Integrate passive flex contact implicitly

Contact of a flex with `passive` collisions enabled was applied as an
explicit spring of fixed stiffness 1e4, which the timestep bounds: any
stiffness worth having oscillates faster than the step can resolve, so
the force was too soft to keep sheets apart and interpenetration was
routine.

Carry its curvature in the effective metric M + K instead, alongside the
flex's own stretch and bending stiffness. The contact block k*J^T*J is
appended to the per-vertex candidate list already assembled for the flex
stencils, so it costs additional entries in an existing matrix rather
than a new one, and the accompanying shift -h*K*v is what damps the
stiff modes. At a 2 ms timestep this holds roughly 50x the stiffness an
explicit force of the same step could.

With the timestep no longer setting the bound, the stiffness is chosen
as a natural frequency scaled by the participating vertex mass rather
than left at a fixed 1e4, so one value suits models of any scale.

Passive handling is scoped to contacts whose every dof is a flex vertex
carried by the metric: flex against flex, flex against itself, and flex
against static geometry, which contributes no dofs of its own. For those
the Hessian is assembled in full. Contact with a body that can move
would have that body's dofs dropped from it, and is left on the
constraint solver.

The feature now requires an integrator whose constraint solve runs in
that metric, and is rejected with an error otherwise.

Add model/flex/drape.xml as the example model, replacing sphere_passive,
whose contacts no longer demonstrated the feature.

src/engine/engine_core_constraint.c

@@ -2549,11 +2549,26 @@ static int mj_nc(const mjModel* m, mjData* d, int* nnz) {
   for (int i=0; i < ncon; i++) {
     mjContact* con = d->contact + i;
 
-    // skip if passive
-    if ((con->flex[0] > -1 && m->flex_passive[con->flex[0]]) ||
-        (con->flex[1] > -1 && m->flex_passive[con->flex[1]])) {
-      con->efc_address = -1;
-      con->exclude = 4;
+    // Passive handling covers flex-flex contact, self-collision included, and contact between a
+    // flex and STATIC geometry. What those have in common is that every dof the contact touches is
+    // a flex vertex carried by the effective metric -- a static geom has none -- so the Hessian
+    // k*J^T*J is assembled in full rather than truncated. A flex against a MOVING body keeps the
+    // constraint path: its dofs would be dropped from the Hessian, and the passive force is a
+    // normal penalty with no friction cone, which is the wrong trade where a gripper closes on
+    // cloth. A contact is passive if either flex asks for it.
+    {
+      int f0 = con->flex[0], f1 = con->flex[1];
+      int wants = (f0 > -1 && m->flex_passive[f0]) || (f1 > -1 && m->flex_passive[f1]);
+      int ok = (f0 > -1 && f1 > -1);   // flex-flex, or a flex with itself
+      for (int s = 0; s < 2 && !ok; s++) {
+        if (con->flex[s] < 0 && con->geom[s] > -1) {
+          ok = (m->body_weldid[m->geom_bodyid[con->geom[s]]] == 0);   // welded to the world
+        }
+      }
+      if (wants && ok) {
+        con->efc_address = -1;
+        con->exclude = 4;
+      }
     }
 
