From c751eb0dda7ca1f6c07473188f94634515a433d9 Mon Sep 17 00:00:00 2001
From: castor639 <316019788+castor639@users.noreply.github.com>
Date: Mon, 31 Aug 2026 20:10:03 +0000
Subject: [PATCH 1/2] [multibody] Throw if CalcSpatialInertia frame is not in
 the plant

Validate that frame_F belongs to this MultibodyTree before computing
spatial inertia, so an unowned FixedOffsetFrame throws instead of
segfaulting.

Fixes #22636.
---
 multibody/plant/multibody_plant.h                        | 1 +
 .../plant/test/multibody_plant_momentum_energy_test.cc   | 9 +++++++++
 multibody/tree/multibody_tree.cc                         | 3 +++
 3 files changed, 13 insertions(+)

diff --git a/multibody/plant/multibody_plant.h b/multibody/plant/multibody_plant.h
index a173ad91ffcc..1a5fb6892a8f 100644
--- a/multibody/plant/multibody_plant.h
+++ b/multibody/plant/multibody_plant.h
@@ -4165,6 +4165,7 @@ class MultibodyPlant final : public internal::MultibodyTreeSystem<T> {
   ///  the expressed-in frame for the returned spatial inertia.
   /// @param[in] body_indexes Array of selected bodies.  This method does not
   ///  distinguish between welded bodies, joint-connected bodies, etc.
+  /// @throws std::exception if `frame_F` does not belong to `this` plant.
   /// @throws std::exception if body_indexes contains an invalid BodyIndex or
   ///  if there is a repeated BodyIndex.
   /// @note The mass and inertia of the world_body() does not contribute to the
diff --git a/multibody/plant/test/multibody_plant_momentum_energy_test.cc b/multibody/plant/test/multibody_plant_momentum_energy_test.cc
index 856735702468..cdebf2888875 100644
--- a/multibody/plant/test/multibody_plant_momentum_energy_test.cc
+++ b/multibody/plant/test/multibody_plant_momentum_energy_test.cc
@@ -7,6 +7,7 @@
 #include "drake/common/test_utilities/eigen_matrix_compare.h"
 #include "drake/common/test_utilities/expect_throws_message.h"
 #include "drake/multibody/plant/multibody_plant.h"
+#include "drake/multibody/tree/fixed_offset_frame.h"
 #include "drake/multibody/tree/multibody_tree_indexes.h"
 #include "drake/multibody/tree/revolute_joint.h"
 #include "drake/multibody/tree/rigid_body.h"
@@ -335,6 +336,14 @@ TEST_F(TwoDofPlanarPendulumTest, CalcSpatialInertia) {
   DRAKE_EXPECT_THROWS_MESSAGE(
       plant_.CalcSpatialInertia(*context_, frame_A, body_indexes),
       "CalcSpatialInertia\\(\\): contains a repeated BodyIndex.*");
+
+  // Verify an exception is thrown if frame_F was never added to the plant
+  // (regression test for #22636).
+  const FixedOffsetFrame<double> orphan_frame(
+      "orphan_frame", frame_A, math::RigidTransformd());
+  DRAKE_EXPECT_THROWS_MESSAGE(
+      plant_.CalcSpatialInertia(*context_, orphan_frame, {body_A.index()}),
+      ".*does not belong to the supplied MultibodyTree.*");
 }
 
 }  // namespace
diff --git a/multibody/tree/multibody_tree.cc b/multibody/tree/multibody_tree.cc
index c4064aff87bb..b177ed4640c9 100644
--- a/multibody/tree/multibody_tree.cc
+++ b/multibody/tree/multibody_tree.cc
@@ -2634,6 +2634,9 @@ template <typename T>
 SpatialInertia<T> MultibodyTree<T>::CalcSpatialInertia(
     const systems::Context<T>& context, const Frame<T>& frame_F,
     const std::vector<LinkIndex>& link_indexes) const {
+  // Ensure frame_F belongs to this tree; otherwise pose queries can segfault.
+  frame_F.HasThisParentTreeOrThrow(this);
+
   // Check if there are repeated LinkIndex in link_indexes by converting the
   // vector to a set (to eliminate duplicates) and see if their sizes differ.
   const std::set<LinkIndex> without_duplicate_bodies(link_indexes.begin(),

From f6bde33d00255d6c844d3e6d096b73763302af76 Mon Sep 17 00:00:00 2001
From: castor639 <316019788+castor639@users.noreply.github.com>
Date: Tue, 1 Sep 2026 22:31:56 +0000
Subject: [PATCH 2/2] [bindings] Regenerate MultibodyPlant docstrings for
 CalcSpatialInertia

---
 bindings/generated_docstrings/multibody_plant.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/bindings/generated_docstrings/multibody_plant.h b/bindings/generated_docstrings/multibody_plant.h
index bd39e54e858f..4cb65a96b2d8 100644
--- a/bindings/generated_docstrings/multibody_plant.h
+++ b/bindings/generated_docstrings/multibody_plant.h
@@ -4528,6 +4528,9 @@ Parameter ``body_indexes``:
     Array of selected bodies. This method does not distinguish between
     welded bodies, joint-connected bodies, etc.
 
+Raises:
+    RuntimeError if ``frame_F`` does not belong to ``this`` plant.
+
 Raises:
     RuntimeError if body_indexes contains an invalid BodyIndex or if
     there is a repeated BodyIndex.