diff --git a/Humans.slnx b/Humans.slnx
index 5de636fea..6c3d9286d 100644
--- a/Humans.slnx
+++ b/Humans.slnx
@@ -2,6 +2,7 @@
   <Project Path="docs/docs.csproj" />
   <Folder Name="/src/">
     <Project Path="src/Humans.Analyzers/Humans.Analyzers.csproj" />
+    <Project Path="src/Humans.Interfaces/Humans.Interfaces.csproj" />
     <Project Path="src/Humans.Domain/Humans.Domain.csproj" />
     <Project Path="src/Humans.Application/Humans.Application.csproj" />
     <Project Path="src/Humans.Infrastructure/Humans.Infrastructure.csproj" />
diff --git a/src/Humans.Application/Humans.Application.csproj b/src/Humans.Application/Humans.Application.csproj
index c8cd9fe35..f0cb8851e 100644
--- a/src/Humans.Application/Humans.Application.csproj
+++ b/src/Humans.Application/Humans.Application.csproj
@@ -6,6 +6,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\Humans.Domain\Humans.Domain.csproj" />
+    <ProjectReference Include="..\Humans.Interfaces\Humans.Interfaces.csproj" />
   </ItemGroup>
 
   <ItemGroup>
diff --git a/src/Humans.Application/Architecture/DontFixAttribute.cs b/src/Humans.Interfaces/Architecture/DontFixAttribute.cs
similarity index 100%
rename from src/Humans.Application/Architecture/DontFixAttribute.cs
rename to src/Humans.Interfaces/Architecture/DontFixAttribute.cs
diff --git a/src/Humans.Application/Architecture/GrandfatheredAttribute.cs b/src/Humans.Interfaces/Architecture/GrandfatheredAttribute.cs
similarity index 100%
rename from src/Humans.Application/Architecture/GrandfatheredAttribute.cs
rename to src/Humans.Interfaces/Architecture/GrandfatheredAttribute.cs
diff --git a/src/Humans.Application/Architecture/SurfaceBudgetAttribute.cs b/src/Humans.Interfaces/Architecture/SurfaceBudgetAttribute.cs
similarity index 100%
rename from src/Humans.Application/Architecture/SurfaceBudgetAttribute.cs
rename to src/Humans.Interfaces/Architecture/SurfaceBudgetAttribute.cs
diff --git a/src/Humans.Interfaces/Humans.Interfaces.csproj b/src/Humans.Interfaces/Humans.Interfaces.csproj
new file mode 100644
index 000000000..3c9b59ea0
--- /dev/null
+++ b/src/Humans.Interfaces/Humans.Interfaces.csproj
@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    Lowest-level project: system marker interfaces and architecture attributes
+    only. No ProjectReferences, no NuGet packages. Namespaces intentionally
+    remain Humans.Application.* (namespace ≠ assembly) so call sites and the
+    Humans.Analyzers full-name constants are unaffected by the move.
+  -->
+
+</Project>
diff --git a/src/Humans.Application/Interfaces/IApplicationService.cs b/src/Humans.Interfaces/Interfaces/IApplicationService.cs
similarity index 100%
rename from src/Humans.Application/Interfaces/IApplicationService.cs
rename to src/Humans.Interfaces/Interfaces/IApplicationService.cs
diff --git a/src/Humans.Application/Interfaces/IFanout.cs b/src/Humans.Interfaces/Interfaces/IFanout.cs
similarity index 100%
rename from src/Humans.Application/Interfaces/IFanout.cs
rename to src/Humans.Interfaces/Interfaces/IFanout.cs
diff --git a/src/Humans.Application/Interfaces/IInvalidator.cs b/src/Humans.Interfaces/Interfaces/IInvalidator.cs
similarity index 100%
rename from src/Humans.Application/Interfaces/IInvalidator.cs
rename to src/Humans.Interfaces/Interfaces/IInvalidator.cs
diff --git a/src/Humans.Application/Interfaces/IOrchestrator.cs b/src/Humans.Interfaces/Interfaces/IOrchestrator.cs
similarity index 100%
rename from src/Humans.Application/Interfaces/IOrchestrator.cs
rename to src/Humans.Interfaces/Interfaces/IOrchestrator.cs
diff --git a/src/Humans.Application/Interfaces/Repositories/IRepository.cs b/src/Humans.Interfaces/Interfaces/Repositories/IRepository.cs
similarity index 100%
rename from src/Humans.Application/Interfaces/Repositories/IRepository.cs
rename to src/Humans.Interfaces/Interfaces/Repositories/IRepository.cs
diff --git a/tests/Humans.Application.Tests/Architecture/ServiceBoundaryArchitectureTests.cs b/tests/Humans.Application.Tests/Architecture/ServiceBoundaryArchitectureTests.cs
index a99daf81a..9857fe3d5 100644
--- a/tests/Humans.Application.Tests/Architecture/ServiceBoundaryArchitectureTests.cs
+++ b/tests/Humans.Application.Tests/Architecture/ServiceBoundaryArchitectureTests.cs
@@ -138,8 +138,11 @@ internal static IEnumerable<string> ScanApplicationServiceEntityReadReturns()
         }
     }
 
+    // Anchored to a type that lives in Humans.Application — the marker
+    // interfaces (IApplicationService, IRepository, …) moved to the
+    // Humans.Interfaces assembly, keeping their namespaces.
     private static IEnumerable<Type> ApplicationInterfaceTypes() =>
-        typeof(IApplicationService).Assembly.GetTypes()
+        typeof(IUserRepository).Assembly.GetTypes()
             .Where(t => t.IsInterface)
             .Where(t => t.Namespace?.StartsWith("Humans.Application.Interfaces", StringComparison.Ordinal) == true);
 
diff --git a/tests/Humans.Application.Tests/Services/DependencyCycleResolutionTests.cs b/tests/Humans.Application.Tests/Services/DependencyCycleResolutionTests.cs
index eeb4ce82c..a82fbef38 100644
--- a/tests/Humans.Application.Tests/Services/DependencyCycleResolutionTests.cs
+++ b/tests/Humans.Application.Tests/Services/DependencyCycleResolutionTests.cs
@@ -176,7 +176,7 @@ public void NoCircularConstructorDependencies_AcrossApplicationServices()
     {
         var assemblies = new[]
         {
-            typeof(IApplicationService).Assembly,
+            typeof(UserService).Assembly,
             typeof(HumansDbContext).Assembly,
             typeof(Humans.Web.Controllers.HomeController).Assembly,
         };