diff --git a/lang/src/mir/builder/MirBuilderBox.hako b/lang/src/mir/builder/MirBuilderBox.hako
index aa67054d..2978b5cc 100644
--- a/lang/src/mir/builder/MirBuilderBox.hako
+++ b/lang/src/mir/builder/MirBuilderBox.hako
@@ -29,6 +29,12 @@ static box MirBuilderBox {
 
   // Main entry
   method emit_from_program_json_v0(program_json, opts) {
+    // Debug tag (dev toggle only)
+    using "hako.mir.builder.internal.builder_config" as BuilderConfigBox
+    if BuilderConfigBox.trace_enabled() == 1 {
+      print("[mirbuilder/entry:build]")
+    }
+
     if program_json == null {
       print("[mirbuilder/input/null] program_json is null")
       return null
diff --git a/lang/src/mir/builder/internal/builder_config_box.hako b/lang/src/mir/builder/internal/builder_config_box.hako
index bfbe1143..1c9ba4b1 100644
--- a/lang/src/mir/builder/internal/builder_config_box.hako
+++ b/lang/src/mir/builder/internal/builder_config_box.hako
@@ -30,6 +30,7 @@ static box BuilderConfigBox {
 
   // Loop/JsonFrag related
   loop_jsonfrag_on() { return self._is_on("HAKO_MIR_BUILDER_LOOP_JSONFRAG") }
+  loop_force_jsonfrag_on() { return self._is_on("HAKO_MIR_BUILDER_LOOP_FORCE_JSONFRAG") }
   jsonfrag_normalize_on() { return self._is_on("HAKO_MIR_BUILDER_JSONFRAG_NORMALIZE") }
   skip_loops_on() { return self._is_on("HAKO_MIR_BUILDER_SKIP_LOOPS") }
 
diff --git a/lang/src/mir/builder/internal/loop_opts_adapter_box.hako b/lang/src/mir/builder/internal/loop_opts_adapter_box.hako
index dd9d8b15..142ffb57 100644
--- a/lang/src/mir/builder/internal/loop_opts_adapter_box.hako
+++ b/lang/src/mir/builder/internal/loop_opts_adapter_box.hako
@@ -27,6 +27,7 @@ static box LoopOptsBox {
       // Block 2 (body): i_next = i+1, jump back to loop
       // Block 3 (exit): ret i (final loop variable value)
       // Value IDs: r1=i0(0), r2=limit(N), r3=phi(i), r4=cmp, r5=i_next, r10=const(1)
+      // PHI semantics: r3 receives r1 from block 0 (entry), r5 from block 2 (backedge)
       local mir = "{\"functions\":[{\"name\":\"main\",\"params\":[],\"locals\":[],\"blocks\":[" +
         "{\"id\":0,\"instructions\":[" +
           "{\"op\":\"const\",\"dst\":1,\"value\":{\"type\":\"i64\",\"value\":0}}," +
diff --git a/tools/hakorune_emit_mir.sh b/tools/hakorune_emit_mir.sh
index 6aabf7d9..ba1a8d27 100644
--- a/tools/hakorune_emit_mir.sh
+++ b/tools/hakorune_emit_mir.sh
@@ -66,9 +66,13 @@ fi
 
 try_selfhost_builder() {
   local prog_json="$1" out_path="$2"
+
+  # Builder box selection (default: hako.mir.builder)
+  local builder_box="${HAKO_MIR_BUILDER_BOX:-hako.mir.builder}"
+
   local tmp_hako; tmp_hako=$(mktemp --suffix .hako)
   cat >"$tmp_hako" <<'HCODE'
-using "hako.mir.builder" as MirBuilderBox
+using "__BUILDER_BOX__" as MirBuilderBox
 static box Main { method main(args) {
   local prog_json = env.get("HAKO_BUILDER_PROGRAM_JSON")
   if prog_json == null { print("[builder/selfhost-first:fail:nojson]"); return 1 }
@@ -81,6 +85,8 @@ static box Main { method main(args) {
   return 0
 } }
 HCODE
+  # Substitute builder box name after heredoc to avoid shell interpolation issues
+  sed -i "s|__BUILDER_BOX__|$builder_box|g" "$tmp_hako"
   local tmp_stdout; tmp_stdout=$(mktemp)
   trap 'rm -f "$tmp_hako" "$tmp_stdout" || true' RETURN
 
@@ -96,23 +102,28 @@ HCODE
     if [ -f "$ROOT/nyash.toml" ]; then
       toml_status="present"
     fi
-    echo "[builder/selfhost-first:trace] prog_json_len=$prog_len tokens=Loop:$loop_count,Compare:$cmp_count cwd=$cwd nyash.toml=$toml_status" >&2
+    echo "[builder/selfhost-first:trace] builder_box=$builder_box prog_json_len=$prog_len tokens=Loop:$loop_count,Compare:$cmp_count cwd=$cwd nyash.toml=$toml_status" >&2
   fi
 
