diff --git a/include/numsim-materials/umat/umat_interface.h b/include/numsim-materials/umat/umat_interface.h
index b10ce03..3e1859c 100644
--- a/include/numsim-materials/umat/umat_interface.h
+++ b/include/numsim-materials/umat/umat_interface.h
@@ -207,10 +207,9 @@ class umat_registry {
     std::unique_ptr<context_type> ctx;
     std::unique_ptr<evaluator_type> solid;
     std::unique_ptr<ps_evaluator_type> ps;
-    /// How many constants the context was built from. The graph is built once
-    /// and reused, so a later call arriving with a different count would mean
-    /// the cached parameters no longer describe this material.
-    std::size_t nprops{0};
+    /// What the context was built from; the graph is reused, so different
+    /// constants on a later call would no longer describe this material.
+    std::vector<double> props;
   };
 
   static std::unordered_map<std::string, thread_state, transparent_string_hash,
@@ -229,17 +228,26 @@ class umat_registry {
     const auto key = normalise_cmname(cmname.data(), cmname.size(), buf);
     auto& cache = thread_cache();
     if (auto it = cache.find(key); it != cache.end()) {
-      // PROPS cannot vary for a given material name — two *MATERIAL blocks must
-      // have distinct names — so a changed count means the deck contradicts the
-      // cached graph. Checking the size is one comparison; checking the values
-      // is not worth it per integration point.
-      if (it->second.nprops != props.size())
+      // PROPS cannot vary for one material name, so anything different
+      // contradicts the graph the constants were baked into. Values, not just
+      // the count: same length with different numbers is the case that reaches
+      // a material. Two faults, two messages — the check only ever explains.
+      if (it->second.props.size() != props.size())
         throw fatal_error(
-            "numsim UMAT: material '" + std::string(key) +
-            "' was built from " + std::to_string(it->second.nprops) +
-            " constants but this call supplies " +
-            std::to_string(props.size()) +
-            " — PROPS must be constant for a given material name");
+            "numsim UMAT: material '" + std::string(key) + "' was built from " +
+            std::to_string(it->second.props.size()) +
+            " constants but this call supplies " + std::to_string(props.size()) +
+            " — NPROPS cannot vary for a given material name");
+
+      for (std::size_t i = 0; i < props.size(); ++i)
+        if (it->second.props[i] != props[i])
+          throw fatal_error(
+              "numsim UMAT: material '" + std::string(key) + "' constant " +
+              std::to_string(i + 1) + " was baked into the graph as " +
+              std::to_string(it->second.props[i]) + " but this call supplies " +
+              std::to_string(props[i]) +
+              " — PROPS must be constant for a given material name; use "
+              "distinct *MATERIAL names for distinct constants");
       return it->second;
     }
 
@@ -269,7 +277,7 @@ class umat_registry {
       thread_state ts;
       ts.ctx = std::make_unique<context_type>();
       m.build(*ts.ctx, props);
-      ts.nprops = props.size();
+      ts.props.assign(props.begin(), props.end());
       if (!ts.ctx->is_finalized())
         throw fatal_error(
             "the builder returned without calling finalize() on the context");
diff --git a/tests/test_umat_interface.cpp b/tests/test_umat_interface.cpp
index 72dafc2..5f7078c 100644
--- a/tests/test_umat_interface.cpp
+++ b/tests/test_umat_interface.cpp
@@ -166,6 +166,9 @@ struct Registration {
     // against an already-built name the NPROPS-consistency check fires first
     // and require_props is never reached.
     registry::instance().register_model("COLDNAME", build_deck_elastic, de);
+    // One test only: it warms the cache itself, so a shared name would let
+    // test order decide the outcome.
+    registry::instance().register_model("VALUEPROBE", build_deck_elastic, de);
 
     // Deliberately lower-case, to prove the registry folds case on both sides.
     registry::config lc;
@@ -765,7 +768,43 @@ TEST(UmatInterface, ChangingNpropsForTheSameNameIsFatal) {
   call_umat("STIFF", stress, statev.data(), ddsdde, stran, dstran, 0.0, 0.1, 3,
             3, 6, 0, &pnewdt, nullptr, nullptr, nullptr, nullptr, three, 3);
   EXPECT_EQ(FatalProbe::count, 1);
-  EXPECT_NE(FatalProbe::last.find("constant"), std::string::npos)
+  // Both counts, so NPROPS=3 is not left to be matched against one number.
+  EXPECT_NE(FatalProbe::last.find("2 constants"), std::string::npos)
+      << FatalProbe::last;
+  EXPECT_NE(FatalProbe::last.find("supplies 3"), std::string::npos)
+      << FatalProbe::last;
+}
+
+/// Same count, different numbers — the case that reaches a material. A count
+/// check accepts it and serves the first call's stiffness forever: a converged
+/// analysis with the wrong moduli.
+TEST(UmatInterface, ChangingPropsValuesForTheSameNameIsFatal) {
+  FatalProbe probe;
+  std::vector<T> statev(1, 0.0);
+  const T stran[6] = {0, 0, 0, 0, 0, 0};
+  const T dstran[6] = {0.001, 0, 0, 0, 0, 0};
+  T stress[6] = {0}, ddsdde[36] = {0}, pnewdt = 1.0;
+  const T soft[2] = {100.0, 40.0};
+  const T stiff[2] = {300.0, 140.0};
+
+  // Builds the context, and shows which constants it was built from.
+  call_umat("VALUEPROBE", stress, statev.data(), ddsdde, stran, dstran, 0.0, 0.1,
+            3, 3, 6, 0, &pnewdt, nullptr, nullptr, nullptr, nullptr, soft, 2);
+  ASSERT_EQ(FatalProbe::count, 0);
+  ASSERT_NEAR(ddsdde[0], 100.0 + 4.0 * 40.0 / 3.0, 1e-9);
+
+  // Same name, same count, different values.
+  call_umat("VALUEPROBE", stress, statev.data(), ddsdde, stran, dstran, 0.0, 0.1,
+            3, 3, 6, 0, &pnewdt, nullptr, nullptr, nullptr, nullptr, stiff, 2);
+  EXPECT_EQ(FatalProbe::count, 1)
+      << "a same-length PROPS with different numbers must be reported, not "
+         "silently served from the cached graph";
+  // Must name which constant disagrees, and both values.
+  EXPECT_NE(FatalProbe::last.find("constant 1"), std::string::npos)
+      << FatalProbe::last;
+  EXPECT_NE(FatalProbe::last.find("100.0"), std::string::npos)
+      << FatalProbe::last;
+  EXPECT_NE(FatalProbe::last.find("300.0"), std::string::npos)
       << FatalProbe::last;
 }