diff --git a/src/main/java/com/google/api/generator/engine/ast/AssignmentExpr.java b/src/main/java/com/google/api/generator/engine/ast/AssignmentExpr.java
index 0e32d58c42..9223e41783 100644
--- a/src/main/java/com/google/api/generator/engine/ast/AssignmentExpr.java
+++ b/src/main/java/com/google/api/generator/engine/ast/AssignmentExpr.java
@@ -15,6 +15,7 @@
 package com.google.api.generator.engine.ast;
 
 import com.google.auto.value.AutoValue;
+import com.google.common.base.Preconditions;
 
 @AutoValue
 public abstract class AssignmentExpr implements Expr {
@@ -67,6 +68,15 @@ public AssignmentExpr build() {
                   lhsType.reference().name(), rhsType.reference().name()));
         }
       }
+
+      if (!assignmentExpr.variableExpr().isDecl()) {
+        Preconditions.checkState(
+            !assignmentExpr.variableExpr().isFinal(),
+            String.format(
+                "Cannot assign a value to final variable '%s'.",
+                assignmentExpr.variableExpr().variable().name()));
+      }
+
       return assignmentExpr;
     }
   }
diff --git a/src/test/java/com/google/api/generator/engine/ast/AssignmentExprTest.java b/src/test/java/com/google/api/generator/engine/ast/AssignmentExprTest.java
index 56f91f7fec..b40590f231 100644
--- a/src/test/java/com/google/api/generator/engine/ast/AssignmentExprTest.java
+++ b/src/test/java/com/google/api/generator/engine/ast/AssignmentExprTest.java
@@ -137,6 +137,32 @@ public void writeAssignmentExpr_invalidBoxedPrimitiveType() {
     assertInvalidAssignmentExpr(lVariableExpr, rVariableExpr);
   }
 
+  @Test
+  public void writeAssignmentExpr_validIsDeclFinalVariableExpr() {
+    Variable lVariable = Variable.builder().setName("x").setType(TypeNode.INT).build();
+    VariableExpr lVariableExpr =
+        VariableExpr.builder().setVariable(lVariable).setIsDecl(true).setIsFinal(true).build();
+
+    ValueExpr valueExpr =
+        ValueExpr.withValue(PrimitiveValue.builder().setType(TypeNode.INT).setValue("0").build());
+    assertValidAssignmentExpr(lVariableExpr, valueExpr);
+  }
+
+  @Test
+  public void writeAssignmentExpr_invalidIsNotDeclFinalVariableExpr() {
+    Variable lVariable = Variable.builder().setName("x").setType(TypeNode.INT).build();
+    VariableExpr lVariableExpr =
+        VariableExpr.builder().setVariable(lVariable).setIsDecl(false).setIsFinal(true).build();
+
+    ValueExpr valueExpr =
+        ValueExpr.withValue(PrimitiveValue.builder().setType(TypeNode.INT).setValue("0").build());
+    assertThrows(
+        IllegalStateException.class,
+        () -> {
+          AssignmentExpr.builder().setVariableExpr(lVariableExpr).setValueExpr(valueExpr).build();
+        });
+  }
+
   private static void assertInvalidAssignmentExpr(VariableExpr variableExpr, Expr valueExpr) {
     assertThrows(
         TypeMismatchException.class,