diff --git a/test/runtests.jl b/test/runtests.jl
index e1c75c2..1d044c8 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -1,12 +1,63 @@
-# This file is a part of JuliaFEM.
-# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
 
 using JuliaFEM, Test
 
+# The active test tree mirrors `src/`. Every subdirectory below has its
+# own `runtests.jl` that bundles the topic into a single `@testset`.
+# Adding a new topic = adding the directory to `topics` below.
+const TOPICS = [
+    "basis",
+    "fields",
+    "physics",
+    "elements",
+    "materials",
+    "topology",
+    "geometry",
+    "mesh",
+    "quadrature",
+    "domains/continuum",
+    "domains/heat",
+    "domains/darcy",
+    "domains/thermo_elastic",
+    "dofs",
+    "interface",
+    "assemblers",
+    "sparse",
+    "io",
+    "validation",
+    "reference",
+    "docs",
+]
+
+# If ARGS has no positional topic names, run every entry in `TOPICS`.
+# Otherwise run only listed topics (in `TOPICS` order). Unknown names warn;
+# if none match, run the full suite with a warning.
+#
+# `Pkg.test(; test_args=["assemblers", ...])` forwards those strings to ARGS.
+function _topics_from_args()
+    requested = String[s for s in ARGS if !startswith(s, '-')]
+    isempty(requested) && return TOPICS
+    known = Set(TOPICS)
+    unknown = filter(!in(known), requested)
+    if !isempty(unknown)
+        @warn "Unknown test topics (skipped): $(collect(unknown))"
+    end
+    want = Set(requested)
+    chosen = filter(in(want), TOPICS)
+    if isempty(chosen)
+        @warn "No valid topics in ARGS $(requested); running full suite"
+        return TOPICS
+    end
+    return chosen
+end
+
 @testset "JuliaFEM.jl" begin
     @testset "Package loads" begin
         @test isdefined(JuliaFEM, :AbstractBasis)
         @test isdefined(JuliaFEM, :AbstractMesh)
+        @test isdefined(JuliaFEM, :AbstractInterfaceMesh)
+        @test isdefined(JuliaFEM, :InterfaceMesh)
         @test isdefined(JuliaFEM, :AbstractTopology)
         @test isdefined(JuliaFEM, :AbstractMaterial)
         @test isdefined(JuliaFEM, :AbstractPhysics)
@@ -15,9 +66,9 @@ using JuliaFEM, Test
     end
 
     @testset "Basic types instantiate" begin
-        # Test that basic types can be referenced
         @test AbstractBasis isa Type
         @test AbstractMesh isa Type
+        @test AbstractInterfaceMesh isa Type
         @test AbstractTopology isa Type
         @test AbstractMaterial isa Type
         @test AbstractPhysics isa Type
@@ -26,7 +77,6 @@ using JuliaFEM, Test
     end
 
     @testset "Concrete types exist" begin
-        # Test that concrete implementations exist
         @test Mesh isa Type
         @test LinearElastic isa Type
         @test Hex8 isa Type
@@ -34,56 +84,10 @@ using JuliaFEM, Test
         @test LocalField isa Type
     end
 
-    # Field tests
-    @testset "Fields" begin
-        include("fields/test_local_field.jl")
+    for topic in _topics_from_args()
+        include(joinpath(topic, "runtests.jl"))
     end
-
-    # Physics tests
-    @testset "Physics" begin
-        include("physics/test_strain.jl")
-    end
-
-    # Element interpolation tests
-    @testset "Element Interpolation" begin
-        include("elements/test_interpolate_local_fields.jl")
-    end
-
-    # Material tests
-    @testset "Materials" begin
-        include("materials/test_state_variables.jl")
-        include("materials/test_traits.jl")
-        include("materials/test_global_material_cache.jl")
-        include("materials/test_plasticity_integration.jl")
-        include("materials/test_cantilever_material_cache_zero_allocations.jl")
-        include("materials/test_assembly_workspace_refactor.jl")
-    end
-
-    # Topology tests
-    @testset "Topology" begin
-        include("topology/test_segments.jl")
-        include("topology/test_triangles.jl")
-        include("topology/test_quadrilaterals.jl")
-        include("topology/test_topology_entities_tetrahedron.jl")
-        include("topology/test_hexahedra.jl")
-        include("topology/test_pyramids.jl")
-        include("topology/test_wedges.jl")
-        # include("topology/test_topology_type_extraction.jl")  # DISABLED: obsolete API (entities don't carry topology)
-        include("topology/test_topological_invariance.jl")
-        include("topology/test_entity_dimensions.jl")
-    end
-
-    # Domain-specific tests
-    @testset "Continuum Domain" begin
-        include("domains/continuum/runtests.jl")
-    end
-
-    # Validation tests
-    include("validation/test_cantilever_regression.jl")
-    include("validation/test_plasticity_simple.jl")
 end
 
-# Note: All legacy tests have been moved to test/broken/
-# They need to be updated to work with the new modular architecture.
-# To run a specific test, use:
-#   include("test/broken/test_name.jl")
+# Tests that target the older / Legacy API or experimental specs live under
+# `llm/design/legacy-tests/` and are not picked up by this runner.