     // skip if excluded

src/engine/engine_derivative.c

@@ -17,6 +17,7 @@
 #include <mujoco/mjdata.h>
 #include <mujoco/mjmodel.h>
 #include <mujoco/mjsan.h>  // IWYU pragma: keep
+#include "engine/engine_core_constraint.h"
 #include "engine/engine_core_smooth.h"
 #include "engine/engine_core_util.h"
 #include "engine/engine_crossplatform.h"
@@ -1630,6 +1631,62 @@ mjtBool mjd_flexInterpAssemblable(const mjModel* m) {
 // does ANY flex contribute assemblable implicit stiffness? (cheap existence check for the
 // solver gate: stretch stiffness on a standard flex, or -- when Krot will be supplied -- an
 // operator-processed interp flex)
+// does any flex hand its contacts to the passive path? Such a flex wants the metric for the
+// contact stiffness alone, which is a different reason from carrying elasticity -- the two must
+// stay distinguishable, because an empty CSR is a VALID state for an elastic model (the matrix-free
+// operators carry bending and interp) but means "nothing at all" for a contact-only one.
+static mjtBool flexPassiveContact_any(const mjModel* m) {
+  for (int f = 0; f < m->nflex; f++) {
+    if (!m->flex_interp[f] && !m->flex_rigid[f] && m->flex_dim[f] >= 2 && m->flex_passive[f]) {
+      return 1;
+    }
+  }
+  return 0;
+}
+
+// res += scale * K_contact * vec, where K_contact = sum_c k_c * J_c^T J_c over passive flex
+// contacts. The linearly-implicit scheme is (M + h^2 K) a = f - h K v; the shift is assembled
+// per class (see mjd_effShift), so a class that contributes to K must contribute here too.
+// Contributing only to K makes the contact stiffer WITHOUT the velocity correction that damps it,
+// which is a ringing contact rather than a quiet one.
+void mjd_flexContact_mul(const mjModel* m, mjData* d, mjtNum* res, const mjtNum* vec,
+                         mjtNum scale) {
+  if (!d->ncon) {
+    return;
+  }
+  int nv = m->nv;
+  mj_markStack(d);
+  mjtNum* jacdif = mjSTACKALLOC(d, 3*nv, mjtNum);
+  mjtNum* jac1 = mjSTACKALLOC(d, 3*nv, mjtNum);
+  mjtNum* jac2 = mjSTACKALLOC(d, 3*nv, mjtNum);
+  mjtNum* jacn = mjSTACKALLOC(d, 3*nv, mjtNum);
+  int* chain = mjSTACKALLOC(d, nv, int);
+  for (int i = 0; i < d->ncon; i++) {
+    const mjContact* con = d->contact + i;
+    if (con->exclude != 4) {
+      continue;
+    }
+    mjtNum k = mjd_flexContactStiffness(m, d, con);
+    if (k <= 0) {
+      continue;
+    }
+    int NV = mj_contactJacobian(m, d, con, con->dim, jacdif, NULL, jac1, jac2, NULL, NULL, chain);
+    if (!NV) {
+      continue;
+    }
+    mju_mulMatMat(jacn, con->frame, jacdif, con->dim > 1 ? 3 : 1, 3, NV);
+    mjtNum Jv = 0;
+    for (int a = 0; a < NV; a++) {
+      Jv += jacn[a] * vec[chain[a]];
+    }
+    mjtNum s = scale * k * Jv;
+    for (int a = 0; a < NV; a++) {
+      res[chain[a]] += s * jacn[a];
+    }
+  }
+  mj_freeStack(d);
+}
+
 mjtBool mjd_flexStiff_any(const mjModel* m, int flg_interp) {
   for (int f = 0; f < m->nflex; f++) {
     if (flg_interp && flexInterp_processed(m, f)) {
@@ -1645,6 +1702,14 @@ mjtBool mjd_flexStiff_any(const mjModel* m, int flg_interp) {
 
 
 // does this standard flex contribute implicit stiffness under the given term flags?
+// A flex participates in the metric structure if it carries elasticity OR if its contacts are
+// handled passively: the contact stiffness is a passive term in its own right and may be the only
+// stiffness such a flex has, so the vertex slots (and hence the CSR rows, and hence the covered
+// dofs) must exist for it either way. Gating participation on elasticity alone would let an
+// unrelated modelling choice decide whether contact can be implicit.
+static mjtBool flexMetric_participates(const mjModel* m, int f, int flg_bend, int flg_stretch,
+                                       int flg_contact);
+
 static mjtBool flexStiff_active(const mjModel* m, int f, int flg_bend, int flg_stretch) {
   if (m->flex_interp[f] || m->flex_rigid[f] || m->flex_dim[f] < 2) {
     return 0;
@@ -1655,6 +1720,91 @@ static mjtBool flexStiff_active(const mjModel* m, int f, int flg_bend, int flg_s
   return bend || stretch;
 }
 
