTests
         result.Should().BeOfType<BadRequestObjectResult>();
     }
 
+    [Fact]
+    public async Task Create_DuplicateName_ReturnsStable409()
+    {
+        var service = new Mock<IVendorCompanyRosterService>();
+        service.Setup(x => x.CreateRosterAsync(It.IsAny<CreateVendorRosterDTO>(), "42", It.IsAny<CancellationToken>()))
+            .ThrowsAsync(new VendorRosterDuplicateNameException(
+                "database detail",
+                new InvalidOperationException("IX_VendorCompanies_NormalizedName")));
+
+        var result = await NewController(service).Create(
+            new CreateVendorRosterDTO { Name = "Acme", Email = "acme@example.com" },
+            CancellationToken.None);
+
+        var conflict = result.Should().BeOfType<ConflictObjectResult>().Subject;
+        conflict.StatusCode.Should().Be(StatusCodes.Status409Conflict);
+        var json = System.Text.Json.JsonSerializer.Serialize(conflict.Value);
+        json.Should().Contain("duplicate_vendor_company_name");
+        json.Should().Contain("Another vendor company already uses that name.");
+        json.Should().NotContain("IX_VendorCompanies_NormalizedName");
+        json.Should().NotContain("database detail");
+    }
+
     [Fact]
     public async Task Reconcile_Unauthenticated_Returns401()
     {
diff --git a/Api.SeaHavenIndustries.Tests/VendorCompanyRosterDataServiceTests.cs b/Api.SeaHavenIndustries.Tests/VendorCompanyRosterDataServiceTests.cs
index 2997c41..6114c31 100644
--- a/Api.SeaHavenIndustries.Tests/VendorCompanyRosterDataServiceTests.cs
+++ b/Api.SeaHavenIndustries.Tests/VendorCompanyRosterDataServiceTests.cs
@@ -74,6 +74,26 @@ public class VendorCompanyRosterDataServiceTests
         context.Vendors.Where(v => v.CompanyId == company.Id).Should().OnlyContain(v => v.CompanyName == "Atomic Co");
     }
 
+    [Fact]
+    public async Task CreateRoster_DuplicateNormalizedName_ThrowsWithoutPersisting()
+    {
+        var dbName = Guid.NewGuid().ToString();
+        using var context = NewContext(dbName);
+        await SeedCompanyWithTechnicianAsync(context, "Eletrizante", "Existing");
+        var service = new VendorCompanyRosterDataService(context);
+
+        var call = () => service.CreateRosterAsync(new VendorCompanyRosterWriteModel
+        {
+            Name = "  ELETRIZANTE  ",
+            Email = "duplicate@example.com",
+            ActorUserId = "42"
+        }, CancellationToken.None);
+
+        await call.Should().ThrowAsync<VendorRosterDuplicateNameException>();
+        context.VendorCompanies.Should().ContainSingle();
+        context.Vendors.Should().ContainSingle();
+    }
+
     [Fact]
     public async Task GetRoster_ByVendorId_ResolvesCompanyAndAllTechnicians()
     {
diff --git a/Api.SeaHavenIndustries/Controllers/VendorCompanyRosterController.cs b/Api.SeaHavenIndustries/Controllers/VendorCompanyRosterController.cs
index 8180b3b..fa5e4fa 100644
--- a/Api.SeaHavenIndustries/Controllers/VendorCompanyRosterController.cs
+++ b/Api.SeaHavenIndustries/Controllers/VendorCompanyRosterController.cs
@@ -82,6 +82,15 @@ namespace Api.SeaHavenIndustries.Controllers
             {
                 return Unauthorized(new Response { Status = "Error", Message = "User not authenticated" });
             }
+            catch (VendorRosterDuplicateNameException dup)
+            {
+                return Conflict(new
+                {
+                    Status = "Conflict",
+                    Message = _logger.Sanitize(dup, "Another vendor company already uses that name."),
+                    Code = "duplicate_vendor_company_name"
+                });
+            }
             catch (Exception ex)
             {
                 return StatusCode(500, new Response { Status = "Error", Message = _logger.Sanitize(ex) });
diff --git a/SeaHaven.DataServices/Implementation/VendorCompanyRosterDataService.cs b/SeaHaven.DataServices/Implementation/VendorCompanyRosterDataService.cs
index 2a23676..aacdf9c 100644
--- a/SeaHaven.DataServices/Implementation/VendorCompanyRosterDataService.cs
+++ b/SeaHaven.DataServices/Implementation/VendorCompanyRosterDataService.cs
@@ -219,11 +219,24 @@ namespace SeaHaven.DataServices.Implementation
         {
             var now = DateTime.UtcNow;
             var trimmedName = roster.Name.Trim();
+            var normalizedName = trimmedName.ToLowerInvariant();
+
+            // NormalizedName has an unfiltered unique index, so deleted companies still
+            // reserve their names. Detect the ordinary collision before tracking a new
+            // aggregate; the save guard below covers the concurrent-create race.
+            var nameTaken = await _context.VendorCompanies
+                .AnyAsync(company => company.NormalizedName == normalizedName, cancellationToken);
+
+            if (nameTaken)
+                throw new VendorRosterDuplicateNameException(
+                    "Another vendor company already uses that name.",
+                    new InvalidOperationException(
+                        $"NormalizedName '{normalizedName}' is already in use."));
 
             var company = new VendorCompany
             {
                 Name = trimmedName,
-                NormalizedName = trimmedName.ToLowerInvariant(),
+                NormalizedName = normalizedName,
                 CompanyPhone = roster.CompanyPhone,
                 Email = roster.Email,
                 Address = roster.Address,
@@ -263,12 +276,28 @@ namespace SeaHaven.DataServices.Implementation
                 await _context.Vendors.AddAsync(vendor, cancellationToken);
             }
 
-            await _context.SaveChangesAsync(cancellationToken);
+            try
+            {
+                await _context.SaveChangesAsync(cancellationToken);
+            }
+            catch (DbUpdateException ex) when (IsUniqueConstraintViolation(ex))
+            {
+                throw new VendorRosterDuplicateNameException(
+                    "Another vendor company already uses that name.",
+                    ex);
+            }
 
             return await GetRosterAsync(null, company.Id, cancellationToken)
                 ?? throw new InvalidOperationException("Roster could not be reloaded after create.");
         }
 
+        private static bool IsUniqueConstraintViolation(DbUpdateException ex)
+        {
+            var message = ex.InnerException?.Message ?? ex.Message;
+            return message.Contains("duplicate", StringComparison.OrdinalIgnoreCase)
+                || message.Contains("UNIQUE", StringComparison.OrdinalIgnoreCase);
+        }
+
         public async Task<VendorCompanyRosterReadModel> AddTechniciansAsync(
             VendorCompanyRosterAddWriteModel roster,
             CancellationToken cancellationToken)