From c49bc7c7c0306dc194acf649a9c99243478fa1da Mon Sep 17 00:00:00 2001
From: Alessio Quaglino <quaglino@google.com>
Date: Wed, 25 Mar 2026 06:49:43 -0700
Subject: [PATCH] Warn when a non-rigid flex has no constraints or passive
 forces.

Add a warning in user_model.cc if a flex is not rigid and has no edge equality constraints or passive forces (edge stiffness, edge damping, or vertex damping). Such flexes would not affect the simulation. A test case is added to verify the warning.

PiperOrigin-RevId: 889215981
Change-Id: I481128e1456921b632719547c676d201fdfbe931
---
 src/user/user_model.cc      |  6 ++++++
 test/user/user_flex_test.cc | 17 +++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index 7e95353a..2af21381 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -3538,6 +3538,12 @@ void mjCModel::CopyObjects(mjModel* m) {
       }
     }
 
+    if (!pfl->rigid && m->flex_edgeequality[i] == 0 &&
+        !pfl->edgestiffness && !pfl->edgedamping && !pfl->damping) {
+      mju_warning("flex '%s' is not rigid and has no equality constraints "
+                  "or passive forces", pfl->name.c_str());
+    }
+
     // copy bvh data (flex aabb computed dynamically in mjData)
     if (pfl->tree.Nbvh()) {
       memcpy(m->bvh_child + 2*bvh_adr, pfl->tree.Child().data(), 2*pfl->tree.Nbvh()*sizeof(int));
diff --git a/test/user/user_flex_test.cc b/test/user/user_flex_test.cc
index 457ea0af..017bb320 100644
--- a/test/user/user_flex_test.cc
+++ b/test/user/user_flex_test.cc
@@ -991,5 +991,22 @@ TEST_F(UserFlexTest, FlexAttachConstraintPreserved) {
   mj_deleteVFS(vfs.get());
 }
 
+TEST_F(UserFlexTest, FlexNoConstraintsWarning) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+  <worldbody>
+    <flexcomp name="test" type="grid" count="4 4 1" spacing=".2 .2 .2"
+              dim="2" radius=".1"/>
+  </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  EXPECT_THAT(error.data(),
+              HasSubstr("no equality constraints or passive forces"));
+  mj_deleteModel(m);
+}
+
 }  // namespace
 }  // namespace mujoco