+// Passive flex contact stiffness, expressed as a natural frequency scaled by the participating
+// mass: k = omega^2 * m_min. A frequency travels across models where an absolute stiffness does
+// not, which is why the same number works from a millimetre cloth to a metre-scale bag. The min is
+// over NONZERO masses: flex vertices pinned to a rigid attachment carry mass 0 (their inertia is in
+// the rigid body) and a plain min would give k = 0.
+#define mjFLEXCONTACT_OMEGA2 5e7
+
+mjtNum mjd_flexContactStiffness(const mjModel* m, const mjData* d, const mjContact* con) {
+  mjtNum mmin = 0;
+  for (int side = 0; side < 2; side++) {
+    int f = con->flex[side];
+    if (f < 0) {
+      continue;
+    }
+    int gv[8], ngv = 0;
+    if (con->vert[side] >= 0) {
+      gv[ngv++] = m->flex_vertadr[f] + con->vert[side];
+    } else if (con->elem[side] >= 0) {
+      int nvrt = m->flex_dim[f] + 1;
+      const int* e = m->flex_elem + m->flex_elemdataadr[f] + nvrt*con->elem[side];
+      for (int j = 0; j < nvrt && ngv < 8; j++) {
+        gv[ngv++] = m->flex_vertadr[f] + e[j];
+      }
+    }
+    for (int j = 0; j < ngv; j++) {
+      int b = m->flex_vertbodyid[gv[j]];
+      if (m->body_dofnum[b] != 3) {
+        continue;
+      }
+      int da = m->body_dofadr[b];
+      mjtNum mv = d->M[m->M_rowadr[da] + m->M_rownnz[da] - 1];   // diagonal: the point mass
+      if (mv > 0 && (mmin == 0 || mv < mmin)) {
+        mmin = mv;
+      }
+    }
+  }
+  return mjFLEXCONTACT_OMEGA2 * mmin;   // 0 if every participant is massless: no stiffness, no NaN
+}
+
+// The flex vertex slots a passive contact couples: the vertex itself for a vertex side, the
+// element's vertices for an element side. Duplicates dropped, and slots outside the metric skipped.
+static int contactFlexSlots(const mjModel* m, const mjContact* con, const int* vslot,
+                            int* out, int cap) {
+  int n = 0;
+  for (int side = 0; side < 2; side++) {
+    int f = con->flex[side];
+    if (f < 0) {
+      continue;
+    }
+    int gv[8], ngv = 0;
+    if (con->vert[side] >= 0) {
+      gv[ngv++] = m->flex_vertadr[f] + con->vert[side];
+    } else if (con->elem[side] >= 0) {
+      int nvrt = m->flex_dim[f] + 1;
+      const int* e = m->flex_elem + m->flex_elemdataadr[f] + nvrt*con->elem[side];
+      for (int j = 0; j < nvrt && ngv < 8; j++) {
+        gv[ngv++] = m->flex_vertadr[f] + e[j];
+      }
+    }
+    for (int j = 0; j < ngv; j++) {
+      int s = vslot[gv[j]];
+      if (s < 0) {
+        continue;
+      }
+      int dup = 0;
+      for (int q = 0; q < n; q++) {
+        if (out[q] == s) { dup = 1; break; }
+      }
+      if (!dup && n < cap) {
+        out[n++] = s;
+      }
+    }
+  }
+  return n;
+}
+
+static mjtBool flexMetric_participates(const mjModel* m, int f, int flg_bend, int flg_stretch,
+                                       int flg_contact) {
+  if (flexStiff_active(m, f, flg_bend, flg_stretch)) {
+    return 1;
+  }
+  return flg_contact && m->flex_passive[f] && !m->flex_rigid[f] && !m->flex_interp[f] &&
+         m->flex_dim[f] >= 2;
+}
+
 