   set +e
   # Run from repo root to ensure nyash.toml is available for using resolution
+  # Capture both stdout and stderr (2>&1) instead of discarding stderr
   (cd "$ROOT" && \
     HAKO_MIR_BUILDER_INTERNAL=1 HAKO_MIR_BUILDER_REGISTRY=1 \
+    HAKO_MIR_BUILDER_TRACE="${HAKO_SELFHOST_TRACE:-}" \
     HAKO_MIR_BUILDER_LOOP_JSONFRAG="${HAKO_MIR_BUILDER_LOOP_JSONFRAG:-}" \
     HAKO_MIR_BUILDER_LOOP_FORCE_JSONFRAG="${HAKO_MIR_BUILDER_LOOP_FORCE_JSONFRAG:-}" \
     HAKO_MIR_BUILDER_JSONFRAG_NORMALIZE="${HAKO_MIR_BUILDER_JSONFRAG_NORMALIZE:-}" \
     HAKO_MIR_BUILDER_JSONFRAG_PURIFY="${HAKO_MIR_BUILDER_JSONFRAG_PURIFY:-}" \
     HAKO_MIR_BUILDER_NORMALIZE_TAG="${HAKO_MIR_BUILDER_NORMALIZE_TAG:-}" \
     HAKO_MIR_BUILDER_DEBUG="${HAKO_MIR_BUILDER_DEBUG:-}" \
+    NYASH_DISABLE_PLUGINS=1 NYASH_FILEBOX_MODE="core-ro" HAKO_PROVIDER_POLICY="safe-core-first" \
     NYASH_ENABLE_USING=1 HAKO_ENABLE_USING=1 \
     NYASH_PARSER_STAGE3=1 HAKO_PARSER_STAGE3=1 NYASH_PARSER_ALLOW_SEMICOLON=1 \
+    NYASH_USE_NY_COMPILER=0 HAKO_USE_NY_COMPILER=0 NYASH_DISABLE_NY_COMPILER=1 HAKO_DISABLE_NY_COMPILER=1 \
+    NYASH_MACRO_DISABLE=1 HAKO_MACRO_DISABLE=1 \
     HAKO_BUILDER_PROGRAM_JSON="$prog_json" \
-    "$NYASH_BIN" --backend vm "$tmp_hako" 2>/dev/null | tee "$tmp_stdout" >/dev/null)
+    "$NYASH_BIN" --backend vm "$tmp_hako" 2>&1 | tee "$tmp_stdout" >/dev/null)
   local rc=$?
   set -e
 
@@ -123,17 +134,34 @@ HCODE
       echo "[builder/selfhost-first:fail:detail] Last 80 lines of output:" >&2
       tail -n 80 "$tmp_stdout" >&2 || true
     fi
-    return 1
+    # Don't return immediately - check for fallback below
   fi
 
-  if ! grep -q "\[builder/selfhost-first:ok\]" "$tmp_stdout"; then
+  if [ $rc -eq 0 ] && ! grep -q "\[builder/selfhost-first:ok\]" "$tmp_stdout"; then
     if [ "${HAKO_SELFHOST_NO_DELEGATE:-0}" = "1" ]; then
       echo "[builder/selfhost-first:fail:no-ok-marker]" >&2
       echo "[builder/selfhost-first:fail:detail] Last 80 lines of output:" >&2
       tail -n 80 "$tmp_stdout" >&2 || true
     fi
+    rc=1
+  fi
+
+  # Try min builder fallback if enabled and initial builder failed
+  if [ "${HAKO_SELFHOST_TRY_MIN:-0}" = "1" ] && [ $rc -ne 0 ] && [ "$builder_box" != "hako.mir.builder.min" ]; then
+    if [ "${HAKO_SELFHOST_NO_DELEGATE:-0}" = "1" ]; then
+      echo "[builder/selfhost-first:trying-min-fallback]" >&2
+    fi
+
+    # Retry with min builder
+    HAKO_MIR_BUILDER_BOX="hako.mir.builder.min" try_selfhost_builder "$prog_json" "$out_path"
+    return $?
+  fi
+
+  # Return original failure if no fallback or if fallback not triggered
+  if [ $rc -ne 0 ]; then
     return 1
   fi
+
   local mir
   mir=$(awk '/\[MIR_OUT_BEGIN\]/{flag=1;next}/\[MIR_OUT_END\]/{flag=0}flag' "$tmp_stdout")
   if [ -z "$mir" ]; then return 1; fi
diff --git a/tools/smokes/v2/profiles/quick/core/phase2100/stageb_loop_jsonfrag_crate_exe_canary_vm.sh b/tools/smokes/v2/profiles/quick/core/phase2100/stageb_loop_jsonfrag_crate_exe_canary_vm.sh
index 236ef8a3..9f9ea601 100644
--- a/tools/smokes/v2/profiles/quick/core/phase2100/stageb_loop_jsonfrag_crate_exe_canary_vm.sh
+++ b/tools/smokes/v2/profiles/quick/core/phase2100/stageb_loop_jsonfrag_crate_exe_canary_vm.sh
@@ -59,13 +59,22 @@ if ! NYASH_LLVM_BACKEND=crate NYASH_LLVM_VERIFY=1 NYASH_LLVM_VERIFY_IR=1 \
   exit 0
 fi
 
-# Run and just ensure it executes (RC arbitrary here because structure-only lower)
+# Run and check exit code
 set +e
 "$EXE_OUT" >/dev/null 2>&1
 rc=$?
 set -e
-if [[ "$rc" -ge 0 ]]; then
+
+# Expected: rc=10 (limit value from loop)
+if [[ "$rc" -eq 10 ]]; then
   echo "[PASS] stageb_loop_jsonfrag_crate_exe_canary_vm"
   exit 0
 fi
-echo "[FAIL] stageb_loop_jsonfrag_crate_exe_canary_vm (unexpected run failure)"; exit 1
+
+# If rc != 10, this is a failure
+echo "[FAIL] stageb_loop_jsonfrag_crate_exe_canary_vm (expected rc=10, got $rc)"
+if [ -f "$IR_DUMP" ] && [ -s "$IR_DUMP" ]; then
+  echo "[DEBUG] First 120 lines of LLVM IR for diagnosis:" >&2
+  head -n 120 "$IR_DUMP" >&2 || true
+fi
+exit 1