diff --git a/src/mir/control_tree/normalized_shadow/builder.rs b/src/mir/control_tree/normalized_shadow/builder.rs
index b540b379..716ea5b9 100644
--- a/src/mir/control_tree/normalized_shadow/builder.rs
+++ b/src/mir/control_tree/normalized_shadow/builder.rs
@@ -207,13 +207,13 @@ impl StepTreeNormalizedShadowLowererBox {
             env_params.clone(),
         );
 
-        // Phase 123-125: Return node lowering
-        // If Phase 123-125 patterns are not supported, return Ok(None)
+        // Phase 123-128: Return node lowering
+        // If Phase 123-128 patterns are not supported, return Ok(None)
         match Self::lower_return_from_tree(
             &step_tree.root,
             &mut main_func.body,
             &mut next_value_id,
-            &env,
+            &mut env,
             &step_tree.contract,
         ) {
             Ok(()) => {
@@ -222,9 +222,10 @@ impl StepTreeNormalizedShadowLowererBox {
             Err(msg)
                 if msg.starts_with("[phase123/")
                     || msg.starts_with("[phase124/")
-                    || msg.starts_with("[phase125/") =>
+                    || msg.starts_with("[phase125/")
+                    || msg.starts_with("[phase128/") =>
             {
-                // Phase 123-125 limitation - out of scope
+                // Phase 123-128 limitation - out of scope
                 return Ok(None);
             }
             Err(msg) => {
@@ -245,20 +246,21 @@ impl StepTreeNormalizedShadowLowererBox {
         Ok(Some((module, meta)))
     }
 
-    /// Phase 123-124 P1-P3: Lower node from StepTree
+    /// Phase 123-128 P1-P3: Lower node from StepTree
     ///
-    /// ## Support (Phase 123-124)
+    /// ## Support (Phase 123-128)
     ///
     /// - Return(Integer literal): Generate Const + Ret(Some(vid))
     /// - Return(void): Ret(None)
     /// - Return(Variable): Phase 124 - lookup from env (dev-only)
     /// - Return(other): Fail-Fast with structured error
     /// - If(minimal compare): Generate Compare + Branch + Ret (P3)
+    /// - Assign(int literal): Phase 128 - update env with Const
     fn lower_return_from_tree(
         node: &crate::mir::control_tree::step_tree::StepNode,
         body: &mut Vec<crate::mir::join_ir::JoinInst>,
         next_value_id: &mut u32,
-        env: &std::collections::BTreeMap<String, crate::mir::ValueId>,
+        env: &mut std::collections::BTreeMap<String, crate::mir::ValueId>,
         contract: &crate::mir::control_tree::step_tree_contract_box::StepTreeContract,
     ) -> Result<(), String> {
         use crate::ast::{ASTNode, LiteralValue};
@@ -271,6 +273,14 @@ impl StepTreeNormalizedShadowLowererBox {
                 // Process nodes in order
                 for n in nodes {
                     match n {
+                        StepNode::Stmt {
+                            kind: StepStmtKind::Assign { target, value_ast },
+                            ..
+                        } => {
+                            // Phase 128: Process assign statement
+                            Self::lower_assign_stmt(target, value_ast, body, next_value_id, env)?;
+                            // Continue to next node
+                        }
                         StepNode::Stmt {
                             kind: StepStmtKind::Return { value_ast },
                             ..
@@ -306,25 +316,90 @@ impl StepTreeNormalizedShadowLowererBox {
         }
     }
 
-    /// Phase 123-124 P3: Lower If node with minimal compare
+    /// Phase 128: Lower assign statement (int literal only)
+    ///
+    /// ## Support
+    ///
+    /// - target: Variable name
+    /// - value: Integer literal only (Phase 128)
+    ///
+    /// ## Not Supported (Fail-Fast)
+    ///
+    /// - value: Any expression other than Integer literal
+    ///
+    /// ## Effect
+    ///
+    /// - Generates Const instruction
+    /// - Updates env[target] to new ValueId
+    fn lower_assign_stmt(
+        target: &Option<String>,
+        value_ast: &Option<crate::mir::control_tree::step_tree::AstNodeHandle>,
+        body: &mut Vec<crate::mir::join_ir::JoinInst>,
+        next_value_id: &mut u32,
+        env: &mut std::collections::BTreeMap<String, crate::mir::ValueId>,
+    ) -> Result<(), String> {
+        use crate::ast::{ASTNode, LiteralValue};
+        use crate::mir::join_ir::{ConstValue, JoinInst, MirLikeInst};
+        use crate::mir::ValueId;
+
+        // Check target
+        let target_name = target
+            .as_ref()
+            .ok_or_else(|| "[phase128/assign/target] Assign target must be a variable".to_string())?;
+
+        // Check value_ast
+        let value_ast = value_ast.as_ref().ok_or_else(|| {
+            "[phase128/assign/value] Assign value AST is missing".to_string()
+        })?;
+
+        // Parse value - Phase 128: int literal only
+        match value_ast.0.as_ref() {
+            ASTNode::Literal {
+                value: LiteralValue::Integer(i),
+                ..
+            } => {
+                // Generate Const instruction
+                let dst_vid = ValueId(*next_value_id);
+                *next_value_id += 1;
+
+                body.push(JoinInst::Compute(MirLikeInst::Const {
+                    dst: dst_vid,
+                    value: ConstValue::Integer(*i),
+                }));
+
+                // Update env
+                env.insert(target_name.clone(), dst_vid);
+
+                Ok(())
+            }
+            _ => {
+                // Phase 128 limitation: only int literal supported
+                Err(format!(
+                    "[phase128/assign/unsupported] Phase128 supports int literal only  Hint: Assign RHS must be an integer literal (e.g., x = 42)"
+                ))
+            }
+        }
+    }
+
+    /// Phase 123-128 P3: Lower If node with minimal compare
     ///
     /// ## Support
     ///
     /// - Minimal binary comparison: Variable vs Integer literal
     /// - then/else: Return(Integer literal) or Return(Variable) (Phase 124)
-    /// - Merge: Not yet implemented (will use join_k tail-call in future)
+    /// - Merge: Phase 128 - env update + join continuation (else保持)
     ///
     /// ## Not Supported (Fail-Fast)
     ///
     /// - Compound expressions (&&, ||)
     /// - Method calls
     /// - Complex expressions
-    /// - Non-return statements in branches
+    /// - Non-return statements in branches (except Assign in Phase 128)
     fn lower_if_node(
         node: &crate::mir::control_tree::step_tree::StepNode,
         body: &mut Vec<crate::mir::join_ir::JoinInst>,
         next_value_id: &mut u32,
-        env: &std::collections::BTreeMap<String, crate::mir::ValueId>,
+        env: &mut std::collections::BTreeMap<String, crate::mir::ValueId>,
         contract: &crate::mir::control_tree::step_tree_contract_box::StepTreeContract,
     ) -> Result<(), String> {
         use crate::ast::{ASTNode, BinaryOperator};