diff --git a/BeamAdapter_test/component/controller/InterventionalRadiologyController_test.cpp b/BeamAdapter_test/component/controller/InterventionalRadiologyController_test.cpp
index 580342fb2..1728e42a5 100644
--- a/BeamAdapter_test/component/controller/InterventionalRadiologyController_test.cpp
+++ b/BeamAdapter_test/component/controller/InterventionalRadiologyController_test.cpp
@@ -465,6 +465,20 @@ TEST_F(InterventionalRadiologyController_test, action_movesControlledInstrument)
     EXPECT_NEAR(ctrl->d_xTip.getValue()[0], x0Before, 1e-9);
 }
 
+/// A negative controlledInstrument must be rejected (bounds check), not used as a negative index.
+TEST_F(InterventionalRadiologyController_test, action_negativeControlledInstrument_isRejected)
+{
+    loadScene(singleInstrumentScene("controlledInstrument='-1'"));
+    auto* ctrl = getController();
+    ASSERT_NE(ctrl, nullptr);
+
+    const Real x0Before = ctrl->d_xTip.getValue()[0];
+
+    // applyAction returns early for an out-of-range (negative) instrument: no crash, no change.
+    ctrl->applyAction(BeamAdapterAction::MOVE_FORWARD);
+    EXPECT_NEAR(ctrl->d_xTip.getValue()[0], x0Before, 1e-9);
+}
+
 
 ///////////////////////////////////// Getters / helpers ///////////////////////////////////////////
 
@@ -595,4 +609,20 @@ TEST_F(InterventionalRadiologyController_test, deploy_runsStable)
     EXPECT_EQ(ctrl->getComponentState(), ComponentState::Valid);
 }
 
+/// A negative controlledInstrument during stepping must fall back to instrument 0 (no negative index).
+TEST_F(InterventionalRadiologyController_test, deploy_negativeControlledInstrument_fallsBackToZero)
+{
+    loadAndInitRoot(deploymentScene("xtip='0' step='0.5' speed='10' controlledInstrument='-1'"));
+
+    auto* ctrl = getController();
+    ASSERT_NE(ctrl, nullptr);
+    ASSERT_EQ(ctrl->getComponentState(), ComponentState::Valid);
+
+    for (int i = 0; i < 10; ++i)
+        sofa::simulation::node::animate(m_root.get(), 0.01);
+
+    // the step is applied to the fallback instrument 0 instead of dereferencing index -1
+    EXPECT_GT(ctrl->d_xTip.getValue()[0], 0.0);
+}
+
 } // namespace beamadapter_test
diff --git a/src/BeamAdapter/component/controller/InterventionalRadiologyController.inl b/src/BeamAdapter/component/controller/InterventionalRadiologyController.inl
index affde0f77..efba3afbe 100644
--- a/src/BeamAdapter/component/controller/InterventionalRadiologyController.inl
+++ b/src/BeamAdapter/component/controller/InterventionalRadiologyController.inl
@@ -372,7 +372,7 @@ void InterventionalRadiologyController<DataTypes>::onBeginAnimationStep(const do
     if(m_FF || m_RW)
     {
         int id = d_controlledInstrument.getValue();
-        if (id >= (int)xInstrTip.size())
+        if (id < 0 || id >= (int)xInstrTip.size())
         {
             msg_warning()<<"Controlled Instument num "<<id<<" does not exist (size ="<< xInstrTip.size() <<") use instrument 0 instead";
             id=0;
@@ -406,7 +406,7 @@ template <class DataTypes>
 void InterventionalRadiologyController<DataTypes>::applyAction(BeamAdapterAction action)
 {
     int id = d_controlledInstrument.getValue();
-    if (id >= int(m_instrumentsList.size()))
+    if (id < 0 || id >= int(m_instrumentsList.size()))
     {
         msg_warning() << "Controlled Instrument num " << id << " does not exist (size =" << m_instrumentsList.size() << ").";
         return;
@@ -500,6 +500,14 @@ void InterventionalRadiologyController<DataTypes>::computeInstrumentsCurvAbs(typ
         type::vector<sofa::Size> density_I;
         m_instrumentsList[i]->getSamplingParameters(xP_noticeable_I, density_I); // sampling of the different section of this instrument
 