 // assemble the standard-flex implicit stiffness K = (s1 + s2*damping) * (K_bend + K_stretch)
 // into dof-level CSR (same terms mjd_flexBend_mul / mjd_flexStretch_mul apply matrix-free; the
@@ -1669,7 +1819,7 @@ static mjtBool flexStiff_active(const mjModel* m, int f, int flg_bend, int flg_s
 // so one CSR replaces all three matrix-free operators uniformly.
 int mjd_flexStiff_assemble(const mjModel* m, mjData* d, int* rownnz, int* rowadr,
                            int* colind, mjtNum* val, mjtNum s1, mjtNum s2,
-                           int flg_bend, int flg_stretch, const mjtNum* Krot) {
+                           int flg_bend, int flg_stretch, int flg_contact, const mjtNum* Krot) {
   int nv = m->nv;
   mj_markStack(d);
 
@@ -1680,7 +1830,7 @@ int mjd_flexStiff_assemble(const mjModel* m, mjData* d, int* rownnz, int* rowadr
     vslot[i] = -1;
   }
   for (int f = 0; f < m->nflex; f++) {
-    if (!flexStiff_active(m, f, flg_bend, flg_stretch)) {
+    if (!flexMetric_participates(m, f, flg_bend, flg_stretch, flg_contact)) {
       continue;
     }
     for (int lv = 0; lv < m->flex_vertnum[f]; lv++) {
@@ -1777,6 +1927,22 @@ int mjd_flexStiff_assemble(const mjModel* m, mjData* d, int* rownnz, int* rowadr
     }
   }
 
+  // passive contacts (counting): a contact makes its participating vertices mutual neighbours, so
+  // the CSR gains the blocks its Hessian k*J^T*J will occupy. This is the only source of structure
+  // for a flex whose contacts are passive but which carries no elasticity.
+  if (flg_contact) {
+    for (int i = 0; i < d->ncon; i++) {
+      const mjContact* con = d->contact + i;
+      if (con->exclude != 4) {
+        continue;
+      }
+      int cs[8], ncs = contactFlexSlots(m, con, vslot, cs, 8);
+      for (int a = 0; a < ncs; a++) {
+        ncand[cs[a]] += ncs;
+      }
+    }
+  }
+
   // gather candidate neighbor lists (vertex slots, with duplicates)
   int* cadr = mjSTACKALLOC(d, nvert + 1, int);
   cadr[0] = 0;
@@ -1842,6 +2008,22 @@ int mjd_flexStiff_assemble(const mjModel* m, mjData* d, int* rownnz, int* rowadr
     }
   }
 
