Updates URDF importer to 2.4.31 to continue support for merge-joints (#4000)

# Description

Latest URDF importer in Isaac Sim 5.1 removed the support for
merge-joints, which caused a breaking change for some users. Although
this will maintain as the behavior going forward, we are updating the
importer version to a patch fix of the URDF importer for the short term
that still provides the merge-joints support.

In the future, we will look into providing more tooling for supporting
similar behaviors in addition to the importer itself.

Fixes #3943 


## Type of change

- Bug fix (non-breaking change which fixes an issue)
- Breaking change (existing functionality will not work without user
modification)

## Checklist

- [x] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

<!--
As you go through the checklist above, you can mark something as done by
putting an x character in it

For example,
- [x] I have done this task
- [ ] I have not done this task
-->

---------
Signed-off-by: Kelly Guo <kellyg@nvidia.com>

apps/isaaclab.python.kit

@@ -38,7 +38,7 @@ keywords = ["experience", "app", "usd"]
 
 # Isaac Sim Extra
 "isaacsim.asset.importer.mjcf" = {}
-"isaacsim.asset.importer.urdf" = {}
+"isaacsim.asset.importer.urdf" = {version = "2.4.31", exact = true}
 "omni.physx.bundle" = {}
 "omni.physx.tensors" = {}
 "omni.replicator.core" = {}

source/isaaclab/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.48.9"
+version = "0.49.0"
 
 # Description
 title = "Isaac Lab framework for Robot Learning"

source/isaaclab/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.49.0 (2025-11-10)
+~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Updated the URDF Importer version to 2.4.31 to avoid issues with merging joints on the latest URDF importer in Isaac Sim 5.1
+
 
 0.48.9 (2025-11-21)
 ~~~~~~~~~~~~~~~~~~~

source/isaaclab/isaaclab/sim/converters/urdf_converter.py

@@ -47,8 +47,8 @@ class UrdfConverter(AssetConverterBase):
             cfg: The configuration instance for URDF to USD conversion.
         """
         manager = omni.kit.app.get_app().get_extension_manager()
-        if not manager.is_extension_enabled("isaacsim.asset.importer.urdf"):
-            enable_extension("isaacsim.asset.importer.urdf")
+        if not manager.is_extension_enabled("isaacsim.asset.importer.urdf-2.4.31"):
+            enable_extension("isaacsim.asset.importer.urdf-2.4.31")
         from isaacsim.asset.importer.urdf._urdf import acquire_urdf_interface
 
         self._urdf_interface = acquire_urdf_interface()
@@ -121,6 +121,7 @@ class UrdfConverter(AssetConverterBase):
         import_config.set_collision_from_visuals(self.cfg.collision_from_visuals)
         # consolidating links that are connected by fixed joints
         import_config.set_merge_fixed_joints(self.cfg.merge_fixed_joints)
+        import_config.set_merge_fixed_ignore_inertia(self.cfg.merge_fixed_joints)
         # -- physics settings
         # create fix joint for base link
         import_config.set_fix_base(self.cfg.fix_base)

source/isaaclab/test/sim/test_spawn_from_files.py

@@ -69,8 +69,8 @@ def test_spawn_usd_fails(sim):
 def test_spawn_urdf(sim):
     """Test loading prim from URDF file."""
     # retrieve path to urdf importer extension
-    enable_extension("isaacsim.asset.importer.urdf")
-    extension_path = get_extension_path_from_name("isaacsim.asset.importer.urdf")
+    enable_extension("isaacsim.asset.importer.urdf-2.4.31")
+    extension_path = get_extension_path_from_name("isaacsim.asset.importer.urdf-2.4.31")
     # Spawn franka from URDF
     cfg = sim_utils.UrdfFileCfg(
         asset_path=f"{extension_path}/data/urdf/robots/franka_description/robots/panda_arm_hand.urdf",

source/isaaclab/test/sim/test_urdf_converter.py

@@ -31,8 +31,8 @@ def sim_config():
     # Create a new stage
     stage_utils.create_new_stage()
     # retrieve path to urdf importer extension
-    enable_extension("isaacsim.asset.importer.urdf")
-    extension_path = get_extension_path_from_name("isaacsim.asset.importer.urdf")
+    enable_extension("isaacsim.asset.importer.urdf-2.4.31")
+    extension_path = get_extension_path_from_name("isaacsim.asset.importer.urdf-2.4.31")
     # default configuration
     config = UrdfConverterCfg(
         asset_path=f"{extension_path}/data/urdf/robots/franka_description/robots/panda_arm_hand.urdf",