diff --git a/README.md b/README.md
index 8576d11..c6a8d94 100644
--- a/README.md
+++ b/README.md
@@ -99,7 +99,7 @@ the finalize stage idempotent and the MJCF rooted at the CAD root link.
 ## Re-generating from CAD
 
 ```bash
-uv sync
+uv sync --extra cad
 sudo apt install openscad        # for collider editing (onshape-to-robot)
 ```
 
diff --git a/pyproject.toml b/pyproject.toml
index 9eae228..b593a49 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,7 +8,10 @@ version = "0.3.1"
 description = "Robot asset tool: generate URDF / MJCF / xacro from Onshape CAD, and load robot descriptions"
 readme = "README.md"
 requires-python = ">=3.10"
-dependencies = [
+dependencies = []
+
+[project.optional-dependencies]
+cad = [
     "mujoco>=3.4.0",
     "onshape-to-robot>=1.8.0",
     "pymeshlab>=2025.7.post1",
@@ -26,6 +29,7 @@ robot-assets-urdf-to-xacro = "robot_assets.workflow.urdf_to_xacro:main"
 [dependency-groups]
 test = [
     "pytest>=8.0",
+    "robot-assets[cad]",
 ]
 
 [tool.pytest.ini_options]
diff --git a/robot_assets/workflow/onshape_to_urdf.py b/robot_assets/workflow/onshape_to_urdf.py
index 176f231..f7b8c84 100644
--- a/robot_assets/workflow/onshape_to_urdf.py
+++ b/robot_assets/workflow/onshape_to_urdf.py
@@ -26,8 +26,18 @@
 
 
 def _onshape_to_robot_bin() -> str:
-    """Locate the onshape-to-robot CLI (PATH, else alongside the running interpreter)."""
-    return shutil.which("onshape-to-robot") or str(Path(sys.executable).parent / "onshape-to-robot")
+    """Locate the onshape-to-robot CLI (PATH, else alongside the running interpreter).
+
+    Raises:
+        FileNotFoundError: If the CLI is missing, i.e. the `cad` extra is not installed.
+    """
+    binary = shutil.which("onshape-to-robot") or str(Path(sys.executable).parent / "onshape-to-robot")
+    if not Path(binary).exists():
+        raise FileNotFoundError(
+            "The onshape-to-robot CLI was not found. It ships with the CAD toolchain rather than "
+            "the base install; install it with `pip install robot-assets[cad]`."
+        )
+    return binary
 
 
 def export(robot_dir: Path, *, keep_assets: bool = False, convert: bool = False) -> Path:
diff --git a/robot_assets/workflow/urdf_to_mjcf.py b/robot_assets/workflow/urdf_to_mjcf.py
index ab02e5a..59a0b2a 100644
--- a/robot_assets/workflow/urdf_to_mjcf.py
+++ b/robot_assets/workflow/urdf_to_mjcf.py
@@ -20,7 +20,13 @@
 import tempfile
 import xml.etree.ElementTree as ET
 
-import mujoco
+try:
+    import mujoco
+except ModuleNotFoundError as error:
+    raise ModuleNotFoundError(
+        "MuJoCo is needed to compile MJCF, and ships with the CAD toolchain rather than the "
+        "base install. Install it with `pip install robot-assets[cad]`."
+    ) from error
 
 from . import robot_model
 
diff --git a/uv.lock b/uv.lock
index 07b0d01..82f8833 100644
--- a/uv.lock
+++ b/uv.lock
@@ -616,7 +616,9 @@ wheels = [
 name = "robot-assets"
 version = "0.3.1"
 source = { editable = "." }
-dependencies = [
+
+[package.optional-dependencies]
+cad = [
     { name = "mujoco" },
     { name = "onshape-to-robot" },
     { name = "pymeshlab" },
@@ -625,17 +627,22 @@ dependencies = [
 [package.dev-dependencies]
 test = [
     { name = "pytest" },
+    { name = "robot-assets", extra = ["cad"] },
 ]
 
 [package.metadata]
 requires-dist = [
-    { name = "mujoco", specifier = ">=3.4.0" },
-    { name = "onshape-to-robot", specifier = ">=1.8.0" },
-    { name = "pymeshlab", specifier = ">=2025.7.post1" },
+    { name = "mujoco", marker = "extra == 'cad'", specifier = ">=3.4.0" },
+    { name = "onshape-to-robot", marker = "extra == 'cad'", specifier = ">=1.8.0" },
+    { name = "pymeshlab", marker = "extra == 'cad'", specifier = ">=2025.7.post1" },
 ]
+provides-extras = ["cad"]
 
 [package.metadata.requires-dev]
-test = [{ name = "pytest", specifier = ">=8.0" }]
+test = [
+    { name = "pytest", specifier = ">=8.0" },
+    { name = "robot-assets", extras = ["cad"] },
+]
 
 [[package]]
 name = "tomli"