+  // passive contacts (filling)
+  if (flg_contact) {
+    for (int i = 0; i < d->ncon; i++) {
+      const mjContact* con = d->contact + i;
+      if (con->exclude != 4) {
+        continue;
+      }
+      int cs[8], ncs = contactFlexSlots(m, con, vslot, cs, 8);
+      for (int a = 0; a < ncs; a++) {
+        for (int b = 0; b < ncs; b++) {
+          cand[cadr[cs[a]] + ncand[cs[a]]++] = cs[b];
+        }
+      }
+    }
+  }
+
   // per vertex: sort by neighbor dofadr, unique -> neighbor lists
   int* nadr = mjSTACKALLOC(d, nvert + 1, int);
   int* neigh = mjSTACKALLOC(d, cadr[nvert] > 0 ? cadr[nvert] : 1, int);
@@ -2103,6 +2285,73 @@ int mjd_flexStiff_assemble(const mjModel* m, mjData* d, int* rownnz, int* rowadr
       })
     }
   }
+  // passive contacts (values): k*J^T*J over the contact's flex vertices, where J is the normal-
+  // direction row of the contact Jacobian. Both participants are flex vertices carried by the
+  // metric (mj_makeConstraint hands this path no contact with a moving body), so the block is assembled
+  // in full -- no truncation, and the Hessian matches the force exactly. The force is applied by
+  // mj_contactPassive from the same stiffness helper; the two are one linearization.
+  if (flg_contact && d->ncon) {
+    int nv = m->nv;
+    int* dof2slot = mjSTACKALLOC(d, nv, int);
+    mjtNum* jacdif = mjSTACKALLOC(d, 3*nv, mjtNum);
+    mjtNum* jac1 = mjSTACKALLOC(d, 3*nv, mjtNum);
+    mjtNum* jac2 = mjSTACKALLOC(d, 3*nv, mjtNum);
+    mjtNum* jacn = mjSTACKALLOC(d, 3*nv, mjtNum);
+    int* chain = mjSTACKALLOC(d, nv, int);
+    mjtNum* w = mjSTACKALLOC(d, 3*(nvert > 0 ? nvert : 1), mjtNum);
+    for (int i = 0; i < nv; i++) {
+      dof2slot[i] = -1;
+    }
+    for (int s = 0; s < nvert; s++) {
+      for (int k = 0; k < 3; k++) {
+        dof2slot[vdof[s] + k] = s;
+      }
+    }
+    for (int i = 0; i < d->ncon; i++) {
+      const mjContact* con = d->contact + i;
+      if (con->exclude != 4) {
+        continue;
+      }
+      int cs[8], ncs = contactFlexSlots(m, con, vslot, cs, 8);
+      if (ncs < 1) {
+        continue;
+      }
+      mjtNum k = mjd_flexContactStiffness(m, d, con);
+      if (k <= 0) {
+        continue;
+      }
+      int NV = mj_contactJacobian(m, d, con, con->dim, jacdif, NULL, jac1, jac2, NULL, NULL, chain);
+      if (NV == 0) {
+        continue;
+      }
+      // rotate into the contact frame and keep the normal row
+      mju_mulMatMat(jacn, con->frame, jacdif, con->dim > 1 ? 3 : 1, 3, NV);
+      for (int a = 0; a < ncs; a++) {
+        mju_zero(w + 3*cs[a], 3);
+      }
+      for (int a = 0; a < NV; a++) {
+        int s = dof2slot[chain[a]];
+        if (s >= 0) {
+          w[3*s + (chain[a] - vdof[s])] = jacn[a];
+        }
+      }
+      for (int a = 0; a < ncs; a++) {
+        for (int b = 0; b < ncs; b++) {
+          int pos;
+          FLEXSTIFF_BLOCK(cs[a], cs[b], pos);
+          if (pos < 0) {
+            continue;
+          }
+          for (int r = 0; r < 3; r++) {
+            for (int c = 0; c < 3; c++) {
+              val[rowadr[vdof[cs[a]] + r] + 3*pos + c] += s1 * k * w[3*cs[a] + r] * w[3*cs[b] + c];
+            }
+          }
+        }
+      }
+    }
+  }
+
   #undef FLEXSTIFF_BLOCK
   #undef FLEXINTERP_WALK
 
@@ -3166,6 +3415,7 @@ void mjd_effShift(const mjModel* m, mjData* d) {
   mjd_flexInterp_mul(m, d, d->efm_c, d->qvel, h, 0, d->flexelem_krot);
   mjd_flexBend_mul(m, d, d->efm_c, d->qvel, -h, 0);
   mjd_flexStretch_mul(m, d, d->efm_c, d->qvel, -h, 0);
+  mjd_flexContact_mul(m, d, d->efm_c, d->qvel, -h);
 }
 
