diff --git a/src/mir/basic_block.rs b/src/mir/basic_block.rs
index 37d5040e..88953c72 100644
--- a/src/mir/basic_block.rs
+++ b/src/mir/basic_block.rs
@@ -102,6 +102,20 @@ impl BasicBlock {
         }
     }
 
+    /// Add instruction before terminator (for edge-copy in PHI-off mode)
+    /// If no terminator exists, behaves like add_instruction()
+    pub fn add_instruction_before_terminator(&mut self, instruction: MirInstruction) {
+        // Update effect mask
+        self.effects = self.effects | instruction.effects();
+
+        // Non-terminator instructions always go into instructions vec
+        if !self.is_terminator(&instruction) {
+            self.instructions.push(instruction);
+        } else {
+            panic!("Cannot add terminator via add_instruction_before_terminator");
+        }
+    }
+
     /// Check if an instruction is a terminator
     fn is_terminator(&self, instruction: &MirInstruction) -> bool {
         matches!(
diff --git a/src/runner/json_v0_bridge/lowering/merge.rs b/src/runner/json_v0_bridge/lowering/merge.rs
index 3a20a312..9628dae0 100644
--- a/src/runner/json_v0_bridge/lowering/merge.rs
+++ b/src/runner/json_v0_bridge/lowering/merge.rs
@@ -35,11 +35,12 @@ pub(super) fn merge_values(
     }
     let dst = f.next_value_id();
     if no_phi {
+        // Insert edge-copy BEFORE terminator (critical fix for ValueId undefined errors)
         if let Some(bb) = f.get_block_mut(pred_a) {
-            bb.add_instruction(MirInstruction::Copy { dst, src: val_a });
+            bb.add_instruction_before_terminator(MirInstruction::Copy { dst, src: val_a });
         }
         if let Some(bb) = f.get_block_mut(pred_b) {
-            bb.add_instruction(MirInstruction::Copy { dst, src: val_b });
+            bb.add_instruction_before_terminator(MirInstruction::Copy { dst, src: val_b });
         }
     } else if let Some(bb) = f.get_block_mut(merge_bb) {
         bb.insert_instruction_after_phis(MirInstruction::Phi {