diff --git a/lang/src/mir/builder/MirBuilderBox.hako b/lang/src/mir/builder/MirBuilderBox.hako
index eb730210..24fbd1c5 100644
--- a/lang/src/mir/builder/MirBuilderBox.hako
+++ b/lang/src/mir/builder/MirBuilderBox.hako
@@ -234,13 +234,13 @@ static box MirBuilderBox {
                 }
                 if lhs_val2 != null && rhs_val2 != null && then_val != null && else_val != null {
                   // MIR: bb0(const lhs, const rhs, compare op -> v3, branch v3 then bb1 else bb2), bb1(const then, ret), bb2(const else, ret)
-                  local mir_if = "{\"functions\":{\"Main.main\":{\"params\":[],\"locals\":[],\"blocks\":[" +
-                    "{\"label\":\"bb0\",\"instructions\":[{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":" + lhs_val2 + "}}," +
+                  local mir_if = "{\"functions\":[{\"name\":\"main\",\"params\":[],\"locals\":[],\"blocks\":[" +
+                    "{\"id\":0,\"instructions\":[{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":" + lhs_val2 + "}}," +
                     "{\"op\":\"const\",\"dst\":2,\"value\":{\"type\":\"i64\",\"value\":" + rhs_val2 + "}}," +
                     "{\"op\":\"compare\",\"operation\":\"" + op2 + "\",\"lhs\":1,\"rhs\":2,\"dst\":3}," +
                     "{\"op\":\"branch\",\"cond\":3,\"then\":1,\"else\":2}]}," +
-                    "{\"label\":\"bb1\",\"instructions\":[{\"op\":\"const\",\"dst\":4,\"value\":{\"type\":\"i64\",\"value\":" + then_val + "}},{\"op\":\"ret\",\"value\":4}]}," +
-                    "{\"label\":\"bb2\",\"instructions\":[{\"op\":\"const\",\"dst\":5,\"value\":{\"type\":\"i64\",\"value\":" + else_val + "}},{\"op\":\"ret\",\"value\":5}]}] }},\"blocks\":3}"
+                    "{\"id\":1,\"instructions\":[{\"op\":\"const\",\"dst\":4,\"value\":{\"type\":\"i64\",\"value\":" + then_val + "}},{\"op\":\"ret\",\"value\":4}]}," +
+                    "{\"id\":2,\"instructions\":[{\"op\":\"const\",\"dst\":5,\"value\":{\"type\":\"i64\",\"value\":" + else_val + "}},{\"op\":\"ret\",\"value\":5}]}]}]}"
                   return mir_if
                 }
               }
@@ -305,11 +305,11 @@ static box MirBuilderBox {
                 }
               }
               if lhs_val != null && rhs_val != null {
-                local mir_bin = "{\"functions\":{\"Main.main\":{\"params\":[],\"locals\":[],\"blocks\":[{\"label\":\"bb0\",\"instructions\":[" +
+                local mir_bin = "{\"functions\":[{\"name\":\"main\",\"params\":[],\"locals\":[],\"blocks\":[{\"id\":0,\"instructions\":[" +
                   "{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":" + lhs_val + "}}," +
                   "{\"op\":\"const\",\"dst\":2,\"value\":{\"type\":\"i64\",\"value\":" + rhs_val + "}}," +
                   "{\"op\":\"binop\",\"operation\":\"" + op + "\",\"lhs\":1,\"rhs\":2,\"dst\":3}," +
-                  "{\"op\":\"ret\",\"value\":3}]}] }},\"blocks\":1}"
+                  "{\"op\":\"ret\",\"value\":3}]}]}]}"
                 return mir_bin
               }
             }
@@ -328,7 +328,7 @@ static box MirBuilderBox {
               loop(j < n) { local ch = s.substring(j,j+1); if ch >= "0" && ch <= "9" { had = 1 j = j + 1 } else { break } }
               if had == 1 {
                 local num = s.substring(i, j)
-                local mir = "{\"functions\":{\"Main.main\":{\"params\":[],\"locals\":[],\"blocks\":[{\"label\":\"bb0\",\"instructions\":[{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":" + num + "}},{\"op\":\"ret\",\"value\":1}] }] }},\"blocks\":1}"
+                local mir = "{\"functions\":[{\"name\":\"main\",\"params\":[],\"locals\":[],\"blocks\":[{\"id\":0,\"instructions\":[{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":" + num + "}},{\"op\":\"ret\",\"value\":1}]}]}]}"
                 return mir
               }
             }
diff --git a/lang/src/mir/builder/internal/lower_if_compare_box.hako b/lang/src/mir/builder/internal/lower_if_compare_box.hako
index 56aa3bc3..c9f1425d 100644
--- a/lang/src/mir/builder/internal/lower_if_compare_box.hako
+++ b/lang/src/mir/builder/internal/lower_if_compare_box.hako
@@ -54,13 +54,14 @@ static box LowerIfCompareBox {
     local else_val = JsonFragBox.read_int_after(s, kv_else + 8)
     if else_val == null { return null }
 
-    local mir = "{\"functions\":{\"Main.main\":{\"params\":[],\"locals\":[],\"blocks\":[" +
-      "{\"label\":\"bb0\",\"instructions\":[{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":" + lhs_val + "}}," +
+    // Emit MIR v0 JSON as string (functions[]/name="main"/blocks.id)
+    local mir = "{\"functions\":[{\"name\":\"main\",\"params\":[],\"locals\":[],\"blocks\":[" +
+      "{\"id\":0,\"instructions\":[{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":" + lhs_val + "}}," +
       "{\"op\":\"const\",\"dst\":2,\"value\":{\"type\":\"i64\",\"value\":" + rhs_val + "}}," +
       "{\"op\":\"compare\",\"operation\":\"" + op + "\",\"lhs\":1,\"rhs\":2,\"dst\":3}," +
       "{\"op\":\"branch\",\"cond\":3,\"then\":1,\"else\":2}]}," +
-      "{\"label\":\"bb1\",\"instructions\":[{\"op\":\"const\",\"dst\":4,\"value\":{\"type\":\"i64\",\"value\":" + then_val + "}},{\"op\":\"ret\",\"value\":4}]}," +
-      "{\"label\":\"bb2\",\"instructions\":[{\"op\":\"const\",\"dst\":5,\"value\":{\"type\":\"i64\",\"value\":" + else_val + "}},{\"op\":\"ret\",\"value\":5}]}] }},\"blocks\":3}"
+      "{\"id\":1,\"instructions\":[{\"op\":\"const\",\"dst\":4,\"value\":{\"type\":\"i64\",\"value\":" + then_val + "}},{\"op\":\"ret\",\"value\":4}]}," +
+      "{\"id\":2,\"instructions\":[{\"op\":\"const\",\"dst\":5,\"value\":{\"type\":\"i64\",\"value\":" + else_val + "}},{\"op\":\"ret\",\"value\":5}]}]}]}"
     return mir
   }
 }