 
@@ -3197,16 +3447,17 @@ void mjd_effBuild(const mjModel* m, mjData* d, int active, int flg_factor) {
   const mjtNum* krot = mjd_flexInterpAssemblable(m) ? d->flexelem_krot : NULL;
   d->efm_K_rownnz = EFMALLOC(int, nv);
   d->efm_K_rowadr = EFMALLOC(int, nv);
-  if (mjd_flexStiff_any(m, krot != NULL)) {
+  if (mjd_flexStiff_any(m, krot != NULL) || flexPassiveContact_any(m)) {
     d->nefmK = mjd_flexStiff_assemble(m, d, d->efm_K_rownnz, d->efm_K_rowadr,
-                                      NULL, NULL, h*h, h, /*bend*/ 1, /*stretch*/ 1, krot);
+                                      NULL, NULL, h*h, h, /*bend*/ 1, /*stretch*/ 1,
+                                      /*contact*/ 1, krot);
   }
   if (d->nefmK) {
     d->efm_K_colind = EFMALLOC(int, d->nefmK);
     d->efm_K_val    = EFMALLOC(mjtNum, d->nefmK);
     mjd_flexStiff_assemble(m, d, d->efm_K_rownnz, d->efm_K_rowadr,
                            d->efm_K_colind, d->efm_K_val, h*h, h,
-                           /*bend*/ 1, /*stretch*/ 1, krot);
+                           /*bend*/ 1, /*stretch*/ 1, /*contact*/ 1, krot);
     // per-step factor of the flex block of (M + K): the stiffness is constant during the
     // step, so one factorization here turns every preconditioner application into a direct
     // solve (the stiff flex block stops being iterated on). Consumers that only multiply
@@ -3219,6 +3470,7 @@ void mjd_effBuild(const mjModel* m, mjData* d, int active, int flg_factor) {
     mju_zeroInt(d->efm_K_rownnz, nv);
     mju_zeroInt(d->efm_K_rowadr, nv);
   }
+
   d->efm_active = 1;
 
   // fill the shift with the current velocity (refreshed again in the velocity stage)

src/engine/engine_derivative.h

@@ -77,9 +77,18 @@ MJAPI void mjd_flexStretch_mul(const mjModel* m, mjData* d, mjtNum* res, const m
 // dof-level CSR; phase 1 (colind==NULL) fills rownnz/rowadr and returns total nnz, phase 2
 // fills colind/val. Interp flexes are assembled iff Krot (mjd_flexInterp_cacheKrot cache) is
 // non-NULL and the centered fast path applies (check mjd_flexInterpAssemblable first).
+// Stiffness of a passive flex contact: omega^2 scaled by the smallest nonzero participating mass.
+// The force (mj_contactPassive) and the Hessian (mjd_flexStiff_assemble) MUST use this same value,
+// or the linearization the metric carries does not match the force being applied.
+// res += scale * K_contact * vec (the shift counterpart of the contact stiffness in the metric)
+MJAPI void mjd_flexContact_mul(const mjModel* m, mjData* d, mjtNum* res, const mjtNum* vec,
+                               mjtNum scale);
+
+MJAPI mjtNum mjd_flexContactStiffness(const mjModel* m, const mjData* d, const mjContact* con);
+
 MJAPI int mjd_flexStiff_assemble(const mjModel* m, mjData* d, int* rownnz, int* rowadr,
                                  int* colind, mjtNum* val, mjtNum s1, mjtNum s2,
-                                 int flg_bend, int flg_stretch, const mjtNum* Krot);
+                                 int flg_bend, int flg_stretch, int flg_contact, const mjtNum* Krot);
 
 // can all interp flexes be assembled to dof-level CSR? (centered fast path everywhere)
 MJAPI mjtBool mjd_flexInterpAssemblable(const mjModel* m);

src/engine/engine_forward.c

@@ -1591,6 +1591,19 @@ void mj_RungeKutta(const mjModel* m, mjData* d, int N) {
 
 
 // return 1 if any flex needs implicit stiffness treatment (interp or bending)
+// a flex whose contacts are handled passively wants the effective metric in its own right: the
+// contact stiffness is a passive term like any other, and it is the only stiffness such a flex may
+// have. Gating it on the flex's ELASTICITY would let an unrelated modelling choice decide whether
+// contact is implicit.
+static mjtBool flex_has_passive_contact(const mjModel* m) {
+  for (int f=0; f < m->nflex; f++) {
+    if (!m->flex_rigid[f] && m->flex_passive[f]) {
+      return 1;
+    }
+  }
+  return 0;
+}
+
 static mjtBool flex_has_implicit_stiffness(const mjModel* m) {
   for (int f=0; f < m->nflex; f++) {
     if (m->flex_rigid[f]) {
@@ -1634,7 +1647,7 @@ int mj_flexCG(const mjModel* m) {
   return m->opt.solver == mjSOL_CG &&
          (m->opt.integrator == mjINT_IMPLICIT || m->opt.integrator == mjINT_IMPLICITFAST) &&
          m->opt.cone != mjCONE_ELLIPTIC && !mjENABLED(mjENBL_SLEEP) &&
-         flex_has_implicit_stiffness(m);
+         (flex_has_implicit_stiffness(m) || flex_has_passive_contact(m));
 }
 
