diff --git a/src/domain/materials/MaterialRegistry.cpp b/src/domain/materials/MaterialRegistry.cpp
index 757d247..32a9ce1 100644
--- a/src/domain/materials/MaterialRegistry.cpp
+++ b/src/domain/materials/MaterialRegistry.cpp
@@ -35,10 +35,15 @@ void MaterialRegistry::registerMaterialsFromJson(const std::string& jsonPath)
             return;
         }
 
+        std::cerr << "[MaterialRegistry] Parsed " << json.size()
+                  << " material entries from " << jsonPath << std::endl;
         for (const auto& item : json) {
             Material m = item.get<Material>();
+            std::cerr << "[MaterialRegistry] Registering material id=" << m.id
+                      << std::endl;
             registerMaterial(m.id, std::move(m));
         }
+        std::cerr << "[MaterialRegistry] Final count=" << count() << std::endl;
     } catch (const std::exception& e) {
         std::cerr << "[MaterialRegistry] Error parsing " << jsonPath
                   << ": " << e.what() << std::endl;
diff --git a/src/domain/physics/ParticleRegistry.cpp b/src/domain/physics/ParticleRegistry.cpp
index 185e680..5baf919 100644
--- a/src/domain/physics/ParticleRegistry.cpp
+++ b/src/domain/physics/ParticleRegistry.cpp
@@ -32,10 +32,15 @@ void ParticleRegistry::registerParticlesFromJson(const std::string& jsonPath)
                       << jsonPath << std::endl;
             return;
         }
+        std::cerr << "[ParticleRegistry] Parsed " << json.size()
+                  << " particle entries from " << jsonPath << std::endl;
         for (const auto& item : json) {
             Particle p = item.get<Particle>();
+            std::cerr << "[ParticleRegistry] Registering particle id=" << p.id
+                      << std::endl;
             registerParticle(p.id, std::move(p));
         }
+        std::cerr << "[ParticleRegistry] Final count=" << count() << std::endl;
     } catch (const std::exception& e) {
         std::cerr << "[ParticleRegistry] Error parsing " << jsonPath
                   << ": " << e.what() << std::endl;
diff --git a/tests/unit/domain/test_MaterialRegistry.cpp b/tests/unit/domain/test_MaterialRegistry.cpp
index 8e261e4..338cd7e 100644
--- a/tests/unit/domain/test_MaterialRegistry.cpp
+++ b/tests/unit/domain/test_MaterialRegistry.cpp
@@ -256,6 +256,14 @@ TEST_F(MaterialRegistryTest, LoadMaterialsFromJson)
         out.close();
     }
 
+    {
+        std::ifstream in(tmpPath);
+        std::string contents((std::istreambuf_iterator<char>(in)),
+                             std::istreambuf_iterator<char>());
+        std::cerr << "[TEST] Material JSON path=" << tmpPath
+                  << " contents=" << contents << std::endl;
+    }
+
     MaterialRegistry jsonReg;
     jsonReg.registerMaterialsFromJson(tmpPath);
 
diff --git a/tests/unit/domain/test_ParticleRegistry.cpp b/tests/unit/domain/test_ParticleRegistry.cpp
index ec51887..3131bd0 100644
--- a/tests/unit/domain/test_ParticleRegistry.cpp
+++ b/tests/unit/domain/test_ParticleRegistry.cpp
@@ -193,6 +193,14 @@ TEST_F(ParticleRegistryTest, LoadFromJson)
         out.close();
     }
 
+    {
+        std::ifstream in(tmpPath);
+        std::string contents((std::istreambuf_iterator<char>(in)),
+                             std::istreambuf_iterator<char>());
+        std::cerr << "[TEST] Particle JSON path=" << tmpPath
+                  << " contents=" << contents << std::endl;
+    }
+
     ParticleRegistry jsonReg;
     jsonReg.registerParticlesFromJson(tmpPath);