+        // an instrument must provide at least one noticeable point; otherwise the loop below and the
+        // final key point access (xP_noticeable_I.size()-1) would read out of bounds.
+        if (xP_noticeable_I.empty())
+        {
+            msg_error() << "Instrument " << i << " provides no sampling parameters (no noticeable point). Skipping it.";
+            continue;
+        }
+
         // check each interval of noticeable point to see if they go out (>0) and use corresponding density to sample the interval.
         for (int j=0; j<(int)(xP_noticeable_I.size()-1); j++)
         {
@@ -603,10 +611,8 @@ void InterventionalRadiologyController<DataTypes>::interventionalRadiologyCollis
     Real xAbsCurv = m_nodeCurvAbs[node];
     int firstInstruOnx = m_idInstrumentCurvAbsTable[node][0];
 
-    type::vector<unsigned int> segRemove;
-
-    for (unsigned int it=0; it<m_instrumentsList.size(); it++)
-        segRemove.push_back(0);
+    // -1 means "nothing to remove" for that instrument; using 0 as sentinel would drop a point at index 0.
+    type::vector<int> segRemove(m_instrumentsList.size(), -1);
 
     for (int i = static_cast<int>(xPointList.size()) - 1; i>=0; i--)
     {
@@ -659,7 +665,7 @@ void InterventionalRadiologyController<DataTypes>::interventionalRadiologyCollis
 
     for (unsigned int it=0; it<m_instrumentsList.size(); it++)
     {
-        if(segRemove[it]!=0)
+        if(segRemove[it]!=-1)
             removeEdge.push_back(segRemove[it]);
     }
 
@@ -850,6 +856,11 @@ void InterventionalRadiologyController<DataTypes>::applyInterventionalRadiologyC
                 break;
         }
 
+        // If no previous node was found beyond xCurvAbs (see the "Case 1" warning above), the loop
+        // ends with prev_xId == m_nodeCurvAbs.size(): clamp to the last node to avoid out-of-bounds access.
+        if (prev_xId >= m_nodeCurvAbs.size())
+            prev_xId = m_nodeCurvAbs.size() - 1;
+
         sofa::Index prev_globalNodeId = prev_numberOfUnactiveNodes + prev_xId;
         const Real prev_xCurvAbs = m_nodeCurvAbs[prev_xId];
 
@@ -950,7 +961,8 @@ void InterventionalRadiologyController<DataTypes>::applyInterventionalRadiologyC
     {
         RealConstIterator it = rigidCurvAbs->begin();
 
-        for (unsigned int i=0; i<newCurvAbs.size(); i++)
+        // firstSimulatedNode + i indexes into the dofs, so it must stay within [0, numberOfNodes-1]
+        for (unsigned int i=0; i<newCurvAbs.size() && (firstSimulatedNode + i) < numberOfNodes; i++)
         {
             if (newCurvAbs[i] < ((*it)+ std::numeric_limits<float>::epsilon()) && newCurvAbs[i] > ((*it)- std::numeric_limits<float>::epsilon())) // node= border of the rigid segment
             {
@@ -1011,7 +1023,9 @@ void InterventionalRadiologyController<DataTypes>::totalLengthIsChanging(const t
         if (newTable[i].size() == 1)
         {
             modifiedNodeCurvAbs[i] -= dLength;
-            if (i > 1 && modifiedNodeCurvAbs[i] < modifiedNodeCurvAbs[i - 1])
+            // keep the curv abscissa monotonic: clamp against the previous node (including node 0, the origin),
+            // otherwise applyInterventionalRadiologyController's interpolation would receive unsorted abscissa.
+            if (i > 0 && modifiedNodeCurvAbs[i] < modifiedNodeCurvAbs[i - 1])
             {
                 modifiedNodeCurvAbs[i] = modifiedNodeCurvAbs[i - 1];
             }