 
@@ -1776,6 +1789,15 @@ void mj_implicit(const mjModel* m, mjData* d) {
 void mj_forwardSkip(const mjModel* m, mjData* d, int skipstage, int skipsensor) {
   TM_START;
 
+  // Passive flex contact is stiff by construction (see mjd_flexContactStiffness: omega^2 scaled by
+  // the participating mass), which is only stable because its Hessian is carried in the effective
+  // metric. Integrated explicitly it diverges immediately, so say so rather than let the model
+  // explode: the feature requires an integrator whose constraint solve runs in that metric.
+  if (flex_has_passive_contact(m) && !mj_flexCG(m)) {
+    mjERROR("passive flex contact requires the effective metric: use integrator=\"implicit\" or "
+            "\"implicitfast\" with solver=\"CG\", pyramidal cones and sleep disabled");
+  }
+
   // position-dependent
   if (skipstage < mjSTAGE_POS) {
     mj_fwdPosition(m, d);

src/engine/engine_passive.c

@@ -22,6 +22,7 @@
 #include "engine/engine_callback.h"
 #include "engine/engine_core_constraint.h"
 #include "engine/engine_core_util.h"
+#include "engine/engine_derivative.h"
 #include "engine/engine_crossplatform.h"
 #include "engine/engine_inline.h"
 #include "engine/engine_memory.h"
@@ -36,8 +37,6 @@
 
 //----------------------------- passive forces -----------------------------------------------------
 
-// stiffness for passive contacts
-static const mjtNum kContactStiffness = 1e4;
 
 // local edge-based vertex indexing for 2D and 3D elements, 2D and 3D elements
 // have 3 and 6 edges, respectively so the missing indexes are set to 0
@@ -962,8 +961,10 @@ int mj_contactPassive(const mjModel* m, mjData* d) {
     // rotate Jacobian differences to contact frame
     mju_mulMatMat(jac, con->frame, jacdifp, dim > 1 ? 3 : 1, 3, NV);
 
-    // compute passive contact force (dim = 1)
-    mjtNum scl = -kContactStiffness*con->dist;
+    // compute passive contact force (dim = 1). The stiffness is mass-scaled and shared with the
+    // Hessian the effective metric carries for this contact; the pair is one linearization, so the
+    // two must not drift apart.
+    mjtNum scl = -mjd_flexContactStiffness(m, d, con)*con->dist;
     if (!issparse) {
       mju_addToScl(d->qfrc_spring, jac, scl, nv);
     } else {

src/engine/engine_setconst.c

@@ -1416,10 +1416,11 @@ static void setEfm0Factor(mjModel* m, mjData* d) {
   int* K_rownnz = mjSTACKALLOC(d, nv, int);
   int* K_rowadr = mjSTACKALLOC(d, nv, int);
   int nK = mjd_flexStiff_assemble(m, d, K_rownnz, K_rowadr, NULL, NULL, h*h, h,
-                                  /*flg_bend=*/1, /*flg_stretch=*/0, NULL);
+                                  /*flg_bend=*/1, /*flg_stretch=*/0, /*flg_contact=*/0,
+                                  NULL);
   int* K_colind = mjSTACKALLOC(d, nK > 0 ? nK : 1, int);
   mjtNum* K_val = mjSTACKALLOC(d, nK > 0 ? nK : 1, mjtNum);
-  mjd_flexStiff_assemble(m, d, K_rownnz, K_rowadr, K_colind, K_val, h*h, h, 1, 0, NULL);
+  mjd_flexStiff_assemble(m, d, K_rownnz, K_rowadr, K_colind, K_val, h*h, h, 1, 0, 0, NULL);
 
   // inverse map: dof address -> compact factor row (monotone: slots follow dof order)
   int* dofrow = mjSTACKALLOC(d, nv, int);