diff --git a/include/sleipnir/autodiff/Variable.hpp b/include/sleipnir/autodiff/Variable.hpp
index 0e3980915..32b423b0f 100644
--- a/include/sleipnir/autodiff/Variable.hpp
+++ b/include/sleipnir/autodiff/Variable.hpp
@@ -12,14 +12,17 @@
 
 namespace sleipnir {
 
+// Forward declarations for friend declarations in Variable
+class SLEIPNIR_DLLEXPORT Jacobian;
+namespace detail {
+class SLEIPNIR_DLLEXPORT ExpressionGraph;
+}  // namespace detail
+
 /**
  * An autodiff variable pointing to an expression node.
  */
 class SLEIPNIR_DLLEXPORT Variable {
  public:
-  /// The expression node.
-  detail::ExpressionPtr expr = detail::Zero();
-
   /**
    * Constructs a Variable initialized to zero.
    */
@@ -168,6 +171,38 @@ class SLEIPNIR_DLLEXPORT Variable {
    * variables.
    */
   void Update();
+
+ private:
+  /// The expression node.
+  detail::ExpressionPtr expr = detail::Zero();
+
+  friend SLEIPNIR_DLLEXPORT Variable abs(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable acos(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable asin(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable atan(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable atan2(const Variable& y,
+                                           const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable cos(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable cosh(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable erf(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable exp(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable hypot(const Variable& x,
+                                           const Variable& y);
+  friend SLEIPNIR_DLLEXPORT Variable hypot(const Variable& x, const Variable& y,
+                                           const Variable& z);
+  friend SLEIPNIR_DLLEXPORT Variable log(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable log10(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable pow(const Variable& base,
+                                         const Variable& power);
+  friend SLEIPNIR_DLLEXPORT Variable sign(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable sin(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable sinh(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable sqrt(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable tan(const Variable& x);
+  friend SLEIPNIR_DLLEXPORT Variable tanh(const Variable& x);
+
+  friend class SLEIPNIR_DLLEXPORT Jacobian;
+  friend class SLEIPNIR_DLLEXPORT detail::ExpressionGraph;
 };
 
 using VectorXvar = Eigen::Vector<Variable, Eigen::Dynamic>;
diff --git a/src/autodiff/Jacobian.cpp b/src/autodiff/Jacobian.cpp
index 424a06573..eb40b87e4 100644
--- a/src/autodiff/Jacobian.cpp
+++ b/src/autodiff/Jacobian.cpp
@@ -20,14 +20,14 @@ Jacobian::Jacobian(VectorXvar variables, VectorXvar wrt) noexcept
   m_cachedTriplets.reserve(m_variables.rows() * m_wrt.rows() * 0.01);
 
   for (int row = 0; row < m_variables.rows(); ++row) {
-    if (m_variables(row).expr->type == ExpressionType::kLinear) {
+    if (m_variables(row).Type() == ExpressionType::kLinear) {
       // If the row is linear, compute its gradient once here and cache its
       // triplets. Constant rows are ignored because their gradients have no
       // nonzero triplets.
       m_graphs[row].ComputeAdjoints([&](int col, double adjoint) {
         m_cachedTriplets.emplace_back(row, col, adjoint);
       });
-    } else if (m_variables(row).expr->type > ExpressionType::kLinear) {
+    } else if (m_variables(row).Type() > ExpressionType::kLinear) {
       // If the row is quadratic or nonlinear, add it to the list of nonlinear
       // rows to be recomputed in Calculate().
       m_nonlinearRows.emplace_back(row);
diff --git a/test/src/autodiff/VariableTest.cpp b/test/src/autodiff/VariableTest.cpp
index e6f9b4353..8cc240f73 100644
--- a/test/src/autodiff/VariableTest.cpp
+++ b/test/src/autodiff/VariableTest.cpp
@@ -7,5 +7,5 @@ TEST(VariableTest, DefaultConstructor) {
   sleipnir::Variable a;
 
   EXPECT_EQ(0.0, a.Value());
-  EXPECT_EQ(sleipnir::ExpressionType::kConstant, a.expr->type);
+  EXPECT_EQ(sleipnir::ExpressionType::kConstant, a.Type());
 }